/*

 * Copyright (c) 2013, 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

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 */

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.CompoundEnumeration;

import sun.misc.Resource;

import sun.misc.URLClassPath;

import sun.misc.VM;

import sun.reflect.CallerSensitive;

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>

 *

 * <h3> <a name="name">Binary names</a> </h3>

 *

 * <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>}.

     *

     * @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.

     *

     * @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>.