java注解实例

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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 plaincopy
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 plaincopy
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 plaincopy
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 plaincopy
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 plaincopy
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 plaincopy
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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