Java注解深入解析-优快云博客

本文详细介绍了Java注解的工作原理及使用方式，包括注解如何被处理和读取，特别是不同保留策略对注解可见性的影响。通过示例代码解释了如何定义自定义注解及其属性，并展示了如何通过反射API在运行时获取这些注解。

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注解是不干扰原来的代码的执行的，起到一种标识作用，让第三方的注解工具来提

取注解，来完成所需的任务。

package two.zj;

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;
@Documented
@Retention<span style="color:#ff0000;">(</span><span style="background-color: rgb(255, 0, 0);">RetentionPolicy.RUNTIME</span><span style="color:#ff0000;">)</span>
@Target(ElementType.METHOD)
public @interface TestZJ {
	public int id() default 1;
	public String name() default "nametest";
	public String value() ;
	public abstract int [] intt();
	public abstract TestEnum[] tenum();
	
	
}

package two;


import two.zj.TestEnum;
import two.zj.TestZJ;
public class UserAnn {
<span style="white-space:pre">	</span>/**
<span style="white-space:pre">	</span> * @param user
<span style="white-space:pre">	</span> * @return user
<span style="white-space:pre">	</span> */
<span style="white-space:pre">	</span>@TestZJ(id=22,name="liyy", intt = { 0,1,2,3 }, value = "", tenum = {TestEnum.TEST1})
<span style="white-space:pre">	</span>public User getUser(User user) {
<span style="white-space:pre">		</span>return user;
<span style="white-space:pre">	</span>}


}

package two;

import java.lang.reflect.Method;

import two.zj.TestZJ;

public class Testmain {

public static void main(String[] args) throws ClassNotFoundException {
// Class class1 = Class.forName("two.UserAnn");
Class class1 = UserAnn.class;
Method [] methods = class1.getDeclaredMethods();
for(Method method:methods){
TestZJ annotations = method.getAnnotation(TestZJ.class);
if(annotations !=null){
System.out.println(annotations.id());
}
}
}
}

当注解类TestZJ中的@Retention(RetentionPolicy.RUNTIME) 为source 或者class 时候反射是获取不到注解类的，因为jdk用说明

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
}