[JAVA] 注解学习@interface

一直都看框架级的代码中都是各种annotation，一起来看看到底怎么弄的

例子1：直接定义一个annotation，并使用之：

package com.base.annotation.example;

import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;

/**
 * Created by guangyi on 15/12/8.
 */
@Retention(RetentionPolicy.RUNTIME)
public @interface MyTarget {
    String value();
}

package com.base.annotation.example;

import java.lang.reflect.Method;

/**
 * Created by guangyi on 15/12/8.
 */
public class MyTargetTest {
    @MyTarget("test")
    public void doSomething() {
        System.out.println("hello world");
    }
    public static void main(String[] args) throws  NoSuchMethodException{
        Method method = MyTargetTest.class.getMethod("doSomething", null);
        if(method.isAnnotationPresent(MyTarget.class)) {
            System.out.println(method.getAnnotation(MyTarget.class)); // 打印@com.base.annotation.example.MyTarget(value=test)
        }
    }
}

例子2：复杂类型的，各种注解类型

package com.base.annotation.example.example2;

/**
 * Created by guangyi on 15/12/8.
 */

public class EnumTest {
    public static enum TrafficLamp {
        RED, GREEN, YELLOW
    }
}

package com.base.annotation.example.example2;

import com.base.annotation.example.MyTarget;

import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;

/**
 * Created by guangyi on 15/12/8.
 */
@Retention(RetentionPolicy.RUNTIME)
public @interface MyAnnotation {
    String hello() default "hello";
    String world();
    int[] array() default {1, 2, 3, 4};
    EnumTest.TrafficLamp lamp();
    MyTarget myAnnotation() default @MyTarget("ddd");
    Class style() default String.class;
}

package com.base.annotation.example.example2;

import com.base.annotation.example.MyTarget;

import java.lang.annotation.Annotation;
import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;

/**
 * Created by guangyi on 15/12/8.
 */
@MyAnnotation(hello = "beijing", world = "shanghai", array = {5, 6, 7, 8}, lamp = EnumTest.TrafficLamp.GREEN, style = int.class)
public class MyTest {
    @MyAnnotation(myAnnotation = @MyTarget("baby"), world = "shanghai", array = {10, 11, 12}, lamp = EnumTest.TrafficLamp.YELLOW)
    @Deprecated
    @SuppressWarnings("")
    public void output() {
        System.out.println("output something!");
    }

    public static void main(String[] args) throws  NoSuchMethodException, InvocationTargetException, IllegalAccessException{
        MyTest myTest = new MyTest();
        Class<MyTest> c = MyTest.class;
        Method method = c.getMethod("output", new Class[]{});
        if(MyTest.class.isAnnotationPresent(MyAnnotation.class)) {
            System.out.println("have annotation");
        }
        if(method.isAnnotationPresent(MyAnnotation.class)) {
            method.invoke(myTest, null);
            MyAnnotation myAnnotation = method.getAnnotation(MyAnnotation.class);
            String hello = myAnnotation.hello();
            String world = myAnnotation.world();
            System.out.println(hello + ", " + world);
            System.out.println(myAnnotation.array().length);
            System.out.println(myAnnotation.myAnnotation().value());
            System.out.println(myAnnotation.style());
            System.out.println(myAnnotation.lamp());
        }
        Annotation[] annotations = method.getAnnotations();
        for (Annotation annotation : annotations) {
            System.out.println(annotation.annotationType().getName());
        }
    }
}

示例3：看原始注解的定义，可以看到也是使用注解，另外，使用value作为注解属性名，则可以省略注解名字不写，例如：@Rentention(RententionPolicy.RUNTIME)：

package java.lang;

import java.lang.annotation.*;
import static java.lang.annotation.ElementType.*;

@Target({TYPE, FIELD, METHOD, PARAMETER, CONSTRUCTOR, LOCAL_VARIABLE})
@Retention(RetentionPolicy.SOURCE)
public @interface SuppressWarnings {
    String[] value();
}

package java.lang;

import java.lang.annotation.*;
import static java.lang.annotation.ElementType.*;

@Documented
@Retention(RetentionPolicy.RUNTIME)
@Target(value={CONSTRUCTOR, FIELD, LOCAL_VARIABLE, METHOD, PACKAGE, PARAMETER, TYPE})
public @interface Deprecated {
}

package java.lang.annotation;

@Documented
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.ANNOTATION_TYPE)
public @interface Retention {
    RetentionPolicy value();
}

package java.lang.annotation;

@Documented
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.ANNOTATION_TYPE)
public @interface Target {
    ElementType[] value();
}

参考：http://blog.csdn.net/liuwenbo0920/article/details/7290586/