自定义ClassLoader

重写父类方法findClass

package classloader;

import java.io.FileInputStream;
import java.io.IOException;
import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;

public class MyClassLoader extends ClassLoader{

	
	private String url;
	public MyClassLoader(String url) {
		super();
		this.url = url;
	}
	@Override
	public Class<?> findClass(String name)  {
		byte[] classDate = null;
		try {
			FileInputStream f = new FileInputStream(url);

			int num = f.available();
			classDate = new byte[num];
			f.read(classDate);
		} catch (IOException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		}
		Class x = defineClass(name, classDate, 0, classDate.length);
		return x;
	}
	
	public static void main(String args[]) throws ClassNotFoundException, SecurityException, NoSuchMethodException, IllegalArgumentException, IllegalAccessException, InvocationTargetException {
		ClassLoader loader = new MyClassLoader("E:/workspace/woca/bin/classloader/NewClassloader.class");
		Class c = loader.loadClass("classloader.NewClassloader");
		Object myarg[] = {null};
		Class arg[] = {(new String[1]).getClass()};
		Method m =c.getMethod("main", (new String[1]).getClass());
		m.invoke(null, myarg);
	}


}

java classloader api

    protected synchronized Class<?> loadClass(String name, boolean resolve)
	throws ClassNotFoundException
    {
	// First, check if the class has already been loaded
	Class c = findLoadedClass(name);
	if (c == null) {//双亲委派模式加载
	    try {
		if (parent != null) {
		    c = parent.loadClass(name, false);
		} else {
		    c = findBootstrapClassOrNull(name);
		}
	    } catch (ClassNotFoundException e) {
                // ClassNotFoundException thrown if class not found
                // from the non-null parent class loader
            }
            if (c == null) {
	        // If still not found, then invoke findClass in order
	        // to find the class.
	        c = findClass(name);
	    }
	}
	if (resolve) {
	    resolveClass(c);
	}
	return c;
    }