java class reload 动态加载

在运行时动态加载jar包容易，但是如果想动态替换运行时的类，需要定义自己的classloader , 用自己的classloader 来load这个类。具体代码如下。

public class UClassloader extends URLClassLoader{
	   public UClassloader(URL[] urls) {
		super(urls);
	}

    public static void main(String args[]) throws IOException, SecurityException, ClassNotFoundException, IllegalArgumentException, InstantiationException, IllegalAccessException, InvocationTargetException, NoSuchMethodException, InterruptedException{
    	File f = new File("/Users/fcc/workspace/edm/myclass.jar");    	
        UClassloader u = new UClassloader(new URL[]{f.toURI().toURL()});
        Class<?> cs = u.loadClass("org.apache.lucene.demo.classloader.MyClass");
        Interface instance = (Interface)cs.newInstance();
        instance.say();
        u.close();
        Thread.sleep(15*1000);        
        UClassloader uAgain = new UClassloader(new URL[]{f.toURI().toURL()});
        Constructor<?> csAgain = uAgain.loadClass("org.apache.lucene.demo.classloader.MyClass").getConstructor();
        Interface instanceAgain = (Interface)csAgain.newInstance();
        instanceAgain.say();
        uAgain.close();
    }

 注意，想从jar包中加载的类千万不能在现有的JVM中存在， 否则自定义的classloader不会从jar包中加载，而从现有的JVM中查找，加载。