java注解应用实例 - Annotation, 自定义注解, 注解类规则

本文介绍了java的自定义注解及注解类编写的规则, 并通过实例来说明下如何使用java的注解. 实例演示了注解在类,构造方法,方法和字段的使用. 可以从这里下载到完成的工程代码: http://dl.iteye.com/topics/download/f74972df-234f-30c9-aadd-ca2ed1376bc2
自定义注解类编写的一些规则:

1. Annotation型定义为@interface, 所有的Annotation会自动继承java.lang.Annotation这一接口,并且不能再去继承别的类或是接口.

2. 参数成员只能用public或默认(default)这两个访问权修饰

3. 参数成员只能用基本类型byte,short,char,int,long,float,double,boolean八种基本数据类型和String、Enum、Class、annotations等数据类型,以及这一些类型的数组.

4. 要获取类方法和字段的注解信息，必须通过Java的反射技术来获取 Annotation对象,因为你除此之外没有别的获取注解对象的方法

5. 注解也可以没有定义成员, 不过这样注解就没啥用了

自定义注解类时, 可以指定目标 (类、方法、字段, 构造函数等) , 注解的生命周期(运行时,class文件或者源码中有效), 是否将注解包含在javadoc中及是否允许子类继承父类中的注解, 具体如下:

1. @Target 表示该注解目标,可能的 ElemenetType 参数包括:

ElemenetType.CONSTRUCTOR 构造器声明

ElemenetType.FIELD 域声明(包括 enum 实例)

ElemenetType.LOCAL_VARIABLE 局部变量声明

ElemenetType.METHOD 方法声明

ElemenetType.PACKAGE 包声明

ElemenetType.PARAMETER 参数声明

ElemenetType.TYPE 类，接口(包括注解类型)或enum声明

2. @Retention 表示该注解的生命周期,可选的 RetentionPolicy 参数包括

RetentionPolicy.SOURCE 注解将被编译器丢弃

RetentionPolicy.CLASS 注解在class文件中可用，但会被VM丢弃

RetentionPolicy.RUNTIME VM将在运行期也保留注释，因此可以通过反射机制读取注解的信息

3. @Documented 指示将此注解包含在 javadoc 中

4. @Inherited 指示允许子类继承父类中的注解

好, 该介绍的介绍了, 看下自定义的注解应用实例:

1. 首先看下定义的注解类:

类注解定义, MyClassAnnotation.java:

[java] view plain copy

package com.ross.annotation;

import java.lang.annotation.*;

/**

* Author: Jiangtao He; Email: ross.jiangtao.he@gmail.com

* Date: 2012-1-29

* Since: MyJavaExpert v1.0

* Description: class annotation

*/

@Retention (RetentionPolicy.RUNTIME)

@Target (ElementType.TYPE)

public @interface MyClassAnnotation

{

String uri();

String desc();

}

默认构造方法注解定义,MyConstructorAnnotation.java:

[java] view plain copy

package com.ross.annotation;

import java.lang.annotation.ElementType;

import java.lang.annotation.Retention;

import java.lang.annotation.RetentionPolicy;

import java.lang.annotation.Target;

/**

* Author: Jiangtao He; Email: ross.jiangtao.he@gmail.com

* Date: 2012-1-29

* Since: MyJavaExpert v1.0

* Description: Constructor annotation

*/

@Retention (RetentionPolicy.RUNTIME)

@Target (ElementType.CONSTRUCTOR)

public @interface MyConstructorAnnotation

{

String uri();

String desc();

}

方法注解定义,MyMethodAnnotation.java:

[java] view plain copy

package com.ross.annotation;

import java.lang.annotation.ElementType;

import java.lang.annotation.Retention;

import java.lang.annotation.RetentionPolicy;

import java.lang.annotation.Target;

/**

* Author: Jiangtao He; Email: ross.jiangtao.he@gmail.com

* Date: 2012-1-29

* Since: MyJavaExpert v1.0

* Description: method annotation

*/

@Retention (RetentionPolicy.RUNTIME)

@Target (ElementType.METHOD)

public @interface MyMethodAnnotation

{

String uri();

String desc();

}

字段注解定义, MyFieldAnnotation.java:

[java] view plain copy

package com.ross.annotation;

import java.lang.annotation.ElementType;

import java.lang.annotation.Retention;

import java.lang.annotation.RetentionPolicy;

import java.lang.annotation.Target;

/**

* Author: Jiangtao He; Email: ross.jiangtao.he@gmail.com

* Date: 2012-1-29

* Since: MyJavaExpert v1.0

* Description: field annotation

*/

@Retention (RetentionPolicy.RUNTIME)

@Target (ElementType.FIELD)

public @interface MyFieldAnnotation

{

String uri();

String desc();

}

2. 再看下我们注解的应用和测试:

在类上面使用了MyClassAnnotation注解, 默认构造方法上使用了MyConstructorAnnotation注解, 自定义方法上使用了MyMethodAnnotation注解, 自定义字段上使用了MyFieldAnnotation注解, 在Mail函数中则实现了访问这些注解,并打印注解信息.

MySample.java:

[java] view plain copy

package com.ross.annotation;

import java.lang.reflect.*;

/**

* Author: Jiangtao He; Email: ross.jiangtao.he@gmail.com

* Date: 2012-1-29

* Since: MyJavaExpert v1.0

* Description: This class is used to show how to use the annotation of each level

*/

@MyClassAnnotation (uri = "com.ross.MySample" , desc = "The class name" )

public class MySample

{

@MyFieldAnnotation (uri = "com.ross.MySample#id" , desc = "The class field")

public String id;

/**

* Description: default constructor

*/

@MyConstructorAnnotation (uri = "com.ross.MySample#MySample" , desc = "The default constuctor" )

public MySample()

{

}

/**

* Description: normal method

*/

@MyMethodAnnotation (uri = "com.ross.MySample#setId" , desc = "The class method" )

public void setId(String id)

{

this .id = id;

}

/**

* Description: MyAnnotation test

* @throws NoSuchMethodException

* @throws SecurityException

* @throws NoSuchFieldException

*/

public static void main(String[] args) throws SecurityException,

NoSuchMethodException, NoSuchFieldException

{

MySample oMySample = new MySample();

// get class annotation

MyClassAnnotation oMyAnnotation = MySample.class

.getAnnotation(MyClassAnnotation.class );

System.out.println("Class's uri: " + oMyAnnotation.uri() + "; desc: "

+ oMyAnnotation.desc());

// get constructor annotation

Constructor oConstructor = oMySample.getClass().getConstructor();

MyConstructorAnnotation oMyConstructorAnnotation = (MyConstructorAnnotation) oConstructor

.getAnnotation(MyConstructorAnnotation.class );

System.out.println("Constructor's uri: "

+ oMyConstructorAnnotation.uri() + "; desc: "

+ oMyConstructorAnnotation.desc());

// get method annotation

Method oMethod = oMySample.getClass().getDeclaredMethod("setId" ,String. class );

MyMethodAnnotation oMyMethodAnnotation = oMethod

.getAnnotation(MyMethodAnnotation.class );

System.out.println("Method's uri: " + oMyMethodAnnotation.uri()

+ "; desc: " + oMyMethodAnnotation.desc());

// get field annotation

Field oField = oMySample.getClass().getDeclaredField("id" );

MyFieldAnnotation oMyFieldAnnotation = oField

.getAnnotation(MyFieldAnnotation.class );

System.out.println("Field's uri: " + oMyFieldAnnotation.uri()

+ "; desc: " + oMyFieldAnnotation.desc());

}

}

控制台打印结果:

[plain] view plain copy

Class's uri: com.ross.MySample; desc: The class name

Constructor's uri: com.ross.MySample#MySample; desc: The default constuctor

Method's uri: com.ross.MySample#setId; desc: The class method

Field's uri: com.ross.MySample#id; desc: The class field

至此本实例就完成了, 其实就是抓住两点一个是定义注解类,另外一个是如何访问注解, 就算是学会了.

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