java annotation 入门

一. 最常见的annotation
@Override:用在方法之上，用来告诉别人这一个方法是改写父类的 
@DePRecated:建议别人不要使用旧的API的时候用的,编译的时候会用产生警告信息,可以设定在程序里的所有的元素上. 
@SuppressWarnings:暂时把一些警告信息消息关闭 
@Entity:表示该类是可持久化的类 

二. 设计一个自己的Annotation
      先看代码再讲话
1. 只有一个参数的Annotation实现

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package chb.test.annotation;    
import java.lang.annotation.Documented;    
import java.lang.annotation.ElementType;    
import java.lang.annotation.Retention;    
import java.lang.annotation.RetentionPolicy;    
import java.lang.annotation.Target;    
@Target(ElementType.TYPE)    
@Retention(RetentionPolicy.RUNTIME)    
@Documented   
public @interface MyAnnotation1 {    
        String value();    
}   
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package chb.test.annotation;   
import java.lang.annotation.Documented;   
import java.lang.annotation.ElementType;   
import java.lang.annotation.Retention;   
import java.lang.annotation.RetentionPolicy;   
import java.lang.annotation.Target;   
@Target(ElementType.TYPE)   
@Retention(RetentionPolicy.RUNTIME)   
@Documented  
public @interface MyAnnotation1 {   
        String value();   
}  
package chb.test.annotation;
import java.lang.annotation.Documented;
import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
public @interface MyAnnotation1 {
        String value();
} 

2. 有两个参数的Annotation实现

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package chb.test.annotation;    
import java.lang.annotation.Documented;    
import java.lang.annotation.ElementType;    
import java.lang.annotation.Retention;    
import java.lang.annotation.RetentionPolicy;    
import java.lang.annotation.Target;    
@Target(ElementType.METHOD)    
@Retention(RetentionPolicy.RUNTIME)    
@Documented   
public @interface MyAnnotation2 {    
        String description();    
        boolean isAnnotation();    
}   
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package chb.test.annotation;   
import java.lang.annotation.Documented;   
import java.lang.annotation.ElementType;   
import java.lang.annotation.Retention;   
import java.lang.annotation.RetentionPolicy;   
import java.lang.annotation.Target;   
@Target(ElementType.METHOD)   
@Retention(RetentionPolicy.RUNTIME)   
@Documented  
public @interface MyAnnotation2 {   
        String description();   
        boolean isAnnotation();   
}  
package chb.test.annotation;
import java.lang.annotation.Documented;
import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;
@Target(ElementType.METHOD)
@Retention(RetentionPolicy.RUNTIME)
@Documented
public @interface MyAnnotation2 {
        String description();
        boolean isAnnotation();
} 

3. Annotation实验类

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package chb.test.annotation;    
@MyAnnotation1("this is annotation1")    
public class AnnotationDemo {    
        @MyAnnotation2(description="this is annotation2",isAnnotation=true)    
        public void sayHello(){    
                System.out.println("hello world!");    
        }    
}   
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package chb.test.annotation;   
@MyAnnotation1("this is annotation1")   
public class AnnotationDemo {   
        @MyAnnotation2(description="this is annotation2",isAnnotation=true)   
        public void sayHello(){   
                System.out.println("hello world!");   
        }   
}  
package chb.test.annotation;
@MyAnnotation1("this is annotation1")
public class AnnotationDemo {
        @MyAnnotation2(description="this is annotation2",isAnnotation=true)
        public void sayHello(){
                System.out.println("hello world!");
        }
} 

4.Annotation测试说明类

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package chb.test.annotation;    
import java.lang.reflect.Method;    
import org.junit.Test;    
public class TestAnnotation {    
        @Test   
        public void test() throws ClassNotFoundException, SecurityException, NoSuchMethodException{    
                Class<?> cls = Class.forName("chb.test.annotation.AnnotationDemo");    
                boolean flag = cls.isAnnotationPresent(MyAnnotation1.class);    
                if(flag){    
                        System.out.println("判断类是annotation");    
                        MyAnnotation1 annotation1 = cls.getAnnotation(MyAnnotation1.class);    
                        System.out.println(annotation1.value());    
                }    
                    
                Method method = cls.getMethod("sayHello");    
                flag = method.isAnnotationPresent(MyAnnotation2.class) ;    
                if(flag){    
                        System.out.println("判断方法也是annotation");    
                        MyAnnotation2 annotation2 = method.getAnnotation(MyAnnotation2.class);    
                        System.out.println(annotation2.description()+"\t"+annotation2.isAnnotation());    
                }    
        }    
            
}   
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package chb.test.annotation;   
import java.lang.reflect.Method;   
import org.junit.Test;   
public class TestAnnotation {   
        @Test  
        public void test() throws ClassNotFoundException, SecurityException, NoSuchMethodException{   
                Class<?> cls = Class.forName("chb.test.annotation.AnnotationDemo");   
                boolean flag = cls.isAnnotationPresent(MyAnnotation1.class);   
                if(flag){   
                        System.out.println("判断类是annotation");   
                        MyAnnotation1 annotation1 = cls.getAnnotation(MyAnnotation1.class);   
                        System.out.println(annotation1.value());   
                }   
                   
                Method method = cls.getMethod("sayHello");   
                flag = method.isAnnotationPresent(MyAnnotation2.class) ;   
                if(flag){   
                        System.out.println("判断方法也是annotation");   
                        MyAnnotation2 annotation2 = method.getAnnotation(MyAnnotation2.class);   
                        System.out.println(annotation2.description()+"\t"+annotation2.isAnnotation());   
                }   
        }   
           
}  
package chb.test.annotation;
import java.lang.reflect.Method;
import org.junit.Test;
public class TestAnnotation {
        @Test
        public void test() throws ClassNotFoundException, SecurityException, NoSuchMethodException{
                Class<?> cls = Class.forName("chb.test.annotation.AnnotationDemo");
                boolean flag = cls.isAnnotationPresent(MyAnnotation1.class);
                if(flag){
                        System.out.println("判断类是annotation");
                        MyAnnotation1 annotation1 = cls.getAnnotation(MyAnnotation1.class);
                        System.out.println(annotation1.value());
                }
                
                Method method = cls.getMethod("sayHello");
                flag = method.isAnnotationPresent(MyAnnotation2.class) ;
                if(flag){
                        System.out.println("判断方法也是annotation");
                        MyAnnotation2 annotation2 = method.getAnnotation(MyAnnotation2.class);
                        System.out.println(annotation2.description()+"\t"+annotation2.isAnnotation());
                }
        }
        
} 

实验结果，控制台打出如下信息：

判断类是annotation
this is annotation1
判断方法也是annotation
this is annotation2     true

三.简介及说明

1. MyAnnotation1中的@Target(ElementType.TYPE)
      ＠Target里面的ElementType是用来指定Annotation类型可以用在哪些元素上的.例如：
       TYPE(类型)、FIELD(属性)、METHOD(方法)、PARAMETER(参数)、CONSTRUCTOR(构造函数)、LOCAL_VARIABLE(局部变量),、PACKAGE(包),其中的TYPE(类型)是指可以用在Class,Interface,Enum和Annotation类型上。
2. MyAnnotation1中的@Retention(RetentionPolicy.RUNTIME)
      RetentionPolicy 共有三种策略，分别为：
SOURCE:这个Annotation类型的信息只会保留在程序源码里，源码如果经过了编译之后，Annotation的数据就会消失,并不会保留在编译好的.class文件里面 
CLASS:这个Annotation类型的信息保留在程序源码里,同时也会保留在编译好的.class文件里面,在执行的时候，并不会把这些信息加载到JVM中。注：默认策略为CLASS类型 
RUNTIME:表示在源码、编译好的.class文件中保留信息，在执行的时候会把这一些信息加载到JVM中去的 
3. MyAnnotation1中的@Documented
目的就是将这一Annotation的信息显示在JAVA API文档上，如果没有增加@Documented的话，JAVA API文档上不会显示相关annotation信息

4. MyAnnotation1中的@interface
   关键字，表示该类为Annotation定义
5. MyAnnotation1中的 String value();
   表示有一个成员参数，名字为value，访问权为默认(default)修饰符，注意以下两点：
访问权只能用public和默认（default）修饰 
参数成员只能用基本类型byte,short,char,int,long,float,double,boolean八种基本数据类型和String,Enum,Class,annotations等数据类型,以及这一些类型的数组 
6.AnnotationDemo中的@MyAnnotation1("this is annotation1")
   因为MyAnnotation1只有一个参数，因此可以直接在括号中写上value值。注：如果Annotation只有一个参数，则建议最好将该参数名称定义为value
7.TestAnnotation中的cls.isAnnotationPresent(MyAnnotation1.class)
    判断该类是否使用了MyAnnotation1的注释
8. TestAnnotation中的MyAnnotation1 annotation1 = cls.getAnnotation(MyAnnotation1.class)
    返回该类针对MyAnnotation1的注释
9. TestAnnotation中的method.isAnnotationPresent(MyAnnotation2.class) 
    判断该方法是否使用了MyAnnotation2的注释