JAVA设计模式-动态代理(Proxy)源码分析

2018-09-29 04:03:11来源：博客园阅读 ()

在文章：JAVA设计模式-动态代理(Proxy)示例及说明中，为动态代理设计模式举了一个小小的例子，那么这篇文章就来分析一下源码的实现。

一，Proxy.newProxyInstance方法

 1 @CallerSensitive
 2     public static Object newProxyInstance(ClassLoader loader,
 3                                           Class<?>[] interfaces,
 4                                           InvocationHandler h)
 5         throws IllegalArgumentException{
 6         // null检查，h为null就抛出NullPointerException
 7         Objects.requireNonNull(h);
 8         // 将接口类对象数组clone一份。
 9         final Class<?>[] intfs = interfaces.clone();
10         
11         //执行权限检查
12         final SecurityManager sm = System.getSecurityManager();
13         if (sm != null) {
14             checkProxyAccess(Reflection.getCallerClass(), loader, intfs);
15         }
16 
17         /*
18          * Look up or generate the designated proxy class.
19          */
20          // 查找或者是生成一个特定的代理类对象
21         Class<?> cl = getProxyClass0(loader, intfs);
22 
23         /*
24          * Invoke its constructor with the designated invocation handler.
25          */
26         try {
27             if (sm != null) {
28                 checkNewProxyPermission(Reflection.getCallerClass(), cl);
29             }
30             // 是static final 修饰的，源码： private static final Class<?>[] constructorParams ={ InvocationHandler.class };
31             // 从代理类对象中查找参数为InvocationHandler的构造器
32             final Constructor<?> cons = cl.getConstructor(constructorParams);
33             final InvocationHandler ih = h;
34             // 检测构造器是否是Public修饰，如果不是则强行转换为可以访问的。
35             if (!Modifier.isPublic(cl.getModifiers())) {
36                 AccessController.doPrivileged(new PrivilegedAction<Void>() {
37                     public Void run() {
38                         cons.setAccessible(true);
39                         return null;
40                     }
41                 });
42             }
43             // 通过反射，将h作为参数，实例化代理类，返回代理类实例。
44             return cons.newInstance(new Object[]{h});
45         } catch (IllegalAccessException|InstantiationException e) {
46             throw new InternalError(e.toString(), e);
47         } catch (InvocationTargetException e) {
48             Throwable t = e.getCause();
49             if (t instanceof RuntimeException) {
50                 throw (RuntimeException) t;
51             } else {
52                 throw new InternalError(t.toString(), t);
53             }
54         } catch (NoSuchMethodException e) {
55             throw new InternalError(e.toString(), e);
56         }
57     }

　　newProxyInstance是Proxy的静态方法，代码并不难理解出去权限关的代码外，就剩下两步：

　　1，获取代理类对象（21行）

　　2，利用反射技术实例化代理类，并返回实例化对象（44行）

　　接下来分析21行中的getProxyClass0方法

二，Proxy.getProxyClass0方法

 1 /**
 2      * 生成一个代理类对象，
 3      * Generate a proxy class.  Must call the checkProxyAccess method
 4      * to perform permission checks before calling this.
 5      */
 6     private static Class<?> getProxyClass0(ClassLoader loader,
 7                                            Class<?>... interfaces) {
 8         // 接口类对象数组不能大于65535个，否则抛出异常
 9         if (interfaces.length > 65535) {
10             throw new IllegalArgumentException("interface limit exceeded");
11         }
12         // 从代理类对象缓存中，根据类加载器和接口类对象数组查找代理类对象，
13         // If the proxy class defined by the given loader implementing
14         // the given interfaces exists, this will simply return the cached copy;
15         // otherwise, it will create the proxy class via the ProxyClassFactory
16         return proxyClassCache.get(loader, interfaces);
17     }

　　在这个方法中，是直接从一个叫proxyClassCache缓存中读取的，来看一下这个缓存的声明：

//static final
    private static final WeakCache<ClassLoader, Class<?>[], Class<?>> proxyClassCache = new WeakCache<>(new KeyFactory(), new ProxyClassFactory());

　　这里涉及到三个类：WeakCache,KeyFactory,ProxyClassFactory，其中后面两个类都是Proxy类的静态内部类，从类名可以大概猜测到，keyFactory是用来生产key的，ProxyClassFactory是用来生产代理类对象的，这个稍后会提到。

　　首先看一下WeakCache类的大概结构：

 1 final class WeakCache<K, P, V> {
 2 
 3     private final ReferenceQueue<K> refQueue= new ReferenceQueue<>();
 4     // the key type is Object for supporting null key
 5     // key的类型为Object，支持null key,这里的null key并不是真的可以使用null最为key,而是一个new Objdec()对象实例。ConcurrentHashMap,不允许键或值null，而HashMap可以。ConcurrentHashMap是线程安全的，HashMap不是。
 6     private final ConcurrentMap<Supplier<V>, Boolean> reverseMap= new ConcurrentHashMap<>();
 7     
 8     private final BiFunction<K, P, ?> subKeyFactory;
 9     private final BiFunction<K, P, V> valueFactory;
10 
11     //构造方法
12     public WeakCache(BiFunction<K, P, ?> subKeyFactory,
13                      BiFunction<K, P, V> valueFactory) {
14         this.subKeyFactory = Objects.requireNonNull(subKeyFactory);
15         this.valueFactory = Objects.requireNonNull(valueFactory);
16     }
17 
18     
19     public V get(K key, P parameter) {
20        //下面会详细介绍这个方法
21     }
22 
23   ......
24 }

　　上面的源代码中写明，代理类对象是从proxyClassCache镇南关读取的，接下来就分析WeakCache的get方法

三，WeakCache.get方法

 1 // key是类加载器，parameter为接口类对象数组
 2     public V get(K key, P parameter) {
 3         // 接口类对象数组null检查。
 4         Objects.requireNonNull(parameter);
 5         
 6         // 删除过时的条目
 7         expungeStaleEntries();
 8         // 生成缓存key对象实例，如果key = null，cacheKey = new Object();
 9         Object cacheKey = CacheKey.valueOf(key, refQueue);
10 
11         // lazily install the 2nd level valuesMap for the particular cacheKey
12         // 从缓存map中读取指定cacheKey的缓存数据valuesMap
13         ConcurrentMap<Object, Supplier<V>> valuesMap = map.get(cacheKey);
14         
15         if (valuesMap == null) {
16             //如果valuesMap为null,则新增
17             //putIfAbsent方法解释：如果值存在则返回值，并且不对原来的值做任何更改，如果不存在则新增，并返回null
18             //Absent的意思是缺席，不在
19             ConcurrentMap<Object, Supplier<V>> oldValuesMap= map.putIfAbsent(cacheKey,valuesMap = new ConcurrentHashMap<>());
20             if (oldValuesMap != null) {
21                 valuesMap = oldValuesMap;
22             }
23         }
24 
25         // create subKey and retrieve the possible Supplier<V> stored by that
26         // subKey from valuesMap
27         // 获取subKey，这里用到了上面提到的Proxy的静态内部类KeyFactory:subKeyFactory.apply(ket,parameter)
28         Object subKey = Objects.requireNonNull(subKeyFactory.apply(key, parameter));
29         // 从valuesMap中获取supplier
30         Supplier<V> supplier = valuesMap.get(subKey);
31         Factory factory = null;
32 
33         while (true) {
34             if (supplier != null) {
35                 // supplier might be a Factory or a CacheValue<V> instance
36                 // 4,从工厂中获取代理类对象
37                 V value = supplier.get();
38                 if (value != null) {
39                     // 5，返回
40                     return value;
41                 }
42             }
43             // else no supplier in cache
44             // or a supplier that returned null (could be a cleared CacheValue
45             // or a Factory that wasn't successful in installing the CacheValue)
46 
47             // lazily construct a Factory
48             if (factory == null) {
49                 //1,实例化工厂
50                 factory = new Factory(key, parameter, subKey, valuesMap);
51             }
52 
53             if (supplier == null) {
54                 //2，保存到valuesMap中
55                 supplier = valuesMap.putIfAbsent(subKey, factory);
56                 if (supplier == null) {
57                     // successfully installed Factory
58                     // 3，赋值
59                     supplier = factory;
60                 }
61                 // else retry with winning supplier
62             } else {
63                 if (valuesMap.replace(subKey, supplier, factory)) {
64                     // successfully replaced
65                     // cleared CacheEntry / unsuccessful Factory
66                     // with our Factory
67                     supplier = factory;
68                 } else {
69                     // retry with current supplier
70                     supplier = valuesMap.get(subKey);
71                 }
72             }
73         }
74     }

　　因为程序中Proxy.newProxyInstance是第一次执行，所以while循环开始的时候，supplier，valuesMap都是null。在这个前提下，我为代码的执行顺序做了一个编号，从1-5执行。

　　可以看到第5步，也就是源代码的第40行将结果返回，那么，代理类对象就是在第4步，也就是第37行生成的。而且也可以从第3步，也就是第59行发现supplier就是factory。

　　那么接下来，就分析一下Factory.get方法。

四，Factory.get方法

　　Factory类是WeakCache的内部类。这个类中出去构造方法外，就是get方法了，下面就将这个类的代码给出：

 1 private final class Factory implements Supplier<V> {
 2 
 3         private final K key;
 4         private final P parameter;
 5         private final Object subKey;
 6         private final ConcurrentMap<Object, Supplier<V>> valuesMap;
 7 
 8         Factory(K key, P parameter, Object subKey,
 9                 ConcurrentMap<Object, Supplier<V>> valuesMap) {
10             this.key = key;
11             this.parameter = parameter;
12             this.subKey = subKey;
13             this.valuesMap = valuesMap;
14         }
15 
16         @Override
17         public synchronized V get() { // serialize access
18             // re-check
19             // 检查
20             Supplier<V> supplier = valuesMap.get(subKey);
21             if (supplier != this) {
22                 // something changed while we were waiting:
23                 // might be that we were replaced by a CacheValue
24                 // or were removed because of failure ->
25                 // return null to signal WeakCache.get() to retry
26                 // the loop
27                 return null;
28             }
29             // else still us (supplier == this)
30 
31             // create new value
32             V value = null;
33             try {
34                 // valueFactory就是WeakCache的valueFactory属性，因为Factory是WeakCache的内部类，所以可以直接访问WeakCache的valueFactory属性
35                 value = Objects.requireNonNull(valueFactory.apply(key, parameter));
36             } finally {
37                 if (value == null) { // remove us on failure
38                     valuesMap.remove(subKey, this);
39                 }
40             }
41             // the only path to reach here is with non-null value
42             assert value != null;
43 
44             // wrap value with CacheValue (WeakReference)
45             CacheValue<V> cacheValue = new CacheValue<>(value);
46 
47             // try replacing us with CacheValue (this should always succeed)
48             if (valuesMap.replace(subKey, this, cacheValue)) {
49                 // put also in reverseMap
50                 reverseMap.put(cacheValue, Boolean.TRUE);
51             } else {
52                 throw new AssertionError("Should not reach here");
53             }
54 
55             // successfully replaced us with new CacheValue -> return the value
56             // wrapped by it
57             return value;
58         }
59     }

　　关键代码第35行：valueFactory.apply(key, parameter)

　　这里的valueFactory就是Proxy的静态内部类ProxyClassFactory，上面也提到过，那么就接着分析ProxyClassFactory的apply方法吧。

五，ProxyClassFactory.apply方法

  1 /**
  2      * 一个利用给定的类加载器和接口类数组生成，定义并返回代理类对象的工厂方法
  3      * A factory function that generates, defines and returns the proxy class given
  4      * the ClassLoader and array of interfaces.
  5      */
  6     private static final class ProxyClassFactory
  7         implements BiFunction<ClassLoader, Class<?>[], Class<?>>
  8     {
  9         // prefix for all proxy class names
 10         // 所有代理类对象的前缀
 11         private static final String proxyClassNamePrefix = "$Proxy";
 12 
 13         // next number to use for generation of unique proxy class names
 14         // 用于生成唯一代理类名称的下一个数字
 15         private static final AtomicLong nextUniqueNumber = new AtomicLong();
 16 
 17         @Override
 18         public Class<?> apply(ClassLoader loader, Class<?>[] interfaces) {
 19             
 20             Map<Class<?>, Boolean> interfaceSet = new IdentityHashMap<>(interfaces.length);
 21             // 
 22             for (Class<?> intf : interfaces) {
 23                 /*
 24                  * Verify that the class loader resolves the name of this
 25                  * interface to the same Class object.
 26                  */
 27                 Class<?> interfaceClass = null;
 28                 try {
 29                     // 加载接口类，获得接口类的类对象，第二个参数为false表示不进行实例化
 30                     interfaceClass = Class.forName(intf.getName(), false, loader);
 31                 } catch (ClassNotFoundException e) {
 32                 }
 33                 if (interfaceClass != intf) {
 34                     throw new IllegalArgumentException(
 35                         intf + " is not visible from class loader");
 36                 }
 37                 /*
 38                  * Verify that the Class object actually represents an
 39                  * interface.
 40                  */
 41                 if (!interfaceClass.isInterface()) {
 42                     throw new IllegalArgumentException(
 43                         interfaceClass.getName() + " is not an interface");
 44                 }
 45                 /*
 46                  * Verify that this interface is not a duplicate.
 47                  */
 48                 if (interfaceSet.put(interfaceClass, Boolean.TRUE) != null) {
 49                     throw new IllegalArgumentException(
 50                         "repeated interface: " + interfaceClass.getName());
 51                 }
 52             }
 53             // package to define proxy class in
 54             // 代理类的包名
 55             String proxyPkg = null;     
 56             int accessFlags = Modifier.PUBLIC | Modifier.FINAL;
 57 
 58             /*
 59              * Record the package of a non-public proxy interface so that the
 60              * proxy class will be defined in the same package.  Verify that
 61              * all non-public proxy interfaces are in the same package.
 62              */ 
 63             for (Class<?> intf : interfaces) {
 64                 int flags = intf.getModifiers();
 65                 if (!Modifier.isPublic(flags)) {
 66                     accessFlags = Modifier.FINAL;
 67                     String name = intf.getName();
 68                     int n = name.lastIndexOf('.');
 69                     String pkg = ((n == -1) ? "" : name.substring(0, n + 1));
 70                     if (proxyPkg == null) {
 71                         proxyPkg = pkg;
 72                     } else if (!pkg.equals(proxyPkg)) {
 73                         throw new IllegalArgumentException(
 74                             "non-public interfaces from different packages");
 75                     }
 76                 }
 77             }
 78 
 79             if (proxyPkg == null) {
 80                 // if no non-public proxy interfaces, use com.sun.proxy package
 81                 proxyPkg = com.sun.proxy package
 82                 proxyPkg = ReflectUtil.PROXY_PACKAGE + ".";
 83             }
 84 
 85             /*
 86              * 生成代理类的类名
 87              * Choose a name for the proxy class to generate.
 88              */
 89             long num = nextUniqueNumber.getAndIncrement();
 90             String proxyName = proxyPkg + proxyClassNamePrefix + num;
 91 
 92             /*
 93              * Generate the specified proxy class.
 94              */
 95             //生成代理类class文件
 96             byte[] proxyClassFile = ProxyGenerator.generateProxyClass(proxyName, interfaces, accessFlags);
 97             try {
 98                 // 返回代理类对象
 99                 return defineClass0(loader, proxyName, proxyClassFile, 0, proxyClassFile.length);
100             } catch (ClassFormatError e) {
101                 /*
102                  * A ClassFormatError here means that (barring bugs in the
103                  * proxy class generation code) there was some other
104                  * invalid aspect of the arguments supplied to the proxy
105                  * class creation (such as virtual machine limitations
106                  * exceeded).
107                  */
108                 throw new IllegalArgumentException(e.toString());
109             }
110         }
111     }
112 }

　　在代码的第96行，生成了代理类的class文件，并且在99行返回了我们需要的代理类对象。那么怎么找到这个生成的代理类class文件呢？两个步骤：

　　1，添加一行代码：

//实例化调用处理类（编好的故事）
        MyStoryInvocationHandler handler = new MyStoryInvocationHandler(liHong);
        
        //添加这一句是生成代理类的class文件，前提是你需要在工程根目录下创建com/sun/proxy目录
        System.getProperties().put("sun.misc.ProxyGenerator.saveGeneratedFiles","true");

　　2，创建com/sun/proxy目录：

　　当这个程序执行完之后就会有$Proxy0.class文件出现：

六，反编译$Proxy0.class

  1 package com.sun.proxy;
  2 
  3 import com.zcz.proxyTest.testtwo.Singer;
  4 import java.lang.reflect.InvocationHandler;
  5 import java.lang.reflect.Method;
  6 import java.lang.reflect.Proxy;
  7 import java.lang.reflect.UndeclaredThrowableException;
  8 
  9 //继承了Proxy类，实现了Singer接口
 10 public final class $Proxy0 extends Proxy implements Singer{
 11   private static Method m1;
 12   private static Method m4;
 13   private static Method m2;
 14   private static Method m3;
 15   private static Method m0;
 16   
 17   //构造方法，直接调用了父类，也就是Proxy的构造方法，参数paramInvocationHandler就是我们的MyStoryInvocationHandler实例化对象handler
 18   public $Proxy0(InvocationHandler paramInvocationHandler)throws {
 19     super(paramInvocationHandler);
 20   }
 21   
 22   public final boolean equals(Object paramObject)
 23     throws {
 24     try
 25     {
 26       return ((Boolean)this.h.invoke(this, m1, new Object[] { paramObject })).booleanValue();
 27     }
 28     catch (Error|RuntimeException localError)
 29     {
 30       throw localError;
 31     }
 32     catch (Throwable localThrowable)
 33     {
 34       throw new UndeclaredThrowableException(localThrowable);
 35     }
 36   }
 37   
 38   //实现了sayGoodBye
 39   public final void sayGoodBye(String paramString)
 40     throws {
 41     try
 42     {
 43       // 这里的h就是我们的MyStoryInvocationHandler实例化对象handler，原因在下方解释。
 44       // 这里直接调用了MyStoryInvocationHandler的invoke方法
 45       this.h.invoke(this, m4, new Object[] { paramString });
 46       return;
 47     }
 48     catch (Error|RuntimeException localError)
 49     {
 50       throw localError;
 51     }
 52     catch (Throwable localThrowable)
 53     {
 54       throw new UndeclaredThrowableException(localThrowable);
 55     }
 56   }
 57   
 58   public final String toString()
 59     throws 
 60   {
 61     try
 62     {
 63       return (String)this.h.invoke(this, m2, null);
 64     }
 65     catch (Error|RuntimeException localError)
 66     {
 67       throw localError;
 68     }
 69     catch (Throwable localThrowable)
 70     {
 71       throw new UndeclaredThrowableException(localThrowable);
 72     }
 73   }
 74   //实现了orderSong方法
 75   public final void orderSong(String paramString)
 76     throws 
 77   {
 78     try
 79     {
 80      // 这里的h就是我们的MyStoryInvocationHandler实例化对象handler，原因在下方解释。
 81       // 这里直接调用了MyStoryInvocationHandler的invoke方法
 82       this.h.invoke(this, m3, new Object[] { paramString });
 83       return;
 84     }
 85     catch (Error|RuntimeException localError)
 86     {
 87       throw localError;
 88     }
 89     catch (Throwable localThrowable)
 90     {
 91       throw new UndeclaredThrowableException(localThrowable);
 92     }
 93   }
 94   
 95   public final int hashCode()
 96     throws 
 97   {
 98     try
 99     {
100       return ((Integer)this.h.invoke(this, m0, null)).intValue();
101     }
102     catch (Error|RuntimeException localError)
103     {
104       throw localError;
105     }
106     catch (Throwable localThrowable)
107     {
108       throw new UndeclaredThrowableException(localThrowable);
109     }
110   }
111   
112   //静态代码块，做初始化操作
113   static
114   {
115     try
116     {
117       m1 = Class.forName("java.lang.Object").getMethod("equals", new Class[] { Class.forName("java.lang.Object") });
118       //通过反射，获取sayGoodBye方法对象实例
119       m4 = Class.forName("com.zcz.proxyTest.testtwo.Singer").getMethod("sayGoodBye", new Class[] { Class.forName("java.lang.String") });
120       m2 = Class.forName("java.lang.Object").getMethod("toString", new Class[0]);
121       //通过放射，获取orderSong方法对象实例
122       m3 = Class.forName("com.zcz.proxyTest.testtwo.Singer").getMethod("orderSong", new Class[] { Class.forName("java.lang.String") });
123       m0 = Class.forName("java.lang.Object").getMethod("hashCode", new Class[0]);
124       return;
125     }
126     catch (NoSuchMethodException localNoSuchMethodException)
127     {
128       throw new NoSuchMethodError(localNoSuchMethodException.getMessage());
129     }
130     catch (ClassNotFoundException localClassNotFoundException)
131     {
132       throw new NoClassDefFoundError(localClassNotFoundException.getMessage());
133     }
134   }
135 }

　　从第一条中，可以得知，代理类实例化的代码是：cons.newInstance(new Object[]{h})。这里是通过反射调用代理类对象的构造方法，传入了参数h（我们的MyStoryInvocationHandler实例化对象handler）。这个构造方法，就是上述反编译代码里的构造方法，而上述反编译代码里的构造方法调用了Proxy类的构造方法，来看一下Proxy类的构造方法：

 1     /**
 2      * the invocation handler for this proxy instance.
 3      * @serial
 4      */
 5     protected InvocationHandler h;  
 6     private Proxy() {
 7     }
 8     protected Proxy(InvocationHandler h) {
 9         Objects.requireNonNull(h);
10         this.h = h;
11     }

　　这里将我们传入的handler直接赋值给了InvocationHandler h。上述反编译代码中的45行和82行使用的this.h 就是我们传入的handler。

　　所以proxy.orderSong("就是现在");方法在执行的时候会去调用MyStoryInvocationHandler类的invoke方法。