下文所使用的java版本信息
$ java -version java version "1.8.0_20" Java(TM) SE Runtime Environment (build 1.8.0_20-b26) Java HotSpot(TM) 64-Bit Server VM (build 25.20-b23, mixed mode)
java中的注解是一種繼承自接口`java.lang.annotation.Annotation`的特殊接口。參見下面的[JLS][2]和JDK文檔。
An annotation type declaration specifies a new annotation type, a special kind of interface type. To distinguish an annotation type declaration from a normal interface declaration, the keyword interface is preceded by an at-sign (@). ... The direct superinterface of every annotation type is java.lang.annotation.Annotation.
/** * The common interface extended by all annotation types. Note that an * interface that manually extends this one does <i>not</i> define * an annotation type. Also note that this interface does not itself * define an annotation type. * * More information about annotation types can be found in section 9.6 of * <cite>The Java™ Language Specification</cite>. * * The {@link java.lang.reflect.AnnotatedElement} interface discusses * compatibility concerns when evolving an annotation type from being * non-repeatable to being repeatable. * * @author Josh Bloch * @since 1.5 */ public interface Annotation { ... }
下面看一下具體示例。
定義一個注解：
import java.lang.annotation.ElementType; import java.lang.annotation.Retention; import java.lang.annotation.RetentionPolicy; import java.lang.annotation.Target; /** * Created by Administrator on 2015/1/18. */ @Target(ElementType.TYPE) @Retention(RetentionPolicy.RUNTIME) public @interface TestAnnotation { int count() default 1; }
經過編譯之后，注解`TestAnnotation`的字節碼是這樣的：
Classfile /e:/workspace/intellij/SpringTest/target/classes/TestAnnotation.class Last modified 2015-1-18; size 379 bytes MD5 checksum 200dc3a75216b7a88ae17873d5dffd4f Compiled from "TestAnnotation.java" public interface TestAnnotation extends java.lang.annotation.Annotation minor version: 0 major version: 52 flags: ACC_PUBLIC, ACC_INTERFACE, ACC_ABSTRACT, ACC_ANNOTATION Constant pool: #1 = Class #14 // TestAnnotation #2 = Class #15 // java/lang/Object #3 = Class #16 // java/lang/annotation/Annotation #4 = Utf8 SourceFile #5 = Utf8 TestAnnotation.java #6 = Utf8 RuntimeVisibleAnnotations #7 = Utf8 Ljava/lang/annotation/Target; #8 = Utf8 value #9 = Utf8 Ljava/lang/annotation/ElementType; #10 = Utf8 TYPE #11 = Utf8 Ljava/lang/annotation/Retention; #12 = Utf8 Ljava/lang/annotation/RetentionPolicy; #13 = Utf8 RUNTIME #14 = Utf8 TestAnnotation #15 = Utf8 java/lang/Object #16 = Utf8 java/lang/annotation/Annotation { } SourceFile: "TestAnnotation.java" RuntimeVisibleAnnotations: 0: #7(#8=[e#9.#10]) 1: #11(#8=e#12.#13)
從反編譯后的信息中可以看出，注解就是一個繼承自`java.lang.annotation.Annotation`的接口。
那么接口怎么能夠設置屬性呢？簡單來說就是java通過動態代理的方式為你生成了一個實現了"接口"`TestAnnotation`的實例（對于當前的實體來說，例如類、方法、屬性域等，這個代理對象是單例的），然后對該代理實例的屬性賦值，這樣就可以在程序運行時（如果將注解設置為運行時可見的話）通過反射獲取到注解的配置信息。
具體來說是怎么實現的呢？
寫一個使用該注解的類：
import java.io.IOException; /** * Created by Administrator on 2015/1/18. */ @TestAnnotation(count = 0x7fffffff) public class TestMain { public static void main(String[] args) throws InterruptedException, NoSuchFieldException, IllegalAccessException, IOException { TestAnnotation annotation = TestMain.class.getAnnotation(TestAnnotation.class); System.out.println(annotation.count()); System.in.read(); } }
反編譯一下這段代碼：
Classfile /e:/workspace/intellij/SpringTest/target/classes/TestMain.class Last modified 2015-1-20; size 1006 bytes MD5 checksum a2d5367ea568240f078d5fb1de917550 Compiled from "TestMain.java" public class TestMain minor version: 0 major version: 52 flags: ACC_PUBLIC, ACC_SUPER Constant pool: #1 = Methodref #10.#34 // java/lang/Object."<init>":()V #2 = Class #35 // TestMain #3 = Class #36 // TestAnnotation #4 = Methodref #37.#38 // java/lang/Class.getAnnotation:(Ljava/lang/Class;)Ljava/lang/annotation/Annotation; #5 = Fieldref #39.#40 // java/lang/System.out:Ljava/io/PrintStream; #6 = InterfaceMethodref #3.#41 // TestAnnotation.count:()I #7 = Methodref #42.#43 // java/io/PrintStream.println:(I)V #8 = Fieldref #39.#44 // java/lang/System.in:Ljava/io/InputStream; #9 = Methodref #45.#46 // java/io/InputStream.read:()I #10 = Class #47 // java/lang/Object #11 = Utf8 <init> #12 = Utf8 ()V #13 = Utf8 Code #14 = Utf8 LineNumberTable #15 = Utf8 LocalVariableTable #16 = Utf8 this #17 = Utf8 LTestMain; #18 = Utf8 main #19 = Utf8 ([Ljava/lang/String;)V #20 = Utf8 args #21 = Utf8 [Ljava/lang/String; #22 = Utf8 annotation #23 = Utf8 LTestAnnotation; #24 = Utf8 Exceptions #25 = Class #48 // java/lang/InterruptedException #26 = Class #49 // java/lang/NoSuchFieldException #27 = Class #50 // java/lang/IllegalAccessException #28 = Class #51 // java/io/IOException #29 = Utf8 SourceFile #30 = Utf8 TestMain.java #31 = Utf8 RuntimeVisibleAnnotations #32 = Utf8 count #33 = Integer 2147483647 #34 = NameAndType #11:#12 // "<init>":()V #35 = Utf8 TestMain #36 = Utf8 TestAnnotation #37 = Class #52 // java/lang/Class #38 = NameAndType #53:#54 // getAnnotation:(Ljava/lang/Class;)Ljava/lang/annotation/Annotation; #39 = Class #55 // java/lang/System #40 = NameAndType #56:#57 // out:Ljava/io/PrintStream; #41 = NameAndType #32:#58 // count:()I #42 = Class #59 // java/io/PrintStream #43 = NameAndType #60:#61 // println:(I)V #44 = NameAndType #62:#63 // in:Ljava/io/InputStream; #45 = Class #64 // java/io/InputStream #46 = NameAndType #65:#58 // read:()I #47 = Utf8 java/lang/Object #48 = Utf8 java/lang/InterruptedException #49 = Utf8 java/lang/NoSuchFieldException #50 = Utf8 java/lang/IllegalAccessException #51 = Utf8 java/io/IOException #52 = Utf8 java/lang/Class #53 = Utf8 getAnnotation #54 = Utf8 (Ljava/lang/Class;)Ljava/lang/annotation/Annotation; #55 = Utf8 java/lang/System #56 = Utf8 out #57 = Utf8 Ljava/io/PrintStream; #58 = Utf8 ()I #59 = Utf8 java/io/PrintStream #60 = Utf8 println #61 = Utf8 (I)V #62 = Utf8 in #63 = Utf8 Ljava/io/InputStream; #64 = Utf8 java/io/InputStream #65 = Utf8 read { public 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 7: 0 LocalVariableTable: Start Length Slot Name Signature 0 5 0 this LTestMain; public static void main(java.lang.String[]) throws java.lang.InterruptedException, java.lang.NoSuchFieldException, java.lang.IllegalAccessException, java.io.IOException; descriptor: ([Ljava/lang/String;)V flags: ACC_PUBLIC, ACC_STATIC Code: stack=2, locals=2, args_size=1 0: ldc #2 // class TestMain 2: ldc #3 // class TestAnnotation 4: invokevirtual #4 // Method java/lang/Class.getAnnotation:(Ljava/lang/Class;)Ljava/lang/annotation/Annotation; 7: checkcast #3 // class TestAnnotation 10: astore_1 11: getstatic #5 // Field java/lang/System.out:Ljava/io/PrintStream; 14: aload_1 15: invokeinterface #6, 1 // InterfaceMethod TestAnnotation.count:()I 20: invokevirtual #7 // Method java/io/PrintStream.println:(I)V 23: getstatic #8 // Field java/lang/System.in:Ljava/io/InputStream; 26: invokevirtual #9 // Method java/io/InputStream.read:()I 29: pop 30: return LineNumberTable: line 10: 0 line 11: 11 line 12: 23 line 13: 30 LocalVariableTable: Start Length Slot Name Signature 0 31 0 args [Ljava/lang/String; 11 20 1 annotation LTestAnnotation; Exceptions: throws java.lang.InterruptedException, java.lang.NoSuchFieldException, java.lang.IllegalAccessException, java.io.IOException } SourceFile: "TestMain.java" RuntimeVisibleAnnotations: 0: #23(#32=I#33)
最后一行的代碼說明，注解`TestAnnotation`的屬性設置是在編譯時就確定了的。（對屬性的說明在[這里][1]）。
然后，運行上面的程序，通過CLHSDB在eden區找到注解實例，
hsdb> scanoops 0x00000000e1b80000 0x00000000e3300000 TestAnnotation 0x00000000e1d6c360 com/sun/proxy/$Proxy1
類型`com/sun/proxy/$Proxy1`是jdk動態代理生成對象時的默認類型，其中`com.sun.proxy`是默認的包名，定義于`ReflectUtil`類的`PROXY_PACKAGE`字段中。代理類名`$PROXY1`包含兩部分，其中前綴`$PROXY`是jdk種默認的代理類類名前綴（參見`java.lang.reflect.Proxy`類的javadoc），后的1是自增的結果。
下面看一下這個代理類的內容。運行java程序時添加參數`-Dsun.misc.ProxyGenerator.saveGeneratedFiles=true`可以將轉儲出jdk動態代理類的class文件。若是項目較大或是使用了各種框架的話，慎用此參數。
Classfile /e:/workspace/intellij/SpringTest/target/classes/com/sun/proxy/$Proxy1.class Last modified 2015-1-19; size 2062 bytes MD5 checksum 7321e44402258ba9e061275e313c5c9f public final class com.sun.proxy.$Proxy1 extends java.lang.reflect.Proxy implements TestAnnotation minor version: 0 major version: 49 flags: ACC_PUBLIC, ACC_FINAL ...
太長了，只截取一部分。從中可以看到，這個代理類實現了繼承自`java.lang.reflect.Proxy`類，又實現了“接口”TestAnnotation。
接下來查看一下代理對象的內容：
hsdb> inspect 0x00000000e1d6c360 instance of Oop for com/sun/proxy/$Proxy1 @ 0x00000000e1d6c360 @ 0x00000000e1d6c360 (size = 16) _mark: 1 _metadata._compressed_klass: InstanceKlass for com/sun/proxy/$Proxy1 h: Oop for sun/reflect/annotation/AnnotationInvocationHandler @ 0x00000000e1ce7670 Oop for sun/reflect/annotation/Annota tionInvocationHandler @ 0x00000000e1ce7670
其中，0xe1ce74e0是成員變量h的地址（h是定義在`java.lang.reflect.Proxy`類中的），通過查看類`AnnotationInvocationHandler`的源碼可以知道注解的代理實例的值就存儲在它的成員變量`memberValues`中，然后繼續向下挖就好了：
hsdb> inspect 0x00000000e1ce7670 instance of Oop for sun/reflect/annotation/AnnotationInvocationHandler @ 0x00000000e1ce7670 @ 0x00000000e1ce7670 (size = 24) _mark: 1 _metadata._compressed_klass: InstanceKlass for sun/reflect/annotation/AnnotationInvocationHandler type: Oop for java/lang/Class @ 0x00000000e1ccc5f8 Oop for java/lang/Class @ 0x00000000e1ccc5f8 memberValues: Oop for java/util/LinkedHashMap @ 0x00000000e1ce7548 Oop for java/util/LinkedHashMap @ 0x00000000e1ce7548 memberMethods: null null hsdb> inspect 0x00000000e1ce7548 instance of Oop for java/util/LinkedHashMap @ 0x00000000e1ce7548 @ 0x00000000e1ce7548 (size = 56) _mark: 1 _metadata._compressed_klass: InstanceKlass for java/util/LinkedHashMap keySet: null null values: null null table: ObjArray @ 0x00000000e1ce75b8 Oop for [Ljava/util/HashMap$Node; @ 0x00000000e1ce75b8 entrySet: null null size: 1 modCount: 1 threshold: 1 loadFactor: 0.75 head: Oop for java/util/LinkedHashMap$Entry @ 0x00000000e1ce75d0 Oop for java/util/LinkedHashMap$Entry @ 0x00000000e1ce7 5d0 tail: Oop for java/util/LinkedHashMap$Entry @ 0x00000000e1ce75d0 Oop for java/util/LinkedHashMap$Entry @ 0x00000000e1ce75d0 accessOrder: false hsdb> inspect 0x00000000e1ce75d0 instance of Oop for java/util/LinkedHashMap$Entry @ 0x00000000e1ce75d0 @ 0x00000000e1ce75d0 (size = 40) _mark: 1 _metadata._compressed_klass: InstanceKlass for java/util/LinkedHashMap$Entry hash: 94852264 key: "count" @ 0x00000000e1bd7c90 Oop for java/lang/String @ 0x00000000e1bd7c90 value: Oop for java/lang/Integer @ 0x00000000e1ce7630 Oop for java/lang/Integer @ 0x00000000e1ce7630 next: null null before: null null after: null null hsdb> inspect 0x00000000e1ce7630 instance of Oop for java/lang/Integer @ 0x00000000e1ce7630 @ 0x00000000e1ce7630 (size = 16) _mark: 1 _metadata._compressed_klass: InstanceKlass for java/lang/Integer value: 2147483647
最后可以看到，key=“count”, value=Integer(2147483647 = 0x7fffffff)，正是在TestMain中設置的值.
嗯，就這樣吧。
[1]:
Chapter 4. The class File Format[2]:
Chapter 9. Interfaces