Mega Code Archive
Demonstration of a ClassLoader
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/*
* Copyright (c) Ian F. Darwin, http://www.darwinsys.com/, 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...");
}
}
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