注解元数据分为4部分分别为Target,Documented,Inherited,Retention:

Target>指定被注解的注解仅仅能使用在某个类型上;ElementType指定其类型:能够为方法

字段,类,返回值等等;

声明:

/**

 * 	@author Lean  @date:2014-10-13

 */

@Target(ElementType.METHOD)

public @interface WorkInProgress {}

应用:

/**

 * 	@author Lean  @date:2014-10-13

 */

public class AnnotationSample {

	//当在字段中使用时:The annotation @WorkInProgress is disallowed for this location

	//@WorkInProgress

	private int age;

	@WorkInProgress

	public static boolean doSomeThing() {

		// TODO Auto-generated method stub

		return false;

	}

}

Retention>设置注解可见性;使用到RetentionPolicy枚举

RetentionPolicy.SOURCE>>编译器可见,但对.class文件和执行时不可见;

RetentionPolicy.CLASS>>默认工具可见,对.class文件可见,但执行不可见;

RetentionPolicy.RUNTIME>>执行时可见;不会被.class文件所知,在执行时告诉JVM的值;

下面样例为执行时内省检查,当一个元注解须要多个限定值的时,必须使用{}和逗号隔开,

如@Target({ElementType.METHOD,ElementType.TYPE})

/**

 * 	@author Lean  @date:2014-10-13

 */

@WorkInProgress

public class AnnotationSample {

	private int age;

	@WorkInProgress

	public static boolean doSomeThing() {

		// TODO Auto-generated method stub

		return false;

	}

	public static void main(String[] args) {

		AnnotationSample obj=new AnnotationSample();

		Class clazz=obj.getClass();

		WorkInProgress progress=(WorkInProgress) clazz.getAnnotation(WorkInProgress.class);

		System.out.println(clazz.getName());

		if (clazz.isAnnotationPresent(WorkInProgress.class)) {

			System.out.println("class Annotationed WorkInProgress!");

		}

		Method[] methods=clazz.getMethods();

		for (Method method : methods) {

			if (method.isAnnotationPresent(WorkInProgress.class)) {

				System.out.println("method Annotationed WorkInProgress!");

			}

		}

	}

}

@Retention(RetentionPolicy.RUNTIME)

@Target({ElementType.METHOD,ElementType.TYPE})

@interface WorkInProgress {}

Documented>为加入注解的类书写文档,编译执行后执行javadoc的dos命令.就可以在该命令行

位置看到所生成的文档.



Inherited>当对某个类进行注解的时候,希望对继承他的子类也进行注解.默认情况下

没有使用该@Inherited注解方式,系统会觉得子类不须要继承该功能,例如以下:

/**

 * 	@author Lean  @date:2014-10-13

 */

@WorkInProgress

public class AnnotationSample {

	public static void main(String[] args) throws IllegalAccessException {

		AnnotationSample obj=new AnnotationSample();

		Class clazz=obj.getClass();

		if (clazz.isAnnotationPresent(WorkInProgress.class)) {

			System.out.println("class Annotationed WorkInProgress!");

		}

		Class childClass=AnnotationChildClass.class;

		if (childClass.isAnnotationPresent(WorkInProgress.class)) {

			System.out.println("child class Annotationed WorkInProgress!");

		}

	}

}

@Inherited

@Documented

@Retention(RetentionPolicy.RUNTIME)

@Target({ElementType.METHOD,ElementType.TYPE})

@interface WorkInProgress {}

class AnnotationChildClass extends AnnotationSample{

}