注解是如何保证注值的，原理是啥？

[esolve]

我试了一下这篇文章里的注解 http://www.cnblogs.com/peida/archive/2013/04/24/3036689.html 发现并没有能注值

[esolve]

@Target(ElementType.FIELD)
@Retention(RetentionPolicy.RUNTIME)
@Documented
public @interface FruitName {
String value() default "";
}



public class Apple {

@FruitName("Apple")
private String appleName;



public void setAppleName(String appleName) {
this.appleName = appleName;
}
public String getAppleName() {
return appleName;
}

public void displayName(){
System.out.println("水果的名字是："+getAppleName());
}
}

这种没有能够注值啊

[Cbdy]

注解是 java 的一种元数据，本身不提供功能

[esolve]

@Cbdy 那注值是如何实现的？

[LaudOak]

要配合反射注值吧

[esolve]

@LaudOak 有没有原理分析的文章啊
大致是如何通过反射使得注解能够注值的。。

[LaudOak]

```
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;
import java.lang.reflect.Field;

public class Reflect {

@Target(ElementType.FIELD)
@Retention(RetentionPolicy.RUNTIME)
@Documented
public @interface FruitName {
String value() default "";
}

public static class Apple {
@FruitName("apple")
public String name;
}

public static void main(String... args) throws Exception {
Apple apple = new Apple();
Class clz = apple.getClass();
Field[] fields = clz.getDeclaredFields();
for (Field field : fields) {
if (field.isAnnotationPresent(FruitName.class)) {
field.set(apple, field.getAnnotation(FruitName.class).value());
}
}
System.out.println(apple.name);
}

}

```

http://tutorials.jenkov.com/java-reflection/annotations.html
https://keyholesoftware.com/2014/09/15/java-annotations-using-reflection/
http://www.journaldev.com/1789/java-reflection-example-tutorial
https://docs.oracle.com/javase/tutorial/reflect/member/fieldValues.html

[aristotll]

RetentionPolicy.RUNTIME 的通过反射来取