Java注解

1.简介
(1).概念
为程序元素(字段、方法、类、包)设置额外信息，然后在编译或者运行时使用。

(2).功能

• 作为特定的标记，用于告诉编译器一些信息:@Override
• 编译时动态处理，动态生成代码:@Dada
• 运行时动态处理，作为额外信息的载体:@Controller @Service @Repository

2.元注解(注解上的注解)
(1).@Target
定义注解的使用范围

• ANNOTATION_TYPE:注解
• PACKAGE:包
• TYPE:类、接口、枚举
• METHOD:方法
• CONSTRUCTOR:构造方法
• FIELD:属性
• PARAMETER:参数

public enum ElementType {
    /** Class, interface (including annotation type), or enum declaration */
    TYPE,

    /** Field declaration (includes enum constants) */
    FIELD,

    /** Method declaration */
    METHOD,

    /** Formal parameter declaration */
    PARAMETER,

    /** Constructor declaration */
    CONSTRUCTOR,

    /** Local variable declaration */
    LOCAL_VARIABLE,

    /** Annotation type declaration */
    ANNOTATION_TYPE,

    /** Package declaration */
    PACKAGE,

    /**
     * Type parameter declaration
     *
     * @since 1.8
     */
    TYPE_PARAMETER,

    /**
     * Use of a type
     *
     * @since 1.8
     */
    TYPE_USE
}

(2).@Retention
定义注解的生命周期

• SOURCE:只在源码中存在，编译后的.class文件中就不存在了
• CLASS:存在于源码和.class文件中，如JDK自带的注解
• RUNTIME:在运行阶段还会起作用的注解，能够影响程序的运行逻辑

public enum RetentionPolicy {
    /**
     * Annotations are to be discarded by the compiler.
     */
    SOURCE,

    /**
     * Annotations are to be recorded in the class file by the compiler
     * but need not be retained by the VM at run time.  This is the default
     * behavior.
     */
    CLASS,

    /**
     * Annotations are to be recorded in the class file by the compiler and
     * retained by the VM at run time, so they may be read reflectively.
     *
     * @see java.lang.reflect.AnnotatedElement
     */
    RUNTIME
}

(3).@Documented
生成Javadoc时会包含的注解

(4).@Inherited
允许子类继承，子类只继承类上的注解

3.自定义注解
(1).定义注解

@Target({ElementType.METHOD,ElementType.TYPE})
@Retention(RetentionPolicy.RUNTIME)
@Inherited
@Documented
//1.使用@interface关键字定义注解
public @interface DescriptionAnnotation { 
  //2.成员以无参数方法方式声明
  String desc();   
  
  String author();
  
  //3.可以用default为成员制定一个默认值
  int age() default 18;  
}

(2).使用注解

@DescriptionAnnotation(desc="这是一个类上的注解")
public class Person {
    //因为@DescriptionAnnotation的作用范围没有字段ElementType.FIELD，所以报错
    //@DescriptionAnnotation(desc="这是一个方法上的注解",author = "Sherry")
    private String name;

    @DescriptionAnnotation(desc="这是一个方法上的注解",author = "Sherry")
    public String getName() {
        return name;
    }

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

(3).解析注解

public class Test {
    public static void main(String[] args) throws ClassNotFoundException {
        Class personClass = Class.forName("com.steven.annotation.Person");
        //1.获取类上所有注解
        Annotation[] annotations = personClass.getAnnotations();
        for (Annotation annotation : annotations) {
            if (annotation instanceof DescriptionAnnotation) {
                DescriptionAnnotation descriptionAnnotation = (DescriptionAnnotation) annotation;
                System.out.println("desc:" + descriptionAnnotation.desc() + ",author:" + descriptionAnnotation.author());
            }
        }

        //2.获取方法上的注解
        Method[] methods = personClass.getMethods();
        for (Method method : methods) {
            Annotation annotation = method.getAnnotation(DescriptionAnnotation.class);
            if (annotation instanceof DescriptionAnnotation) {
                DescriptionAnnotation descriptionAnnotation = (DescriptionAnnotation) annotation;
                System.out.println("desc:" + descriptionAnnotation.desc() + ",author:" + descriptionAnnotation.author());
            }
        }
    }
}

desc:这是一个类上的注解,author:Steven
desc:这是一个方法上的注解,author:Sherry