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")
	@Test
	public 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”