Java 注解应用

Java 注解是什么，在这里只写我自己的理解

注解是像钩子一样，写在代码中，可用反射处理我们写的代码里的注解信息。比如可以字段验证，结合AOP处理统一处理权限等操作。Spring MVC中就用到了很多注解。

元注解:指定义注解的注解。

包括@Retention @Target @Document @Inherited四种。

1.@Retention: 定义注解的保留策略
@Retention(RetentionPolicy.SOURCE) //注解仅存在于源码中，在class字节码文件中不包含；检查性的操作，比如 @Override 和 @SuppressWarnings
@Retention(RetentionPolicy.CLASS) // 默认的保留策略，注解会在class字节码文件中存在，但运行时无法获得，编译时进行一些预处理操作，比如生成辅助代码
@Retention(RetentionPolicy.RUNTIME) // 注解会在class字节码文件中存在，在运行时可以通过反射获取到运行时去动态获取注解信息；
生命周期长度 SOURCE < CLASS < RUNTIME 。

2.@Target：定义注解的作用目标
@Target(ElementType.TYPE) //接口、类、枚举、注解
@Target(ElementType.FIELD) //字段、枚举的常量
@Target(ElementType.METHOD) //方法
@Target(ElementType.PARAMETER) //方法参数
@Target(ElementType.CONSTRUCTOR) //构造函数
@Target(ElementType.LOCAL_VARIABLE)//局部变量
@Target(ElementType.ANNOTATION_TYPE)//注解
@Target(ElementType.PACKAGE) ///包

3.@Document：说明该注解将被包含在javadoc中
4.@Inherited：说明子类可以继承父类中的该注解

注解定义

package main.java.annotation;
import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;

/**
 * @author jiangqiang
 * @version 2.0.1
 * @date 2018/4/26
 **/
@Target(ElementType.TYPE)//Class, interface (including annotation type), or enum declaration
@Retention(RetentionPolicy.RUNTIME)
public @interface ClassAnnotation {
    String author() default  "jiangqiang";
    String version() default "1.0";
    String description()  default  "";
}

package main.java.annotation;

import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;

/**
 * @author jiangqiang
 * @version 2.0.1
 * @date 2018/4/25
 **/
@Target(ElementType.FIELD)//Field declaration (includes enum constants)
@Retention(RetentionPolicy.RUNTIME)
public @interface FieldAnnotation {
   int length() default 0;
   String description() default "";
}

package main.java.annotation;

import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;

/**
 * @author jiangqiang
 * @version 2.0.1
 * @date 2018/4/26
 **/
@Target(ElementType.METHOD)//Method declaration
@Retention(RetentionPolicy.RUNTIME)
public @interface MethodAnnotation {
    String author() default "jianqiang";
    String version() default "1.0";
    String description() default "";
}

注解使用

package main.java.example;

import main.java.annotation.ClassAnnotation;
import main.java.annotation.MethodAnnotation;

/**
 * @author jiangqiang
 * @version 2.0.1
 * @date 2018/4/26
 **/
@ClassAnnotation(author = "demo",version = "10.2",description = "测试注解类")
public class AnnotationExample {

    @MethodAnnotation(author = "jiangqiang",version = "1.3",description = "测试注解方法")
    public void AnnotationMethod(Entity entity,String content){
        System.out.println("name:"+entity.getName()+";description:"+entity.getDescription());
        System.out.println("content:"+content);
    }
}

package main.java.example;

import main.java.annotation.FieldAnnotation;

/**
 * @author jiangqiang
 * @version 2.0.1
 * @date 2018/4/25
 **/
public class Entity {
    @FieldAnnotation(length = 2,description = "名称")
    private String name;
    @FieldAnnotation(length = 5,description = "描述")
    private String description;

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public String getDescription() {
        return description;
    }

    public void setDescription(String description) {
        this.description = description;
    }
}

注解应用

import main.java.annotation.FieldAnnotation;

import java.lang.reflect.Field;
import java.lang.reflect.Method;

/**
 * Hello world!
 *
 */
public class App 
{
    public static void main( String[] args ) throws Exception
    {
        //类注解
        Class<AnnotationExample> annotationExampleClass = AnnotationExample.class;
        ClassAnnotation classAnnotation = annotationExampleClass.getAnnotation(ClassAnnotation.class);
        System.out.println(classAnnotation.author());
        System.out.println(classAnnotation.version());
        System.out.println(classAnnotation.description());

        //方法注解
        Method method = annotationExampleClass.getMethod("AnnotationMethod",new Class[]{Entity.class,String.class});
        MethodAnnotation methodAnnotation = method.getAnnotation(MethodAnnotation.class);
        System.out.println(methodAnnotation.author());
        System.out.println(methodAnnotation.version());
        System.out.println(methodAnnotation.description());

        //字段注解
        Class<Entity> entityClass= Entity.class;
        Field[] fields = entityClass.getDeclaredFields();
        for(Field field:fields){
           FieldAnnotation fieldAnnotation = field.getAnnotation(FieldAnnotation.class);
            System.out.println(fieldAnnotation.length());
            System.out.println(fieldAnnotation.description());
        }
        //根据字段名获取
        Field field = entityClass.getDeclaredField("name");
        FieldAnnotation entityField =  field.getAnnotation(FieldAnnotation.class);
        System.out.println(entityField.length());
        System.out.println(entityField.description());
    }
}

Java 注解 应用

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