什么是Java Proxy机制

动态代理其实就是java.lang.reflect.Proxy类动态的根据您指定的所有接口生成一个class byte,该class会继承Proxy类,并实现所有你指定的接口(你在参数中传入的接口数组);然后再利用您指定的classloader将 class byte加载进系统,最后生成这样一个类的对象,并初始化该对象的一些值,如invocationHandler,以即所有的接口对应的Method成员。 初始化之后将对象返回给调用的客户端。这样客户端拿到的就是一个实现你所有的接口的Proxy对象。请看实例分析:

一 业务接口类

public interface BusinessProcessor {
 public void processBusiness();
}

二 业务实现类

public class BusinessProcessorImpl implements BusinessProcessor {
 public void processBusiness() {
  System.out.println("processing business.....");
 }
}

三 业务代理类

import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;

public class BusinessProcessorHandler implements InvocationHandler {

 private Object target = null;

 BusinessProcessorHandler(Object target){
  this.target = target;
 }

 public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
  System.out.println("You can do something here before process your business");
  Object result = method.invoke(target, args);
  System.out.println("You can do something here after process your business");
  return result;
 }

}

四 客户端应用类

import java.lang.reflect.Field;
import java.lang.reflect.Method;
import java.lang.reflect.Modifier;
import java.lang.reflect.Proxy;

public class Test {

 public static void main(String[] args) {
  BusinessProcessorImpl bpimpl = new BusinessProcessorImpl();
  BusinessProcessorHandler handler = new BusinessProcessorHandler(bpimpl);
  BusinessProcessor bp = (BusinessProcessor)Proxy.newProxyInstance(bpimpl.getClass().getClassLoader(), bpimpl.getClass().getInterfaces(), handler);
  bp.processBusiness();
 }
}

现在我们看一下打印结果:

You can do something here before process your business
processing business.....
You can do something here after process your business

通过结果我们就能够很简单的看出Proxy的作用了,它能够在你的核心业务方法前后做一些你所想做的辅助工作,如log日志,安全机制等等。

分析一下上面的类的工作原理。

类1、2没什么好说的。先看看BusinessProcessorHandler吧。实现了InvocationHandler接口的invoke方法。其实这个类就是最终Proxy调用的固定接口方法。Proxy不管客户端的业务方法是怎么实现的。当客户端调用Proxy时,它只会调用InvocationHandlerinvoke接口,所以我们的真正实现的方法就必须在invoke方法中去调用。关系如下:

BusinessProcessorImpl bpimpl = new BusinessProcessorImpl();
BusinessProcessorHandler handler = new BusinessProcessorHandler(bpimpl);

BusinessProcessor bp = (BusinessProcessor)Proxy.newProxyInstance(....);

bp.processBusiness() --> invocationHandler.invoke() --> bpimpl.processBusiness();

那么bp到底是怎么样一个对象呢。我们改一下main方法看一下就知道了:

public static void main(String[] args) {
    BusinessProcessorImpl bpimpl = new BusinessProcessorImpl();
    BusinessProcessorHandler handler = new BusinessProcessorHandler(bpimpl);
    BusinessProcessor bp = (BusinessProcessor)Proxy.newProxyInstance(bpimpl.getClass().getClassLoader(), bpimpl.    getClass().getInterfaces(), handler);
    bp.processBusiness();
    System.out.println(bp.getClass().getName());
}

输出结果:

You can do something here before process your business
processing business.....
You can do something here after process your business
$Proxy0

bp原来是个$Proxy0这个类的对象。那么这个类到底是长什么样子呢?好的。我们再写二个方法去把这个类打印出来看个究竟,我们在main下面写如下两个静态方法。

package com.system.net.proxyjdk;

import java.lang.reflect.Field;
import java.lang.reflect.Method;
import java.lang.reflect.Modifier;
import java.lang.reflect.Proxy;

public class Test2 {

    public static void main(String[] args) {
        BusinessProcessorImpl bpimpl = new BusinessProcessorImpl();
        BusinessProcessorHandler handler = new BusinessProcessorHandler(bpimpl);
        BusinessProcessor bp = (BusinessProcessor) Proxy.newProxyInstance(bpimpl.getClass().getClassLoader(), bpimpl.getClass().getInterfaces(), handler);
        bp.processBusiness();
        System.out.println(bp.getClass().getName());
        Class clz = bp.getClass();
        printClassDefinition(clz);
    }

    public static String getModifier(int modifier) {
        String result = "";
        switch (modifier) {
            case Modifier.PRIVATE:
                result = "private";
            case Modifier.PUBLIC:
                result = "public";
            case Modifier.PROTECTED:
                result = "protected";
            case Modifier.ABSTRACT:
                result = "abstract";
            case Modifier.FINAL:
                result = "final";
            case Modifier.NATIVE:
                result = "native";
            case Modifier.STATIC:
                result = "static";
            case Modifier.SYNCHRONIZED:
                result = "synchronized";
            case Modifier.STRICT:
                result = "strict";
            case Modifier.TRANSIENT:
                result = "transient";
            case Modifier.VOLATILE:
                result = "volatile";
            case Modifier.INTERFACE:
                result = "interface";
        }
        return result;
    }

    public static void printClassDefinition(Class clz) {

        String clzModifier = getModifier(clz.getModifiers());
        if (clzModifier != null && !clzModifier.equals("")) {
            clzModifier = clzModifier + " ";
        }
        String superClz = clz.getSuperclass().getName();
        if (superClz != null && !superClz.equals("")) {
            superClz = "extends " + superClz;
        }

        Class[] interfaces = clz.getInterfaces();

        String inters = "";
        for (int i = 0; i < interfaces.length; i++) {
            if (i == 0) {
                inters += "implements ";
            }
            inters += interfaces[i].getName();
        }

        System.out.println(clzModifier + clz.getName() + " " + superClz + " " + inters);
        System.out.println("{");

        Field[] fields = clz.getDeclaredFields();
        for (int i = 0; i < fields.length; i++) {
            String modifier = getModifier(fields[i].getModifiers());
            if (modifier != null && !modifier.equals("")) {
                modifier = modifier + " ";
            }
            String fieldName = fields[i].getName();
            String fieldType = fields[i].getType().getName();
            System.out.println("    " + modifier + fieldType + " " + fieldName + ";");
        }

        System.out.println();

        Method[] methods = clz.getDeclaredMethods();
        for (int i = 0; i < methods.length; i++) {
            Method method = methods[i];

            String modifier = getModifier(method.getModifiers());
            if (modifier != null && !modifier.equals("")) {
                modifier = modifier + " ";
            }

            String methodName = method.getName();

            Class returnClz = method.getReturnType();
            String retrunType = returnClz.getName();

            Class[] clzs = method.getParameterTypes();
            String paraList = "(";
            for (int j = 0; j < clzs.length; j++) {
                paraList += clzs[j].getName();
                if (j != clzs.length - 1) {
                    paraList += ", ";
                }
            }
            paraList += ")";

            clzs = method.getExceptionTypes();
            String exceptions = "";
            for (int j = 0; j < clzs.length; j++) {
                if (j == 0) {
                    exceptions += "throws ";
                }

                exceptions += clzs[j].getName();

                if (j != clzs.length - 1) {
                    exceptions += ", ";
                }
            }

            exceptions += ";";

            String methodPrototype = modifier + retrunType + " " + methodName + paraList + exceptions;

            System.out.println("    " + methodPrototype);

        }
        System.out.println("}");
    }
}

现在我们再看看输出结果:

You can do something here before process your business
processing business.....
You can do something here after process your business
$Proxy0
$Proxy0 extends java.lang.reflect.Proxy implements com.tom.proxy.dynamic.BusinessProcessor
{
    java.lang.reflect.Method m4;
    java.lang.reflect.Method m2;
    java.lang.reflect.Method m0;
    java.lang.reflect.Method m3;
    java.lang.reflect.Method m1;

    void processBusiness();
    int hashCode();
    boolean equals(java.lang.Object);
    java.lang.String toString();
}

很明显,Proxy.newProxyInstance方法会做如下几件事:

  1. 根据传入的第二个参数interfaces动态生成一个类,实现interfaces中的接口,该例中即BusinessProcessor接口的processBusiness方法。并且继承了Proxy类,重写了hashcode,toString,equals等三个方法。具体实现可参看 ProxyGenerator.generateProxyClass(...); 该例中生成了$Proxy0类

  2. 通过传入的第一个参数classloder将刚生成的类加载到jvm中。即将$Proxy0类load

  3. 利用第三个参数,调用$Proxy0的$Proxy0(InvocationHandler)构造函数 创建$Proxy0的对象,并且用interfaces参数遍历其所有接口的方法,并生成Method对象初始化对象的几个Method成员变量

  4. 将$Proxy0的实例返回给客户端。

现在好了。我们再看客户端怎么调就清楚了。

  1. 客户端拿到的是$Proxy0的实例对象,由于$Proxy0继承了BusinessProcessor,因此转化为BusinessProcessor没任何问题。
    BusinessProcessor bp = (BusinessProcessor)Proxy.newProxyInstance(....);

  2. bp.processBusiness();

实际上调用的是$Proxy0.processBusiness();那么$Proxy0.processBusiness()的实现就是通过InvocationHandler去调用invoke`方法!







发表于: 8月前   最后更新时间: 8月前   游览量:665
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