【第一部分】
首先了解一下java1.5起默认的三个annotation类型：
@override：只能用在方法上，用来告诉人们这个方法是改写的父类的
@Deprecated：建议别人不要使用旧的api的时候使用的，编译的时候会产生警告信息，可以设定在程序的所有元素上。
@SuppressWarnings：这一类型可以暂时把一些警告信息消除。
【第二部分】
先讲一下怎么自己设计一个annotation，最好的就是读以下jdk自带的annotation源文件
1、源文件Documented.class
- /*
- * @(#)Documented.java 1.6 05/11/17
- *
- * Copyright 2006 Sun Microsystems, Inc. All rights reserved.
- * SUN PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
- */
- package java.lang.annotation;
- /**
- * Indicates that annotations with a type are to be documented by javadoc
- * and similar tools by default. This type should be used to annotate the
- * declarations of types whose annotations affect the use of annotated
- * elements by their clients. If a type declaration is annotated with
- * Documented, its annotations become part of the public API
- * of the annotated elements.
- *
- * @author Joshua Bloch
- * @version 1.6, 11/17/05
- * @since 1.5
- */
- @Documented
- @Retention(RetentionPolicy.RUNTIME)
- @Target(ElementType.ANNOTATION_TYPE)
- public @interface Documented {
- }
@Documented：作用是在生成javadoc文档的时候将该Annotation也写入到文档中。
2、Target.class
- /*
- * @(#)Target.java 1.6 05/11/17
- *
- * Copyright 2006 Sun Microsystems, Inc. All rights reserved.
- * SUN PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
- */
- package java.lang.annotation;
- /**
- * Indicates the kinds of program element to which an annotation type
- * is applicable. If a Target meta-annotation is not present on an
- * annotation type declaration, the declared type may be used on any
- * program element. If such a meta-annotation is present, the compiler
- * will enforce the specified usage restriction.
- *
- * For example, this meta-annotation indicates that the declared type is
- * itself a meta-annotation type. It can only be used on annotation type
- * declarations:
- * <pre>
- * @Target(ElementType.ANNOTATION_TYPE)
- * public @interface MetaAnnotationType {
- * ...
- * }
- * </pre>
- * This meta-annotation indicates that the declared type is intended solely
- * for use as a member type in complex annotation type declarations. It
- * cannot be used to annotate anything directly:
- * <pre>
- * @Target({})
- * public @interface MemberType {
- * ...
- * }
- * </pre>
- * It is a compile-time error for a single ElementType constant to
- * appear more than once in a Target annotation. For example, the
- * following meta-annotation is illegal:
- * <pre>
- * @Target({ElementType.FIELD, ElementType.METHOD, ElementType.FIELD})
- * public @interface Bogus {
- * ...
- * }
- * </pre>
- */
- @Documented
- @Retention(RetentionPolicy.RUNTIME)
- @Target(ElementType.ANNOTATION_TYPE)
- public @interface Target {
- ElementType[] value();
- }
@Target 指示注释类型所适用的程序元素的种类。如果不指定则可以用在任一程序元素上。取值为枚举的数据类型Java.lang.annotation.ElementType，枚举项含义为：
ANNOTATION_TYPE 注释类型声明
CONSTRUCTOR 构造方法声明
FIELD 字段声明（包括枚举常量）
LOCAL_VARIABLE 局部变量声明
METHOD 方法声明
PACKAGE 包声明
PARAMETER 参数声明
TYPE 类、接口（包括注释类型）或枚举声明
3、Retention.class
- /*
- * @(#)Retention.java 1.6 05/11/17
- *
- * Copyright 2006 Sun Microsystems, Inc. All rights reserved.
- * SUN PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
- */
- package java.lang.annotation;
- /**
- * Indicates how long annotations with the annotated type are to
- * be retained. If no Retention annotation is present on
- * an annotation type declaration, the retention policy defaults to
- * <tt>RetentionPolicy.CLASS</tt>.
- *
- * <p>A Target meta-annotation has effect only if the meta-annotated
- * type is use directly for annotation. It has no effect if the meta-annotated
- * type is used as a member type in another annotation type.
- *
- * @author Joshua Bloch
- * @since 1.5
- */
- @Documented
- @Retention(RetentionPolicy.RUNTIME)
- @Target(ElementType.ANNOTATION_TYPE)
- public @interface Retention {
- RetentionPolicy value();
- }
@Retention 指示注释类型的注释要保留多久。默认为 RetentionPolicy.CLASS。取值为枚举：
java.lang.annotation.RetentionPolicy：
CLASS 编译器将把注释记录在类文件中，但在运行时 VM 不需要保留注释。
RUNTIME 编译器将把注释记录在类文件中，在运行时 VM 将保留注释，因此可以反射性地读取。
SOURCE 编译器要丢弃的注释。
【第三部分】下面我们自己来写一个注解类使用
1、Description
- import java.lang.annotation.Documented;
- import java.lang.annotation.ElementType;
- import java.lang.annotation.Retention;
- import java.lang.annotation.RetentionPolicy;
- import java.lang.annotation.Target;
- @Target(ElementType.TYPE)
- @Documented//作用是在生成javadoc文档的时候将该Annotation也写入到文档中。
- @Retention(RetentionPolicy.RUNTIME)//指示注释类型的注释要保留多久。默认为 RetentionPolicy.CLASS。取值为枚举
- /**
- * 注解中定义的属性如果名称为 value， 此属性在使用时可以省写属性名
- */
- public @interface Description {
- String value() ;
- }
2、Name
- import java.lang.annotation.Documented;
- import java.lang.annotation.ElementType;
- import java.lang.annotation.Retention;
- import java.lang.annotation.RetentionPolicy;
- import java.lang.annotation.Target;
- @Documented
- @Target(ElementType.METHOD)
- @Retention(RetentionPolicy.RUNTIME)
- /**
- * 使用了默认值
- */
- public @interface Name {
- String work() default "JAVA";
- String community() default "BLOG";
- }
3、hzu_Opensource类
- @Description("吴海旭的社区测试")
- public class hzu_Opensource {
- @Name(work="sales",community="ITeye")
- public String getName(){
- return null ;
- }
- @Name(community="csdn",work="IT")
- public String getName2(){
- return "csdn" ;
- }
- @Name
- public String getName3(){
- return null ;
- }
- }
4、测试类TestAnnotation
- import java.lang.reflect.Method;
- import java.util.HashSet;
- import java.util.Set;
- public class TestAnnotation {
- @SuppressWarnings("unchecked")
- public static void main(String[] args) throws Exception {
- String CLASS_NAME = "hzu_Opensource" ;
- Class test = Class.forName(CLASS_NAME) ;
- boolean flag = test.isAnnotationPresent(Description.class) ;
- if(flag){
- Description des = (Description)test.getAnnotation(Description.class) ;
- System.out.println("描述: " + des.value());
- System.out.println("---------------");
- }
- Method[] method = test.getMethods() ;
- Set<Method> set = new HashSet<Method>() ;
- for(int i=0;i<method.length;i++){
- boolean otherFlag = method[i].isAnnotationPresent(Name.class) ;
- if(otherFlag) set.add(method[i]) ;
- }
- for(Method m:set){
- Name name = m.getAnnotation(Name.class) ;
- System.out.println(name.work());
- System.out.println("社区：" + name.community());
- }
- }
- }
结果为：
- 描述: 吴海旭的社区测试
- ---------------
- JAVA
- 社区：BLOG
- IT
- 社区：csdn
- sales
- 社区：ITeye
