/*

 * 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 = 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) {

        throw new UnsupportedOperationException();

    }

    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;

        }

    }

}