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
}
}
}
</source>
