/*

  * Copyright (c) 1994, 2011, Oracle and/or its affiliates. All rights reserved.

  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.

  *

  * This code is free software; you can redistribute it and/or modify it

  * under the terms of the GNU General Public License version 2 only, as

  * published by the Free Software Foundation.  Oracle designates this

  * particular file as subject to the "Classpath" exception as provided

  * by Oracle in the LICENSE file that accompanied this code.

  *

  * This code is distributed in the hope that it will be useful, but WITHOUT

  * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or

  * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License

  * version 2 for more details (a copy is included in the LICENSE file that

  * accompanied this code).

  *

  * You should have received a copy of the GNU General Public License version

  * 2 along with this work; if not, write to the Free Software Foundation,

  * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.

  *

  * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA

  * or visit www.oracle.com if you need additional information or have any

  * questions.

  */

 package java.lang;

 import java.io.InputStream;

 import java.io.IOException;

 import java.io.File;

 import java.lang.reflect.Constructor;

 import java.lang.reflect.InvocationTargetException;

 import java.net.MalformedURLException;

 import java.net.URL;

 import java.security.AccessController;

 import java.security.AccessControlContext;

 import java.security.CodeSource;

 import java.security.Policy;

 import java.security.PrivilegedAction;

 import java.security.PrivilegedActionException;

 import java.security.PrivilegedExceptionAction;

 import java.security.ProtectionDomain;

 import java.security.cert.Certificate;

 import java.util.Collections;

 import java.util.Enumeration;

 import java.util.HashMap;

 import java.util.HashSet;

 import java.util.Set;

 import java.util.Stack;

 import java.util.Map;

 import java.util.Vector;

 import java.util.Hashtable;

 import java.util.WeakHashMap;

 import java.util.concurrent.ConcurrentHashMap;

 import sun.misc.ClassFileTransformer;

 import sun.misc.CompoundEnumeration;

 import sun.misc.Resource;

 import sun.misc.URLClassPath;

 import sun.misc.VM;

 import sun.reflect.Reflection;

 import sun.security.util.SecurityConstants;

 /**

  * A class loader is an object that is responsible for loading classes. The

  * class <tt>ClassLoader</tt> is an abstract class.  Given the <a

  * href="#name">binary name</a> of a class, a class loader should attempt to

  * locate or generate data that constitutes a definition for the class.  A

  * typical strategy is to transform the name into a file name and then read a

  * "class file" of that name from a file system.

  *

  * <p> Every {@link Class <tt>Class</tt>} object contains a {@link

  * Class#getClassLoader() reference} to the <tt>ClassLoader</tt> that defined

  * it.

  *

  * <p> <tt>Class</tt> objects for array classes are not created by class

  * loaders, but are created automatically as required by the Java runtime.

  * The class loader for an array class, as returned by {@link

  * Class#getClassLoader()} is the same as the class loader for its element

  * type; if the element type is a primitive type, then the array class has no

  * class loader.

  *

  * <p> Applications implement subclasses of <tt>ClassLoader</tt> in order to

  * extend the manner in which the Java virtual machine dynamically loads

  * classes.

  *

  * <p> Class loaders may typically be used by security managers to indicate

  * security domains.

  *

  * <p> The <tt>ClassLoader</tt> class uses a delegation model to search for

  * classes and resources.  Each instance of <tt>ClassLoader</tt> has an

  * associated parent class loader.  When requested to find a class or

  * resource, a <tt>ClassLoader</tt> instance will delegate the search for the

  * class or resource to its parent class loader before attempting to find the

  * class or resource itself.  The virtual machine's built-in class loader,

  * called the "bootstrap class loader", does not itself have a parent but may

  * serve as the parent of a <tt>ClassLoader</tt> instance.

  *

  * <p> Class loaders that support concurrent loading of classes are known as

  * <em>parallel capable</em> class loaders and are required to register

  * themselves at their class initialization time by invoking the

  * {@link

  * #registerAsParallelCapable <tt>ClassLoader.registerAsParallelCapable</tt>}

  * method. Note that the <tt>ClassLoader</tt> class is registered as parallel

  * capable by default. However, its subclasses still need to register themselves

  * if they are parallel capable. <br>

  * In environments in which the delegation model is not strictly

  * hierarchical, class loaders need to be parallel capable, otherwise class

  * loading can lead to deadlocks because the loader lock is held for the

  * duration of the class loading process (see {@link #loadClass

  * <tt>loadClass</tt>} methods).

  *

  * <p> Normally, the Java virtual machine loads classes from the local file

  * system in a platform-dependent manner.  For example, on UNIX systems, the

  * virtual machine loads classes from the directory defined by the

  * <tt>CLASSPATH</tt> environment variable.

  *

  * <p> However, some classes may not originate from a file; they may originate

  * from other sources, such as the network, or they could be constructed by an

  * application.  The method {@link #defineClass(String, byte[], int, int)

  * <tt>defineClass</tt>} converts an array of bytes into an instance of class

  * <tt>Class</tt>. Instances of this newly defined class can be created using

  * {@link Class#newInstance <tt>Class.newInstance</tt>}.

  *

  * <p> The methods and constructors of objects created by a class loader may

  * reference other classes.  To determine the class(es) referred to, the Java

  * virtual machine invokes the {@link #loadClass <tt>loadClass</tt>} method of

  * the class loader that originally created the class.

  *

  * <p> For example, an application could create a network class loader to

  * download class files from a server.  Sample code might look like:

  *

  * <blockquote><pre>

  *   ClassLoader loader = new NetworkClassLoader(host, port);

  *   Object main = loader.loadClass("Main", true).newInstance();

  *        . . .

  * </pre></blockquote>

  *

  * <p> The network class loader subclass must define the methods {@link

  * #findClass <tt>findClass</tt>} and <tt>loadClassData</tt> to load a class

  * from the network.  Once it has downloaded the bytes that make up the class,

  * it should use the method {@link #defineClass <tt>defineClass</tt>} to

  * create a class instance.  A sample implementation is:

  *

  * <blockquote><pre>

  *     class NetworkClassLoader extends ClassLoader {

  *         String host;

  *         int port;

  *

  *         public Class findClass(String name) {

  *             byte[] b = loadClassData(name);

  *             return defineClass(name, b, 0, b.length);

  *         }

  *

  *         private byte[] loadClassData(String name) {

  *             // load the class data from the connection

  *              . . .

  *         }

  *     }

  * </pre></blockquote>

  *

  * <h4> <a name="name">Binary names</a> </h4>

  *

  * <p> Any class name provided as a {@link String} parameter to methods in

  * <tt>ClassLoader</tt> must be a binary name as defined by

  * <cite>The Java&trade; Language Specification</cite>.

  *

  * <p> Examples of valid class names include:

  * <blockquote><pre>

  *   "java.lang.String"

  *   "javax.swing.JSpinner$DefaultEditor"

  *   "java.security.KeyStore$Builder$FileBuilder$1"

  *   "java.net.URLClassLoader$3$1"

  * </pre></blockquote>

  *

  * @see      #resolveClass(Class)

  * @since 1.0

  */

 public abstract class ClassLoader {

     private static native void registerNatives();

     static {

         registerNatives();

     }

     // The parent class loader for delegation

     // Note: VM hardcoded the offset of this field, thus all new fields

     // must be added *after* it.

     private final ClassLoader parent;

     /**

      * Encapsulates the set of parallel capable loader types.

      */

     private static class ParallelLoaders {

         private ParallelLoaders() {}

         // the set of parallel capable loader types

         private static final Set<Class<? extends ClassLoader>> loaderTypes =

             Collections.newSetFromMap(

                 new WeakHashMap<Class<? extends ClassLoader>, Boolean>());

         static {

             synchronized (loaderTypes) { loaderTypes.add(ClassLoader.class); }

         }

         /**

          * Registers the given class loader type as parallel capabale.

          * Returns {@code true} is successfully registered; {@code false} if

          * loader's super class is not registered.

          */

         static boolean register(Class<? extends ClassLoader> c) {

             synchronized (loaderTypes) {

                 if (loaderTypes.contains(c.getSuperclass())) {

                     // register the class loader as parallel capable

                     // if and only if all of its super classes are.

                     // Note: given current classloading sequence, if

                     // the immediate super class is parallel capable,

                     // all the super classes higher up must be too.

                     loaderTypes.add(c);

                     return true;

                 } else {

                     return false;

                 }

             }

         }

         /**

          * Returns {@code true} if the given class loader type is

          * registered as parallel capable.

          */

         static boolean isRegistered(Class<? extends ClassLoader> c) {

             synchronized (loaderTypes) {

                 return loaderTypes.contains(c);

             }

         }

     }

     // Maps class name to the corresponding lock object when the current

     // class loader is parallel capable.

     // Note: VM also uses this field to decide if the current class loader

     // is parallel capable and the appropriate lock object for class loading.

     private final ConcurrentHashMap<String, Object> parallelLockMap;

     // Hashtable that maps packages to certs

     private final Map <String, Certificate[]> package2certs;

     // Shared among all packages with unsigned classes

     private static final Certificate[] nocerts = new Certificate[0];

     // The classes loaded by this class loader. The only purpose of this table

     // is to keep the classes from being GC'ed until the loader is GC'ed.

     private final Vector<Class<?>> classes = new Vector<>();

     // The "default" domain. Set as the default ProtectionDomain on newly

     // created classes.

     private final ProtectionDomain defaultDomain =

         new ProtectionDomain(new CodeSource(null, (Certificate[]) null),

                              null, this, null);

     // The initiating protection domains for all classes loaded by this loader

     private final Set<ProtectionDomain> domains;

     // Invoked by the VM to record every loaded class with this loader.

     void addClass(Class c) {

         classes.addElement(c);

     }

     // The packages defined in this class loader.  Each package name is mapped

     // to its corresponding Package object.

     // @GuardedBy("itself")

     private final HashMap<String, Package> packages = new HashMap<>();

     private static Void checkCreateClassLoader() {

         SecurityManager security = System.getSecurityManager();

         if (security != null) {

             security.checkCreateClassLoader();

         }

         return null;

     }

     private ClassLoader(Void unused, ClassLoader parent) {

         this.parent = parent;

         if (ParallelLoaders.isRegistered(this.getClass())) {

             parallelLockMap = new ConcurrentHashMap<>();

             package2certs = new ConcurrentHashMap<>();

             domains =

                 Collections.synchronizedSet(new HashSet<ProtectionDomain>());

             assertionLock = new Object();

         } else {

             // no finer-grained lock; lock on the classloader instance

             parallelLockMap = null;

             package2certs = new Hashtable<>();

             domains = new HashSet<>();

             assertionLock = this;

         }

     }

     /**

      * Creates a new class loader using the specified parent class loader for

      * delegation.

      *

      * <p> If there is a security manager, its {@link

      * SecurityManager#checkCreateClassLoader()

      * <tt>checkCreateClassLoader</tt>} method is invoked.  This may result in

      * a security exception.  </p>

      *

      * @param  parent

      *         The parent class loader

      *

      * @throws  SecurityException

      *          If a security manager exists and its

      *          <tt>checkCreateClassLoader</tt> method doesn't allow creation

      *          of a new class loader.

      *

      * @since  1.2

      */

     protected ClassLoader(ClassLoader parent) {

         this(checkCreateClassLoader(), parent);

     }

     /**

      * Creates a new class loader using the <tt>ClassLoader</tt> returned by

      * the method {@link #getSystemClassLoader()

      * <tt>getSystemClassLoader()</tt>} as the parent class loader.

      *

      * <p> If there is a security manager, its {@link

      * SecurityManager#checkCreateClassLoader()

      * <tt>checkCreateClassLoader</tt>} method is invoked.  This may result in

      * a security exception.  </p>

      *

      * @throws  SecurityException

      *          If a security manager exists and its

      *          <tt>checkCreateClassLoader</tt> method doesn't allow creation

      *          of a new class loader.

      */

     protected ClassLoader() {

         this(checkCreateClassLoader(), getSystemClassLoader());

     }

     // -- Class --

     /**

      * Loads the class with the specified <a href="#name">binary name</a>.

      * This method searches for classes in the same manner as the {@link

      * #loadClass(String, boolean)} method.  It is invoked by the Java virtual

      * machine to resolve class references.  Invoking this method is equivalent

      * to invoking {@link #loadClass(String, boolean) <tt>loadClass(name,

      * false)</tt>}.  </p>

      *

      * @param  name

      *         The <a href="#name">binary name</a> of the class

      *

      * @return  The resulting <tt>Class</tt> object

      *

      * @throws  ClassNotFoundException

      *          If the class was not found

      */

     public Class<?> loadClass(String name) throws ClassNotFoundException {

         return loadClass(name, false);

     }

     /**

      * Loads the class with the specified <a href="#name">binary name</a>.  The

      * default implementation of this method searches for classes in the

      * following order:

      *

      * <p><ol>

      *

      *   <li><p> Invoke {@link #findLoadedClass(String)} to check if the class

      *   has already been loaded.  </p></li>

      *

      *   <li><p> Invoke the {@link #loadClass(String) <tt>loadClass</tt>} method

      *   on the parent class loader.  If the parent is <tt>null</tt> the class

      *   loader built-in to the virtual machine is used, instead.  </p></li>

      *

      *   <li><p> Invoke the {@link #findClass(String)} method to find the

      *   class.  </p></li>

      *

      * </ol>

      *

      * <p> If the class was found using the above steps, and the

      * <tt>resolve</tt> flag is true, this method will then invoke the {@link

      * #resolveClass(Class)} method on the resulting <tt>Class</tt> object.

      *

      * <p> Subclasses of <tt>ClassLoader</tt> are encouraged to override {@link

      * #findClass(String)}, rather than this method.  </p>

      *

      * <p> Unless overridden, this method synchronizes on the result of

      * {@link #getClassLoadingLock <tt>getClassLoadingLock</tt>} method

      * during the entire class loading process.

      *

      * @param  name

      *         The <a href="#name">binary name</a> of the class

      *

      * @param  resolve

      *         If <tt>true</tt> then resolve the class

      *

      * @return  The resulting <tt>Class</tt> object

      *

      * @throws  ClassNotFoundException

      *          If the class could not be found

      */

     protected Class<?> loadClass(String name, boolean resolve)

         throws ClassNotFoundException

     {

         synchronized (getClassLoadingLock(name)) {

             // First, check if the class has already been loaded

             Class c = findLoadedClass(name);

             if (c == null) {

                 long t0 = System.nanoTime();

                 try {

                     if (parent != null) {

                         c = parent.loadClass(name, false);

                     } else {

                         c = findBootstrapClassOrNull(name);

                     }

                 } catch (ClassNotFoundException e) {

                     // ClassNotFoundException thrown if class not found

                     // from the non-null parent class loader

                 }

                 if (c == null) {

                     // If still not found, then invoke findClass in order

                     // to find the class.

                     long t1 = System.nanoTime();

                     c = findClass(name);

                     // this is the defining class loader; record the stats

                     sun.misc.PerfCounter.getParentDelegationTime().addTime(t1 - t0);

                     sun.misc.PerfCounter.getFindClassTime().addElapsedTimeFrom(t1);

                     sun.misc.PerfCounter.getFindClasses().increment();

                 }

             }

             if (resolve) {

                 resolveClass(c);

             }

             return c;

         }

     }

     /**

      * Returns the lock object for class loading operations.

      * For backward compatibility, the default implementation of this method

      * behaves as follows. If this ClassLoader object is registered as

      * parallel capable, the method returns a dedicated object associated

      * with the specified class name. Otherwise, the method returns this

      * ClassLoader object. </p>

      *

      * @param  className

      *         The name of the to-be-loaded class

      *

      * @return the lock for class loading operations

      *

      * @throws NullPointerException

      *         If registered as parallel capable and <tt>className</tt> is null

      *

      * @see #loadClass(String, boolean)

      *

      * @since  1.7

      */

     protected Object getClassLoadingLock(String className) {

         Object lock = this;

         if (parallelLockMap != null) {

             Object newLock = new Object();

             lock = parallelLockMap.putIfAbsent(className, newLock);

             if (lock == null) {

                 lock = newLock;

             }

         }

         return lock;

     }

     // This method is invoked by the virtual machine to load a class.

     private Class loadClassInternal(String name)

         throws ClassNotFoundException

     {

         // For backward compatibility, explicitly lock on 'this' when

         // the current class loader is not parallel capable.

         if (parallelLockMap == null) {

             synchronized (this) {

                  return loadClass(name);

             }

         } else {

             return loadClass(name);

         }

     }

     // Invoked by the VM after loading class with this loader.

     private void checkPackageAccess(Class cls, ProtectionDomain pd) {

         final SecurityManager sm = System.getSecurityManager();

         if (sm != null) {

             final String name = cls.getName();

             final int i = name.lastIndexOf('.');

             if (i != -1) {

                 AccessController.doPrivileged(new PrivilegedAction<Void>() {

                     public Void run() {

                         sm.checkPackageAccess(name.substring(0, i));

                         return null;

                     }

                 }, new AccessControlContext(new ProtectionDomain[] {pd}));

             }

         }

         domains.add(pd);

     }

     /**

      * Finds the class with the specified <a href="#name">binary name</a>.

      * This method should be overridden by class loader implementations that

      * follow the delegation model for loading classes, and will be invoked by

      * the {@link #loadClass <tt>loadClass</tt>} method after checking the

      * parent class loader for the requested class.  The default implementation

      * throws a <tt>ClassNotFoundException</tt>.  </p>

      *

      * @param  name

      *         The <a href="#name">binary name</a> of the class

      *

      * @return  The resulting <tt>Class</tt> object

      *

      * @throws  ClassNotFoundException

      *          If the class could not be found

      *

      * @since  1.2

      */

     protected Class<?> findClass(String name) throws ClassNotFoundException {

         throw new ClassNotFoundException(name);

     }

     /**

      * Converts an array of bytes into an instance of class <tt>Class</tt>.

      * Before the <tt>Class</tt> can be used it must be resolved.  This method

      * is deprecated in favor of the version that takes a <a

      * href="#name">binary name</a> as its first argument, and is more secure.

      *

      * @param  b

      *         The bytes that make up the class data.  The bytes in positions

      *         <tt>off</tt> through <tt>off+len-1</tt> should have the format

      *         of a valid class file as defined by

      *         <cite>The Java&trade; Virtual Machine Specification</cite>.

      *

      * @param  off

      *         The start offset in <tt>b</tt> of the class data

      *

      * @param  len

      *         The length of the class data

      *

      * @return  The <tt>Class</tt> object that was created from the specified

      *          class data

      *

      * @throws  ClassFormatError

      *          If the data did not contain a valid class

      *

      * @throws  IndexOutOfBoundsException

      *          If either <tt>off</tt> or <tt>len</tt> is negative, or if

      *          <tt>off+len</tt> is greater than <tt>b.length</tt>.

      *

      * @throws  SecurityException

      *          If an attempt is made to add this class to a package that

      *          contains classes that were signed by a different set of

      *          certificates than this class, or if an attempt is made

      *          to define a class in a package with a fully-qualified name

      *          that starts with "{@code java.}".

      *

      * @see  #loadClass(String, boolean)

      * @see  #resolveClass(Class)

      *

      * @deprecated  Replaced by {@link #defineClass(String, byte[], int, int)

      * defineClass(String, byte[], int, int)}

      */

     @Deprecated

     protected final Class<?> defineClass(byte[] b, int off, int len)

         throws ClassFormatError

     {

         return defineClass(null, b, off, len, null);

     }

     /**

      * Converts an array of bytes into an instance of class <tt>Class</tt>.

      * Before the <tt>Class</tt> can be used it must be resolved.

      *

      * <p> This method assigns a default {@link java.security.ProtectionDomain

      * <tt>ProtectionDomain</tt>} to the newly defined class.  The

      * <tt>ProtectionDomain</tt> is effectively granted the same set of

      * permissions returned when {@link

      * java.security.Policy#getPermissions(java.security.CodeSource)

      * <tt>Policy.getPolicy().getPermissions(new CodeSource(null, null))</tt>}

      * is invoked.  The default domain is created on the first invocation of

      * {@link #defineClass(String, byte[], int, int) <tt>defineClass</tt>},

      * and re-used on subsequent invocations.

      *

      * <p> To assign a specific <tt>ProtectionDomain</tt> to the class, use

      * the {@link #defineClass(String, byte[], int, int,

      * java.security.ProtectionDomain) <tt>defineClass</tt>} method that takes a

      * <tt>ProtectionDomain</tt> as one of its arguments.  </p>

      *

      * @param  name

      *         The expected <a href="#name">binary name</a> of the class, or

      *         <tt>null</tt> if not known

      *

      * @param  b

      *         The bytes that make up the class data.  The bytes in positions

      *         <tt>off</tt> through <tt>off+len-1</tt> should have the format

      *         of a valid class file as defined by

      *         <cite>The Java&trade; Virtual Machine Specification</cite>.

      *

      * @param  off

      *         The start offset in <tt>b</tt> of the class data

      *

      * @param  len

      *         The length of the class data

      *

      * @return  The <tt>Class</tt> object that was created from the specified

      *          class data.

      *

      * @throws  ClassFormatError

      *          If the data did not contain a valid class

      *

      * @throws  IndexOutOfBoundsException

      *          If either <tt>off</tt> or <tt>len</tt> is negative, or if

      *          <tt>off+len</tt> is greater than <tt>b.length</tt>.

      *

      * @throws  SecurityException

      *          If an attempt is made to add this class to a package that

      *          contains classes that were signed by a different set of

      *          certificates than this class (which is unsigned), or if

      *          <tt>name</tt> begins with "<tt>java.</tt>".

      *

      * @see  #loadClass(String, boolean)

      * @see  #resolveClass(Class)

      * @see  java.security.CodeSource

      * @see  java.security.SecureClassLoader

      *

      * @since  1.1

      */

     protected final Class<?> defineClass(String name, byte[] b, int off, int len)

         throws ClassFormatError

     {

         return defineClass(name, b, off, len, null);

     }

     /* Determine protection domain, and check that:

         - not define java.* class,

         - signer of this class matches signers for the rest of the classes in

           package.

     */

     private ProtectionDomain preDefineClass(String name,

                                             ProtectionDomain pd)

     {

         if (!checkName(name))

             throw new NoClassDefFoundError("IllegalName: " + name);

         if ((name != null) && name.startsWith("java.")) {

             throw new SecurityException

                 ("Prohibited package name: " +

                  name.substring(0, name.lastIndexOf('.')));

         }

         if (pd == null) {

             pd = defaultDomain;

         }

         if (name != null) checkCerts(name, pd.getCodeSource());

         return pd;

     }

     private String defineClassSourceLocation(ProtectionDomain pd)

     {

         CodeSource cs = pd.getCodeSource();

         String source = null;

         if (cs != null && cs.getLocation() != null) {

             source = cs.getLocation().toString();

         }

         return source;

     }

     private Class defineTransformedClass(String name, byte[] b, int off, int len,

                                          ProtectionDomain pd,

                                          ClassFormatError cfe, String source)

       throws ClassFormatError

     {

         // Class format error - try to transform the bytecode and

         // define the class again

         //

         ClassFileTransformer[] transformers =

             ClassFileTransformer.getTransformers();

         Class c = null;

         if (transformers != null) {

             for (ClassFileTransformer transformer : transformers) {

                 try {

                     // Transform byte code using transformer

                     byte[] tb = transformer.transform(b, off, len);

                     c = defineClass1(name, tb, 0, tb.length,

                                      pd, source);

                     break;

                 } catch (ClassFormatError cfe2)     {

                     // If ClassFormatError occurs, try next transformer

                 }

             }

         }

         // Rethrow original ClassFormatError if unable to transform

         // bytecode to well-formed

         //

         if (c == null)

             throw cfe;

         return c;

     }

     private void postDefineClass(Class c, ProtectionDomain pd)

     {

         if (pd.getCodeSource() != null) {

             Certificate certs[] = pd.getCodeSource().getCertificates();

             if (certs != null)

                 setSigners(c, certs);

         }

     }

     /**

      * Converts an array of bytes into an instance of class <tt>Class</tt>,

      * with an optional <tt>ProtectionDomain</tt>.  If the domain is

      * <tt>null</tt>, then a default domain will be assigned to the class as

      * specified in the documentation for {@link #defineClass(String, byte[],

      * int, int)}.  Before the class can be used it must be resolved.

      *

      * <p> The first class defined in a package determines the exact set of

      * certificates that all subsequent classes defined in that package must

      * contain.  The set of certificates for a class is obtained from the

      * {@link java.security.CodeSource <tt>CodeSource</tt>} within the

      * <tt>ProtectionDomain</tt> of the class.  Any classes added to that

      * package must contain the same set of certificates or a

      * <tt>SecurityException</tt> will be thrown.  Note that if

      * <tt>name</tt> is <tt>null</tt>, this check is not performed.

      * You should always pass in the <a href="#name">binary name</a> of the

      * class you are defining as well as the bytes.  This ensures that the

      * class you are defining is indeed the class you think it is.

      *

      * <p> The specified <tt>name</tt> cannot begin with "<tt>java.</tt>", since

      * all classes in the "<tt>java.*</tt> packages can only be defined by the

      * bootstrap class loader.  If <tt>name</tt> is not <tt>null</tt>, it

      * must be equal to the <a href="#name">binary name</a> of the class

      * specified by the byte array "<tt>b</tt>", otherwise a {@link

      * <tt>NoClassDefFoundError</tt>} will be thrown.  </p>

      *

      * @param  name

      *         The expected <a href="#name">binary name</a> of the class, or

      *         <tt>null</tt> if not known

      *

      * @param  b

      *         The bytes that make up the class data. The bytes in positions

      *         <tt>off</tt> through <tt>off+len-1</tt> should have the format

      *         of a valid class file as defined by

      *         <cite>The Java&trade; Virtual Machine Specification</cite>.

      *

      * @param  off

      *         The start offset in <tt>b</tt> of the class data

      *

      * @param  len

      *         The length of the class data

      *

      * @param  protectionDomain

      *         The ProtectionDomain of the class

      *

      * @return  The <tt>Class</tt> object created from the data,

      *          and optional <tt>ProtectionDomain</tt>.

      *

      * @throws  ClassFormatError

      *          If the data did not contain a valid class

      *

      * @throws  NoClassDefFoundError

      *          If <tt>name</tt> is not equal to the <a href="#name">binary

      *          name</a> of the class specified by <tt>b</tt>

      *

      * @throws  IndexOutOfBoundsException

      *          If either <tt>off</tt> or <tt>len</tt> is negative, or if

      *          <tt>off+len</tt> is greater than <tt>b.length</tt>.

      *

      * @throws  SecurityException

      *          If an attempt is made to add this class to a package that

      *          contains classes that were signed by a different set of

      *          certificates than this class, or if <tt>name</tt> begins with

      *          "<tt>java.</tt>".

      */

     protected final Class<?> defineClass(String name, byte[] b, int off, int len,

                                          ProtectionDomain protectionDomain)

         throws ClassFormatError

     {

         protectionDomain = preDefineClass(name, protectionDomain);

         Class c = null;

         String source = defineClassSourceLocation(protectionDomain);

         try {

             c = defineClass1(name, b, off, len, protectionDomain, source);

         } catch (ClassFormatError cfe) {

             c = defineTransformedClass(name, b, off, len, protectionDomain, cfe,

                                        source);

         }

         postDefineClass(c, protectionDomain);

         return c;

     }

     /**

      * Converts a {@link java.nio.ByteBuffer <tt>ByteBuffer</tt>}

      * into an instance of class <tt>Class</tt>,

      * with an optional <tt>ProtectionDomain</tt>.  If the domain is

      * <tt>null</tt>, then a default domain will be assigned to the class as

      * specified in the documentation for {@link #defineClass(String, byte[],

      * int, int)}.  Before the class can be used it must be resolved.

      *

      * <p>The rules about the first class defined in a package determining the

      * set of certificates for the package, and the restrictions on class names

      * are identical to those specified in the documentation for {@link

      * #defineClass(String, byte[], int, int, ProtectionDomain)}.

      *

      * <p> An invocation of this method of the form

      * <i>cl</i><tt>.defineClass(</tt><i>name</i><tt>,</tt>

      * <i>bBuffer</i><tt>,</tt> <i>pd</i><tt>)</tt> yields exactly the same

      * result as the statements

      *

      * <blockquote><tt>

      * ...<br>

      * byte[] temp = new byte[</tt><i>bBuffer</i><tt>.{@link

      * java.nio.ByteBuffer#remaining remaining}()];<br>

      *     </tt><i>bBuffer</i><tt>.{@link java.nio.ByteBuffer#get(byte[])

      * get}(temp);<br>

      *     return {@link #defineClass(String, byte[], int, int, ProtectionDomain)

      * </tt><i>cl</i><tt>.defineClass}(</tt><i>name</i><tt>, temp, 0,

      * temp.length, </tt><i>pd</i><tt>);<br>

      * </tt></blockquote>

      *

      * @param  name

      *         The expected <a href="#name">binary name</a>. of the class, or

      *         <tt>null</tt> if not known

      *

      * @param  b

      *         The bytes that make up the class data. The bytes from positions

      *         <tt>b.position()</tt> through <tt>b.position() + b.limit() -1

      *         </tt> should have the format of a valid class file as defined by

      *         <cite>The Java&trade; Virtual Machine Specification</cite>.

      *

      * @param  protectionDomain

      *         The ProtectionDomain of the class, or <tt>null</tt>.

      *

      * @return  The <tt>Class</tt> object created from the data,

      *          and optional <tt>ProtectionDomain</tt>.

      *

      * @throws  ClassFormatError

      *          If the data did not contain a valid class.

      *

      * @throws  NoClassDefFoundError

      *          If <tt>name</tt> is not equal to the <a href="#name">binary

      *          name</a> of the class specified by <tt>b</tt>

      *

      * @throws  SecurityException

      *          If an attempt is made to add this class to a package that

      *          contains classes that were signed by a different set of

      *          certificates than this class, or if <tt>name</tt> begins with

      *          "<tt>java.</tt>".

      *

      * @see      #defineClass(String, byte[], int, int, ProtectionDomain)

      *

      * @since  1.5

      */

     protected final Class<?> defineClass(String name, java.nio.ByteBuffer b,

                                          ProtectionDomain protectionDomain)

         throws ClassFormatError

     {

         int len = b.remaining();

         // Use byte[] if not a direct ByteBufer:

         if (!b.isDirect()) {

             if (b.hasArray()) {

                 return defineClass(name, b.array(),

                                    b.position() + b.arrayOffset(), len,

                                    protectionDomain);

             } else {

                 // no array, or read-only array

                 byte[] tb = new byte[len];

                 b.get(tb);  // get bytes out of byte buffer.

                 return defineClass(name, tb, 0, len, protectionDomain);

             }

         }

         protectionDomain = preDefineClass(name, protectionDomain);

         Class c = null;

         String source = defineClassSourceLocation(protectionDomain);

         try {

             c = defineClass2(name, b, b.position(), len, protectionDomain,

                              source);

         } catch (ClassFormatError cfe) {

             byte[] tb = new byte[len];

             b.get(tb);  // get bytes out of byte buffer.

             c = defineTransformedClass(name, tb, 0, len, protectionDomain, cfe,

                                        source);

         }

         postDefineClass(c, protectionDomain);

         return c;

     }

     private native Class defineClass0(String name, byte[] b, int off, int len,

                                       ProtectionDomain pd);

     private native Class defineClass1(String name, byte[] b, int off, int len,

                                       ProtectionDomain pd, String source);

     private native Class defineClass2(String name, java.nio.ByteBuffer b,

                                       int off, int len, ProtectionDomain pd,

                                       String source);

     // true if the name is null or has the potential to be a valid binary name

     private boolean checkName(String name) {

         if ((name == null) || (name.length() == 0))

             return true;

         if ((name.indexOf('/') != -1)

             || (!VM.allowArraySyntax() && (name.charAt(0) == '[')))

             return false;

         return true;

     }

     private void checkCerts(String name, CodeSource cs) {

         int i = name.lastIndexOf('.');

         String pname = (i == -1) ? "" : name.substring(0, i);

         Certificate[] certs = null;

         if (cs != null) {

             certs = cs.getCertificates();

         }

         Certificate[] pcerts = null;

         if (parallelLockMap == null) {

             synchronized (this) {

                 pcerts = package2certs.get(pname);

                 if (pcerts == null) {

                     package2certs.put(pname, (certs == null? nocerts:certs));

                 }

             }

         } else {

             pcerts = ((ConcurrentHashMap<String, Certificate[]>)package2certs).

                 putIfAbsent(pname, (certs == null? nocerts:certs));

         }

         if (pcerts != null && !compareCerts(pcerts, certs)) {

             throw new SecurityException("class \""+ name +

                  "\"'s signer information does not match signer information of other classes in the same package");

         }

     }

     /**

      * check to make sure the certs for the new class (certs) are the same as

      * the certs for the first class inserted in the package (pcerts)

      */

     private boolean compareCerts(Certificate[] pcerts,

                                  Certificate[] certs)

     {

         // certs can be null, indicating no certs.

         if ((certs == null) || (certs.length == 0)) {

             return pcerts.length == 0;

         }

         // the length must be the same at this point

         if (certs.length != pcerts.length)

             return false;

         // go through and make sure all the certs in one array

         // are in the other and vice-versa.

         boolean match;

         for (int i = 0; i < certs.length; i++) {

             match = false;

             for (int j = 0; j < pcerts.length; j++) {

                 if (certs[i].equals(pcerts[j])) {

                     match = true;

                     break;

                 }

             }

             if (!match) return false;

         }

         // now do the same for pcerts

         for (int i = 0; i < pcerts.length; i++) {

             match = false;

             for (int j = 0; j < certs.length; j++) {

                 if (pcerts[i].equals(certs[j])) {

                     match = true;

                     break;

                 }

             }

             if (!match) return false;

         }

         return true;

     }

     /**

      * Links the specified class.  This (misleadingly named) method may be

      * used by a class loader to link a class.  If the class <tt>c</tt> has

      * already been linked, then this method simply returns. Otherwise, the

      * class is linked as described in the "Execution" chapter of

      * <cite>The Java&trade; Language Specification</cite>.

      * </p>

      *

      * @param  c

      *         The class to link

      *

      * @throws  NullPointerException

      *          If <tt>c</tt> is <tt>null</tt>.

      *

      * @see  #defineClass(String, byte[], int, int)

      */

     protected final void resolveClass(Class<?> c) {

         resolveClass0(c);

     }

     private native void resolveClass0(Class c);

     /**

      * Finds a class with the specified <a href="#name">binary name</a>,

      * loading it if necessary.

      *

      * <p> This method loads the class through the system class loader (see

      * {@link #getSystemClassLoader()}).  The <tt>Class</tt> object returned

      * might have more than one <tt>ClassLoader</tt> associated with it.

      * Subclasses of <tt>ClassLoader</tt> need not usually invoke this method,

      * because most class loaders need to override just {@link

      * #findClass(String)}.  </p>

      *

      * @param  name

      *         The <a href="#name">binary name</a> of the class

      *

      * @return  The <tt>Class</tt> object for the specified <tt>name</tt>

      *

      * @throws  ClassNotFoundException

      *          If the class could not be found

      *

      * @see  #ClassLoader(ClassLoader)

      * @see  #getParent()

      */

     protected final Class<?> findSystemClass(String name)

         throws ClassNotFoundException

     {

         ClassLoader system = getSystemClassLoader();

         if (system == null) {

             if (!checkName(name))

                 throw new ClassNotFoundException(name);

             Class cls = findBootstrapClass(name);

             if (cls == null) {

                 throw new ClassNotFoundException(name);

             }

             return cls;

         }

         return system.loadClass(name);

     }

     /**

      * Returns a class loaded by the bootstrap class loader;

      * or return null if not found.

      */

     private Class findBootstrapClassOrNull(String name)

     {

         if (!checkName(name)) return null;

         return findBootstrapClass(name);

     }

     // return null if not found

     private native Class findBootstrapClass(String name);

     /**

      * Returns the class with the given <a href="#name">binary name</a> if this

      * loader has been recorded by the Java virtual machine as an initiating

      * loader of a class with that <a href="#name">binary name</a>.  Otherwise

      * <tt>null</tt> is returned.  </p>

      *

      * @param  name

      *         The <a href="#name">binary name</a> of the class

      *

      * @return  The <tt>Class</tt> object, or <tt>null</tt> if the class has

      *          not been loaded

      *

      * @since  1.1

      */

     protected final Class<?> findLoadedClass(String name) {

         if (!checkName(name))

             return null;

         return findLoadedClass0(name);

     }

     private native final Class findLoadedClass0(String name);

     /**

      * Sets the signers of a class.  This should be invoked after defining a

      * class.  </p>

      *

      * @param  c

      *         The <tt>Class</tt> object

      *

      * @param  signers

      *         The signers for the class

      *

      * @since  1.1

      */

     protected final void setSigners(Class<?> c, Object[] signers) {

         c.setSigners(signers);

     }

     // -- Resource --

     /**

      * Finds the resource with the given name.  A resource is some data

      * (images, audio, text, etc) that can be accessed by class code in a way

      * that is independent of the location of the code.

      *

      * <p> The name of a resource is a '<tt>/</tt>'-separated path name that

      * identifies the resource.

      *

      * <p> This method will first search the parent class loader for the

      * resource; if the parent is <tt>null</tt> the path of the class loader

      * built-in to the virtual machine is searched.  That failing, this method

      * will invoke {@link #findResource(String)} to find the resource.  </p>

      *

      * @param  name

      *         The resource name

      *

      * @return  A <tt>URL</tt> object for reading the resource, or

      *          <tt>null</tt> if the resource could not be found or the invoker

      *          doesn't have adequate  privileges to get the resource.

      *

      * @since  1.1

      */

     public URL getResource(String name) {

         URL url;

         if (parent != null) {

             url = parent.getResource(name);

         } else {

             url = getBootstrapResource(name);

         }

         if (url == null) {

             url = findResource(name);

         }

         return url;

     }

     /**

      * Finds all the resources with the given name. A resource is some data

      * (images, audio, text, etc) that can be accessed by class code in a way

      * that is independent of the location of the code.

      *

      * <p>The name of a resource is a <tt>/</tt>-separated path name that

      * identifies the resource.

      *

      * <p> The search order is described in the documentation for {@link

      * #getResource(String)}.  </p>

      *

      * @param  name

      *         The resource name

      *

      * @return  An enumeration of {@link java.net.URL <tt>URL</tt>} objects for

      *          the resource.  If no resources could  be found, the enumeration

      *          will be empty.  Resources that the class loader doesn't have

      *          access to will not be in the enumeration.

      *

      * @throws  IOException

      *          If I/O errors occur

      *

      * @see  #findResources(String)

      *

      * @since  1.2

      */

     public Enumeration<URL> getResources(String name) throws IOException {

         Enumeration[] tmp = new Enumeration[2];

         if (parent != null) {

             tmp[0] = parent.getResources(name);

         } else {

             tmp[0] = getBootstrapResources(name);

         }

         tmp[1] = findResources(name);

         return new CompoundEnumeration<>(tmp);

     }

     /**

      * Finds the resource with the given name. Class loader implementations

      * should override this method to specify where to find resources.  </p>

      *

      * @param  name

      *         The resource name

      *

      * @return  A <tt>URL</tt> object for reading the resource, or

      *          <tt>null</tt> if the resource could not be found

      *

      * @since  1.2

      */

     protected URL findResource(String name) {

         return null;

     }

     /**

      * Returns an enumeration of {@link java.net.URL <tt>URL</tt>} objects

      * representing all the resources with the given name. Class loader

      * implementations should override this method to specify where to load

      * resources from.  </p>

      *

      * @param  name

      *         The resource name

      *

      * @return  An enumeration of {@link java.net.URL <tt>URL</tt>} objects for

      *          the resources

      *

      * @throws  IOException

      *          If I/O errors occur

      *

      * @since  1.2

      */

     protected Enumeration<URL> findResources(String name) throws IOException {

         return java.util.Collections.emptyEnumeration();

     }

     // index 0: java.lang.ClassLoader.class

     // index 1: the immediate caller of index 0.

     // index 2: the immediate caller of index 1.

     private static native Class<? extends ClassLoader> getCaller(int index);

     /**

      * Registers the caller as parallel capable.</p>

      * The registration succeeds if and only if all of the following

      * conditions are met: <br>

      * 1. no instance of the caller has been created</p>

      * 2. all of the super classes (except class Object) of the caller are

      * registered as parallel capable</p>

      * Note that once a class loader is registered as parallel capable, there

      * is no way to change it back. </p>

      *

      * @return  true if the caller is successfully registered as

      *          parallel capable and false if otherwise.

      *

      * @since   1.7

      */

     protected static boolean registerAsParallelCapable() {

         return ParallelLoaders.register(getCaller(1));

     }

     /**

      * Find a resource of the specified name from the search path used to load

      * classes.  This method locates the resource through the system class

      * loader (see {@link #getSystemClassLoader()}).  </p>

      *

      * @param  name

      *         The resource name

      *

      * @return  A {@link java.net.URL <tt>URL</tt>} object for reading the

      *          resource, or <tt>null</tt> if the resource could not be found

      *

      * @since  1.1

      */

     public static URL getSystemResource(String name) {

         ClassLoader system = getSystemClassLoader();

         if (system == null) {

             return getBootstrapResource(name);

         }

         return system.getResource(name);

     }

     /**

      * Finds all resources of the specified name from the search path used to

      * load classes.  The resources thus found are returned as an

      * {@link java.util.Enumeration <tt>Enumeration</tt>} of {@link

      * java.net.URL <tt>URL</tt>} objects.

      *

      * <p> The search order is described in the documentation for {@link

      * #getSystemResource(String)}.  </p>

      *

      * @param  name

      *         The resource name

      *

      * @return  An enumeration of resource {@link java.net.URL <tt>URL</tt>}

      *          objects

      *

      * @throws  IOException

      *          If I/O errors occur

      * @since  1.2

      */

     public static Enumeration<URL> getSystemResources(String name)

         throws IOException

     {

         ClassLoader system = getSystemClassLoader();

         if (system == null) {

             return getBootstrapResources(name);

         }

         return system.getResources(name);

     }

     /**

      * Find resources from the VM's built-in classloader.

      */

     private static URL getBootstrapResource(String name) {

         URLClassPath ucp = getBootstrapClassPath();

         Resource res = ucp.getResource(name);

         return res != null ? res.getURL() : null;

     }

     /**

      * Find resources from the VM's built-in classloader.

      */

     private static Enumeration<URL> getBootstrapResources(String name)

         throws IOException

     {

         final Enumeration<Resource> e =

             getBootstrapClassPath().getResources(name);

         return new Enumeration<URL> () {

             public URL nextElement() {

                 return e.nextElement().getURL();

             }

             public boolean hasMoreElements() {

                 return e.hasMoreElements();

             }

         };

     }

     // Returns the URLClassPath that is used for finding system resources.

     static URLClassPath getBootstrapClassPath() {

         return sun.misc.Launcher.getBootstrapClassPath();

     }

     /**

      * Returns an input stream for reading the specified resource.

      *

      * <p> The search order is described in the documentation for {@link

      * #getResource(String)}.  </p>

      *

      * @param  name

      *         The resource name

      *

      * @return  An input stream for reading the resource, or <tt>null</tt>

      *          if the resource could not be found

      *

      * @since  1.1

      */

     public InputStream getResourceAsStream(String name) {

         URL url = getResource(name);

         try {

             return url != null ? url.openStream() : null;

         } catch (IOException e) {

             return null;

         }

     }

     /**

      * Open for reading, a resource of the specified name from the search path

      * used to load classes.  This method locates the resource through the

      * system class loader (see {@link #getSystemClassLoader()}).  </p>

      *

      * @param  name

      *         The resource name

      *

      * @return  An input stream for reading the resource, or <tt>null</tt>

      *          if the resource could not be found

      *

      * @since  1.1

      */

     public static InputStream getSystemResourceAsStream(String name) {

         URL url = getSystemResource(name);

         try {

             return url != null ? url.openStream() : null;

         } catch (IOException e) {

             return null;

         }

     }

     // -- Hierarchy --

     /**

      * Returns the parent class loader for delegation. Some implementations may

      * use <tt>null</tt> to represent the bootstrap class loader. This method

      * will return <tt>null</tt> in such implementations if this class loader's

      * parent is the bootstrap class loader.

      *

      * <p> If a security manager is present, and the invoker's class loader is

      * not <tt>null</tt> and is not an ancestor of this class loader, then this

      * method invokes the security manager's {@link

      * SecurityManager#checkPermission(java.security.Permission)

      * <tt>checkPermission</tt>} method with a {@link

      * RuntimePermission#RuntimePermission(String)

      * <tt>RuntimePermission("getClassLoader")</tt>} permission to verify

      * access to the parent class loader is permitted.  If not, a

      * <tt>SecurityException</tt> will be thrown.  </p>

      *

      * @return  The parent <tt>ClassLoader</tt>

      *

      * @throws  SecurityException

      *          If a security manager exists and its <tt>checkPermission</tt>

      *          method doesn't allow access to this class loader's parent class

      *          loader.

      *

      * @since  1.2

      */

     public final ClassLoader getParent() {

         if (parent == null)

             return null;

         SecurityManager sm = System.getSecurityManager();

         if (sm != null) {

             ClassLoader ccl = getCallerClassLoader();

             if (ccl != null && !isAncestor(ccl)) {

                 sm.checkPermission(SecurityConstants.GET_CLASSLOADER_PERMISSION);

             }

         }

         return parent;

     }

     /**

      * Returns the system class loader for delegation.  This is the default

      * delegation parent for new <tt>ClassLoader</tt> instances, and is

      * typically the class loader used to start the application.

      *

      * <p> This method is first invoked early in the runtime's startup

      * sequence, at which point it creates the system class loader and sets it

      * as the context class loader of the invoking <tt>Thread</tt>.

      *

      * <p> The default system class loader is an implementation-dependent

      * instance of this class.

      *

      * <p> If the system property "<tt>java.system.class.loader</tt>" is defined

      * when this method is first invoked then the value of that property is

      * taken to be the name of a class that will be returned as the system

      * class loader.  The class is loaded using the default system class loader

      * and must define a public constructor that takes a single parameter of

      * type <tt>ClassLoader</tt> which is used as the delegation parent.  An

      * instance is then created using this constructor with the default system

      * class loader as the parameter.  The resulting class loader is defined

      * to be the system class loader.

      *

      * <p> If a security manager is present, and the invoker's class loader is

      * not <tt>null</tt> and the invoker's class loader is not the same as or

      * an ancestor of the system class loader, then this method invokes the

      * security manager's {@link

      * SecurityManager#checkPermission(java.security.Permission)

      * <tt>checkPermission</tt>} method with a {@link

      * RuntimePermission#RuntimePermission(String)

      * <tt>RuntimePermission("getClassLoader")</tt>} permission to verify

      * access to the system class loader.  If not, a

      * <tt>SecurityException</tt> will be thrown.  </p>

      *

      * @return  The system <tt>ClassLoader</tt> for delegation, or

      *          <tt>null</tt> if none

      *

      * @throws  SecurityException

      *          If a security manager exists and its <tt>checkPermission</tt>

      *          method doesn't allow access to the system class loader.

      *

      * @throws  IllegalStateException

      *          If invoked recursively during the construction of the class

      *          loader specified by the "<tt>java.system.class.loader</tt>"

      *          property.

      *

      * @throws  Error

      *          If the system property "<tt>java.system.class.loader</tt>"

      *          is defined but the named class could not be loaded, the

      *          provider class does not define the required constructor, or an

      *          exception is thrown by that constructor when it is invoked. The

      *          underlying cause of the error can be retrieved via the

      *          {@link Throwable#getCause()} method.

      *

      * @revised  1.4

      */

     public static ClassLoader getSystemClassLoader() {

         initSystemClassLoader();

         if (scl == null) {

             return null;

         }

         SecurityManager sm = System.getSecurityManager();

         if (sm != null) {

             ClassLoader ccl = getCallerClassLoader();

             if (ccl != null && ccl != scl && !scl.isAncestor(ccl)) {

                 sm.checkPermission(SecurityConstants.GET_CLASSLOADER_PERMISSION);

             }

         }

         return scl;

     }

     private static synchronized void initSystemClassLoader() {

         if (!sclSet) {

             if (scl != null)

                 throw new IllegalStateException("recursive invocation");

             sun.misc.Launcher l = sun.misc.Launcher.getLauncher();

             if (l != null) {

                 Throwable oops = null;

                 scl = l.getClassLoader();

                 try {

                     scl = AccessController.doPrivileged(

                         new SystemClassLoaderAction(scl));

                 } catch (PrivilegedActionException pae) {

                     oops = pae.getCause();

                     if (oops instanceof InvocationTargetException) {

                         oops = oops.getCause();

                     }

                 }

                 if (oops != null) {

                     if (oops instanceof Error) {

                         throw (Error) oops;

                     } else {

                         // wrap the exception

                         throw new Error(oops);

                     }

                 }

             }

             sclSet = true;

         }

     }

     // Returns true if the specified class loader can be found in this class

     // loader's delegation chain.

     boolean isAncestor(ClassLoader cl) {

         ClassLoader acl = this;

         do {

             acl = acl.parent;

             if (cl == acl) {

                 return true;

             }

         } while (acl != null);

         return false;

     }

     // Returns the invoker's class loader, or null if none.

     // NOTE: This must always be invoked when there is exactly one intervening

     // frame from the core libraries on the stack between this method's

     // invocation and the desired invoker.

     static ClassLoader getCallerClassLoader() {

         // NOTE use of more generic Reflection.getCallerClass()

         Class caller = Reflection.getCallerClass(3);

         // This can be null if the VM is requesting it

         if (caller == null) {

             return null;

         }

         // Circumvent security check since this is package-private

         return caller.getClassLoader0();

     }

     // The class loader for the system

     // @GuardedBy("ClassLoader.class")

     private static ClassLoader scl;

     // Set to true once the system class loader has been set

     // @GuardedBy("ClassLoader.class")

     private static boolean sclSet;

     // -- Package --

     /**

      * Defines a package by name in this <tt>ClassLoader</tt>.  This allows

      * class loaders to define the packages for their classes. Packages must

      * be created before the class is defined, and package names must be

      * unique within a class loader and cannot be redefined or changed once

      * created.  </p>

      *

      * @param  name

      *         The package name

      *

      * @param  specTitle

      *         The specification title

      *

      * @param  specVersion

      *         The specification version

      *

      * @param  specVendor

      *         The specification vendor

      *

      * @param  implTitle

      *         The implementation title

      *

      * @param  implVersion

      *         The implementation version

      *

      * @param  implVendor

      *         The implementation vendor

      *

      * @param  sealBase

      *         If not <tt>null</tt>, then this package is sealed with

      *         respect to the given code source {@link java.net.URL

      *         <tt>URL</tt>}  object.  Otherwise, the package is not sealed.

      *

      * @return  The newly defined <tt>Package</tt> object

      *

      * @throws  IllegalArgumentException

      *          If package name duplicates an existing package either in this

      *          class loader or one of its ancestors

      *

      * @since  1.2

      */

     protected Package definePackage(String name, String specTitle,

                                     String specVersion, String specVendor,

                                     String implTitle, String implVersion,

                                     String implVendor, URL sealBase)

         throws IllegalArgumentException

     {

         synchronized (packages) {

             Package pkg = getPackage(name);

             if (pkg != null) {

                 throw new IllegalArgumentException(name);

             }

             pkg = new Package(name, specTitle, specVersion, specVendor,

                               implTitle, implVersion, implVendor,

                               sealBase, this);

             packages.put(name, pkg);

             return pkg;

         }

     }

     /**

      * Returns a <tt>Package</tt> that has been defined by this class loader

      * or any of its ancestors.  </p>

      *

      * @param  name

      *         The package name

      *

      * @return  The <tt>Package</tt> corresponding to the given name, or

      *          <tt>null</tt> if not found

      *

      * @since  1.2

      */

     protected Package getPackage(String name) {

         Package pkg;

         synchronized (packages) {

             pkg = packages.get(name);

         }

         if (pkg == null) {

             if (parent != null) {

                 pkg = parent.getPackage(name);

             } else {

                 pkg = Package.getSystemPackage(name);

             }

             if (pkg != null) {

                 synchronized (packages) {

                     Package pkg2 = packages.get(name);

                     if (pkg2 == null) {

                         packages.put(name, pkg);

                     } else {

                         pkg = pkg2;

                     }

                 }

             }

         }

         return pkg;

     }

     /**

      * Returns all of the <tt>Packages</tt> defined by this class loader and

      * its ancestors.  </p>

      *

      * @return  The array of <tt>Package</tt> objects defined by this

      *          <tt>ClassLoader</tt>

      *

      * @since  1.2

      */

     protected Package[] getPackages() {

         Map<String, Package> map;

         synchronized (packages) {

             map = new HashMap<>(packages);

         }

         Package[] pkgs;

         if (parent != null) {

             pkgs = parent.getPackages();

         } else {

             pkgs = Package.getSystemPackages();

         }

         if (pkgs != null) {

             for (int i = 0; i < pkgs.length; i++) {

                 String pkgName = pkgs[i].getName();

                 if (map.get(pkgName) == null) {

                     map.put(pkgName, pkgs[i]);

                 }

             }

         }

         return map.values().toArray(new Package[map.size()]);

     }

     // -- Native library access --

     /**

      * Returns the absolute path name of a native library.  The VM invokes this

      * method to locate the native libraries that belong to classes loaded with

      * this class loader. If this method returns <tt>null</tt>, the VM

      * searches the library along the path specified as the

      * "<tt>java.library.path</tt>" property.  </p>

      *

      * @param  libname

      *         The library name

      *

      * @return  The absolute path of the native library

      *

      * @see  System#loadLibrary(String)

      * @see  System#mapLibraryName(String)

      *

      * @since  1.2

      */

     protected String findLibrary(String libname) {

         return null;

     }

     /**

      * The inner class NativeLibrary denotes a loaded native library instance.

      * Every classloader contains a vector of loaded native libraries in the

      * private field <tt>nativeLibraries</tt>.  The native libraries loaded

      * into the system are entered into the <tt>systemNativeLibraries</tt>

      * vector.

      *

      * <p> Every native library requires a particular version of JNI. This is

      * denoted by the private <tt>jniVersion</tt> field.  This field is set by

      * the VM when it loads the library, and used by the VM to pass the correct

      * version of JNI to the native methods.  </p>

      *

      * @see      ClassLoader

      * @since    1.2

      */

     static class NativeLibrary {

         // opaque handle to native library, used in native code.

         long handle;

         // the version of JNI environment the native library requires.

         private int jniVersion;

         // the class from which the library is loaded, also indicates

         // the loader this native library belongs.

         private Class fromClass;

         // the canonicalized name of the native library.

         String name;

         native void load(String name);

         native long find(String name);

         native void unload();

         public NativeLibrary(Class fromClass, String name) {

             this.name = name;

             this.fromClass = fromClass;

         }

         protected void finalize() {

             synchronized (loadedLibraryNames) {

                 if (fromClass.getClassLoader() != null && handle != 0) {

                     /* remove the native library name */

                     int size = loadedLibraryNames.size();

                     for (int i = 0; i < size; i++) {

                         if (name.equals(loadedLibraryNames.elementAt(i))) {

                             loadedLibraryNames.removeElementAt(i);

                             break;

                         }

                     }

                     /* unload the library. */

                     ClassLoader.nativeLibraryContext.push(this);

                     try {

                         unload();

                     } finally {

                         ClassLoader.nativeLibraryContext.pop();

                     }

                 }

             }

         }

         // Invoked in the VM to determine the context class in

         // JNI_Load/JNI_Unload

         static Class getFromClass() {

             return ClassLoader.nativeLibraryContext.peek().fromClass;

         }

     }

     // All native library names we've loaded.

     private static Vector<String> loadedLibraryNames = new Vector<>();

     // Native libraries belonging to system classes.

     private static Vector<NativeLibrary> systemNativeLibraries

         = new Vector<>();

     // Native libraries associated with the class loader.

     private Vector<NativeLibrary> nativeLibraries = new Vector<>();

     // native libraries being loaded/unloaded.

     private static Stack<NativeLibrary> nativeLibraryContext = new Stack<>();

     // The paths searched for libraries

     private static String usr_paths[];

     private static String sys_paths[];

     private static String[] initializePath(String propname) {

         String ldpath = System.getProperty(propname, "");

         String ps = File.pathSeparator;

         int ldlen = ldpath.length();

         int i, j, n;

         // Count the separators in the path

         i = ldpath.indexOf(ps);

         n = 0;

         while (i >= 0) {

             n++;

             i = ldpath.indexOf(ps, i + 1);

         }

         // allocate the array of paths - n :'s = n + 1 path elements

         String[] paths = new String[n + 1];

         // Fill the array with paths from the ldpath

         n = i = 0;

         j = ldpath.indexOf(ps);

         while (j >= 0) {

             if (j - i > 0) {

                 paths[n++] = ldpath.substring(i, j);

             } else if (j - i == 0) {

                 paths[n++] = ".";

             }

             i = j + 1;

             j = ldpath.indexOf(ps, i);

         }

         paths[n] = ldpath.substring(i, ldlen);

         return paths;

     }

     // Invoked in the java.lang.Runtime class to implement load and loadLibrary.

     static void loadLibrary(Class fromClass, String name,

                             boolean isAbsolute) {

         ClassLoader loader =

             (fromClass == null) ? null : fromClass.getClassLoader();

         if (sys_paths == null) {

             usr_paths = initializePath("java.library.path");

             sys_paths = initializePath("sun.boot.library.path");

         }

         if (isAbsolute) {

             if (loadLibrary0(fromClass, new File(name))) {

                 return;

             }

             throw new UnsatisfiedLinkError("Can't load library: " + name);

         }

         if (loader != null) {

             String libfilename = loader.findLibrary(name);

             if (libfilename != null) {

                 File libfile = new File(libfilename);

                 if (!libfile.isAbsolute()) {

                     throw new UnsatisfiedLinkError(

     "ClassLoader.findLibrary failed to return an absolute path: " + libfilename);

                 }

                 if (loadLibrary0(fromClass, libfile)) {

                     return;

                 }

                 throw new UnsatisfiedLinkError("Can't load " + libfilename);

             }

         }

         for (int i = 0 ; i < sys_paths.length ; i++) {

             File libfile = new File(sys_paths[i], System.mapLibraryName(name));

             if (loadLibrary0(fromClass, libfile)) {

                 return;

             }

         }

         if (loader != null) {

             for (int i = 0 ; i < usr_paths.length ; i++) {

                 File libfile = new File(usr_paths[i],

                                         System.mapLibraryName(name));

                 if (loadLibrary0(fromClass, libfile)) {

                     return;

                 }

             }

         }

         // Oops, it failed

         throw new UnsatisfiedLinkError("no " + name + " in java.library.path");

     }

     private static boolean loadLibrary0(Class fromClass, final File file) {

         boolean exists = AccessController.doPrivileged(

             new PrivilegedAction<Object>() {

                 public Object run() {

                     return file.exists() ? Boolean.TRUE : null;

                 }})

             != null;

         if (!exists) {

             return false;

         }

         String name;

         try {

             name = file.getCanonicalPath();

         } catch (IOException e) {

             return false;

         }

         ClassLoader loader =

             (fromClass == null) ? null : fromClass.getClassLoader();

         Vector<NativeLibrary> libs =

             loader != null ? loader.nativeLibraries : systemNativeLibraries;

         synchronized (libs) {

             int size = libs.size();

             for (int i = 0; i < size; i++) {

                 NativeLibrary lib = libs.elementAt(i);

                 if (name.equals(lib.name)) {

                     return true;

                 }

             }

             synchronized (loadedLibraryNames) {

                 if (loadedLibraryNames.contains(name)) {

                     throw new UnsatisfiedLinkError

                         ("Native Library " +

                          name +

                          " already loaded in another classloader");

                 }

                 /* If the library is being loaded (must be by the same thread,

                  * because Runtime.load and Runtime.loadLibrary are

                  * synchronous). The reason is can occur is that the JNI_OnLoad

                  * function can cause another loadLibrary invocation.

                  *

                  * Thus we can use a static stack to hold the list of libraries

                  * we are loading.

                  *

                  * If there is a pending load operation for the library, we

                  * immediately return success; otherwise, we raise

                  * UnsatisfiedLinkError.

                  */

                 int n = nativeLibraryContext.size();

                 for (int i = 0; i < n; i++) {

                     NativeLibrary lib = nativeLibraryContext.elementAt(i);

                     if (name.equals(lib.name)) {

                         if (loader == lib.fromClass.getClassLoader()) {

                             return true;

                         } else {

                             throw new UnsatisfiedLinkError

                                 ("Native Library " +

                                  name +

                                  " is being loaded in another classloader");

                         }

                     }

                 }

                 NativeLibrary lib = new NativeLibrary(fromClass, name);

                 nativeLibraryContext.push(lib);

                 try {

                     lib.load(name);

                 } finally {

                     nativeLibraryContext.pop();

                 }

                 if (lib.handle != 0) {

                     loadedLibraryNames.addElement(name);

                     libs.addElement(lib);

                     return true;

                 }

                 return false;

             }

         }

     }

     // Invoked in the VM class linking code.

     static long findNative(ClassLoader loader, String name) {

         Vector<NativeLibrary> libs =

             loader != null ? loader.nativeLibraries : systemNativeLibraries;

         synchronized (libs) {

             int size = libs.size();

             for (int i = 0; i < size; i++) {

                 NativeLibrary lib = libs.elementAt(i);

                 long entry = lib.find(name);

                 if (entry != 0)

                     return entry;

             }

         }

         return 0;

     }

     // -- Assertion management --

     final Object assertionLock;

     // The default toggle for assertion checking.

     // @GuardedBy("assertionLock")

     private boolean defaultAssertionStatus = false;

     // Maps String packageName to Boolean package default assertion status Note

     // that the default package is placed under a null map key.  If this field

     // is null then we are delegating assertion status queries to the VM, i.e.,

     // none of this ClassLoader's assertion status modification methods have

     // been invoked.

     // @GuardedBy("assertionLock")

     private Map<String, Boolean> packageAssertionStatus = null;

     // Maps String fullyQualifiedClassName to Boolean assertionStatus If this

     // field is null then we are delegating assertion status queries to the VM,

     // i.e., none of this ClassLoader's assertion status modification methods

     // have been invoked.

     // @GuardedBy("assertionLock")

     Map<String, Boolean> classAssertionStatus = null;

     /**

      * Sets the default assertion status for this class loader.  This setting

      * determines whether classes loaded by this class loader and initialized

      * in the future will have assertions enabled or disabled by default.

      * This setting may be overridden on a per-package or per-class basis by

      * invoking {@link #setPackageAssertionStatus(String, boolean)} or {@link

      * #setClassAssertionStatus(String, boolean)}.  </p>

      *

      * @param  enabled

      *         <tt>true</tt> if classes loaded by this class loader will

      *         henceforth have assertions enabled by default, <tt>false</tt>

      *         if they will have assertions disabled by default.

      *

      * @since  1.4

      */

     public void setDefaultAssertionStatus(boolean enabled) {

         synchronized (assertionLock) {

             if (classAssertionStatus == null)

                 initializeJavaAssertionMaps();

             defaultAssertionStatus = enabled;

         }

     }

     /**

      * Sets the package default assertion status for the named package.  The

      * package default assertion status determines the assertion status for

      * classes initialized in the future that belong to the named package or

      * any of its "subpackages".

      *

      * <p> A subpackage of a package named p is any package whose name begins

      * with "<tt>p.</tt>".  For example, <tt>javax.swing.text</tt> is a

      * subpackage of <tt>javax.swing</tt>, and both <tt>java.util</tt> and

      * <tt>java.lang.reflect</tt> are subpackages of <tt>java</tt>.

      *

      * <p> In the event that multiple package defaults apply to a given class,

      * the package default pertaining to the most specific package takes

      * precedence over the others.  For example, if <tt>javax.lang</tt> and

      * <tt>javax.lang.reflect</tt> both have package defaults associated with

      * them, the latter package default applies to classes in

      * <tt>javax.lang.reflect</tt>.

      *

      * <p> Package defaults take precedence over the class loader's default

      * assertion status, and may be overridden on a per-class basis by invoking

      * {@link #setClassAssertionStatus(String, boolean)}.  </p>

      *

      * @param  packageName

      *         The name of the package whose package default assertion status

      *         is to be set. A <tt>null</tt> value indicates the unnamed

      *         package that is "current"

      *         (see section 7.4.2 of

      *         <cite>The Java&trade; Language Specification</cite>.)

      *

      * @param  enabled

      *         <tt>true</tt> if classes loaded by this classloader and

      *         belonging to the named package or any of its subpackages will

      *         have assertions enabled by default, <tt>false</tt> if they will

      *         have assertions disabled by default.

      *

      * @since  1.4

      */

     public void setPackageAssertionStatus(String packageName,

                                           boolean enabled) {

         synchronized (assertionLock) {

             if (packageAssertionStatus == null)

                 initializeJavaAssertionMaps();

             packageAssertionStatus.put(packageName, enabled);

         }

     }

     /**

      * Sets the desired assertion status for the named top-level class in this

      * class loader and any nested classes contained therein.  This setting

      * takes precedence over the class loader's default assertion status, and

      * over any applicable per-package default.  This method has no effect if

      * the named class has already been initialized.  (Once a class is

      * initialized, its assertion status cannot change.)

      *

      * <p> If the named class is not a top-level class, this invocation will

      * have no effect on the actual assertion status of any class. </p>

      *

      * @param  className

      *         The fully qualified class name of the top-level class whose

      *         assertion status is to be set.

      *

      * @param  enabled

      *         <tt>true</tt> if the named class is to have assertions

      *         enabled when (and if) it is initialized, <tt>false</tt> if the

      *         class is to have assertions disabled.

      *

      * @since  1.4

      */

     public void setClassAssertionStatus(String className, boolean enabled) {

         synchronized (assertionLock) {

             if (classAssertionStatus == null)

                 initializeJavaAssertionMaps();

             classAssertionStatus.put(className, enabled);

         }

     }

     /**

      * Sets the default assertion status for this class loader to

      * <tt>false</tt> and discards any package defaults or class assertion

      * status settings associated with the class loader.  This method is

      * provided so that class loaders can be made to ignore any command line or

      * persistent assertion status settings and "start with a clean slate."

      * </p>

      *

      * @since  1.4

      */

     public void clearAssertionStatus() {

         /*

          * Whether or not "Java assertion maps" are initialized, set

          * them to empty maps, effectively ignoring any present settings.

          */

         synchronized (assertionLock) {

             classAssertionStatus = new HashMap<>();

             packageAssertionStatus = new HashMap<>();

             defaultAssertionStatus = false;

         }

     }

     /**

      * Returns the assertion status that would be assigned to the specified

      * class if it were to be initialized at the time this method is invoked.

      * If the named class has had its assertion status set, the most recent

      * setting will be returned; otherwise, if any package default assertion

      * status pertains to this class, the most recent setting for the most

      * specific pertinent package default assertion status is returned;

      * otherwise, this class loader's default assertion status is returned.

      * </p>

      *

      * @param  className

      *         The fully qualified class name of the class whose desired

      *         assertion status is being queried.

      *

      * @return  The desired assertion status of the specified class.

      *

      * @see  #setClassAssertionStatus(String, boolean)

      * @see  #setPackageAssertionStatus(String, boolean)

      * @see  #setDefaultAssertionStatus(boolean)

      *

      * @since  1.4

      */

     boolean desiredAssertionStatus(String className) {

         synchronized (assertionLock) {

             // assert classAssertionStatus   != null;

             // assert packageAssertionStatus != null;

             // Check for a class entry

             Boolean result = classAssertionStatus.get(className);

             if (result != null)

                 return result.booleanValue();

             // Check for most specific package entry

             int dotIndex = className.lastIndexOf(".");

             if (dotIndex < 0) { // default package

                 result = packageAssertionStatus.get(null);

                 if (result != null)

                     return result.booleanValue();

             }

             while(dotIndex > 0) {

                 className = className.substring(0, dotIndex);

                 result = packageAssertionStatus.get(className);

                 if (result != null)

                     return result.booleanValue();

                 dotIndex = className.lastIndexOf(".", dotIndex-1);

             }

             // Return the classloader default

             return defaultAssertionStatus;

         }

     }

     // Set up the assertions with information provided by the VM.

     // Note: Should only be called inside a synchronized block

     private void initializeJavaAssertionMaps() {

         // assert Thread.holdsLock(assertionLock);

         classAssertionStatus = new HashMap<>();

         packageAssertionStatus = new HashMap<>();

         AssertionStatusDirectives directives = retrieveDirectives();

         for(int i = 0; i < directives.classes.length; i++)

             classAssertionStatus.put(directives.classes[i],

                                      directives.classEnabled[i]);

         for(int i = 0; i < directives.packages.length; i++)

             packageAssertionStatus.put(directives.packages[i],

                                        directives.packageEnabled[i]);

         defaultAssertionStatus = directives.deflt;

     }

     // Retrieves the assertion directives from the VM.

     private static native AssertionStatusDirectives retrieveDirectives();

 }

 class SystemClassLoaderAction

     implements PrivilegedExceptionAction<ClassLoader> {

     private ClassLoader parent;

     SystemClassLoaderAction(ClassLoader parent) {

         this.parent = parent;

     }

     public ClassLoader run() throws Exception {

         String cls = System.getProperty("java.system.class.loader");

         if (cls == null) {

             return parent;

         }

         Constructor ctor = Class.forName(cls, true, parent)

             .getDeclaredConstructor(new Class[] { ClassLoader.class });

         ClassLoader sys = (ClassLoader) ctor.newInstance(

             new Object[] { parent });

         Thread.currentThread().setContextClassLoader(sys);

         return sys;

     }

 }