JAVA进阶-Annotation

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

Target>指定被注解的注解只能使用在某个类型上;ElementType指定其类型:可以为方法

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文件和运行时不可见;

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

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

以下例子为运行时内省检查,当一个元注解需要多个限定值的时,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>为添加注解的类书写文档,编译运行后执行javadoc的dos命令.即可在该命令行

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,系统会认为子类不需要继承该功能,如下:

/**
 * 	@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{

}