Custom class loader in Java

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/** Custom class loader for native classes

* @author sajal.maity

*

*/

package com.mycomp.component.core;

import java.io.ByteArrayOutputStream;

import java.io.DataInputStream;

import java.io.File;

import java.io.FileInputStream;

import java.io.InputStream;

import java.net.URL;

import java.util.Hashtable;

import java.util.jar.JarEntry;

import java.util.jar.JarFile;

import net.refractions.udig.catalog.URLUtils;

/**

* NativeLoader loads native classes

*

* @author sajal.maity Oct 14, 2010

*

*/

public class NativeLoader extends ClassLoader {

private static String classPath;

static {

URL location = null;

try {

location = CorePlugin.class.getProtectionDomain().getCodeSource().getLocation();

} catch (Exception e) {

e.printStackTrace();

}

File file = URLUtils.urlToFile(location);

classPath = file.getAbsolutePath();

if (classPath.endsWith(".jar") == false) {

classPath = classPath + File.separator + "bin";

}

}

private Hashtable classes = new Hashtable(); // used to cache already

private NativeLoader() {

super(NativeLoader.class.getClassLoader());

}

public static NativeLoader getInstance() {

if (instance == null) {

instance = new NativeLoader();

}

return instance;

}

public static void dispose() {

instance = null;

}

public void finalize() {

System.out.println(this + " is garbage collected.");

}

public String toString() {

return NativeLoader.class.getName();

}

/**

* Uncomment the following method if you want to load all child class using

* the parent class loader

*/

/*

* public Class loadClass(String className) throws ClassNotFoundException {

* return findClass(className); }

*/

public Class findClass(String className) {

byte classByte[];

Class result = null;

result = (Class) classes.get(className); // checks in cached classes

if (result != null) {

return result;

}

try {

return findSystemClass(className);

} catch (Exception e) {

}

if (classPath.endsWith(".jar")) {

try {

result = loadFromJar(className, classPath);

} catch (Exception e) {

e.printStackTrace();

result = null;

}

} else {

result = loadFromFile(className);

}

return result;

}

private Class loadFromJar(String className, String jarFile)

throws Exception {

byte classByte[];

Class result = null;

String name = className.replace('.', '/') + ".class";

try {

JarFile jar = new JarFile(jarFile);

JarEntry entry = jar.getJarEntry(name);

InputStream is = jar.getInputStream(entry);

ByteArrayOutputStream byteStream = new ByteArrayOutputStream();

int nextValue = is.read();

while (-1 != nextValue) {

byteStream.write(nextValue);

nextValue = is.read();

}

classByte = byteStream.toByteArray();

result = defineClass(className, classByte, 0, classByte.length,

null);

classes.put(className, result);

return result;

} catch (Exception e) {

return null;

}

}

public synchronized Class loadFromFile(String name) {

Class result = null;

String filepath = classPath + File.separatorChar;

filepath = filepath + name.replace('.', File.separatorChar) + ".class";

try {

File file = new File(filepath);

int size = (int) file.length();

byte buff[] = new byte[size];

DataInputStream in = new DataInputStream(new FileInputStream(

filepath));

in.readFully(buff);

in.close();

result = defineClass(name, buff, 0, buff.length);

classes.put(name, result);

resolveClass(result);

return result;

} catch (Exception e) {

e.printStackTrace();

return null;

}

}

private static NativeLoader instance;

}