反射与Annotation

Annotation使用前提:需要存在代码容器,才可以实现自定义的Annotation.

Annotation注解可以定义在类或方法上,在学习反射之后,我们可以通过反射取得定义的Annotation信息.

1.反射取得Annotation的信息:

1.取得全部Annotation:public Annotation[] getAnnotations();

2.取得指定的Annotation:public T getAnnotation(Class annotationClass);

对应举例:

import java.io.Serializable;
import java.lang.annotation.Annotation;

@SuppressWarnings("serail")
@Deprecated
class Member implements Serializable{}
public class Test12{
    public static void main(String[] args){
        Annotation[] ant = Member.class.getAnnotations();//取得全部信息
        for (Annotation i:ant){
            System.out.println(i);
        }
    }
}

在方法上使用Annotation:

import java.io.Serializable;
import java.lang.annotation.Annotation;

@SuppressWarnings("serail")
@Deprecated
class Member implements Serializable{
    @Deprecated
    @Override
    public String toString() {
        return super.toString();
    }
}
public class Test12{
    public static void main(String[] args){
        Annotation[] ant = new Annotation[0];
        try {
            ant = Member.class.getMethod("toString").getAnnotations();
        } catch (NoSuchMethodException e) {
            e.printStackTrace();
        }
        for (Annotation i:ant){
            System.out.println(i);
        }
    }
}

 由上可知:Annotation本身存在自身的保存范围,不同的Annotation返回不同.

2.自定义Annotation:
若想自定义Annotation,首先需要解决Annotation的作用范围. 

1.SOURCE:在源代码中出现的Annotation

2.CLASS:在*.class中出现的Annotation

3.RUNTIME:在类执行的时候出现的Annotation

定义一个在运行时生效的Annotation:

import java.io.Serializable;
import java.lang.annotation.Annotation;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;

@Retention(RetentionPolicy.RUNTIME)
@interface MyAnnotation{
    public String name();
    public int age();
}

@Deprecated
@MyAnnotation(name = "lala",age = 20)//强制设置属性值
class Member implements Serializable{

}
public class Test12{
    public static void main(String[] args){
        Annotation[] ant = new Annotation[0];
        ant = Member.class.getAnnotations();
        for (Annotation i:ant){
            System.out.println(i);
        }
    }
}

默认值定义:

import java.io.Serializable;
import java.lang.annotation.Annotation;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;

@Retention(RetentionPolicy.RUNTIME)
@interface MyAnnotation{
    public String name() default "lala";
    public int age() default 20;
}

class Member implements Serializable{

}
public class Test12{
    public static void main(String[] args){
        Annotation[] ant = new Annotation[0];
        ant = Member.class.getAnnotations();
        for (Annotation i:ant){
            System.out.println(i);
        }
    }
}

取得具体信息:

@Retention(RetentionPolicy.RUNTIME)
@interface MyAnnotation{
    public String name() default "lala";
    public int age() default 20;
}
@Deprecated
@MyAnnotation
class Member implements Serializable{

}
public class Test12{
    public static void main(String[] args){
        MyAnnotation myAnnotation = Member.class.getAnnotation(MyAnnotation.class);
        System.out.println("name:"+myAnnotation.name());
        System.out.println("age:"+myAnnotation.age());
    }
}

注:Annotation使用需要特殊环境,不是随意编写的.

3.Annotation与工厂设计模式

对应举例:

import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
@Retention(RetentionPolicy.RUNTIME)
@interface MyAnnotation{
    public Class<?> target();
}
interface Ifruit{
    public void eat();
}
class Apple implements Ifruit{
    public void eat() {
        System.out.println("吃苹果");
    }
}
@MyAnnotation(target = Apple.class)
class Factory{
    public static <T> T getInstance() throws Exception{
        MyAnnotation myAnnotation = Factory.class.getAnnotation(MyAnnotation.class);
        return (T) myAnnotation.target().newInstance();
    }
}
public class Test12{
    public static void main(String[] args) throws Exception {
        Ifruit ifruit = Factory.getInstance();
        ifruit.eat();
    }
}

注:Annotation可以很好地解决程序配置问题.程序可通过Annotation配置信息来实现不同的操作效果.