Java註解（Annotation）原理詳解

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

　　首先寫一個簡單的自定義註解小程序

//先自定義一個運行時註解
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
public @interface HelloAnnotation {

    String say() default "Hi";

}

　　然後在Main函數中解析註解

@HelloAnnotation(say = "Do it!")
public class TestMain {
    public static void main(String[] args) {
        HelloAnnotation annotation = TestMain.class.getAnnotation(HelloAnnotation.class);//獲取TestMain類上的註解對象
        System.out.println(annotation.say());//調用註解對象的say方法，並打印到控制檯
    }
}
輸出結果：Do it!

　　下面將圍繞上面的代碼來研究Java註解（Annotation）的實現原理

1. 註解對象具體是什麼？

　　首先，我們先在main函數第一行斷點，看看HelloAnnotation具體是什麼類的對象

　　可以看到HelloAnnotation註解的實例是jvm生成的動態代理類的對象。

　　這個運行時生成的動態代理對象是可以導出到文件的，方法有兩種

• 在代碼中加入System.setProperty(“sun.misc.ProxyGenerator.saveGeneratedFiles”, “true”);
• 在運行時加入jvm 參數 -Dsun.misc.ProxyGenerator.saveGeneratedFiles=true
  我們使用第一種如下：

@HelloAnnotation(say = "Do it!")
public class TestMain {
    public static void main(String[] args) {
        /* 設置此係統屬性,讓JVM生成的Proxy類寫入文件.保存路徑爲：com/sun/proxy(如果不存在請手工創建) */
        System.setProperty("sun.misc.ProxyGenerator.saveGeneratedFiles", "true");
        HelloAnnotation annotation = TestMain.class.getAnnotation(HelloAnnotation.class);//獲取TestMain類上的註解對象
        System.out.println(annotation.say());//調用註解對象的say方法，並打印到控制檯
    }
}

然後運行程序。

　　可以看到，已經導出了運行時生成的代理類，而且每個分別實現了一個接口。
HelloAnnotation的動態代理類是$Proxy3.class，Intellij自帶了反編譯工具，直接雙擊打開，得到如下的Java代碼：

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());
        }
    }
}

　　從第一行我們可以看到，我們自定義的註解HelloAnnotation是一個接口，而$Proxy1這個Java生成的動態代理類就是它的實現類。
　　我們接着看一下HelloAnnotation的字節碼

 $ 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就是繼承了Annotation的接口。再看第10行，flag字段中，我們可以看到，有個ACC_ANNOTATION標記，說明是一個註解，所以註解本質是一個繼承了Annotation的特殊接口。
　　而Annotation接口聲明瞭以下方法。

public interface Annotation {
    boolean equals(Object var1);
    int hashCode();
    String toString();
    Class<? extends Annotation> annotationType();
}

　　這些方法，已經被$Proxy3實現了。（這就是動態代理的機制）

小結

　　現在我們知道了HelloAnnotation註解（接口）是一個繼承了Annotation接口的特殊接口，而我們通過反射獲取註解時，返回的是Java運行時生成的動態代理對象$Proxy3，該類就是HelloAnnotation註解（接口）的具體實現類。

二、動態代理類$Proxy3如何處理annotation.say()方法的調用

　　無論是否瞭解動態代理，這裏只需要明確一點，動態代理方法的調用最終會傳遞給綁定的InvocationHandler實例的invoke方法處理。我們可以看看源碼

$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);
        }
    }
    ....
}

　　從上面不難看出，say方法最終會執行(String)super.h.invoke(this, m3, (Object[])null);，而這其中的h對象類型就是InvocationHandler接口的某個實現類

　　斷點調試，看看InvocationHandler具體實現類是哪個。

　　可以看到h對象是AnnotationInvocationHandler的實例，讓我們來看看該實現類的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;
                }
            }
        }
    }
    .......
}

　　我們直接從invoke方法第一行開始單步調試，看看invoke方法是如何處理我們annotation.say()方法的調用的。

　　可以看到，say方法的返回值是從一個Map中獲取到的。這個map以key（註解方法名）—value（註解方法對應的值）存儲TestMain類上的註解

　　那memberValues這個Map對象是怎麼生成的，繼續調試
　　通過方法調用棧找到memberValues的本源。

‘do,it’是在parseMemberValue()方法中獲取的，我們繼續跟進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); //此處會調用parseConst方法，繼續跟進到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是通過常量池獲取到，這裏的var3就是常量池中的序號。
        default:
            throw new AnnotationFormatError("Invalid member-value tag in annotation: " + var0);
        }
    }

　　調用完parseConst方法，然後返回到parseMemberValue（）方法

　　可以看到獲取的就是我們定義在TestMain類上註解的say的值——“Do it！”

　　這裏可以通過javap -verbose TestMain查看TestMain字節碼中的常量池。

$ 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! //這裏就是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

　　到此爲止，say方法就完成了。

總結

　　註解本質是一個繼承了Annotation的特殊接口，其具體實現類是Java運行時生成的動態代理類。通過代理對象調用自定義註解（接口）的方法，會最終調用AnnotationInvocationHandler的invoke方法。該方法會從memberValues這個Map中索引出對應的值。而memberValues的來源是Java常量池。