Java动态代理

Posted 2023-02-22 蜡笔小斌

看java核心技术的时候，感觉动态代理没有理解好，于是查找了一些文档，写下这篇文章

首先，你需要熟悉代理模式，至少是静态代理，才可以很快理解这篇文章。

Dynamic Proxy是在运行时生成的class，在生成它时你必须提供一 组interface给它，然后该class就宣称它实现了这些 interface。你当然可以把该class的实 例当作这些interface中的任何一个来用。当然啦，这个Dynamic Proxy其实就是一个Proxy， 它不会替你作实质性的工作，在生成它的实例时你必须提供一个handler，由它接管实际的工 作。

A dynamic proxy class is a class that implements a list of interfaces specified at runtime such that a method invocation through one of the interfaces on an instance of the class will be encoded and dispatched to another object through a uniform interface. It can be used to create a type-safe proxy object for a list of interfaces without requiring pre-generation of the proxy class. Dynamic proxy classes are useful to an application or library that needs to provide type-safe reflective dispatch of invocations on objects that present interface APIs.

通过动态代理技术，可以很方便地实现类似Spring AOP一样的方法拦截器

public interface Foo 
    Object bar(Object obj) throws BazException;


public class FooImpl implements Foo 
    Object bar(Object obj) throws BazException 
        // ...
    


public class DebugProxy implements java.lang.reflect.InvocationHandler 

    private Object obj;

    public static Object newInstance(Object obj) 
        return java.lang.reflect.Proxy.newProxyInstance(
            obj.getClass().getClassLoader(),
            obj.getClass().getInterfaces(),
            new DebugProxy(obj));
    

    private DebugProxy(Object obj) 
        this.obj = obj;
    

    public Object invoke(Object proxy, Method m, Object[] args)
        throws Throwable
    
        Object result;
        try 
            System.out.println("before method " + m.getName());
            result = m.invoke(obj, args);
         catch (InvocationTargetException e) 
            throw e.getTargetException();
         catch (Exception e) 
            throw new RuntimeException("unexpected invocation exception: " +
                                       e.getMessage());
         finally 
            System.out.println("after method " + m.getName());
        
        return result;
    

参考文献

java代理机制
oracle doc