JAVA进阶-注解

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{
	
}