JAVA Annotation

介绍

    尤其是我们在学习Spring时，都绕不开Annotation，而且使用的非常频繁，并且给我们带来很大的便利， 所以我们有必要了解JAVA Annotation。

    在此，我们自定义两个Annotation：一个是Class相关的Annotation；另一个是Method相关的Annotation。然后，我们写一个类来使用那两个Annotation，这样我们就能比较快速的了解Annotation。

自定义一个Class相关的Annotation：Message.java

package shuai.study.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)@Documentedpublic @interface Message {String mess();}

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对于上面的Annotation，你可能不清楚@Target(ElementType.TYPE) 和 @Retention(RetentionPolicy.RUNTIME) 是什么意思，那么我们就可以看一下JDK的源码：

public enum ElementType {    /** Class, interface (including annotation type), or enum declaration */    TYPE,    /** Field declaration (includes enum constants) */    FIELD,    /** Method declaration */    METHOD,    /** Parameter declaration */    PARAMETER,    /** Constructor declaration */    CONSTRUCTOR,    /** Local variable declaration */    LOCAL_VARIABLE,    /** Annotation type declaration */    ANNOTATION_TYPE,    /** Package declaration */    PACKAGE}

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}

JDK的源码就是NB，写得非常清楚！

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另一个Method相关的Annotation：Employee.java

package shuai.study.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)@Documentedpublic @interface Employee {String name() default "XXX";int budget() default 0;}

再写一个类Company.java，来使用上面的两个Annotation：

package shuai.study.annotation;@Message(mess = "NB company publish month star, please pay attention to this message")public class Company {@Employee(name = "SB", budget = 6666)public void monthStar() {System.out.println("向钱看，向厚赚！");}}

最后再来一个测试启动类AnnotationTest.java：

package shuai.study.annotation;import java.lang.reflect.Method;public class AnnotationTest {public static void main(String[] args) throws Exception {Class<?> companyClass = Class.forName("shuai.study.annotation.Company");if (companyClass.isAnnotationPresent(Message.class)) {Message message = companyClass.getAnnotation(Message.class);System.out.println("=== " + message.mess() + " ===");}Method[] methods = companyClass.getMethods();int length = methods.length;for (int index = 0; index < length; index++) {if (methods[index].isAnnotationPresent(Employee.class)) {Employee employee = methods[index].getAnnotation(Employee.class);System.out.println("Employee Name: " + employee.name());System.out.println("Employee Budget: " + employee.budget());}}}}