Java动态代理详解及实例

java动态代理

代理设计模式

定义：为其他对象提供一种代理以控制对这个对象的访问。

动态代理使用

java动态代理机制以巧妙的方式实现了代理模式的设计理念。

代理模式示例代码

public interface subject  
{  
 public void dosomething();  
}  
public class realsubject implements subject  
{  
 public void dosomething()  
 {  
  system.out.println( "call dosomething()" );  
 }  
}  
public class proxyhandler implements invocationhandler  
{  
 private object proxied;  
   
 public proxyhandler( object proxied )  
 {  
  this.proxied = proxied;  
 }  
   
 public object invoke( object proxy, method method, object[] args ) throws throwable  
 {  
  //在转调具体目标对象之前，可以执行一些功能处理

  //转调具体目标对象的方法
  return method.invoke( proxied, args); 
  
  //在转调具体目标对象之后，可以执行一些功能处理
 }  
} 

import java.lang.reflect.invocationhandler;  
import java.lang.reflect.method;  
import java.lang.reflect.proxy;  
import sun.misc.proxygenerator;  
import java.io.*;  
public class dynamicproxy  
{  
 public static void main( string args[] )  
 {  
  realsubject real = new realsubject();  
  subject proxysubject = (subject)proxy.newproxyinstance(subject.class.getclassloader(), 
   new class[]{subject.class}, 
   new proxyhandler(real));
     
  proxysubject.dosomething();
  
  //write proxysubject class binary data to file  
  createproxyclassfile();  
 }  
   
 public static void createproxyclassfile()  
 {  
  string name = "proxysubject";  
  byte[] data = proxygenerator.generateproxyclass( name, new class[] { subject.class } );  
  try 
  {  
   fileoutputstream out = new fileoutputstream( name + ".class" );  
   out.write( data );  
   out.close();  
  }  
  catch( exception e )  
  {  
   e.printstacktrace();  
  }  
 }  
} 

动态代理内部实现

首先来看看类proxy的代码实现 proxy的主要静态变量

// 映射表：用于维护类装载器对象到其对应的代理类缓存
private static map loadertocache = new weakhashmap(); 

// 标记：用于标记一个动态代理类正在被创建中
private static object pendinggenerationmarker = new object(); 

// 同步表：记录已经被创建的动态代理类类型，主要被方法 isproxyclass 进行相关的判断
private static map proxyclasses = collections.synchronizedmap(new weakhashmap()); 

// 关联的调用处理器引用
protected invocationhandler h;

proxy的构造方法

// 由于 proxy 内部从不直接调用构造函数，所以 private 类型意味着禁止任何调用
private proxy() {} 

// 由于 proxy 内部从不直接调用构造函数，所以 protected 意味着只有子类可以调用
protected proxy(invocationhandler h) {this.h = h;} 

proxy静态方法newproxyinstance

public static object newproxyinstance(classloader loader, class<?>[]interfaces,invocationhandler h) throws illegalargumentexception { 
  // 检查 h 不为空，否则抛异常
  if (h == null) { 
    throw new nullpointerexception(); 
  } 

  // 获得与指定类装载器和一组接口相关的代理类类型对象
  class cl = getproxyclass(loader, interfaces); 

  // 通过反射获取构造函数对象并生成代理类实例
  try { 
    constructor cons = cl.getconstructor(constructorparams); 
    return (object) cons.newinstance(new object[] { h }); 
  } catch (nosuchmethodexception e) { throw new internalerror(e.tostring()); 
  } catch (illegalaccessexception e) { throw new internalerror(e.tostring()); 
  } catch (instantiationexception e) { throw new internalerror(e.tostring()); 
  } catch (invocationtargetexception e) { throw new internalerror(e.tostring()); 
  } 
}

类proxy的getproxyclass方法调用proxygenerator的 generateproxyclass方法产生proxysubject.class的二进制数据：

public static byte[] generateproxyclass(final string name, class[] interfaces)

我们可以import sun.misc.proxygenerator，调用 generateproxyclass方法产生binary data，然后写入文件，最后通过反编译工具来查看内部实现原理。 反编译后的proxysubject.java proxy静态方法newproxyinstance

import java.lang.reflect.*;  
public final class proxysubject extends proxy  
  implements subject  
{  
  private static method m1;  
  private static method m0;  
  private static method m3;  
  private static method m2;  
  public proxysubject(invocationhandler invocationhandler)  
  {  
    super(invocationhandler);  
  }  
  public final boolean equals(object obj)  
  {  
    try 
    {  
      return ((boolean)super.h.invoke(this, m1, new object[] {  
        obj  
      })).booleanvalue();  
    }  
    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);  
    }  
  }  
  public final void dosomething()  
  {  
    try 
    {  
      super.h.invoke(this, m3, null);  
      return;  
    }  
    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);  
    }  
  }  
  static  
  {  
    try 
    {  
      m1 = class.forname("java.lang.object").getmethod("equals", new class[] {  
        class.forname("java.lang.object")  
      });  
      m0 = class.forname("java.lang.object").getmethod("hashcode", new class[0]);  
      m3 = class.forname("subject").getmethod("dosomething", new class[0]);  
      m2 = class.forname("java.lang.object").getmethod("tostring", new class[0]);  
    }  
    catch(nosuchmethodexception nosuchmethodexception)  
    {  
      throw new nosuchmethoderror(nosuchmethodexception.getmessage());  
    }  
    catch(classnotfoundexception classnotfoundexception)  
    {  
      throw new noclassdeffounderror(classnotfoundexception.getmessage());  
    }  
  }  
} 

proxygenerator内部是如何生成class二进制数据，可以参考源代码。

private byte[] generateclassfile() {  
 /* 
  * record that proxy methods are needed for the hashcode, equals, 
  * and tostring methods of java.lang.object. this is done before 
  * the methods from the proxy interfaces so that the methods from 
  * java.lang.object take precedence over duplicate methods in the 
  * proxy interfaces. 
  */ 
 addproxymethod(hashcodemethod, object.class);  
 addproxymethod(equalsmethod, object.class);  
 addproxymethod(tostringmethod, object.class);  
 /* 
  * now record all of the methods from the proxy interfaces, giving 
  * earlier interfaces precedence over later ones with duplicate 
  * methods. 
  */ 
 for (int i = 0; i < interfaces.length; i++) {  
   method[] methods = interfaces[i].getmethods();  
   for (int j = 0; j < methods.length; j++) {  
  addproxymethod(methods[j], interfaces[i]);  
   }  
 }  
 /* 
  * for each set of proxy methods with the same signature, 
  * verify that the methods' return types are compatible. 
  */ 
 for (list<proxymethod> sigmethods : proxymethods.values()) {  
   checkreturntypes(sigmethods);  
 }  
 /* ============================================================ 
  * step 2: assemble fieldinfo and methodinfo structs for all of 
  * fields and methods in the class we are generating. 
  */ 
 try {  
   methods.add(generateconstructor());  
   for (list<proxymethod> sigmethods : proxymethods.values()) {  
  for (proxymethod pm : sigmethods) {  
    // add static field for method's method object  
    fields.add(new fieldinfo(pm.methodfieldname,  
   "ljava/lang/reflect/method;",  
    acc_private | acc_static));  
    // generate code for proxy method and add it  
    methods.add(pm.generatemethod());  
  }  
   }  
   methods.add(generatestaticinitializer());  
 } catch (ioexception e) {  
   throw new internalerror("unexpected i/o exception");  
 }  
 /* ============================================================ 
  * step 3: write the final class file. 
  */ 
 /* 
  * make sure that constant pool indexes are reserved for the 
  * following items before starting to write the final class file. 
  */ 
 cp.getclass(dottoslash(classname));  
 cp.getclass(superclassname);  
 for (int i = 0; i < interfaces.length; i++) {  
   cp.getclass(dottoslash(interfaces[i].getname()));  
 }  
 /* 
  * disallow new constant pool additions beyond this point, since 
  * we are about to write the final constant pool table. 
  */ 
 cp.setreadonly();  
 bytearrayoutputstream bout = new bytearrayoutputstream();  
 dataoutputstream dout = new dataoutputstream(bout);  
 try {  
   /* 
    * write all the items of the "classfile" structure. 
    * see jvms section 4.1. 
    */ 
     // u4 magic;  
   dout.writeint(0xcafebabe);  
     // u2 minor_version;  
   dout.writeshort(classfile_minor_version);  
     // u2 major_version;  
   dout.writeshort(classfile_major_version);  
   cp.write(dout);  // (write constant pool)  
     // u2 access_flags;  
   dout.writeshort(acc_public | acc_final | acc_super);  
     // u2 this_class;  
   dout.writeshort(cp.getclass(dottoslash(classname)));  
     // u2 super_class;  
   dout.writeshort(cp.getclass(superclassname));  
     // u2 interfaces_count;  
   dout.writeshort(interfaces.length);  
     // u2 interfaces[interfaces_count];  
   for (int i = 0; i < interfaces.length; i++) {  
  dout.writeshort(cp.getclass(  
    dottoslash(interfaces[i].getname())));  
   }  
     // u2 fields_count;  
   dout.writeshort(fields.size());  
     // field_info fields[fields_count];  
   for (fieldinfo f : fields) {  
  f.write(dout);  
   }  
     // u2 methods_count;  
   dout.writeshort(methods.size());  
     // method_info methods[methods_count];  
   for (methodinfo m : methods) {  
  m.write(dout);  
   }  
       // u2 attributes_count;  
   dout.writeshort(0); // (no classfile attributes for proxy classes)  
 } catch (ioexception e) {  
   throw new internalerror("unexpected i/o exception");  
 }  
 return bout.tobytearray(); 

总结

一个典型的动态代理创建对象过程可分为以下四个步骤：

1、通过实现invocationhandler接口创建自己的调用处理器 ivocationhandler handler = new invocationhandlerimpl(...);

2、通过为proxy类指定classloader对象和一组interface创建动态代理类

class clazz = proxy.getproxyclass(classloader,new class[]{...});

3、通过反射机制获取动态代理类的构造函数，其参数类型是调用处理器接口类型

constructor constructor = clazz.getconstructor(new class[]{invocationhandler.class});

4、通过构造函数创建代理类实例，此时需将调用处理器对象作为参数被传入

interface proxy = (interface)constructor.newinstance(new object[] (handler));

为了简化对象创建过程，proxy类中的newinstance方法封装了2~4，只需两步即可完成代理对象的创建。

生成的proxysubject继承proxy类实现subject接口，实现的subject的方法实际调用处理器的invoke方法，而invoke方法利用反射调用的是被代理对象的的方法（object result=method.invoke(proxied,args)）

美中不足

诚然，proxy已经设计得非常优美，但是还是有一点点小小的遗憾之处，那就是它始终无法摆脱仅支持interface代理的桎梏，因为它的设计注定了这个遗憾。回想一下那些动态生成的代理类的继承关系图，它们已经注定有一个共同的父类叫proxy。java的继承机制注定了这些动态代理类们无法实现对class的动态代理，原因是多继承在java中本质上就行不通。有很多条理由，人们可以否定对 class代理的必要性，但是同样有一些理由，相信支持class动态代理会更美好。接口和类的划分，本就不是很明显，只是到了java中才变得如此的细化。如果只从方法的声明及是否被定义来考量，有一种两者的混合体，它的名字叫抽象类。实现对抽象类的动态代理，相信也有其内在的价值。此外，还有一些历史遗留的类，它们将因为没有实现任何接口而从此与动态代理永世无缘。如此种种，不得不说是一个小小的遗憾。但是，不完美并不等于不伟大，伟大是一种本质，java动态代理就是佐例。

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