JDK动态代理源码解析

1. 动态代码使用示例

public interface Calculator {
    public int add(int i, int j);
}
//被代理类
public class MyCalculator implements Calculator {
    public int add(int i, int j) {
        int result = i + j;
        return result;
    }


}
//
public class CalculatorProxy {
    public static Calculator getProxy(final Calculator calculator){
        //获取加载器
        ClassLoader loader = calculator.getClass().getClassLoader();
        //获取接口
        Class<?>[] interfaces = calculator.getClass().getInterfaces();
        InvocationHandler h = new InvocationHandler() {
            public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
                Object result = null;
                try {
                    // calculator是owner，调用具体的方法
                    // 根据method调用指定的方法
                    result = method.invoke(calculator, args);
                } catch (Exception e) {
                } finally {
                }
                return result;
            }
        };
        Object proxy = Proxy.newProxyInstance(loader, interfaces, h);
        return (Calculator) proxy;
    }
}

//测试
public static void main(String[] args) {    System.getProperties().put("sun.misc.ProxyGenerator.saveGeneratedFiles", "true");
        Calculator proxy = CalculatorProxy.getProxy(new MyCalculator());
        proxy.add(1,1);//到$Proxy0.class ，handler.invoke -> add方法
        System.out.println(proxy.getClass());
    }

2. 源码实现

入口Object proxy = Proxy.newProxyInstance(loader, interfaces, h);

public static Object newProxyInstance(ClassLoader loader, Class<?>[] interfaces,                       InvocationHandler h){
    //省略若干
    // 获取Class对象，其中完成对被代理接口方法的查询封装
    //流程如下—>proxyClassCache.get(loader, interfaces)->supplier.get();即调到WeakCache.Factory#get()方法->Objects.requireNonNull(valueFactory.apply(key, parameter))->到达ProxyClassFactory内部类的apply()
     Class<?> cl = getProxyClass0(loader, intfs);
    // 生成代理对象
     return cons.newInstance(new Object[]{h});
    
} 

注意生成代理对象：T inst = (T) ca.newInstance(initargs); 其中会调用

protected Proxy(InvocationHandler h) {    Objects.requireNonNull(h);    this.h = h;}

将InvocationHandler传进去，供后面使用。

2.1 讲解Proxy#ProxyClassFactory 内部类

 private static final class ProxyClassFactory
        implements BiFunction<ClassLoader, Class<?>[], Class<?>>
 {
     //省略若干
     //代理类名字前缀
        private static final String proxyClassNamePrefix = "$Proxy";
//原子操作
        private static final AtomicLong nextUniqueNumber = new AtomicLong();
     //... 省略包名的处理....
      long num = nextUniqueNumber.getAndIncrement();
     //代理类名称      
     String proxyName = proxyPkg + proxyClassNamePrefix + num;
     
  //核心逻辑。 处理被代理的接口，
     //1.将其中的方法放到methods中，且增加了hashCode、equals、toString及含有InvocationHandler的构造方法,生成的静态代码块也添加进methods中
     //2.根据上面的methods 生成文件
   byte[] proxyClassFile = ProxyGenerator.generateProxyClass(
                proxyName, interfaces, accessFlags);
     
 }

生成的代理class文件如下

//
// Source code recreated from a .class file by IntelliJ IDEA
// (powered by Fernflower decompiler)
//

package com.sun.proxy;

import com.songbl.jdk.Calculator;
import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;
import java.lang.reflect.Proxy;
import java.lang.reflect.UndeclaredThrowableException;

public final class $Proxy0 extends Proxy implements Calculator {
    private static Method m1;
    private static Method m2;
    private static Method m3;
    private static Method m0;

   //构造函数
   // 父类：protected Proxy(InvocationHandler h) {
   //     Objects.requireNonNull(h);
   //     this.h = h;
   // }
    public $Proxy0(InvocationHandler var1) throws  {
        super(var1);
    }

    public final boolean equals(Object var1) throws  {
        try {
            // 执行InvocationHandler的invoke方法，转移到hander的方法中去了。此处的h就是构造函数传递进来的
            return (Boolean)super.h.invoke(this, m1, new Object[]{var1});
        } catch (RuntimeException | Error var3) {
            throw var3;
        } catch (Throwable var4) {
            throw new UndeclaredThrowableException(var4);
        }
    }

    public final String toString() throws  {
        try {
            return (String)super.h.invoke(this, m2, (Object[])null);
        } catch (RuntimeException | Error var2) {
            throw var2;
        } catch (Throwable var3) {
            throw new UndeclaredThrowableException(var3);
        }
    }

    public final int add(int var1, int var2) throws  {
        try {
            return (Integer)super.h.invoke(this, m3, new Object[]{var1, var2});
        } catch (RuntimeException | Error var4) {
            throw var4;
        } catch (Throwable var5) {
            throw new UndeclaredThrowableException(var5);
        }
    }

    public final int hashCode() throws  {
        try {
            return (Integer)super.h.invoke(this, m0, (Object[])null);
        } catch (RuntimeException | Error var2) {
            throw var2;
        } catch (Throwable var3) {
            throw new UndeclaredThrowableException(var3);
        }
    }

    static {
        try {
            m1 = Class.forName("java.lang.Object").getMethod("equals", Class.forName("java.lang.Object"));
            m2 = Class.forName("java.lang.Object").getMethod("toString");
            m3 = Class.forName("com.songbl.jdk.Calculator").getMethod("add", Integer.TYPE, Integer.TYPE);
            m0 = Class.forName("java.lang.Object").getMethod("hashCode");
        } catch (NoSuchMethodException var2) {
            throw new NoSuchMethodError(var2.getMessage());
        } catch (ClassNotFoundException var3) {
            throw new NoClassDefFoundError(var3.getMessage());
        }
    }
}

3 思考

我的理解是这样：代理对象调用方法，到达$Proxy0.class的指定方法，本例是add

Calculator proxy = CalculatorProxy.getProxy(new MyCalculator());
 proxy.add(1,1);//到$Proxy0.class ，handler.invoke -> add方法

那么继续调用h(InvocationHandler，在创建代理对象的时候传进来，cons.newInstance(new Object[]{h}))，进而转移到 Handler的invoke()中

$Proxy0.class
  
public final int add(int var1, int var2) throws  {
        try {
        //h是InvokeHandler 对象，m3是指定的方法，进而完成代理的调用
            return (Integer)super.h.invoke(this, m3, new Object[]{var1, var2});
        } catch (RuntimeException | Error var4) {
            throw var4;
        } catch (Throwable var5) {
            throw new UndeclaredThrowableException(var5);
        }
    }