[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/