classloader.java

这是个开源的JAVA虚拟机,可以直接RUN jar或class文件, 是个国外人写的.想了解JAVA底层的朋友不能不看~!

   * @since 1.2
   */
  public static Enumeration getSystemResources(String name) throws IOException
  {
    return systemClassLoader.getResources(name);
  }

  /**
   * Get a resource as stream using this classloader or one of its parents.
   * First calls <code>getResource()</code> and if that returns a URL to
   * the resource then it calls and returns the InputStream given by
   * <code>URL.openStream()</code>.
   *
   * <p>Subclasses should not override this method but should override
   * <code>findResource()</code> which is called by this method.
   *
   * @param name the name of the resource relative to this classloader
   * @return an InputStream to the resource, or null
   * @since 1.1
   */
  public InputStream getResourceAsStream(String name)
  {
    try
      {
        URL url = getResource(name);
        if (url == null)
          return null;
        return url.openStream();
      }
    catch (IOException e)
      {
        return null;
      }
  }

  /**
   * Get a resource using the system classloader.
   *
   * @param name the name of the resource relative to the system classloader
   * @return an input stream for the resource, or null
   * @since 1.1
   */
  public static final InputStream getSystemResourceAsStream(String name)
  {
    try
      {
        URL url = getSystemResource(name);
        if (url == null)
          return null;
        return url.openStream();
      }
    catch (IOException e)
      {
        return null;
      }
  }

  /**
   * Returns the system classloader. The system classloader (also called
   * the application classloader) is the classloader that was used to
   * load the application classes on the classpath (given by the system
   * property <code>java.class.path</code>. This is set as the context
   * class loader for a thread. The system property
   * <code>java.system.class.loader</code>, if defined, is taken to be the
   * name of the class to use as the system class loader, which must have
   * a public constructor which takes a ClassLoader as a parent; otherwise this
   * uses gnu.java.lang.SystemClassLoader.
   *
   * <p>Note that this is different from the bootstrap classloader that
   * actually loads all the real "system" classes (the bootstrap classloader
   * is the parent of the returned system classloader).
   *
   * <p>A security check will be performed for
   * <code>RuntimePermission("getClassLoader")</code> if the calling class
   * is not a parent of the system class loader.
   *
   * @return the system class loader
   * @throws SecurityException if the security check fails
   * @throws IllegalStateException if this is called recursively
   * @throws Error if <code>java.system.class.loader</code> fails to load
   * @since 1.2
   */
  public static ClassLoader getSystemClassLoader()
  {
    // Check if we may return the system classloader
    SecurityManager sm = System.getSecurityManager();
    if (sm != null)
      {
	Class c = VMSecurityManager.getClassContext()[1];
	ClassLoader cl = c.getClassLoader();
	if (cl != null && cl != systemClassLoader)
	  sm.checkPermission(new RuntimePermission("getClassLoader"));
      }

    return systemClassLoader;
  }

  /**
   * Defines a new package and creates a Package object. The package should
   * be defined before any class in the package is defined with
   * <code>defineClass()</code>. The package should not yet be defined
   * before in this classloader or in one of its parents (which means that
   * <code>getPackage()</code> should return <code>null</code>). All
   * parameters except the <code>name</code> of the package may be
   * <code>null</code>.
   *
   * <p>Subclasses should call this method from their <code>findClass()</code>
   * implementation before calling <code>defineClass()</code> on a Class
   * in a not yet defined Package (which can be checked by calling
   * <code>getPackage()</code>).
   *
   * @param name the name of the Package
   * @param specTitle the name of the specification
   * @param specVendor the name of the specification designer
   * @param specVersion the version of this specification
   * @param implTitle the name of the implementation
   * @param implVendor the vendor that wrote this implementation
   * @param implVersion the version of this implementation
   * @param sealed if sealed the origin of the package classes
   * @return the Package object for the specified package
   * @throws IllegalArgumentException if the package name is null or it
   *         was already defined by this classloader or one of its parents
   * @see Package
   * @since 1.2
   */
  protected Package definePackage(String name, String specTitle,
                                  String specVendor, String specVersion,
                                  String implTitle, String implVendor,
                                  String implVersion, URL sealed)
  {
    if (getPackage(name) != null)
      throw new IllegalArgumentException("Package " + name
                                         + " already defined");
    Package p = new Package(name, specTitle, specVendor, specVersion,
                            implTitle, implVendor, implVersion, sealed);
    synchronized (definedPackages)
      {
        definedPackages.put(name, p);
      }
    return p;
  }

  /**
   * Returns the Package object for the requested package name. It returns
   * null when the package is not defined by this classloader or one of its
   * parents.
   *
   * @param name the package name to find
   * @return the package, if defined
   * @since 1.2
   */
  protected Package getPackage(String name)
  {
    Package p;
    if (parent == null)
      p = VMClassLoader.getPackage(name);
    else
      p = parent.getPackage(name);

    if (p == null)
      {
	synchronized (definedPackages)
	  {
	    p = (Package) definedPackages.get(name);
	  }
      }
    return p;
  }

  /**
   * Returns all Package objects defined by this classloader and its parents.
   *
   * @return an array of all defined packages
   * @since 1.2
   */
  protected Package[] getPackages()
  {
    // Get all our packages.
    Package[] packages;
    synchronized(definedPackages)
      {
        packages = new Package[definedPackages.size()];
        definedPackages.values().toArray(packages);
      }

    // If we have a parent get all packages defined by our parents.
    Package[] parentPackages;
    if (parent == null)
      parentPackages = VMClassLoader.getPackages();
    else
      parentPackages = parent.getPackages();

    Package[] allPackages = new Package[parentPackages.length
					+ packages.length];
    System.arraycopy(parentPackages, 0, allPackages, 0,
                     parentPackages.length);
    System.arraycopy(packages, 0, allPackages, parentPackages.length,
                     packages.length);
    return allPackages;
  }

  /**
   * Called by <code>Runtime.loadLibrary()</code> to get an absolute path
   * to a (system specific) library that was requested by a class loaded
   * by this classloader. The default implementation returns
   * <code>null</code>. It should be implemented by subclasses when they
   * have a way to find the absolute path to a library. If this method
   * returns null the library is searched for in the default locations
   * (the directories listed in the <code>java.library.path</code> system
   * property).
   *
   * @param name the (system specific) name of the requested library
   * @return the full pathname to the requested library, or null
   * @see Runtime#loadLibrary()
   * @since 1.2
   */
  protected String findLibrary(String name)
  {
    return null;
  }

  /**
   * Set the default assertion status for classes loaded by this classloader,
   * used unless overridden by a package or class request.
   *
   * @param enabled true to set the default to enabled
   * @see #setClassAssertionStatus(String, boolean)
   * @see #setPackageAssertionStatus(String, boolean)
   * @see #clearAssertionStatus()
   * @since 1.4
   */
  public void setDefaultAssertionStatus(boolean enabled)
  {
    defaultAssertionStatus = enabled;
  }
  
  /**
   * Set the default assertion status for packages, used unless overridden
   * by a class request. This default also covers subpackages, unless they
   * are also specified. The unnamed package should use null for the name.
   *
   * @param name the package (and subpackages) to affect
   * @param enabled true to set the default to enabled
   * @see #setDefaultAssertionStatus(String, boolean)
   * @see #setClassAssertionStatus(String, boolean)
   * @see #clearAssertionStatus()
   * @since 1.4
   */
  public synchronized void setPackageAssertionStatus(String name,
                                                     boolean enabled)
  {
    if (packageAssertionStatus == null)
      packageAssertionStatus
        = new HashMap(systemPackageAssertionStatus);
    packageAssertionStatus.put(name, Boolean.valueOf(enabled));
  }
  
  /**
   * Set the default assertion status for a class. This only affects the
   * status of top-level classes, any other string is harmless.
   *
   * @param name the class to affect
   * @param enabled true to set the default to enabled
   * @throws NullPointerException if name is null
   * @see #setDefaultAssertionStatus(String, boolean)
   * @see #setPackageAssertionStatus(String, boolean)
   * @see #clearAssertionStatus()
   * @since 1.4
   */
  public synchronized void setClassAssertionStatus(String name,
                                                   boolean enabled)
  {
    if (classAssertionStatus == null)
      classAssertionStatus = new HashMap(systemClassAssertionStatus);
    // The toString() hack catches null, as required.
    classAssertionStatus.put(name.toString(), Boolean.valueOf(enabled));
  }
  
  /**
   * Resets the default assertion status of this classloader, its packages
   * and classes, all to false. This allows overriding defaults inherited
   * from the command line.
   *
   * @see #setDefaultAssertionStatus(boolean)
   * @see #setClassAssertionStatus(String, boolean)
   * @see #setPackageAssertionStatus(String, boolean)
   * @since 1.4
   */
  public synchronized void clearAssertionStatus()
  {
    defaultAssertionStatus = false;
    packageAssertionStatus = new HashMap();
    classAssertionStatus = new HashMap();
  }

  /**
   * Return true if this loader is either the specified class loader
   * or an ancestor thereof.
   * @param loader the class loader to check
   */
  final boolean isAncestorOf(ClassLoader loader)
  {
    while (loader != null)
      {
	if (this == loader)
	  return true;
	loader = loader.parent;
      }
    return false;
  }
}