java class loader code example

Example 1: custom classloader

package test.main;
 
import java.io.DataInputStream;
import java.io.File;
import java.io.IOException;
import java.io.InputStream;
 
public class MyClassLoader extends ClassLoader {
 
	public MyClassLoader(ClassLoader parent) {
		super(parent);
	}
 
	@Override
	public Class<?> loadClass(String name) throws ClassNotFoundException {
		System.out.println("Class Loading Started for " + name);
		if (name.startsWith("test")) {
			return getClass(name);
		}
		return super.loadClass(name);
	}
 
	/**
	 * Loading of class from .class file
	 * happens here You Can modify logic of
	 * this method to load Class 
	 * from Network or any other source
	 * @param name
	 * @return
	 * @throws ClassNotFoundException
	 */
	private Class<?> getClass(String name) throws ClassNotFoundException {
		System.out.println("*********Inside getClass*********");
 
		String file = name.replace('.', File.separatorChar) + ".class";
		System.out.println("Name of File" + file);
		byte[] byteArr = null;
		try {
			// This loads the byte code data from the file
			byteArr = loadClassData(file);
			System.out.println("Size of byte array "+byteArr.length);
			Class<?> c = defineClass(name, byteArr, 0, byteArr.length);
			resolveClass(c);
			return c;
		} catch (IOException e) {
			e.printStackTrace();
			return null;
		}
	}
 
	/**
	 * Loads a given file and converts
	 * it into a Byte Array
	 * @param name
	 * @return
	 * @throws IOException
	 */
	private byte[] loadClassData(String name) throws IOException {
 
		System.out.println("<<<<<<<<<Inside loadClassData>>>>>>");
 
		InputStream stream = getClass().getClassLoader().getResourceAsStream(
				name);
		int size = stream.available();
		byte buff[] = new byte[size];
		DataInputStream in = new DataInputStream(stream);
		// Reading the binary data
		in.readFully(buff);
		in.close();
		return buff;
	}
 
}

Example 2: custom classloader

package test.main;
 
import java.lang.reflect.InvocationTargetException;
 
public class Test {
 
	public static void main(String[] args) throws ClassNotFoundException, InstantiationException, IllegalAccessException, IllegalArgumentException, InvocationTargetException, NoSuchMethodException, Exception {
 
		MyClassLoader loader = new MyClassLoader(
	            Test.class.getClassLoader());
		
		System.out.println("loader name---- " +loader.getParent().getClass().getName());
		
		//This Loads the Class we must always
		//provide binary name of the class
		Class<?> clazz =
	            loader.loadClass("test.frugalis.Frugalis");
		
        System.out.println("Loaded class name: " + clazz.getName());
 
        //Create instance Of the Class and invoke the particular method
        Object instance = clazz.newInstance();
        
        clazz.getMethod("printMyName").invoke(instance);
	}
}

Example 3: custom classloader

loader name---- sun.misc.Launcher$AppClassLoader
Class Loading Started for test.frugalis.Frugalis
*********Inside getClass*********
Name of Filetest\frugalis\Frugalis.class
<<<<<<<<<Inside loadClassData>>>>>>
Size of byte array 519
Class Loading Started for java.lang.Object
Loaded class name: test.frugalis.Frugalis
Class Loading Started for java.lang.System
Class Loading Started for java.io.PrintStream
Welcome Frugalis Minds

Example 4: custom classloader

package test.frugalis;
 
public class Frugalis {
 
	public void printMyName(){
		System.out.println("Welcome Frugalis Minds");
	}
}