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JAVA 动态代理

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标签:col   target   href   IV   需求   AC   struct   exce   const   

与oc的消息转发类似。
 
原文出处: forever

具体场景

为了使代理类和被代理类对第三方有相同的函数,代理类和被代理类一般实现一个公共的interface,该interface定义如下

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public interface Calculator {

    public Integer add(Integer num1, Integer num2);

    public Integer minus(Integer num1, Integer num2);

}

被代理类定义如下

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public class CalculatorImpl implements Calculator {

    @Override

    public Integer add(Integer num1, Integer num2) {

        int ret = num1 + num2;

        System.out.println("in calculatorImpl, res: " + ret);

        return ret;

    }

     

    @Override

    public Integer minus(Integer num1, Integer num2) {

        int ret = num1 - num2;

        System.out.println("int calculatorImpl, res: " + ret);

        return ret;

    }

}

代理需求:在add函数和minus函数调用前后分别输出before invocation和after invocation字样

静态代理解决方案

代码如下:简单直接,无需赘言,如果calculator里边不仅有add和minus,还有divide,product,log,sin…呢,呵呵哒

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public class StaticCalculatorProxy implements Calculator {

    Calculator obj;

     

    public StaticCalculatorProxy(Calculator obj) {

        this.obj = obj; 

    }

    @Override

    public Integer add(Integer num1, Integer num2) {

        System.out.println("in StaticCalculatorProxy, before invocation");

        Integer ret = obj.add(num1, num2);

        System.out.println("in StaticCalculatorProxy, after invocation");

        return ret;

    }

    @Override

    public Integer minus(Integer num1, Integer num2) {

        System.out.println("in StaticCalculatorProxy, before invocation");

        Integer ret = obj.minus(num1, num2);

        System.out.println("in StaticCalculatorProxy, after invocation");

        return ret;

    }

}

动态代理解决方案

首先编写实现InvocationHandler接口的类,用于请求转发,实现如下

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public class CalculatorHandler implements InvocationHandler {

     

    private Object obj; 

     

    public CalculatorHandler(Object obj) {

        this.obj = obj;

    }

    @Override

    public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {

        System.out.println("in calculatorhandler, before invocation");

         

        Object ret = method.invoke(obj, args);  

         

        System.out.println("in calculationhandler, after invocation");

        return ret;

    }

}

生成动态代理

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CalculatorImpl calculatorImpl = new CalculatorImpl();

CalculatorHandler calculatorHandler = newCalculatorHandler(calculatorImpl);

Calculator calculator = (Calculator) Proxy.newProxyInstance(calculatorImpl.getClass().getClassLoader(), calculatorImpl.getClass().getInterfaces(), calculatorHandler);

System.out.println(calculator.add(1,2));

System.out.println(calculator.minus(12));

无论calculator中包含多少函数,动态代理只需实现一次,实际工程中,System.out.println(“in calculatorhandler, before invocation”)可能是加缓存,打日志等操作

动态代理如何工作的

为了搞清楚动态代理如何工作,首先看看生成的动态代理的代码是什么,借助[1]中ProxyUtil代码

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public class ProxyUtils {

    /**

     * Save proxy class to path

     

     * @param path path to save proxy class

     * @param proxyClassName name of proxy class

     * @param interfaces interfaces of proxy class

     * @return

     */

    public static boolean saveProxyClass(String path, String proxyClassName, Class[] interfaces) {

        if (proxyClassName == null || path == null) {

            return false;

        }

        

        byte[] classFile = ProxyGenerator.generateProxyClass(proxyClassName, interfaces);

        FileOutputStream out = null;

        try {

            out = new FileOutputStream(path);

            out.write(classFile);

            out.flush();

            return true;

        catch (Exception e) {

            e.printStackTrace();

        finally {

            try {

                out.close();

            catch (IOException e) {

                e.printStackTrace();

            }

        }

        return false;

    }

}

得到了生成的动态代理代码如下:

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public final class $Proxy0 extends Proxy

    implements Calculator

{

    public $Proxy0(InvocationHandler invocationhandler)

    {

        super(invocationhandler);

    }

    public final boolean equals(Object obj)

    {

        try

        {

            return ((Boolean)super.h.invoke(this, m1, newObject[] {

                obj

            })).booleanValue();

        }

        catch(Error _ex) { }

        catch(Throwable throwable)

        {

            throw new UndeclaredThrowableException(throwable);

        }

    }

    public final String toString()

    {

        try

        {

            return (String)super.h.invoke(this, m2, null);

        }

        catch(Error _ex) { }

        catch(Throwable throwable)

        {

            throw new UndeclaredThrowableException(throwable);

        }

    }

    public final Integer minus(Integer integer, Integer integer1)

    {

        try

        {

            return (Integer)super.h.invoke(this, m4, new Object[] {

                integer, integer1

            });

        }

        catch(Error _ex) { }

        catch(Throwable throwable)

        {

            throw new UndeclaredThrowableException(throwable);

        }

    }

    public final Integer add(Integer integer, Integer integer1)

    {

        try

        {

            return (Integer)super.h.invoke(this, m3, new Object[] {

                integer, integer1

            });

        }

        catch(Error _ex) { }

        catch(Throwable throwable)

        {

            throw new UndeclaredThrowableException(throwable);

        }

    }

    public final int hashCode()

    {

        try

        {

            return ((Integer)super.h.invoke(this, m0, null)).intValue();

        }

        catch(Error _ex) { }

        catch(Throwable throwable)

        {

            throw new UndeclaredThrowableException(throwable);

        }

    }

    private static Method m1;

    private static Method m2;

    private static Method m4;

    private static Method m3;

    private static Method m0;

    static

    {

        try

        {

            m1 = Class.forName("java.lang.Object").getMethod("equals"newClass[] {

                Class.forName("java.lang.Object")

            });

            m2 = Class.forName("java.lang.Object").getMethod("toString"newClass[0]);

            m4 = Class.forName("com.langrx.mq.Calculator").getMethod("minus"new Class[] {

                Class.forName("java.lang.Integer"), Class.forName("java.lang.Integer")

            });

            m3 = Class.forName("com.langrx.mq.Calculator").getMethod("add"newClass[] {

                Class.forName("java.lang.Integer"), Class.forName("java.lang.Integer")

            });

            m0 = Class.forName("java.lang.Object").getMethod("hashCode"newClass[0]);

        }

        catch(NoSuchMethodException nosuchmethodexception)

        {

            throw newNoSuchMethodError(nosuchmethodexception.getMessage());

        }

        catch(ClassNotFoundException classnotfoundexception)

        {

            throw newNoClassDefFoundError(classnotfoundexception.getMessage());

        }

    }

}

有点长,按照初始化顺序慢慢来分析,首先分析静态代码块:

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m1 = Class.forName("java.lang.Object").getMethod("equals"newClass[] {

                    Class.forName("java.lang.Object")

                });

m2 = Class.forName("java.lang.Object").getMethod("toString"newClass[0]);

m4 = Class.forName("com.langrx.mq.Calculator").getMethod("minus"newClass[] {

                    Class.forName("java.lang.Integer"), Class.forName("java.lang.Integer")

                });

m3 = Class.forName("com.langrx.mq.Calculator").getMethod("add"new Class[] {

                    Class.forName("java.lang.Integer"), Class.forName("java.lang.Integer")

                });

m0 = Class.forName("java.lang.Object").getMethod("hashCode"newClass[0]);

得到公共interface中的add函数和minus函数对应的Method方法,同事也得到了equals,toString,hashCode三个函数的Method,所以调用代理类的equals,toString,hashCode也是要执行被代理类的方法的,知道这点很有必要

构造函数

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public $Proxy0(InvocationHandler invocationhandler)

{

        super(invocationhandler);

}

初始化了内部的InvocationHandler变量,也就是下文的super.h

以add为例看一下请求的转发

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public final Integer add(Integer integer, Integer integer1)

{

    try

    {

        return (Integer)super.h.invoke(this, m3, new Object[] {

            integer, integer1

        });

    }

    catch(Error _ex) { }

    catch(Throwable throwable)

    {

        throw new UndeclaredThrowableException(throwable);

    }

}

super.h.invoke就是invocationhandler.invoke就是传入的CalculatorHandler中实现的

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@Override

public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {

    System.out.println("in calculatorhandler, before invocation");

     

    Object ret = method.invoke(obj, args);  

     

    System.out.println("in calculationhandler, after invocation");

    return ret;

}

最终执行的就是CalculatorHandler对应的invoke函数

总结

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Calculator calculator = (Calculator) Proxy.newProxyInstance(calculatorImpl.getClass().getClassLoader(), calculatorImpl.getClass().getInterfaces(), calculatorHandler);

生成动态代理的过程步骤如下[2]:

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InvocationHandler handler = new InvocationHandlerImpl(..); 

  

Class clazz = Proxy.getProxyClass(classLoader, new Class[] { Interface.class, ... }); 

  

Constructor constructor = clazz.getConstructor(new Class[] { InvocationHandler.class }); 

  

Interface Proxy = (Interface)constructor.newInstance(new Object[] { handler });

Proxy.newProxyInstance帮我们做了2,3,4步,直接返回给我们一个动态代理对象,代理对象最终执行InvocationHandler中invoke函数。顺便强推文章[2]

References

  1. https://github.com/android-cn/android-open-project-demo/tree/master/java-dynamic-proxy
  2. https://www.ibm.com/developerworks/cn/java/j-lo-proxy1/index.html

JAVA 动态代理

标签:col   target   href   IV   需求   AC   struct   exce   const   

原文地址:https://www.cnblogs.com/feng9exe/p/9241277.html

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