Java/Language Basics/Class Loader
Demonstration of a ClassLoader
<source lang="java">
/*
* 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..."); }
}
</source>
Runs a jar application from any url
<source lang="java">
/* 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 } }
}
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