from:http://blog.csdn.net/lylwo317/article/details/52163304

序言

注解在Java中到底是什么样的东西？具体是如何实现的？ 
本文将一层一层深入探究注解的实现原理。为了尽可能的将分析的过程呈现出来，所以文章包含了大量的截图和代码。（ps：如果图片看不清楚，请将网页放大来看，chrome可以通过ctrl+鼠标滚轮放大）

前期准备

知识方面

开始分析前，提醒一下，下面的分析必须具备以下知识 
1. 知道如何自定义注解 
2. 理解Java动态代理机制 
3. 了解Java常量池 
如果不具备以上的知识，会看得云里雾里的。上面提到的知识点谷歌百度都可以找到许多相关的文章。

工具方面

  1. Intellij 2016

开始分析

首先写一个简单的自定义注解小程序。

先自定义一个运行时注解

@Target(ElementType.TYPE) @Retention(RetentionPolicy.RUNTIME) public @interface HelloAnnotation {      String say() default "Hi";  }
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然后在Main函数中解析注解

@HelloAnnotation(say = "Do it!") public class TestMain {     public static void main(String[] args) {         HelloAnnotation annotation = TestMain.class.getAnnotation(HelloAnnotation.class);//获取TestMain类上的注解对象         System.out.println(annotation.say());//调用注解对象的say方法，并打印到控制台     } }
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运行程序，输出结果如下：

Do it!
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下面将围绕上面的代码来研究Java注解（Annotation）的实现原理

1. 注解对象具体是什么？

首先，我们先在main函数第一行断点，看看HelloAnnotation具体是什么类的对象

注解

可以看到HelloAnnotation注解的实例是jvm生成的动态代理类的对象。

这个运行时生成的动态代理对象是可以导出到文件的，方法有两种

  1. 在代码中加入System.setProperty("sun.misc.ProxyGenerator.saveGeneratedFiles", "true");
  2. 在运行时加入jvm 参数 -Dsun.misc.ProxyGenerator.saveGeneratedFiles=true

这里使用第一种，↓

这里写图片描述

然后运行程序。

这里写图片描述

可以看到，已经导出了运行时生成的代理类。↑

HelloAnnotation的动态代理类是$Proxy1.class，Intellij自带了反编译工具，直接双击打开，得到如下的Java代码

// // Source code recreated from a .class file by IntelliJ IDEA // (powered by Fernflower decompiler) //  package com.sun.proxy;  import com.kevin.java.annotation.runtimeAnnotation.HelloAnnotation; import java.lang.reflect.InvocationHandler; import java.lang.reflect.Method; import java.lang.reflect.Proxy; import java.lang.reflect.UndeclaredThrowableException;  public final class $Proxy1 extends Proxy implements HelloAnnotation {     private static Method m1;     private static Method m2;     private static Method m4;     private static Method m3;     private static Method m0;      public $Proxy1(InvocationHandler var1) throws  {         super(var1);     }      public final boolean equals(Object var1) throws  {         try {             return ((Boolean)super.h.invoke(this, m1, new Object[]{var1})).booleanValue();         } catch (RuntimeException | Error var3) {             throw var3;         } catch (Throwable var4) {             throw new UndeclaredThrowableException(var4);         }     }      public final String toString() throws  {         try {             return (String)super.h.invoke(this, m2, (Object[])null);         } catch (RuntimeException | Error var2) {             throw var2;         } catch (Throwable var3) {             throw new UndeclaredThrowableException(var3);         }     }      public final Class annotationType() throws  {         try {             return (Class)super.h.invoke(this, m4, (Object[])null);         } catch (RuntimeException | Error var2) {             throw var2;         } catch (Throwable var3) {             throw new UndeclaredThrowableException(var3);         }     }      public final String say() throws  {         try {             return (String)super.h.invoke(this, m3, (Object[])null);         } catch (RuntimeException | Error var2) {             throw var2;         } catch (Throwable var3) {             throw new UndeclaredThrowableException(var3);         }     }      public final int hashCode() throws  {         try {             return ((Integer)super.h.invoke(this, m0, (Object[])null)).intValue();         } catch (RuntimeException | Error var2) {             throw var2;         } catch (Throwable var3) {             throw new UndeclaredThrowableException(var3);         }     }      static {         try {             m1 = Class.forName("java.lang.Object").getMethod("equals", new Class[]{Class.forName("java.lang.Object")});             m2 = Class.forName("java.lang.Object").getMethod("toString", new Class[0]);             m4 = Class.forName("com.kevin.java.annotation.runtimeAnnotation.HelloAnnotation").getMethod("annotationType", new Class[0]);             m3 = Class.forName("com.kevin.java.annotation.runtimeAnnotation.HelloAnnotation").getMethod("say", new Class[0]);             m0 = Class.forName("java.lang.Object").getMethod("hashCode", new Class[0]);         } catch (NoSuchMethodException var2) {             throw new NoSuchMethodError(var2.getMessage());         } catch (ClassNotFoundException var3) {             throw new NoClassDefFoundError(var3.getMessage());         }     } } 
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从第14行我们可以看到，我们自定义的注解HelloAnnotation是一个接口，而$Proxy1这个Java生成的动态代理类就是它的实现类

我们接着看一下HelloAnnotation的字节码

 $ javap -verbose HelloAnnotation  Warning: Binary file HelloAnnotation contains com.kevin.java.annotation.runtimeAnnotation.HelloAnnotation Classfile /home/kevin/Workspace/IdeaProjects/JavaLearn/out/production/JavaLearn/com/kevin/java/annotation/runtimeAnnotation/HelloAnnotation.class   Last modified Aug 6, 2016; size 496 bytes   MD5 checksum a6c87f863669f6ab9050ffa310160ea5   Compiled from "HelloAnnotation.java" public interface com.kevin.java.annotation.runtimeAnnotation.HelloAnnotation extends java.lang.annotation.Annotation   minor version: 0   major version: 52   flags: ACC_PUBLIC, ACC_INTERFACE, ACC_ABSTRACT, ACC_ANNOTATION Constant pool:    #1 = Class              #18            // com/kevin/java/annotation/runtimeAnnotation/HelloAnnotation    #2 = Class              #19            // java/lang/Object    #3 = Class              #20            // java/lang/annotation/Annotation    #4 = Utf8               say    #5 = Utf8               ()Ljava/lang/String;    #6 = Utf8               AnnotationDefault    #7 = Utf8               Hi    #8 = Utf8               SourceFile    #9 = Utf8               HelloAnnotation.java   #10 = Utf8               RuntimeVisibleAnnotations   #11 = Utf8               Ljava/lang/annotation/Target;   #12 = Utf8               value   #13 = Utf8               Ljava/lang/annotation/ElementType;   #14 = Utf8               TYPE   #15 = Utf8               Ljava/lang/annotation/Retention;   #16 = Utf8               Ljava/lang/annotation/RetentionPolicy;   #17 = Utf8               RUNTIME   #18 = Utf8               com/kevin/java/annotation/runtimeAnnotation/HelloAnnotation   #19 = Utf8               java/lang/Object   #20 = Utf8               java/lang/annotation/Annotation {   public abstract java.lang.String say();     descriptor: ()Ljava/lang/String;     flags: ACC_PUBLIC, ACC_ABSTRACT     AnnotationDefault:       default_value: s#7} SourceFile: "HelloAnnotation.java" RuntimeVisibleAnnotations:   0: #11(#12=[e#13.#14])   1: #15(#12=e#16.#17) 
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看到第7行。很明显，HelloAnnotation就是继承了Annotation的接口。再看第10行，flag字段中，我们可以看到，有个ACC_ANNOTATION标记，说明是一个注解，所以注解本质是一个继承了Annotation的特殊接口。

而Annotation接口声明了以下方法。

package java.lang.annotation;  public interface Annotation {     boolean equals(Object var1);      int hashCode();      String toString();      Class<? extends Annotation> annotationType(); } 
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这些方法，已经被$Proxy1实现了。（这就是动态代理的机制）

小结

现在我们知道了HelloAnnotation注解（接口）是一个继承了Annotation接口的特殊接口，而我们通过反射获取注解时，返回的是Java运行时生成的动态代理对象$Proxy1，该类就是HelloAnnotation注解（接口）的具体实现类。

2. 动态代理类$Proxy1是如何处理annotation.say()方法的调用？

无论是否了解动态代理，这里只需要明确一点，动态代理方法的调用最终会传递给绑定的InvocationHandler实例的invoke方法处理。我们可以看看源码

$Proxy1.java

public final class $Proxy1 extends Proxy implements HelloAnnotation {    .....    public final String say() throws  {         try {             return (String)super.h.invoke(this, m3, (Object[])null);         } catch (RuntimeException | Error var2) {             throw var2;         } catch (Throwable var3) {             throw new UndeclaredThrowableException(var3);         }     }     .... }
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Proxy.java

public class Proxy implements java.io.Serializable {      /**      * the invocation handler for this proxy instance.      * @serial      */     protected InvocationHandler h;
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从上面不难看出，say方法最终会执行(String)super.h.invoke(this, m3, (Object[])null);，而这其中的h对象类型就是InvocationHandler接口的某个实现类

断点调试，看看InvocationHandler具体实现类是哪个。

这里写图片描述

可以看到h对象是AnnotationInvocationHandler的实例。让我们来看看该实现类的invoke方法。

class AnnotationInvocationHandler implements InvocationHandler, Serializable {     private static final long serialVersionUID = 6182022883658399397L;     private final Class<? extends Annotation> type;     private final Map<String, Object> memberValues;     private transient volatile Method[] memberMethods = null;      AnnotationInvocationHandler(Class<? extends Annotation> var1, Map<String, Object> var2) {         Class[] var3 = var1.getInterfaces();         if(var1.isAnnotation() && var3.length == 1 && var3[0] == Annotation.class) {             this.type = var1;             this.memberValues = var2;         } else {             throw new AnnotationFormatError("Attempt to create proxy for a non-annotation type.");         }     }      public Object invoke(Object var1, Method var2, Object[] var3) {         String var4 = var2.getName();         Class[] var5 = var2.getParameterTypes();         if(var4.equals("equals") && var5.length == 1 && var5[0] == Object.class) {             return this.equalsImpl(var3[0]);         } else if(var5.length != 0) {             throw new AssertionError("Too many parameters for an annotation method");         } else {             byte var7 = -1;             switch(var4.hashCode()) {             case -1776922004:                 if(var4.equals("toString")) {                     var7 = 0;                 }                 break;             case 147696667:                 if(var4.equals("hashCode")) {                     var7 = 1;                 }                 break;             case 1444986633:                 if(var4.equals("annotationType")) {                     var7 = 2;                 }             }              switch(var7) {             case 0:                 return this.toStringImpl();             case 1:                 return Integer.valueOf(this.hashCodeImpl());             case 2:                 return this.type;             default:                 Object var6 = this.memberValues.get(var4);                 if(var6 == null) {                     throw new IncompleteAnnotationException(this.type, var4);                 } else if(var6 instanceof ExceptionProxy) {                     throw ((ExceptionProxy)var6).generateException();                 } else {                     if(var6.getClass().isArray() && Array.getLength(var6) != 0) {                         var6 = this.cloneArray(var6);                     }                      return var6;                 }             }         }     }     ....... }
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我们直接从invoke方法第一行开始单步调试，看看invoke方法是如何处理我们annotation.say()方法的调用的。

这里再贴一次测试代码，不然就得翻到前面了

@HelloAnnotation(say = "Do it!") public class TestMain {     public static void main(String[] args) {         HelloAnnotation annotation = TestMain.class.getAnnotation(HelloAnnotation.class);         System.out.println(annotation.say());     } }
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这里写图片描述

可以看到，say方法的返回值是从一个Map中获取到的。这个map以key（注解方法名）—value（注解方法对应的值）存储TestMain类上的注解

那memberValues这个Map对象是怎么生成的，继续调试 
通过方法调用栈找到memberValues的本源

这里写图片描述

我们继续跟进parseMemberValue（）方法

这里写图片描述

在parseMemberValue（）中会调用parseConst方法，继续跟进到parseConst方法

这里写图片描述

可以看到，memberValues是通过常量池获取到，return var2.getUTF8At(var3);中的var3就是常量池中的序号。继续执行返回到parseMemberValue（）方法

这里写图片描述

可以看到获取的就是我们定义在TestMain类上注解的say的值——“Do it！”

这里可以通过javap -verbose TestMain查看TestMain字节码中的常量池

$ javap -verbose TestMain                                            Warning: Binary file TestMain contains com.kevin.java.annotation.runtimeAnnotation.TestMain Classfile /home/kevin/Workspace/IdeaProjects/JavaLearn/out/production/JavaLearn/com/kevin/java/annotation/runtimeAnnotation/TestMain.class   Last modified Aug 10, 2016; size 1117 bytes   MD5 checksum 610b7176c7dfdad08bc4862247df7123   Compiled from "TestMain.java" public class com.kevin.java.annotation.runtimeAnnotation.TestMain   minor version: 0   major version: 52   flags: ACC_PUBLIC, ACC_SUPER Constant pool://常量池    #1 = Methodref          #11.#30        // java/lang/Object."<init>":()V    #2 = String             #31            // sun.misc.ProxyGenerator.saveGeneratedFiles    #3 = String             #32            // true    #4 = Methodref          #33.#34        // java/lang/System.setProperty:(Ljava/lang/String;Ljava/lang/String;)Ljava/lang/String;    #5 = Class              #35            // com/kevin/java/annotation/runtimeAnnotation/TestMain    #6 = Class              #36            // com/kevin/java/annotation/runtimeAnnotation/HelloAnnotation    #7 = Methodref          #37.#38        // java/lang/Class.getAnnotation:(Ljava/lang/Class;)Ljava/lang/annotation/Annotation;    #8 = Fieldref           #33.#39        // java/lang/System.out:Ljava/io/PrintStream;    #9 = InterfaceMethodref #6.#40         // com/kevin/java/annotation/runtimeAnnotation/HelloAnnotation.say:()Ljava/lang/String;   #10 = Methodref          #41.#42        // java/io/PrintStream.println:(Ljava/lang/String;)V   #11 = Class              #43            // java/lang/Object   #12 = Utf8               <init>   #13 = Utf8               ()V   #14 = Utf8               Code   #15 = Utf8               LineNumberTable   #16 = Utf8               LocalVariableTable   #17 = Utf8               this   #18 = Utf8               Lcom/kevin/java/annotation/runtimeAnnotation/TestMain;   #19 = Utf8               main   #20 = Utf8               ([Ljava/lang/String;)V   #21 = Utf8               args   #22 = Utf8               [Ljava/lang/String;   #23 = Utf8               annotation   #24 = Utf8               Lcom/kevin/java/annotation/runtimeAnnotation/HelloAnnotation;   #25 = Utf8               SourceFile   #26 = Utf8               TestMain.java   #27 = Utf8               RuntimeVisibleAnnotations   #28 = Utf8               say   #29 = Utf8               Do it!   #30 = NameAndType        #12:#13        // "<init>":()V   #31 = Utf8               sun.misc.ProxyGenerator.saveGeneratedFiles   #32 = Utf8               true   #33 = Class              #44            // java/lang/System   #34 = NameAndType        #45:#46        // setProperty:(Ljava/lang/String;Ljava/lang/String;)Ljava/lang/String;   #35 = Utf8               com/kevin/java/annotation/runtimeAnnotation/TestMain   #36 = Utf8               com/kevin/java/annotation/runtimeAnnotation/HelloAnnotation   #37 = Class              #47            // java/lang/Class   #38 = NameAndType        #48:#49        // getAnnotation:(Ljava/lang/Class;)Ljava/lang/annotation/Annotation;   #39 = NameAndType        #50:#51        // out:Ljava/io/PrintStream;   #40 = NameAndType        #28:#52        // say:()Ljava/lang/String;   #41 = Class              #53            // java/io/PrintStream   #42 = NameAndType        #54:#55        // println:(Ljava/lang/String;)V   #43 = Utf8               java/lang/Object   #44 = Utf8               java/lang/System   #45 = Utf8               setProperty   #46 = Utf8               (Ljava/lang/String;Ljava/lang/String;)Ljava/lang/String;   #47 = Utf8               java/lang/Class   #48 = Utf8               getAnnotation   #49 = Utf8               (Ljava/lang/Class;)Ljava/lang/annotation/Annotation;   #50 = Utf8               out   #51 = Utf8               Ljava/io/PrintStream;   #52 = Utf8               ()Ljava/lang/String;   #53 = Utf8               java/io/PrintStream   #54 = Utf8               println   #55 = Utf8               (Ljava/lang/String;)V {   public com.kevin.java.annotation.runtimeAnnotation.TestMain();     descriptor: ()V     flags: ACC_PUBLIC     Code:       stack=1, locals=1, args_size=1          0: aload_0          1: invokespecial #1                  // Method java/lang/Object."<init>":()V          4: return       LineNumberTable:         line 10: 0       LocalVariableTable:         Start  Length  Slot  Name   Signature             0       5     0  this   Lcom/kevin/java/annotation/runtimeAnnotation/TestMain;    public static void main(java.lang.String[]);     descriptor: ([Ljava/lang/String;)V     flags: ACC_PUBLIC, ACC_STATIC     Code:       stack=2, locals=2, args_size=1          0: ldc           #2                  // String sun.misc.ProxyGenerator.saveGeneratedFiles          2: ldc           #3                  // String true          4: invokestatic  #4                  // Method java/lang/System.setProperty:(Ljava/lang/String;Ljava/lang/String;)Ljava/lang/String;          7: pop          8: ldc           #5                  // class com/kevin/java/annotation/runtimeAnnotation/TestMain         10: ldc           #6                  // class com/kevin/java/annotation/runtimeAnnotation/HelloAnnotation         12: invokevirtual #7                  // Method java/lang/Class.getAnnotation:(Ljava/lang/Class;)Ljava/lang/annotation/Annotation;         15: checkcast     #6                  // class com/kevin/java/annotation/runtimeAnnotation/HelloAnnotation         18: astore_1         19: getstatic     #8                  // Field java/lang/System.out:Ljava/io/PrintStream;         22: aload_1         23: invokeinterface #9,  1            // InterfaceMethod com/kevin/java/annotation/runtimeAnnotation/HelloAnnotation.say:()Ljava/lang/String;         28: invokevirtual #10                 // Method java/io/PrintStream.println:(Ljava/lang/String;)V         31: return       LineNumberTable:         line 13: 0         line 14: 8         line 15: 19         line 16: 31       LocalVariableTable:         Start  Length  Slot  Name   Signature             0      32     0  args   [Ljava/lang/String;            19      13     1 annotation   Lcom/kevin/java/annotation/runtimeAnnotation/HelloAnnotation; } SourceFile: "TestMain.java" RuntimeVisibleAnnotations:   0: #24(#28=s#29) 
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仔细看第40行#29 = Utf8 Do it!，可以看到#29与var3的29对应（也就常量池的索引），对应的值就是Do it!。

以上就是say方法调用的细节。

总结

注解本质是一个继承了Annotation的特殊接口，其具体实现类是Java运行时生成的动态代理类。通过代理对象调用自定义注解（接口）的方法，会最终调用AnnotationInvocationHandler的invoke方法。该方法会从memberValues这个Map中索引出对应的值。而memberValues的来源是Java常量池。