我看JAVA 之 Annotation

注：基于jdk11

注解包结构

名词解释：

• meta-annotation 元数据注解 表示用来声明注解的注解
• marker-annotation 标记注解 表示没有成员的注解

Annotation

 Annotation 是一个被所有注解类型实现的通用接口。但是如果硬编码实现此接口不能定义一个注解类型，同时，此接口本身也不是一个注解类型。 所有注解声明格式为: @interface annotationName{ } 隐式表明某个类型（注解类型）实现了接口java.lang.annotation.Annotation

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

Target

meta-annotation，表示某注解类型应用在哪些语法元素上，默认应用在除TYPE外的所有语法元素上。

@Documented @Retention(RetentionPolicy.RUNTIME) @Target(ElementType.ANNOTATION_TYPE) public @interface Target { /** * Returns an array of the kinds of elements an annotation type * can be applied to. * @return an array of the kinds of elements an annotation type * can be applied to */ ElementType[] value(); }

ElementType

枚举类型 表示某注解可以应用的元素上下文。 

public enum ElementType { TYPE, FIELD, METHOD, PARAMETER, CONSTRUCTOR, LOCAL_VARIABLE, ANNOTATION_TYPE, PACKAGE, /** * Type parameter declaration * @since 1.8 */ TYPE_PARAMETER, /** * Use of a type * @since 1.8 */ TYPE_USE, /** * Module declaration. * @since 9 */ MODULE }

Retention

meta-annotation 表示注解类型持有范围或生命周期。

@Documented @Retention(RetentionPolicy.RUNTIME) @Target(ElementType.ANNOTATION_TYPE) public @interface Retention { /** * Returns the retention policy. * @return the retention policy */ RetentionPolicy value(); }

RetentionPolicy

枚举类型 表示持有注解周期策略。

public enum RetentionPolicy { SOURCE,//应用在源文件上，主要做编译时检查，检查完毕即丢弃。 CLASS,//默认值，被编译器记录在字节码文件中，但不能被运行时持有。 RUNTIME//除了可以被编译器记录在字节码文件中，还可以被运行时持有。参见：java.lang.reflect. AnnotatedElement（表示运行在VM的程序中的可以被注解或注解了的元素） } 

Inherited

meta-annotation 同时也是marker-annotation 表示某个注解类型是可以被继承的 

Documented

meta-annotation 同时也是marker-annotation 表示某个类型标记@Documented注解，那么此类型上所有注解会被JAVA Doc收集为API一部分 

Native

meta-annotation 同时也是marker-annotation(since1.8)，表明一个字段引用的值可能来自于本地代码

Repeatable

meta-annotation(since1.8) 

异常

AnnotationFormatError
AnnotationTypeMismatchException
IncompleteAnnotationException

内部原理

以实现一个简单的orm注解的形式来描述Java的Annotation的内部原理，代码如下：

源代码

package chapter04; import java.lang.annotation.*; /** * a orm's table annotation */ @Documented @Target(ElementType.TYPE) @Retention(RetentionPolicy.RUNTIME) public @interface Table { String name() default ""; String alias() default ""; }

package chapter04; import java.lang.annotation.*; /** * a orm's column annotation */ @Documented @Target(ElementType.FIELD) @Retention(RetentionPolicy.RUNTIME) public @interface Column { boolean primary() default false; String name() default ""; String type() default "varchar"; int length() default 0; String alias() default ""; }

package chapter04; @Table(name = "t_student", alias = "s") public class Student { @Column(primary = true, name = "_id", type = "bigint", length = 20) String id; @Column(name = "name", type = "string", length = 32) String name; } 

package chapter04; public class TestAnno { public static void main(String [] args) throws NoSuchFieldException { System.setProperty("jdk.proxy.ProxyGenerator.saveGeneratedFiles", "true"); Table table = Student.class.getAnnotation(Table.class); System.out.printf("table'name is %s, alias is %s. \n", table.name(), table.alias()); Column column = Student.class.getDeclaredField("id").getAnnotation(Column.class); System.out.printf("column'name is %s, alias is %s. \n", column.name(), column.alias()); } }

通过设置环境变量，可以保存jdk动态生成的代码，查看java.lang.reflect.ProxyGenerator类中的配置， 如下：

 /** debugging flag for saving generated class files */ private static final boolean saveGeneratedFiles = java.security.AccessController.doPrivileged( new GetBooleanAction( "jdk.proxy.ProxyGenerator.saveGeneratedFiles")).booleanValue();

jdk生成的代理代码

package com.sun.proxy; import java.lang.annotation.Retention; import java.lang.annotation.RetentionPolicy; import java.lang.reflect.InvocationHandler; import java.lang.reflect.Method; import java.lang.reflect.Proxy; import java.lang.reflect.UndeclaredThrowableException; public final class $Proxy0 extends Proxy implements Retention { private static Method m1; private static Method m2; private static Method m4; private static Method m0; private static Method m3; public $Proxy0(InvocationHandler var1) throws { super(var1); } public final boolean equals(Object var1) throws { try { return (Boolean)super.h.invoke(this, m1, new Object[]{var1}); } 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 int hashCode() throws { try { return (Integer)super.h.invoke(this, m0, (Object[])null); } catch (RuntimeException | Error var2) { throw var2; } catch (Throwable var3) { throw new UndeclaredThrowableException(var3); } } public final RetentionPolicy value() throws { try { return (RetentionPolicy)super.h.invoke(this, m3, (Object[])null); } catch (RuntimeException | Error var2) { throw var2; } catch (Throwable var3) { throw new UndeclaredThrowableException(var3); } } static { try { m1 = Class.forName("java.lang.Object").getMethod("equals", Class.forName("java.lang.Object")); m2 = Class.forName("java.lang.Object").getMethod("toString"); m4 = Class.forName("java.lang.annotation.Retention").getMethod("annotationType"); m0 = Class.forName("java.lang.Object").getMethod("hashCode"); m3 = Class.forName("java.lang.annotation.Retention").getMethod("value"); } catch (NoSuchMethodException var2) { throw new NoSuchMethodError(var2.getMessage()); } catch (ClassNotFoundException var3) { throw new NoClassDefFoundError(var3.getMessage()); } } }

package com.sun.proxy; import chapter04.Table; 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 Table { private static Method m1; private static Method m3; private static Method m2; private static Method m5; private static Method m4; 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}); } catch (RuntimeException | Error var3) { throw var3; } catch (Throwable var4) { throw new UndeclaredThrowableException(var4); } } public final String name() 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 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, m5, (Object[])null); } catch (RuntimeException | Error var2) { throw var2; } catch (Throwable var3) { throw new UndeclaredThrowableException(var3); } } public final String alias() throws { try { return (String)super.h.invoke(this, m4, (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); } catch (RuntimeException | Error var2) { throw var2; } catch (Throwable var3) { throw new UndeclaredThrowableException(var3); } } static { try { m1 = Class.forName("java.lang.Object").getMethod("equals", Class.forName("java.lang.Object")); m3 = Class.forName("chapter04.Table").getMethod("name"); m2 = Class.forName("java.lang.Object").getMethod("toString"); m5 = Class.forName("chapter04.Table").getMethod("annotationType"); m4 = Class.forName("chapter04.Table").getMethod("alias"); m0 = Class.forName("java.lang.Object").getMethod("hashCode"); } catch (NoSuchMethodException var2) { throw new NoSuchMethodError(var2.getMessage()); } catch (ClassNotFoundException var3) { throw new NoClassDefFoundError(var3.getMessage()); } } } 

package com.sun.proxy; import chapter04.Column; import java.lang.reflect.InvocationHandler; import java.lang.reflect.Method; import java.lang.reflect.Proxy; import java.lang.reflect.UndeclaredThrowableException; public final class $Proxy2 extends Proxy implements Column { private static Method m1; private static Method m7; private static Method m3; private static Method m4; private static Method m2; private static Method m8; private static Method m5; private static Method m6; private static Method m0; public $Proxy2(InvocationHandler var1) throws { super(var1); } public final boolean equals(Object var1) throws { try { return (Boolean)super.h.invoke(this, m1, new Object[]{var1}); } catch (RuntimeException | Error var3) { throw var3; } catch (Throwable var4) { throw new UndeclaredThrowableException(var4); } } public final boolean primary() throws { try { return (Boolean)super.h.invoke(this, m7, (Object[])null); } catch (RuntimeException | Error var2) { throw var2; } catch (Throwable var3) { throw new UndeclaredThrowableException(var3); } } public final String name() 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 String type() throws { try { return (String)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); } } public final Class annotationType() throws { try { return (Class)super.h.invoke(this, m8, (Object[])null); } catch (RuntimeException | Error var2) { throw var2; } catch (Throwable var3) { throw new UndeclaredThrowableException(var3); } } public final int length() throws { try { return (Integer)super.h.invoke(this, m5, (Object[])null); } catch (RuntimeException | Error var2) { throw var2; } catch (Throwable var3) { throw new UndeclaredThrowableException(var3); } } public final String alias() throws { try { return (String)super.h.invoke(this, m6, (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); } catch (RuntimeException | Error var2) { throw var2; } catch (Throwable var3) { throw new UndeclaredThrowableException(var3); } } static { try { m1 = Class.forName("java.lang.Object").getMethod("equals", Class.forName("java.lang.Object")); m7 = Class.forName("chapter04.Column").getMethod("primary"); m3 = Class.forName("chapter04.Column").getMethod("name"); m4 = Class.forName("chapter04.Column").getMethod("type"); m2 = Class.forName("java.lang.Object").getMethod("toString"); m8 = Class.forName("chapter04.Column").getMethod("annotationType"); m5 = Class.forName("chapter04.Column").getMethod("length"); m6 = Class.forName("chapter04.Column").getMethod("alias"); m0 = Class.forName("java.lang.Object").getMethod("hashCode"); } catch (NoSuchMethodException var2) { throw new NoSuchMethodError(var2.getMessage()); } catch (ClassNotFoundException var3) { throw new NoClassDefFoundError(var3.getMessage()); } } } 

如上图，debug运行TestAnno，结合jdk动态生成的代理类可以得出如下结论：

Table、Column是实现了Annotation的特殊接口，而通过反射获取注解返回的是Java运行时生成的动态代理对象$Proxy1（Table的实现类）、$Proxy2（Column实现类）