java/lang/ClassLoader.java

/*
 * Copyright (C) 2014 The Android Open Source Project
 * Copyright (c) 2013, 2015, Oracle and/or its affiliates. All rights reserved.
 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
 *
 * This code is free software; you can redistribute it and/or modify it
 * under the terms of the GNU General Public License version 2 only, as
 * published by the Free Software Foundation.  Oracle designates this
 * particular file as subject to the "Classpath" exception as provided
 * by Oracle in the LICENSE file that accompanied this code.
 *
 * This code is distributed in the hope that it will be useful, but WITHOUT
 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
 * version 2 for more details (a copy is included in the LICENSE file that
 * accompanied this code).
 *
 * You should have received a copy of the GNU General Public License version
 * 2 along with this work; if not, write to the Free Software Foundation,
 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
 *
 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
 * or visit www.oracle.com if you need additional information or have any
 * questions.
 */
package java.lang;

import java.io.InputStream;
import java.io.IOException;
import java.io.File;
import java.lang.reflect.Constructor;
import java.lang.reflect.InvocationTargetException;
import java.net.MalformedURLException;
import java.net.URL;
import java.security.AccessController;
import java.security.AccessControlContext;
import java.security.CodeSource;
import java.security.Policy;
import java.security.PrivilegedAction;
import java.security.PrivilegedActionException;
import java.security.PrivilegedExceptionAction;
import java.security.ProtectionDomain;
import java.security.cert.Certificate;
import java.util.Collections;
import java.util.Enumeration;
import java.util.HashMap;
import java.util.HashSet;
import java.util.Set;
import java.util.Stack;
import java.util.Map;
import java.util.Vector;
import java.util.Hashtable;
import java.util.WeakHashMap;
import java.util.concurrent.ConcurrentHashMap;
import dalvik.system.PathClassLoader;
import java.util.List;
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&nbsp;= new NetworkClassLoader(host,&nbsp;port);
 *   Object main&nbsp;= loader.loadClass("Main", true).newInstance();
 *       &nbsp;.&nbsp;.&nbsp;.
 * </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
 *             &nbsp;.&nbsp;.&nbsp;.
 *         }
 *     }
 * </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 {

    static private class SystemClassLoader {
        public static ClassLoader loader = ClassLoader.createSystemClassLoader();
    }

    /**
     * To avoid unloading individual classes, {@link java.lang.reflect.Proxy}
     * only generates one class for each set of interfaces. This maps sets of
     * interfaces to the proxy class that implements all of them. It is declared
     * here so that these generated classes can be unloaded with their class
     * loader.
     *
     * @hide
     */
    public final Map<List<Class<?>>, Class<?>> proxyCache =
            new HashMap<List<Class<?>>, Class<?>>();

    // 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 ClassLoader createSystemClassLoader() {
        String classPath = System.getProperty("java.class.path", ".");
        String librarySearchPath = System.getProperty("java.library.path", "");

        // String[] paths = classPath.split(":");
        // URL[] urls = new URL[paths.length];
        // for (int i = 0; i < paths.length; i++) {
        // try {
        // urls[i] = new URL("file://" + paths[i]);
        // }
        // catch (Exception ex) {
        // ex.printStackTrace();
        // }
        // }
        //
        // return new java.net.URLClassLoader(urls, null);

        // TODO Make this a java.net.URLClassLoader once we have those?
        return new PathClassLoader(classPath, librarySearchPath, BootClassLoader.getInstance());
    }

    // 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<>();

    /**
     * Pointer to the allocator used by the runtime to allocate metadata such
     * as ArtFields and ArtMethods.
     */
    private transient long allocator;

    /**
     * Pointer to the class table, only used from within the runtime.
     */
    private transient long classTable;

    private static Void checkCreateClassLoader() {
        return null;
    }

    private ClassLoader(Void unused, ClassLoader parent) {
        this.parent = 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) {
        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:
     *
     * <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>
     *
     *
     * @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
     */
    // Android-removed: Remove references to getClassLoadingLock
    //                   Remove perf counters.
    //
    // <p> Unless overridden, this method synchronizes on the result of
    // {@link #getClassLoadingLock <tt>getClassLoadingLock</tt>} method
    // during the entire class loading process.
    protected Class<?> loadClass(String name, boolean resolve)
        throws ClassNotFoundException
    {
            // 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);
                }
            }
            return c;
    }


    /**
     * 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>.
     *
     * @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 UnsupportedOperationException("can't load this type of class file");
    }

    /**
     * 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 UnsupportedOperationException("can't load this type of class file");
    }


    /**
     * Converts an array of bytes into an instance of class <tt>Class</tt>,
     * with an optional <tt>ProtectionDomain</tt>.  If the domain is
     * <tt>null</tt>, then a default domain will be assigned to the class as
     * specified in the documentation for {@link #defineClass(String, byte[],
     * int, int)}.  Before the class can be used it must be resolved.
     *
     * <p> The first class defined in a package determines the exact set of
     * certificates that all subsequent classes defined in that package must
     * contain.  The set of certificates for a class is obtained from the
     * {@link java.security.CodeSource <tt>CodeSource</tt>} within the
     * <tt>ProtectionDomain</tt> of the class.  Any classes added to that
     * package must contain the same set of certificates or a
     * <tt>SecurityException</tt> will be thrown.  Note that if
     * <tt>name</tt> is <tt>null</tt>, this check is not performed.
     * You should always pass in the <a href="#name">binary name</a> of the
     * class you are defining as well as the bytes.  This ensures that the
     * class you are defining is indeed the class you think it is.
     *
     * <p> The specified <tt>name</tt> cannot begin with "<tt>java.</tt>", since
     * all classes in the "<tt>java.*</tt> packages can only be defined by the
     * bootstrap class loader.  If <tt>name</tt> is not <tt>null</tt>, it
     * must be equal to the <a href="#name">binary name</a> of the class
     * specified by the byte array "<tt>b</tt>", otherwise a {@link
     * NoClassDefFoundError <tt>NoClassDefFoundError</tt>} will be thrown. </p>
     *
     * @param  name
     *         The expected <a href="#name">binary name</a> of the class, or
     *         <tt>null</tt> if not known
     *
     * @param  b
     *         The bytes that make up the class data. The bytes in positions
     *         <tt>off</tt> through <tt>off+len-1</tt> should have the format
     *         of a valid class file as defined by
     *         <cite>The Java&trade; Virtual Machine Specification</cite>.
     *
     * @param  off
     *         The start offset in <tt>b</tt> of the class data
     *
     * @param  len
     *         The length of the class data
     *
     * @param  protectionDomain
     *         The ProtectionDomain of the class
     *
     * @return  The <tt>Class</tt> object created from the data,
     *          and optional <tt>ProtectionDomain</tt>.
     *
     * @throws  ClassFormatError
     *          If the data did not contain a valid class
     *
     * @throws  NoClassDefFoundError
     *          If <tt>name</tt> is not equal to the <a href="#name">binary
     *          name</a> of the class specified by <tt>b</tt>
     *
     * @throws  IndexOutOfBoundsException
     *          If either <tt>off</tt> or <tt>len</tt> is negative, or if
     *          <tt>off+len</tt> is greater than <tt>b.length</tt>.
     *
     * @throws  SecurityException
     *          If an attempt is made to add this class to a package that
     *          contains classes that were signed by a different set of
     *          certificates than this class, or if <tt>name</tt> begins with
     *          "<tt>java.</tt>".
     */
    // Android-changed: Remove <tt> from link for NoClassDefFoundError
    protected final Class<?> defineClass(String name, byte[] b, int off, int len,
                                         ProtectionDomain protectionDomain)
        throws ClassFormatError
    {
        throw new UnsupportedOperationException("can't load this type of class file");
    }

    /**
     * Converts a {@link java.nio.ByteBuffer <tt>ByteBuffer</tt>}
     * into an instance of class <tt>Class</tt>,
     * with an optional <tt>ProtectionDomain</tt>.  If the domain is
     * <tt>null</tt>, then a default domain will be assigned to the class as
     * specified in the documentation for {@link #defineClass(String, byte[],
     * int, int)}.  Before the class can be used it must be resolved.
     *
     * <p>The rules about the first class defined in a package determining the
     * set of certificates for the package, and the restrictions on class names
     * are identical to those specified in the documentation for {@link
     * #defineClass(String, byte[], int, int, ProtectionDomain)}.
     *
     * <p> An invocation of this method of the form
     * <i>cl</i><tt>.defineClass(</tt><i>name</i><tt>,</tt>
     * <i>bBuffer</i><tt>,</tt> <i>pd</i><tt>)</tt> yields exactly the same
     * result as the statements
     *
     *<p> <tt>
     * ...<br>
     * byte[] temp = new byte[bBuffer.{@link
     * java.nio.ByteBuffer#remaining remaining}()];<br>
     *     bBuffer.{@link java.nio.ByteBuffer#get(byte[])
     * get}(temp);<br>
     *     return {@link #defineClass(String, byte[], int, int, ProtectionDomain)
     * cl.defineClass}(name, temp, 0,
     * temp.length, pd);<br>
     * </tt></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 from positions
     *         <tt>b.position()</tt> through <tt>b.position() + b.limit() -1
     *         </tt> should have the format of a valid class file as defined by
     *         <cite>The Java&trade; Virtual Machine Specification</cite>.
     *
     * @param  protectionDomain
     *         The ProtectionDomain of the class, or <tt>null</tt>.
     *
     * @return  The <tt>Class</tt> object created from the data,
     *          and optional <tt>ProtectionDomain</tt>.
     *
     * @throws  ClassFormatError
     *          If the data did not contain a valid class.
     *
     * @throws  NoClassDefFoundError
     *          If <tt>name</tt> is not equal to the <a href="#name">binary
     *          name</a> of the class specified by <tt>b</tt>
     *
     * @throws  SecurityException
     *          If an attempt is made to add this class to a package that
     *          contains classes that were signed by a different set of
     *          certificates than this class, or if <tt>name</tt> begins with
     *          "<tt>java.</tt>".
     *
     * @see      #defineClass(String, byte[], int, int, ProtectionDomain)
     *
     * @since  1.5
     */
    protected final Class<?> defineClass(String name, java.nio.ByteBuffer b,
                                         ProtectionDomain protectionDomain)
        throws ClassFormatError
    {
        throw new UnsupportedOperationException("can't load this type of class file");
    }

    /**
     * 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>.
     *
     * @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) {
    }

    /**
     * Finds a class with the specified <a href="#name">binary name</a>,
     * loading it if necessary.
     *
     * <p> This method loads the class through the system class loader (see
     * {@link #getSystemClassLoader()}).  The <tt>Class</tt> object returned
     * might have more than one <tt>ClassLoader</tt> associated with it.
     * Subclasses of <tt>ClassLoader</tt> need not usually invoke this method,
     * because most class loaders need to override just {@link
     * #findClass(String)}.  </p>
     *
     * @param  name
     *         The <a href="#name">binary name</a> of the class
     *
     * @return  The <tt>Class</tt> object for the specified <tt>name</tt>
     *
     * @throws  ClassNotFoundException
     *          If the class could not be found
     *
     * @see  #ClassLoader(ClassLoader)
     * @see  #getParent()
     */
    protected final Class<?> findSystemClass(String name)
        throws ClassNotFoundException
    {
        return Class.forName(name, false, getSystemClassLoader());
    }

    /**
     * Returns a class loaded by the bootstrap class loader;
     * or return null if not found.
     */
    private Class<?> findBootstrapClassOrNull(String name)
    {
        return null;
    }

    /**
     * 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.
     *
     * @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) {
        ClassLoader loader;
        if (this == BootClassLoader.getInstance())
            loader = null;
        else
            loader = this;
        return VMClassLoader.findLoadedClass(loader, name);
    }

    /**
     * Sets the signers of a class.  This should be invoked after defining a
     * class.
     *
     * @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) {
    }


    // -- 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>
     *
     * @apiNote When overriding this method it is recommended that an
     * implementation ensures that any delegation is consistent with the {@link
     * #getResources(java.lang.String) getResources(String)} method.
     *
     * @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>
     *
     * @apiNote When overriding this method it is recommended that an
     * implementation ensures that any delegation is consistent with the {@link
     * #getResource(java.lang.String) getResource(String)} method. This should
     * ensure that the first element returned by the Enumeration's
     * {@code nextElement} method is the same resource that the
     * {@code getResource(String)} method would return.
     *
     * @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 {
        @SuppressWarnings("unchecked")
        Enumeration<URL>[] tmp = (Enumeration<URL>[]) 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.
     *
     * @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.
     *
     * @param  name
     *         The resource name
     *
     * @return  An enumeration of {@link java.net.URL <tt>URL</tt>} objects for
     *          the resources
     *
     * @throws  IOException
     *          If I/O errors occur
     *
     * @since  1.2
     */
    protected Enumeration<URL> findResources(String name) throws IOException {
        return java.util.Collections.emptyEnumeration();
    }

    /**
     * Registers the caller as parallel capable.
     * The registration succeeds if and only if all of the following
     * conditions are met:
     * <ol>
     * <li> no instance of the caller has been created</li>
     * <li> all of the super classes (except class Object) of the caller are
     * registered as parallel capable</li>
     * </ol>
     * <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
     */
    @CallerSensitive
    protected static boolean registerAsParallelCapable() {
        return true;
    }

    /**
     * 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()}).
     *
     * @param  name
     *         The resource name
     *
     * @return  A {@link java.net.URL <tt>URL</tt>} object for reading the
     *          resource, or <tt>null</tt> if the resource could not be found
     *
     * @since  1.1
     */
    public static URL getSystemResource(String name) {
        ClassLoader system = getSystemClassLoader();
        if (system == null) {
            return getBootstrapResource(name);
        }
        return system.getResource(name);
    }

    /**
     * Finds all resources of the specified name from the search path used to
     * load classes.  The resources thus found are returned as an
     * {@link java.util.Enumeration <tt>Enumeration</tt>} of {@link
     * java.net.URL <tt>URL</tt>} objects.
     *
     * <p> The search order is described in the documentation for {@link
     * #getSystemResource(String)}.  </p>
     *
     * @param  name
     *         The resource name
     *
     * @return  An enumeration of resource {@link java.net.URL <tt>URL</tt>}
     *          objects
     *
     * @throws  IOException
     *          If I/O errors occur

     * @since  1.2
     */
    public static Enumeration<URL> getSystemResources(String name)
        throws IOException
    {
        ClassLoader system = getSystemClassLoader();
        if (system == null) {
            return getBootstrapResources(name);
        }
        return system.getResources(name);
    }

    /**
     * Find resources from the VM's built-in classloader.
     */
    private static URL getBootstrapResource(String name) {
        return null;
    }

    /**
     * Find resources from the VM's built-in classloader.
     */
    private static Enumeration<URL> getBootstrapResources(String name)
        throws IOException
    {
        return null;
    }


    /**
     * 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()}).
     *
     * @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
     */
    @CallerSensitive
    public final ClassLoader getParent() {
        return parent;
    }

    /**
     * Returns the system class loader for delegation.  This is the default
     * delegation parent for new <tt>ClassLoader</tt> instances, and is
     * typically the class loader used to start the application.
     *
     * <p> This method is first invoked early in the runtime's startup
     * sequence, at which point it creates the system class loader and sets it
     * as the context class loader of the invoking <tt>Thread</tt>.
     *
     * <p> The default system class loader is an implementation-dependent
     * instance of this class.
     *
     * <p> If the system property "<tt>java.system.class.loader</tt>" is defined
     * when this method is first invoked then the value of that property is
     * taken to be the name of a class that will be returned as the system
     * class loader.  The class is loaded using the default system class loader
     * and must define a public constructor that takes a single parameter of
     * type <tt>ClassLoader</tt> which is used as the delegation parent.  An
     * instance is then created using this constructor with the default system
     * class loader as the parameter.  The resulting class loader is defined
     * to be the system class loader.
     *
     * <p> If a security manager is present, and the invoker's class loader is
     * not <tt>null</tt> and the invoker's class loader is not the same as or
     * an ancestor of the system class loader, then this method invokes the
     * security manager's {@link
     * SecurityManager#checkPermission(java.security.Permission)
     * <tt>checkPermission</tt>} method with a {@link
     * RuntimePermission#RuntimePermission(String)
     * <tt>RuntimePermission("getClassLoader")</tt>} permission to verify
     * access to the system class loader.  If not, a
     * <tt>SecurityException</tt> will be thrown.  </p>
     *
     * @return  The system <tt>ClassLoader</tt> for delegation, or
     *          <tt>null</tt> if none
     *
     * @throws  SecurityException
     *          If a security manager exists and its <tt>checkPermission</tt>
     *          method doesn't allow access to the system class loader.
     *
     * @throws  IllegalStateException
     *          If invoked recursively during the construction of the class
     *          loader specified by the "<tt>java.system.class.loader</tt>"
     *          property.
     *
     * @throws  Error
     *          If the system property "<tt>java.system.class.loader</tt>"
     *          is defined but the named class could not be loaded, the
     *          provider class does not define the required constructor, or an
     *          exception is thrown by that constructor when it is invoked. The
     *          underlying cause of the error can be retrieved via the
     *          {@link Throwable#getCause()} method.
     *
     * @revised  1.4
     */
    @CallerSensitive
    public static ClassLoader getSystemClassLoader() {
        return SystemClassLoader.loader;
    }

    // -- Package --

    /**
     * Defines a package by name in this <tt>ClassLoader</tt>.  This allows
     * class loaders to define the packages for their classes. Packages must
     * be created before the class is defined, and package names must be
     * unique within a class loader and cannot be redefined or changed once
     * created.
     *
     * @param  name
     *         The package name
     *
     * @param  specTitle
     *         The specification title
     *
     * @param  specVersion
     *         The specification version
     *
     * @param  specVendor
     *         The specification vendor
     *
     * @param  implTitle
     *         The implementation title
     *
     * @param  implVersion
     *         The implementation version
     *
     * @param  implVendor
     *         The implementation vendor
     *
     * @param  sealBase
     *         If not <tt>null</tt>, then this package is sealed with
     *         respect to the given code source {@link java.net.URL
     *         <tt>URL</tt>}  object.  Otherwise, the package is not sealed.
     *
     * @return  The newly defined <tt>Package</tt> object
     *
     * @throws  IllegalArgumentException
     *          If package name duplicates an existing package either in this
     *          class loader or one of its ancestors
     *
     * @since  1.2
     */
    protected Package definePackage(String name, String specTitle,
                                    String specVersion, String specVendor,
                                    String implTitle, String implVersion,
                                    String implVendor, URL sealBase)
        throws IllegalArgumentException
    {
        synchronized (packages) {
            Package pkg = packages.get(name);
            if (pkg != null) {
                throw new IllegalArgumentException(name);
            }
            pkg = new Package(name, specTitle, specVersion, specVendor,
                              implTitle, implVersion, implVendor,
                              sealBase, this);
            packages.put(name, pkg);
            return pkg;
        }
    }

    /**
     * Returns a <tt>Package</tt> that has been defined by this class loader
     * or any of its ancestors.
     *
     * @param  name
     *         The package name
     *
     * @return  The <tt>Package</tt> corresponding to the given name, or
     *          <tt>null</tt> if not found
     *
     * @since  1.2
     */
    protected Package getPackage(String name) {
        Package pkg;
        synchronized (packages) {
            pkg = packages.get(name);
        }
        return pkg;
    }

    /**
     * Returns all of the <tt>Packages</tt> defined by this class loader and
     * its ancestors.
     *
     * @return  The array of <tt>Package</tt> objects defined by this
     *          <tt>ClassLoader</tt>
     *
     * @since  1.2
     */
    protected Package[] getPackages() {
        Map<String, Package> map;
        synchronized (packages) {
            map = new HashMap<>(packages);
        }
        Package[] pkgs;
        return map.values().toArray(new Package[map.size()]);
    }


    // -- Native library access --

    /**
     * Returns the absolute path name of a native library.  The VM invokes this
     * method to locate the native libraries that belong to classes loaded with
     * this class loader. If this method returns <tt>null</tt>, the VM
     * searches the library along the path specified as the
     * "<tt>java.library.path</tt>" property.
     *
     * @param  libname
     *         The library name
     *
     * @return  The absolute path of the native library
     *
     * @see  System#loadLibrary(String)
     * @see  System#mapLibraryName(String)
     *
     * @since  1.2
     */
    protected String findLibrary(String libname) {
        return null;
    }

    /**
     * Sets the default assertion status for this class loader.  This setting
     * determines whether classes loaded by this class loader and initialized
     * in the future will have assertions enabled or disabled by default.
     * This setting may be overridden on a per-package or per-class basis by
     * invoking {@link #setPackageAssertionStatus(String, boolean)} or {@link
     * #setClassAssertionStatus(String, boolean)}.
     *
     * @param  enabled
     *         <tt>true</tt> if classes loaded by this class loader will
     *         henceforth have assertions enabled by default, <tt>false</tt>
     *         if they will have assertions disabled by default.
     *
     * @since  1.4
     */
    public void setDefaultAssertionStatus(boolean enabled) {
    }

    /**
     * Sets the package default assertion status for the named package.  The
     * package default assertion status determines the assertion status for
     * classes initialized in the future that belong to the named package or
     * any of its "subpackages".
     *
     * <p> A subpackage of a package named p is any package whose name begins
     * with "<tt>p.</tt>".  For example, <tt>javax.swing.text</tt> is a
     * subpackage of <tt>javax.swing</tt>, and both <tt>java.util</tt> and
     * <tt>java.lang.reflect</tt> are subpackages of <tt>java</tt>.
     *
     * <p> In the event that multiple package defaults apply to a given class,
     * the package default pertaining to the most specific package takes
     * precedence over the others.  For example, if <tt>javax.lang</tt> and
     * <tt>javax.lang.reflect</tt> both have package defaults associated with
     * them, the latter package default applies to classes in
     * <tt>javax.lang.reflect</tt>.
     *
     * <p> Package defaults take precedence over the class loader's default
     * assertion status, and may be overridden on a per-class basis by invoking
     * {@link #setClassAssertionStatus(String, boolean)}.  </p>
     *
     * @param  packageName
     *         The name of the package whose package default assertion status
     *         is to be set. A <tt>null</tt> value indicates the unnamed
     *         package that is "current"
     *         (see section 7.4.2 of
     *         <cite>The Java&trade; Language Specification</cite>.)
     *
     * @param  enabled
     *         <tt>true</tt> if classes loaded by this classloader and
     *         belonging to the named package or any of its subpackages will
     *         have assertions enabled by default, <tt>false</tt> if they will
     *         have assertions disabled by default.
     *
     * @since  1.4
     */
    public void setPackageAssertionStatus(String packageName,
                                          boolean enabled) {
    }

    /**
     * Sets the desired assertion status for the named top-level class in this
     * class loader and any nested classes contained therein.  This setting
     * takes precedence over the class loader's default assertion status, and
     * over any applicable per-package default.  This method has no effect if
     * the named class has already been initialized.  (Once a class is
     * initialized, its assertion status cannot change.)
     *
     * <p> If the named class is not a top-level class, this invocation will
     * have no effect on the actual assertion status of any class. </p>
     *
     * @param  className
     *         The fully qualified class name of the top-level class whose
     *         assertion status is to be set.
     *
     * @param  enabled
     *         <tt>true</tt> if the named class is to have assertions
     *         enabled when (and if) it is initialized, <tt>false</tt> if the
     *         class is to have assertions disabled.
     *
     * @since  1.4
     */
    public void setClassAssertionStatus(String className, boolean enabled) {
    }

    /**
     * Sets the default assertion status for this class loader to
     * <tt>false</tt> and discards any package defaults or class assertion
     * status settings associated with the class loader.  This method is
     * provided so that class loaders can be made to ignore any command line or
     * persistent assertion status settings and "start with a clean slate."
     *
     * @since  1.4
     */
    public void clearAssertionStatus() {
        /*
         * Whether or not "Java assertion maps" are initialized, set
         * them to empty maps, effectively ignoring any present settings.
         */
    }
}


class BootClassLoader extends ClassLoader {

    private static BootClassLoader instance;

    @FindBugsSuppressWarnings("DP_CREATE_CLASSLOADER_INSIDE_DO_PRIVILEGED")
    public static synchronized BootClassLoader getInstance() {
        if (instance == null) {
            instance = new BootClassLoader();
        }

        return instance;
    }

    public BootClassLoader() {
        super(null);
    }

    @Override
    protected Class<?> findClass(String name) throws ClassNotFoundException {
        return Class.classForName(name, false, null);
    }

    @Override
    protected URL findResource(String name) {
        return VMClassLoader.getResource(name);
    }

    @SuppressWarnings("unused")
    @Override
    protected Enumeration<URL> findResources(String resName) throws IOException {
        return Collections.enumeration(VMClassLoader.getResources(resName));
    }

    /**
     * Returns package information for the given package. Unfortunately, the
     * Android BootClassLoader doesn't really have this information, and as a
     * non-secure ClassLoader, it isn't even required to, according to the spec.
     * Yet, we want to provide it, in order to make all those hopeful callers of
     * {@code myClass.getPackage().getName()} happy. Thus we construct a Package
     * object the first time it is being requested and fill most of the fields
     * with dummy values. The Package object is then put into the ClassLoader's
     * Package cache, so we see the same one next time. We don't create Package
     * objects for null arguments or for the default package.
     * <p>
     * There a limited chance that we end up with multiple Package objects
     * representing the same package: It can happen when when a package is
     * scattered across different JAR files being loaded by different
     * ClassLoaders. Rather unlikely, and given that this whole thing is more or
     * less a workaround, probably not worth the effort.
     */
    @Override
    protected Package getPackage(String name) {
        if (name != null && !name.isEmpty()) {
            synchronized (this) {
                Package pack = super.getPackage(name);

                if (pack == null) {
                    pack = definePackage(name, "Unknown", "0.0", "Unknown", "Unknown", "0.0",
                            "Unknown", null);
                }

                return pack;
            }
        }

        return null;
    }

    @Override
    public URL getResource(String resName) {
        return findResource(resName);
    }

    @Override
    protected Class<?> loadClass(String className, boolean resolve)
           throws ClassNotFoundException {
        Class<?> clazz = findLoadedClass(className);

        if (clazz == null) {
            clazz = findClass(className);
        }

        return clazz;
    }

    @Override
    public Enumeration<URL> getResources(String resName) throws IOException {
        return findResources(resName);
    }
}