java annotation简介

开发中自定义annotation的场景不太多，但是很多框架的源码里都用到了自定义annotation，不了解的话就看不懂了，所以也简单地学习一下

试了一下，比较简单，以下通过一个例子来说明

首先是annotation本身的定义

@Target(ElementType.METHOD)@Retention(RetentionPolicy.RUNTIME)@Documentedpublic @interface ValueBind {public FieldType type();public String value();}

这里用了一个枚举

public enum FieldType {STRING, INT}

下面是用到了该标签的类

public class Student {private String id;private String name;private int age;@Overridepublic String toString() {StringBuilder sb = new StringBuilder();sb.append("id: " + this.getId() + "\n");sb.append("name: " + this.getName() + "\n");sb.append("age: " + this.getAge() + "\n");return sb.toString();}public String getId() {return id;}@ValueBind(type = FieldType.STRING, value = "123")public void setId(String id) {this.id = id;}public String getName() {return name;}@ValueBind(type = FieldType.STRING, value = "kyfxbl")public void setName(String name) {this.name = name;}public int getAge() {return age;}@ValueBind(type = FieldType.INT, value = "29")public void setAge(int age) {this.age = age;}}

最后是执行的代码，可以看到效果

public class Main {public static void main(String[] args) throws Exception {Student student = new Student();System.out.println(student.toString());Method[] methodArray = student.getClass().getDeclaredMethods();for (Method method : methodArray) {if (method.isAnnotationPresent(ValueBind.class)) {ValueBind annotation = method.getAnnotation(ValueBind.class);FieldType type = annotation.type();String value = annotation.value();if (FieldType.INT.equals(type)) {method.invoke(student, new Integer(value));} else {method.invoke(student, value);}}}System.out.println(student.toString());}}

我总的感觉，注解本身干不了啥，只能起到一个类型标识的作用（正如其命名annotation一样），然后代码里可以根据注解的类型，再进行需要的处理，相当于annotation起到的是类型标记的作用

不过在annotation里可以定义一些字段，这样可以传递参数过去

另外，注解也是可以“继承”的，比如说，在spring框架中，@Controller、@Service、@Repository，都认为是@Component

@Target({ElementType.TYPE})@Retention(RetentionPolicy.RUNTIME)@Documented@Componentpublic @interface Repository {/** * The value may indicate a suggestion for a logical component name, * to be turned into a Spring bean in case of an autodetected component. * @return the suggested component name, if any */String value() default "";}