@Override
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
System.out.print("say:");
return method.invoke(target, args);
}
}
public static void main(String[] args) {
Subject subject=new SubjectImpl();
Subject subjectProxy=(Subject) Proxy.newProxyInstance(subject.getClass().getClassLoader(), subject.getClass().getInterfaces(), new ProxyInvocationHandler(subject));
subjectProxy.sayHi();
subjectProxy.sayHello();
}
}
只看main方法的话，只有第二行和之前的静态代理不同，同样是生成一个subjectProxy代理对象，只是生成的代码不同了。静态代理是直接new 一个SubjectImplProxy的实例，而动态代理则调用了java.lang.reflect.Proxy.newProxyInstance()方法，我们来看一下这个方法的源码：
public static Object newProxyInstance(ClassLoader loader,
Class<?>[] interfaces,
InvocationHandler h)
throws IllegalArgumentException
{
if (h == null) {
throw new NullPointerException();
}
/*
* Look up or generate the designated proxy class.
*/
Class<?> cl = getProxyClass(loader, interfaces); //获取代理类的Class
/*
* Invoke its constructor with the designated invocation handler.
*/
try {
Constructor cons = cl.getConstructor(constructorParams); //constructorParams是写死的：{ InvocationHandler.class }，上边返回的代理类Class一定是extends Proxy的，而Proxy有一个参数为InvocationHandler的构造函数
return cons.newInstance(new Object[] { h }); //这里通过构造函数将我们自己定义的InvocationHandler的子类传到代理类的实例里，当我们调用代理类的任何方法时，实际上都会调用我们定义的InvocationHandler子类重写的invoke()函数
} catch (NoSuchMethodException e) {
throw new InternalError(e.toString());
} catch (IllegalAccessException e) {
throw new InternalError(e.toString());
} catch (InstantiationException e) {
throw new InternalError(e.toString());
} catch (InvocationTargetException e) {
throw new InternalError(e.toString());
}
}
上面的 Class<?> cl = getProxyClass(loader, interfaces); 调用的getProxyClass方法：
public static Class<?> getProxyClass(ClassLoader loader,
Class<?>... interfaces)
throws IllegalArgumentException
{
if (interfaces.length > 65535) { //因为在class文件中，一个类保存的接口数量是用2个字节来表示的，因此java中一个类最多可以实现65535个接口
throw new IllegalArgumentException("interface limit exceeded");
}
Class<?> proxyClass = null;
/* collect interface names to use as key for proxy class cache */
String[] interfaceNames = new String[interfaces.length];
// for detecting duplicates
Set<Class<?>> interfaceSet = new HashSet<>();
//验证interfaces里的接口是否能被类加载器加载，是否是接口，是否有重复的
for (int i = 0; i < interfaces.length; i++) {
/*
* Verify that the class loader resolves the name of this
* interface to the same Class object.
*/
String interfaceName = interfaces[i].getName();
Class<?> interfaceClass = null;
try {
interfaceClass = Class.forName(interfaceName, false, loader);
} catch (ClassNotFoundException e) {
}
if (interfaceClass != interfaces[i]) {
throw new IllegalArgumentException(
interfaces[i] + " is not visible from class loader");
}