Java/Language Basics/Class Loader

Demonstration of a ClassLoader

/*
 * Copyright (c) Ian F. Darwin, http://www.darwinsys.ru/, 1996-2002.
 * All rights reserved. Software written by Ian F. Darwin and others.
 * $Id: LICENSE,v 1.8 2004/02/09 03:33:38 ian Exp $
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 *
 * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS""
 * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED
 * TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
 * PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS
 * BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
 * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
 * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
 * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
 * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
 * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
 * POSSIBILITY OF SUCH DAMAGE.
 * 
 * Java, the Duke mascot, and all variants of Sun"s Java "steaming coffee
 * cup" logo are trademarks of Sun Microsystems. Sun"s, and James Gosling"s,
 * pioneering role in inventing and promulgating (and standardizing) the Java 
 * language and environment is gratefully acknowledged.
 * 
 * The pioneering role of Dennis Ritchie and Bjarne Stroustrup, of AT&T, for
 * inventing predecessor languages C and C++ is also gratefully acknowledged.
 */
import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;
import java.util.Hashtable;
/**
 * Demonstration of a ClassLoader
 */
public class ClassLoaderDemo1 extends ClassLoader {
  /** The Hashtable to keep track of classes, to avoid re-loading them */
  protected Hashtable cache = new Hashtable();
  /** data"s expected length */
  private final int dataLength = 433;
  /** data, obtained by dumping a compiled .class file */
  private int[] data = { 202, 254, 186, 190, 0, 3, 0, 45, 0, 31, 8, 0, 20, 7,
      0, 17, 7, 0, 25, 7, 0, 26, 7, 0, 27, 10, 0, 4, 0, 9, 9, 0, 5, 0,
      10, 10, 0, 3, 0, 11, 12, 0, 14, 0, 12, 12, 0, 28, 0, 22, 12, 0, 29,
      0, 13, 1, 0, 3, 40, 41, 86, 1, 0, 21, 40, 76, 106, 97, 118, 97, 47,
      108, 97, 110, 103, 47, 83, 116, 114, 105, 110, 103, 59, 41, 86, 1,
      0, 6, 60, 105, 110, 105, 116, 62, 1, 0, 4, 67, 111, 100, 101, 1, 0,
      13, 67, 111, 110, 115, 116, 97, 110, 116, 86, 97, 108, 117, 101, 1,
      0, 4, 68, 101, 109, 111, 1, 0, 9, 68, 101, 109, 111, 46, 106, 97,
      118, 97, 1, 0, 10, 69, 120, 99, 101, 112, 116, 105, 111, 110, 115,
      1, 0, 10, 72, 101, 108, 108, 111, 32, 74, 97, 118, 97, 1, 0, 15,
      76, 105, 110, 101, 78, 117, 109, 98, 101, 114, 84, 97, 98, 108,
      101, 1, 0, 21, 76, 106, 97, 118, 97, 47, 105, 111, 47, 80, 114,
      105, 110, 116, 83, 116, 114, 101, 97, 109, 59, 1, 0, 14, 76, 111,
      99, 97, 108, 86, 97, 114, 105, 97, 98, 108, 101, 115, 1, 0, 10, 83,
      111, 117, 114, 99, 101, 70, 105, 108, 101, 1, 0, 19, 106, 97, 118,
      97, 47, 105, 111, 47, 80, 114, 105, 110, 116, 83, 116, 114, 101,
      97, 109, 1, 0, 16, 106, 97, 118, 97, 47, 108, 97, 110, 103, 47, 79,
      98, 106, 101, 99, 116, 1, 0, 16, 106, 97, 118, 97, 47, 108, 97,
      110, 103, 47, 83, 121, 115, 116, 101, 109, 1, 0, 3, 111, 117, 116,
      1, 0, 7, 112, 114, 105, 110, 116, 108, 110, 1, 0, 4, 116, 101, 115,
      116, 0, 33, 0, 2, 0, 4, 0, 0, 0, 0, 0, 2, 0, 9, 0, 30, 0, 12, 0, 1,
      0, 15, 0, 0, 0, 37, 0, 2, 0, 0, 0, 0, 0, 9, 178, 0, 7, 18, 1, 182,
      0, 8, 177, 0, 0, 0, 1, 0, 21, 0, 0, 0, 10, 0, 2, 0, 0, 0, 9, 0, 8,
      0, 8, 0, 1, 0, 14, 0, 12, 0, 1, 0, 15, 0, 0, 0, 29, 0, 1, 0, 1, 0,
      0, 0, 5, 42, 183, 0, 6, 177, 0, 0, 0, 1, 0, 21, 0, 0, 0, 6, 0, 1,
      0, 0, 0, 7, 0, 1, 0, 24, 0, 0, 0, 2, 0, 18 };
  /** "load", that is, make up, the data for the class */
  private byte[] genClassData(String name) {
    if (dataLength != data.length)
      throw new IllegalArgumentException("data corrupt");
    byte[] bd = new byte[data.length];
    for (int i = 0; i < bd.length; i++)
      bd[i] = (byte) data[i];
    return bd;
  }
  public synchronized Class loadClass(String name, boolean resolve)
      throws ClassNotFoundException {
    /**
     * We can expect to be called to resolve at least demo"s superclass
     * (java.lang.Object). Fortunatetely, we can just use
     * super.findSystemClass() to load such things...
     */
    if (name.startsWith("java.")) {
      System.out.println("loadClass: SystemLoading " + name);
      return findSystemClass(name);
    }
    Class c = (Class) cache.get(name);
    if (c == null) {
      System.out.println("loadClass: About to genClassData " + name);
      byte mydata[] = genClassData(name);
      System.out.println("loadClass: About to defineClass " + name);
      c = defineClass(name, mydata, 0, mydata.length);
      System.out.println("loadClass: storing " + name + " in cache.");
      cache.put(name, c);
    } else
      System.out.println("loadClass: found " + name + " in cache.");
    if (resolve) {
      System.out.println("loadClass: About to resolveClass " + name);
      resolveClass(c);
    }
    return c;
  }
  public static void main(String[] argv) {
    System.out.println("ClassLoaderDemo1 starting");
    ClassLoaderDemo1 loader = new ClassLoaderDemo1();
    Class c = null;
    Object demo;
    try {
      /* Load the "Demo" class from memory */
      System.out.println("About to load class  Demo");
      c = loader.loadClass("Demo", true);
      System.out.println("About to instantiate class Demo");
      demo = c.newInstance();
      System.out.println("Got Demo class loaded: " + demo);
      /* Now try to call a method */
      Method mi = c.getMethod("test", null);
      mi.invoke(demo, null);
    } catch (InvocationTargetException e) {
      // The invoked method threw an exception. We get it
      // wrapped up inside another exception, hence the
      // extra call here:
      e.getTargetException().printStackTrace();
      System.out.println("Could not run test method");
    } catch (Exception e) {
      e.printStackTrace();
      System.out.println("Could not run test method");
    }
    /**
     * Try to load some arbitrary class, to see if our ClassLoader gets
     * called.
     */
    System.out.println("Trying to load an unrelated class");
    java.awt.image.DirectColorModel jnk = new java.awt.image.DirectColorModel(
        24, 8, 8, 8);
    System.out
        .println("Load an unrelated class - was your ClassLoader called?");
    /** Try to instantiate a second ClassLoader */
    System.out.println("Trying to install another ClassLoader");
    ClassLoaderDemo1 loader2 = new ClassLoaderDemo1();
    System.out.println("Instantiated another ClassLoader...");
  }
}

Runs a jar application from any url

/* From http://java.sun.ru/docs/books/tutorial/index.html */
/*
 * Copyright (c) 2006 Sun Microsystems, Inc. All Rights Reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions are met:
 *
 * -Redistribution of source code must retain the above copyright notice, this
 *  list of conditions and the following disclaimer.
 *
 * -Redistribution in binary form must reproduce the above copyright notice,
 *  this list of conditions and the following disclaimer in the documentation
 *  and/or other materials provided with the distribution.
 *
 * Neither the name of Sun Microsystems, Inc. or the names of contributors may
 * be used to endorse or promote products derived from this software without
 * specific prior written permission.
 *
 * This software is provided "AS IS," without a warranty of any kind. ALL
 * EXPRESS OR IMPLIED CONDITIONS, REPRESENTATIONS AND WARRANTIES, INCLUDING
 * ANY IMPLIED WARRANTY OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE
 * OR NON-INFRINGEMENT, ARE HEREBY EXCLUDED. SUN MIDROSYSTEMS, INC. ("SUN")
 * AND ITS LICENSORS SHALL NOT BE LIABLE FOR ANY DAMAGES SUFFERED BY LICENSEE
 * AS A RESULT OF USING, MODIFYING OR DISTRIBUTING THIS SOFTWARE OR ITS
 * DERIVATIVES. IN NO EVENT WILL SUN OR ITS LICENSORS BE LIABLE FOR ANY LOST
 * REVENUE, PROFIT OR DATA, OR FOR DIRECT, INDIRECT, SPECIAL, CONSEQUENTIAL,
 * INCIDENTAL OR PUNITIVE DAMAGES, HOWEVER CAUSED AND REGARDLESS OF THE THEORY
 * OF LIABILITY, ARISING OUT OF THE USE OF OR INABILITY TO USE THIS SOFTWARE,
 * EVEN IF SUN HAS BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGES.
 *
 * You acknowledge that this software is not designed, licensed or intended
 * for use in the design, construction, operation or maintenance of any
 * nuclear facility.
 */
import java.io.IOException;
import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;
import java.lang.reflect.Modifier;
import java.net.JarURLConnection;
import java.net.MalformedURLException;
import java.net.URL;
import java.net.URLClassLoader;
import java.util.jar.Attributes;
/**
 * Runs a jar application from any url. Usage is "java JarRunner url [args..]"
 * where url is the url of the jar file and args is optional arguments to be
 * passed to the application"s main method.
 */
public class JarRunner {
  public static void main(String[] args) {
    if (args.length < 1) {
      usage();
    }
    URL url = null;
    try {
      url = new URL(args[0]);
    } catch (MalformedURLException e) {
      fatal("Invalid URL: " + args[0]);
    }
    // Create the class loader for the application jar file
    JarClassLoader cl = new JarClassLoader(url);
    // Get the application"s main class name
    String name = null;
    try {
      name = cl.getMainClassName();
    } catch (IOException e) {
      System.err.println("I/O error while loading JAR file:");
      e.printStackTrace();
      System.exit(1);
    }
    if (name == null) {
      fatal("Specified jar file does not contain a "Main-Class""
          + " manifest attribute");
    }
    // Get arguments for the application
    String[] newArgs = new String[args.length - 1];
    System.arraycopy(args, 1, newArgs, 0, newArgs.length);
    // Invoke application"s main class
    try {
      cl.invokeClass(name, newArgs);
    } catch (ClassNotFoundException e) {
      fatal("Class not found: " + name);
    } catch (NoSuchMethodException e) {
      fatal("Class does not define a "main" method: " + name);
    } catch (InvocationTargetException e) {
      e.getTargetException().printStackTrace();
      System.exit(1);
    }
  }
  private static void fatal(String s) {
    System.err.println(s);
    System.exit(1);
  }
  private static void usage() {
    fatal("Usage: java JarRunner url [args..]");
  }
}
/**
 * A class loader for loading jar files, both local and remote.
 */
class JarClassLoader extends URLClassLoader {
  private URL url;
  /**
   * Creates a new JarClassLoader for the specified url.
   * 
   * @param url
   *            the url of the jar file
   */
  public JarClassLoader(URL url) {
    super(new URL[] { url });
    this.url = url;
  }
  /**
   * Returns the name of the jar file main class, or null if no "Main-Class"
   * manifest attributes was defined.
   */
  public String getMainClassName() throws IOException {
    URL u = new URL("jar", "", url + "!/");
    JarURLConnection uc = (JarURLConnection) u.openConnection();
    Attributes attr = uc.getMainAttributes();
    return attr != null ? attr.getValue(Attributes.Name.MAIN_CLASS) : null;
  }
  /**
   * Invokes the application in this jar file given the name of the main class
   * and an array of arguments. The class must define a static method "main"
   * which takes an array of String arguemtns and is of return type "void".
   * 
   * @param name
   *            the name of the main class
   * @param args
   *            the arguments for the application
   * @exception ClassNotFoundException
   *                if the specified class could not be found
   * @exception NoSuchMethodException
   *                if the specified class does not contain a "main" method
   * @exception InvocationTargetException
   *                if the application raised an exception
   */
  public void invokeClass(String name, String[] args)
      throws ClassNotFoundException, NoSuchMethodException,
      InvocationTargetException {
    Class c = loadClass(name);
    Method m = c.getMethod("main", new Class[] { args.getClass() });
    m.setAccessible(true);
    int mods = m.getModifiers();
    if (m.getReturnType() != void.class || !Modifier.isStatic(mods)
        || !Modifier.isPublic(mods)) {
      throw new NoSuchMethodException("main");
    }
    try {
      m.invoke(null, new Object[] { args });
    } catch (IllegalAccessException e) {
      // This should not happen, as we have disabled access checks
    }
  }
}