CODEWITHSUNDEEP
javareflectionannotations
peng
peng

Reputation: 31

How to get the annotation above the annotation

I added an annotation to the method, and there are other annotations in this annotation. When I try to get the annotation above by the method, unfortunately, the return is always null. I want to know why?

This is the method I defined

@ApiTest
public void add() {
}

This is the annotation I defined.

@ValueName("hello word")
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.METHOD)
public @interface ApiTest {

}
////////////////////////////////////////////////////////
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.TYPE)
public @interface ValueName {
  String value() default "";
}

The result always returns null when I try to get @ValueName

Class<?> testApi = Class.forName("com.huawei.netty.TestApi");
    Method add = testApi.getMethod("add", null);
    ApiTest apiTest = add.getDeclaredAnnotation(ApiTest.class);
    ValueName valueName = apiTest.getClass().getDeclaredAnnotation(ValueName.class);
    if (valueName != null) {
      System.out.println(valueName.annotationType());
    }

But this way can be obtained

Class<?> apiTest = Class.forName("com.huawei.netty.ApiTest");
    ValueName valueName = apiTest.getDeclaredAnnotation(ValueName.class);
    if (valueName != null) {
      System.out.println(valueName.value());
    }

Can I know why this is?

Upvotes: 1

Views: 348

Answers (1)

Slaw
Slaw

Reputation: 45786

You should be using Annotation#annotationType() to get the class of the annotation instance, rather than Object#getClass(). The latter method is not returning the class you think it is. Try the following:

MetaAnnotation.java

@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.ANNOTATION_TYPE)
public @interface MetaAnnotation {

    String value();

}

CustomAnnotation.java

@MetaAnnotation("Hello, World!")
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.TYPE)
public @interface CustomAnnotation {}

Main.java

@CustomAnnotation
public final class Main {

    public static void main(String[] args) {
        CustomAnnotation ca = Main.class.getAnnotation(CustomAnnotation.class);
        MetaAnnotation   ma = ca.annotationType().getAnnotation(MetaAnnotation.class);

        System.out.println("CustomAnnotation#getClass()       = " + ca.getClass());
        System.out.println("CustomAnnotation#annotationType() = " + ca.annotationType());
        System.out.println("MetaAnnotation#value()            = " + ma.value());
    }

}

When I run the above using OpenJDK 12.0.1, I get the following output:

CustomAnnotation#getClass()       = class com.sun.proxy.$Proxy1
CustomAnnotation#annotationType() = interface CustomAnnotation
MetaAnnotation#value()            = Hello, World!

As you can see, the class of the annotation instance is a proxy class which would not have the annotation you're querying for.

Upvotes: 3

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