java中的注解

注解在java中使用非常普遍，尤其是在spring中。在这里简单了解一下注解

首先新建一个注解类 NameTest.java

package com.qiao.Annotation;import java.lang.annotation.ElementType;import java.lang.annotation.Retention;import java.lang.annotation.RetentionPolicy;import java.lang.annotation.Target;@Retention(value=RetentionPolicy.RUNTIME)@Target(ElementType.METHOD)public @interface NameTest {String value();}

注解上面的@Target 表示注解NameTest的作用范围，其中ElementType是一个枚举类型为

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}

ElementType.METHOD 表示该注解作用在方法上

@Retention表示注解的作用时机，其中RetentionPolicy也是一个枚举类型，为

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}

RetentionPolicy.RUNTIME 表示jvm运行时，此注解可被读出

这两个注解是必须要有的。

注解的使用：

先写一个测试类

public class AnnotationTest {@NameTest(value = "java")public void test(){System.out.println(" bu niu bi ");}}

在写个Demo

import java.lang.annotation.Annotation;import java.lang.reflect.Method;import org.junit.Test;public class Demo {@SuppressWarnings("unchecked")@Testpublic void getAnnotation(){for(Method m : meth){    if(m.isAnnotationPresent(NameTest.class)){NameTest n = (NameTest) m.getAnnotation(NameTest.class);<span style="white-space:pre"></span>System.out.println(n.value());    }}}}

会输出NameTest的value  --->“java”