关键代码

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interface Interface{
    void doSth(String s);
}

class RealObject implments Interface {
    void doSth(String s){
        //...
    }
}

class DynamicProxyHandler implments InvocationHandler {
    private Object proxied;
    public DynamicProxyHandler(Object proxied) {
        this.proxied = proxied;
    }
    
    public Object invoke(Object proxy, Method m, Object[] args){
        /*
        	在此插入代理逻辑
        */
        return m.invoke(proxied, args);
    }
}


public class SimpleDynamicProxy {
 	Interface proxy = (Interface) Proxy.newInstance(
    	Interface.class.getClassLoader(), //一般传这个已加载的类加载器就行
        new Class[]{Interface.class}, //实现的接口
        new DynamicProxyHandler(new RealObject())
    );
    
    proxy.doSth();
}

可能生成如下代理类

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final class $proxy0 extends Proxy implments Interface {
    
    public $Proxy0(InvocationHandler var1) 	{
        super(var1);
    }
    
    public final void doSth(String var){
        try{
            // 这里的h即上文传入的 new DynamicProxyHandler(new RealObject())
            super.h.invoke(this, m4, new Object[]{var});
        }catch{
            ...
        }
    }
    
    static{
        try{
            m4=Class.forName("ProxyDemo$Interface").getMethod("doSth", Class.forName("java.lang.String"))
        }catch{
            ...
        }
    }
    
}

Proxy 类的源码简单说：先是生成构造器，再用构造器newInstance生成动态代理对象。