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
}
}
输出结果：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
我们使用第一种如下：
@HelloAnnotation(say = "Do it!")
public class TestMain {
public static void main(String[] args) {
/* 设置此系统属性,让JVM生成的Proxy类写入文件.保存路径为：com/sun/proxy(如果不存在请手工创建) */
System.setProperty("sun.misc.ProxyGenerator.saveGeneratedFiles", "true");
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
}
}然后运行程序。
可以看到，已经导出了运行时生成的代理类，而且每个分别实现了一个接口。
HelloAnnotation的动态代理类是$Proxy3.class，Intellij自带了反编译工具，直接双击打开，得到如下的Java代码：
public final class $Proxy3 extends Proxy implements HelloAnnotation {
private static Method m3;
private static Method m1;
private static Method m0;
private static Method m4;
private static Method m2;
public $Proxy3(InvocationHandler var1) throws {
super(var1);
}
public final String say() throws {
try {
return (String)super.h.invoke(this, m3, (Object[])null);
} catch (RuntimeException | Error var2) {
throw var2;
} catch (Throwable var3) {
throw new UndeclaredThrowableException(var3);
}
}
public final boolean equals(Object var1) throws {
try {
return ((Boolean)super.h.invoke(this, m1, new Object[]{var1})).booleanValue();
} catch (RuntimeException | Error var3) {
throw var3;
} catch (Throwable var4) {
throw new UndeclaredThrowableException(var4);
}
}
public final int hashCode() throws {
try {
return ((Integer)super.h.invoke(this, m0, (Object[])null)).intValue();
} catch (RuntimeException | Error var2) {
throw var2;
} catch (Throwable var3) {
throw new UndeclaredThrowableException(var3);
}
}
public final Class annotationType() throws {
try {
return (Class)super.h.invoke(this, m4, (Object[])null);
} catch (RuntimeException | Error var2) {
throw var2;
} catch (Throwable var3) {
throw new UndeclaredThrowableException(var3);
}
}
public final String toString() throws {
try {
return (String)super.h.invoke(this, m2, (Object[])null);
} catch (RuntimeException | Error var2) {
throw var2;
} catch (Throwable var3) {
throw new UndeclaredThrowableException(var3);
}
}
static {
try {
m3 = Class.forName("HelloAnnotation").getMethod("say", new Class[0]);
m1 = Class.forName("java.lang.Object").getMethod("equals", new Class[]{Class.forName("java.lang.Object")});
m0 = Class.forName("java.lang.Object").getMethod("hashCode", new Class[0]);
m4 = Class.forName("HelloAnnotation").getMethod("annotationType", new Class[0]);
m2 = Class.forName("java.lang.Object").getMethod("toString", new Class[0]);
} catch (NoSuchMethodException var2) {
throw new NoSuchMethodError(var2.getMessage());
} catch (ClassNotFoundException var3) {
throw new NoClassDefFoundError(var3.getMessage());
}
}
} 从第一行我们可以看到，我们自定义的注解HelloAnnotation是一个接口，而$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
public interface HelloAnnotation extends java.lang.annotation.Annotation SourceFile: "HelloAnnotation.java" RuntimeVisibleAnnotations: 0: #11(#12=[e#13.#14])
1: #15(#12=e#16.#17)
minor version: 0
major version: 51
flags: ACC_PUBLIC, ACC_INTERFACE, ACC_ABSTRACT, ACC_ANNOTATION
Constant pool:
#1 = Class #18 // 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 HelloAnnotation
#19 = Utf8 java/lang/Object
#20 = Utf8 java/lang/annotation/Annotation
{
public abstract java.lang.String say();
flags: ACC_PUBLIC, ACC_ABSTRACT
AnnotationDefault:
default_value: s#7} 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。
public interface Annotation {
boolean equals(Object var1);
int hashCode();
String toString();
Class<? extends Annotation> annotationType();
}这些方法，已经被$Proxy3实现了。（这就是动态代理的机制）
小结
现在我们知道了HelloAnnotation注解（接口）是一个继承了Annotation接口的特殊接口，而我们通过反射获取注解时，返回的是Java运行时生成的动态代理对象$Proxy3，该类就是HelloAnnotation注解（接口）的具体实现类。
二、动态代理类$Proxy3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
$Proxy3.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);
}
}
....
}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 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;
}
}
}
}
.......
} 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。
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。
‘do,it’是在parseMemberValue()方法中获取的，我们继续跟进parseMemberValue（）方法
public static Object parseMemberValue(Class<?> var0, ByteBuffer var1, ConstantPool var2, Class<?> var3) {
Object var4 = null;
byte var5 = var1.get();
switch(var5) {
case 64:
var4 = parseAnnotation(var1, var2, var3, true);
break;
case 91:
return parseArray(var0, var1, var2, var3);
case 99:
var4 = parseClassValue(var1, var2, var3);
break;
case 101:
return parseEnumValue(var0, var1, var2, var3);
default:
var4 = parseConst(var5, var1, var2); //此处会调用parseConst方法，继续跟进到parseConst方法
}
if(!(var4 instanceof ExceptionProxy) && !var0.isInstance(var4)) {
var4 = new AnnotationTypeMismatchExceptionProxy(var4.getClass() + "[" + var4 + "]");
}
return var4;
}
private static Object parseConst(int var0, ByteBuffer var1, ConstantPool var2) {
int var3 = var1.getShort() & '\uffff';
switch(var0) {
case 66:
return Byte.valueOf((byte)var2.getIntAt(var3));
case 67:
return Character.valueOf((char)var2.getIntAt(var3));
case 68:
return Double.valueOf(var2.getDoubleAt(var3));
case 70:
return Float.valueOf(var2.getFloatAt(var3));
case 73:
return Integer.valueOf(var2.getIntAt(var3));
case 74:
return Long.valueOf(var2.getLongAt(var3));
case 83:
return Short.valueOf((short)var2.getIntAt(var3));
case 90:
return Boolean.valueOf(var2.getIntAt(var3) != 0);
case 115:
return var2.getUTF8At(var3); //memberValues是通过常量池获取到，这里的var3就是常量池中的序号。
default:
throw new AnnotationFormatError("Invalid member-value tag in annotation: " + var0);
}
} 调用完parseConst方法，然后返回到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
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! //这里就是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到此为止，say方法就完成了。
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。