Java 注解

public class Annote {

    @Retention(RetentionPolicy.RUNTIME)
    @Target({ElementType.TYPE,ElementType.METHOD,ElementType.FIELD,ElementType.CONSTRUCTOR})
    public @interface RequiredRoles{
        String name();
        int id() default 0;
    }
    
    @RequiredRoles(name="type") //类成员注解
    public class UserAnnotation {
        
        @RequiredRoles(name="param", id=1) //类成员注解
        private Integer age;
        
        @RequiredRoles (name="construct", id=2)//构造方法注解
        public UserAnnotation(){
        }
        
        @RequiredRoles(name="public method", id=3) //类方法注解
        public void a(){
            Map m = new HashMap(0);
        }
        
        @RequiredRoles(name="protected method", id=4) //类方法注解
        protected void b(){
            Map m = new HashMap(0);
        }
        
        @RequiredRoles(name="private method", id=5) //类方法注解
        private void c(){
            Map m = new HashMap(0);
        }
    }
    
    public static void parseTypeAnnotation() throws ClassNotFoundException {  
        Class clazz = Class.forName("Annote$UserAnnotation");        
        Annotation[] annotations = clazz.getAnnotations();
        
        for (Annotation annotation : annotations) {  
            RequiredRoles testA = (RequiredRoles)annotation;
            System.out.println("id= "+testA.id()+" name= "+testA.name());  
        }  
    } 
    
    public static void parseMethodAnnotation(){
        Method[] methods = UserAnnotation.class.getDeclaredMethods();  
        for (Method method : methods) {  
            
            boolean hasAnnotation = method.isAnnotationPresent(RequiredRoles.class);  
            if (hasAnnotation) {                  
                RequiredRoles annotation = method.getAnnotation(RequiredRoles.class);  
                System.out.println("method = " + method.getName()  + " ; id = " + annotation.id() + " ; description = "+ annotation.name());  
            }  
        }  
    }
    
    
    public static void main(String[] args) throws ClassNotFoundException{
        parseTypeAnnotation();
        
        parseMethodAnnotation();
    }
    
}

详细参考：http://blog.sina.com.cn/s/blog_93dc666c0101gzn5.html

1、注解本质是就是对类、属性、方法的注释
2、区别是，如果在声明注解的时候加上了 @Retention(RetentionPolicy.RUNTIME) 属性， 就可以在运行的时候通过代码读取到具体的注解内容，而注释在运行时是读取不到的

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