JDK 动态代理分析

Java的代理有两种:静态代理和动态代理,动态代理又分为 基于jdk的动态代理 和 基于cglib的动态代理 ,两者都是通过动态生成代理类的方法实现的,但是基于jdk的动态代理需要委托类实现接口,基于cglib的动态代理不要求委托类实现接口。

接下来主要分析一下基于jdk的动态代理的实现原理。

一 动态代理例子

首先来看一个动态代理的例子:

# 测试类,主要功能是生成代理类并调用代理方法 TargetFactory.java
public
class TargetFactory { public static void main(String[] args) { System.getProperties().put("sun.misc.ProxyGenerator.saveGeneratedFiles", "true"); TargetFactory tf = new TargetFactory(); Target tt = new Target(); Display dy = (Display) tf.getInstance(tt, new InvokerHandler(tt)); try { dy.f(); } catch (Exception e) { e.printStackTrace(); } } public Object getInstance(Object target, InvocationHandler handler){ return Proxy.newProxyInstance(target.getClass().getClassLoader(), new Class<?>[]{Display.class},handler); } } # 接口 Display.java interface Display { public void f(); public void g(); } # 实现了接口的目标类 Target.java public class Target implements Display{ @Override public void f() { System.out.println("Targer f() method"); } @Override public void g() { System.out.println("Targer g() method");; } }
# 实现了InvocationHandler接口的代理类的调用处理类 InvokerHandler.java
public class InvokerHandler implements InvocationHandler { private Object target; public InvokerHandler(Object t){ target = t; } @Override public Object invoke(Object proxy, Method method, Object[] args) throws Throwable { System.out.println("beforem invoke method"); method.invoke(target, args); System.out.println("after invoke method"); return null; } }  

 

运行上面的例子,结果为:

beforem invoke method
Targer f() method
after invoke method

 

二 代理类分析

我们从生成的代理类入手来进行分析,代理类默认是只存在于内存中的,我们可以通过添加如下代码来将代理类存储在磁盘上:

System.getProperties().put("sun.misc.ProxyGenerator.saveGeneratedFiles", "true");

FAQ1:添加此代码后程序有时会抛出 java.lang.InternalError: I/O exception saving generated file: java.io.FileNotFoundException : test\java\dynamicProxy\$Proxy0.class (系统找不到指定的路径。)。这个问题不得不说一下代理类的生成路径。系统是根据接口的描述符来选择生成路径的,如果有一个接口的描述符都为public的,那么代理类就被放置在用户目录下面,可以通过System.getProperty("user.dir")来获取到。接口中只要有一个是非public的,那么代理类的放置路径就为System.getProperty("user.dir")+File.separator+该接口的包路径。谈到这里,我们可以想象一下如果有两个接口是非public的,而它们属于不同的包,那么将会抛出IllegalArgumentException的异常。

代理类的命名是 “$Proxy”(由Proxy类中的proxyClassNamePrefix字段指定的)+代理类的序号(Proxy类中的nextUniqueNumber字段,从0开始),考虑到多线程的问题在操作nextUniqueNumber时先要获取到nextUniqueNumberLock的对象锁。

获得了代理类的class文件后我们使用jd-gui(free for no commercial)来进行反编译获取到源码,本文的$Proxy0.class 反编译的结果如下

//代理类都继承 Proxy 类 并且实现代理接口Display
public final class $Proxy0 extends Proxy implements Display
{
    //构造函数的入参为 例子中InvokerHandler的实例
    //也就是 Proxy.newProxyInstance(target.getClass().getClassLoader(), new Class<?>[]{Display.class},handler); 的入参 handler
    public $Proxy0(InvocationHandler paramInvocationHandler) throws 
    {
        /*接着调用Proxy的构造函数,把handler赋值给Proxy类的h字段,下面注释为Proxy的构造函数
         *protected Proxy(InvocationHandler h) {
         *    this.h = h;
         *}
        */
        
         super(paramInvocationHandler);   

    }    
    
      private static Method m1;
      private static Method m3;
      private static Method m4;
      private static Method m0;
      private static Method m2;

    static
    {
      try
      {
       //通过反射获取接口中的方法f() 和 g(),这就决定了委托类必须实现接口,不然的话没有办法通过反射来调用委托类中的方法
        m3 = Class.forName("test.java.dynamicProxy.Display").getMethod("g", new Class[0]);
        m4 = Class.forName("test.java.dynamicProxy.Display").getMethod("f", new Class[0]);
        m0 = Class.forName("java.lang.Object").getMethod("hashCode", new Class[0]);
        m2 = Class.forName("java.lang.Object").getMethod("toString", new Class[0]);
        m1 = Class.forName("java.lang.Object").getMethod("equals", new Class[] { Class.forName("java.lang.Object") });
        return;
      }
      catch (NoSuchMethodException localNoSuchMethodException)
      {
        throw new NoSuchMethodError(localNoSuchMethodException.getMessage());
      }
      catch (ClassNotFoundException localClassNotFoundException)
      {
        throw new NoClassDefFoundError(localClassNotFoundException.getMessage());
      }
    }
    //在调用代理类中的接口方法时,代理类会将此方法和方法的参数作为入参来调用paramInvocationHandler的invoke函数,在invoke函数中调用委托类中对应的函数
    public final void g() throws 
    {
      try
      {
        //m3代表的是g方法,null是g的入参,因为g没有入参所以为null
        this.h.invoke(this, m3, null);
        return;
      }
      catch (Error|RuntimeException localError)
      {
        throw localError;
      }
      catch (Throwable localThrowable)
      {
        throw newpublic final void f() throws 
    {
      try
      {  
        this.h.invoke(this, m4, null);
        return;
      }
      catch (Error|RuntimeException localError)
      {
        throw localError;
      }
      catch (Throwable localThrowable)
      {
        throw new UndeclaredThrowableException(localThrowable);
      }
    }
    
    public final boolean equals(Object paramObject) throws 
    {
      try
      {
        return ((Boolean)this.h.invoke(this, m1, new Object[] { paramObject })).booleanValue();
      }
      catch (Error|RuntimeException localError)
      {
        throw localError;
      }
      catch (Throwable localThrowable)
      {
        throw new UndeclaredThrowableException(localThrowable);
      }
    }
    
    public final int hashCode() throws 
    {
      try
      {
        return ((Integer)this.h.invoke(this, m0, null)).intValue();
      }
      catch (Error|RuntimeException localError)
      {
        throw localError;
      }
      catch (Throwable localThrowable)
      {
        throw new UndeclaredThrowableException(localThrowable);
      }
    }
    
    public final String toString() throws 
    {
      try
      {
        return (String)this.h.invoke(this, m2, null);
      }
      catch (Error|RuntimeException localError)
      {
        throw localError;
      }
      catch (Throwable localThrowable)
      {
        throw new UndeclaredThrowableException(localThrowable);
      }
    }
}

 基于jdk动态代理所生成的代理类和静态代理类一样都要实现接口,动态代理类需要继承Proxy(不知道为何要继承此类)。动态代理invoke函数中的对于委托类的方法调用是反射调用,效率上比这静态代理要差一些。

在编码时,静态代理类需要用户实现每一个接口方法,而动态代理只需要实现 InvocationHandler 中的invoke函数,因此动态代理使得代码比较简洁,所有对method的预处理都在invoke函数中完成。

   

 三 代理类的产生

上面使用反编译的手段来分析了代理类的源码,下面要介绍一下代理类到底是怎么生成的

 

 1. Proxy.newProxyInstance方法

newProxyInstance 是 Proxy类中的静态方法,它的作用就是根据入参来返回一个代理类的实例,下面来介绍一下入参:

Proxy.newProxyInstance(target.getClass().getClassLoader(), new Class<?>[]{Display.class},handler);

target.getClass().getClassLoader():是用来加载代理类的class loader

new Class<?>[]{Display.class}:是需要代理的接口,一个代理类可以代理多个接口,所以这里是个数组

handler: 传递委托类方法调用的调用处理类,在例子一种对应的是InvokerHandler的实例
我们来看一下 newProxyInstance的源码:

 public static Object newProxyInstance(ClassLoader loader,
                      Class<?>[] interfaces,
                      InvocationHandler h)
    throws IllegalArgumentException
    {
    if (h == null) {
        throw new NullPointerException();
    }
    //生成代理类   
    Class cl = getProxyClass(loader, interfaces);

    try {
        //获取构造函数,生成并返回代理类的实例
        //根据第二节中对生成代理类的分析,构造函数的参数类型为 { InvocationHandler.class }
        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());
    }
    }

 

2. getProxyClass

这个函数主要实现了下列的功能:

  • 对需要代理的接口进行合法性验证(接口对传入newProxyInstance的Class loader是否可见,是否是接口类型,接口去重)

在此只关注一下接口的去重。此函数中采用HashSet的方法来简单进行去重,代码如下:

Set interfaceSet = new HashSet();

if (interfaceSet.contains(interfaceClass)) {
    throw new IllegalArgumentException(
         "repeated interface: " + interfaceClass.getName());
}
  • 使用本地缓存loaderToCache来缓存已经产生的代理类

先来看一下本地缓存的初始化,它采用WeakHashMap这个特殊的Map类型,关于WeakHashMap在此不再赘述

     private static Map loaderToCache = new WeakHashMap();

loaderToCache 的类型是<ClassLoader,<Object,Class>>,涉及到缓存操作的代码如下,代码比较简单,又有完整的注释,这里不予过多分析,以免画蛇添足之嫌。

Map cache;
synchronized (loaderToCache) {
    cache = (Map) loaderToCache.get(loader);
    if (cache == null) {
    cache = new HashMap();
    loaderToCache.put(loader, cache);
    }
    /*
     * This mapping will remain valid for the duration of this
     * method, without further synchronization, because the mapping
     * will only be removed if the class loader becomes unreachable.
     */
}

/*
 * Look up the list of interfaces in the proxy class cache using
 * the key.  This lookup will result in one of three possible
 * kinds of values:
 *     null, if there is currently no proxy class for the list of
 *         interfaces in the class loader,
 *     the pendingGenerationMarker object, if a proxy class for the
 *         list of interfaces is currently being generated,
 *     or a weak reference to a Class object, if a proxy class for
 *         the list of interfaces has already been generated.
 */
synchronized (cache) {
    /*
     * Note that we need not worry about reaping the cache for
     * entries with cleared weak references because if a proxy class
     * has been garbage collected, its class loader will have been
     * garbage collected as well, so the entire cache will be reaped
     * from the loaderToCache map.
     */
    do {
    Object value = cache.get(key);
    if (value instanceof Reference) {
        proxyClass = (Class) ((Reference) value).get();
    }
    if (proxyClass != null) {
        // proxy class already generated: return it
        return proxyClass;
    } else if (value == pendingGenerationMarker) {
        // proxy class being generated: wait for it
        try {
        cache.wait();
        } catch (InterruptedException e) {
        /*
         * The class generation that we are waiting for should
         * take a small, bounded time, so we can safely ignore
         * thread interrupts here.
         */
        }
        continue;
    } else {
        /*
         * No proxy class for this list of interfaces has been
         * generated or is being generated, so we will go and
         * generate it now.  Mark it as pending generation.
         */
        cache.put(key, pendingGenerationMarker);
        break;
    }
    } while (true);
}
  • 产生缓存中没有的代理类,并将该类存入缓存

如果缓存中没有所需的代理类,则由下面的这个函数来根据需要代理的接口产生。

byte[] proxyClassFile = ProxyGenerator.generateProxyClass(proxyName, interfaces);

 

3. ProxyGenerator

 这个类是jdk动态代理的核心类,class文件的生成就是在这个类中完成的,在分析这个类之前,首先来看一下class file的 格式,这里只简要的提一下,可以在jvm规范中找到详细的解释。

ClassFile {
    u4 magic; //此处必须为0xCAFEBABE
    u2 minor_version;
    u2 major_version;
    u2 constant_pool_count;
    cp_info constant_pool[constant_pool_count-1];
    u2 access_flags;
    u2 this_class;
    u2 super_class;
    u2 interfaces_count;
    u2 interfaces[interfaces_count];
    u2 fields_count;                  //代理类中的field info没有属性
    field_info fields[fields_count];
    u2 methods_count;
    method_info methods[methods_count];//代理类中的method info只有Code属性和Exceptions属性
    u2 attributes_count; // 代理类没有属性,所以attributes_count=0
    attribute_info attributes[attributes_count];
}

 

java class file中最复杂的就是各种各样的attribute,而在代理类中只存在两种属性"Code" 和 "Exceptions",由此可以看出,该类也是比较简单的。

constant_pool[constant_pool_count-1] 是class file中不可缺少的元素,这里需要提一下,在class file中引用constant_pool中的元素时下标是从1 开始的,比如constant_pool_count是39 那么只能使用constant_pool[1] --> constant_pool[38]的

元素。

在ProxyGenerator 中使用静态内部类ConstantPool来管理constant pool,在ConstantPool中使用private List<Entry> pool = new ArrayList<Entry>(32) 来存储constant pool entries,用private Map<Object,Short> map = new HashMap<Object,Short>(16) 来存储entries 与下标之间的对应关系,这样的设计避免了需要轮询pool来查找需要存入的条目是否已经存在了。

 

4. ProxyGenerator.generateClassFile

在ProxyGenerator中generateClassFile 是入口函数,该函数可以对照class file的结构来阅读。

    private byte[] generateClassFile() {

        /* ============================================================
         * Step 1: Assemble ProxyMethod objects for all methods to
         * generate proxy dispatching code for.
         */

        /*
         * 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.
         */
         
         /* hashCodeMethod equalsMethod toStringMethod 存在于每个生成的代理类中
         * addProxyMethod 中会扫描每个接口中的方法,对于函数签名一致的方法则判断
         * 抛出的异常类型是否一致,如果不一致且没有继承关系,则不抛出异常。如果不
         * 一致但异常有继承关系的,则抛出子类的异常
         * 例如  接口1中 void f() throws Exception
         *       接口2中 void f() throws IOException
         * 代理类中      void f() throws IOException
         */
        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.
         */
         /*这里需要强调一下,签名一致的函数返回值类型不一样且没有继承关系,则是不兼容的
         * 如果返回值类型不一致而返回值存在继承关系的,在代理类中返回值类型为子类
         * 例如: 接口1  superclass f();
         *        接口2  subclass f();
         *    代理类中   subclass f();  其中 subclass extends superclass
         */
        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());//生成构造函数的字节码

            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
                    //生成equals,toString,hashCode 和 接口方法的字节码
                    methods.add(pm.generateMethod());
                }
            }
            //生成静态代码块的字节码
            methods.add(generateStaticInitializer());

        } catch (IOException e) {
            throw new InternalError("unexpected I/O Exception");
        }

        if (methods.size() > 65535) {
            throw new IllegalArgumentException("method limit exceeded");
        }
        if (fields.size() > 65535) {
            throw new IllegalArgumentException("field limit exceeded");
        }

        /* ============================================================
         * 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();
    }

 

 

posted @ 2014-07-02 15:12  cruze_lee  阅读(1090)  评论(1编辑  收藏  举报