Java注解（Annotation）原理详解
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
- 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";
}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
}
}运行程序，输出结果如下：
Do it!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
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
- 在代码中加入
System.setProperty("sun.misc.ProxyGenerator.saveGeneratedFiles", "true"); - 在运行时加入jvm 参数
-Dsun.misc.ProxyGenerator.saveGeneratedFiles=true
这里使用第一种，↓
然后运行程序。
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());
}
}
}
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)
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 annotationType();
}
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);
}
}
....
}Proxy.java
public class Proxy implements java.io.Serializable {
/**
* the invocation handler for this proxy instance.
* @serial
*/
protected InvocationHandler h;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。
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 Classextends Annotation> type;
private final Map<String, Object> memberValues;
private transient volatile Method[] memberMethods = null;
AnnotationInvocationHandler(Classextends 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;
}
}
}
}
.......
}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());
}
}
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
我们继续跟进parseMemberValue（）方法
在parseMemberValue（）中会调用parseConst方法，继续跟进到parseConst方法
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
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."":()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 // "":()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."":()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)
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。