/*

  * 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.net.URL;

 import java.util.Enumeration;

 import java.util.NoSuchElementException;

 import org.apidesign.bck2brwsr.core.JavaScriptBody;

 /**

  * 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 {

     @JavaScriptBody(args = {}, body = "")

     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;

     /**

      * 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) {

         throw new SecurityException();

     }

     /**

      * 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() {

         throw new SecurityException();

     }

     // -- 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) {

                 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.

                     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;

         return lock;

     }

     /**

      * 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

     {

         throw new SecurityException();

     }

     /**

      * 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

     {

         throw new SecurityException();

     }

     /**

      * 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);

     /**

      * 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);

         throw new UnsupportedOperationException();

     }

     // -- 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 new CompoundEnumeration(new Enumeration[0]);

     }

     // 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 false;

 //    }

     /**

      * 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 = null; // getSystemClassLoader();

         if (system == null) {

             return getBootstrapResources(name);

         }

         return system.getResources(name);

     }

     /**

      * 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() {

         throw new SecurityException();

     }

     /**

      * 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() {

         throw new SecurityException();

     }

     // 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;

     }

     private boolean checkName(String name) {

         throw new UnsupportedOperationException(); 

     }

     private Class findBootstrapClassOrNull(String name) {

         throw new UnsupportedOperationException();

     }

     private static URL getBootstrapResource(String name) {

         throw new UnsupportedOperationException();

     }

     private static Enumeration<URL> getBootstrapResources(String name) {

         URL u = Object.class.getResource("/" + name);

         return new OneOrZeroEnum(u);

     }

     private static class OneOrZeroEnum implements Enumeration<URL> {

         private URL u;

         public OneOrZeroEnum(URL u) {

             this.u = u;

         }

         public boolean hasMoreElements() {

             return u != null;

         }

         public URL nextElement() {

             URL r = u;

             if (r == null) {

                 throw new NoSuchElementException();

             }

             u = null;

             return r;

         }

     }

     private static class CompoundEnumeration implements Enumeration<URL> {

         private URL next;

         private int index;

         private final Enumeration[] arr;

         public CompoundEnumeration(Enumeration[] arr) {

             this.arr = arr;

             this.index = 0;

         }

         public boolean hasMoreElements() {

             if (next == null) {

                 if (arr[index].hasMoreElements()) {

                     next = (URL) arr[index].nextElement();

                 } else {

                     if (index < arr.length) {

                         index++;

                         return hasMoreElements();

                     }

                 }

             }

             return next != null;

         }

         public URL nextElement() {

             if (!hasMoreElements()) {

                 throw new NoSuchElementException();

             }

             URL r = next;

             next = null;

             return r;

         }

     }

 }