为什么泛型类型参数的注释对于嵌套类型不可见? [英] Why annotation on generic type argument is not visible for nested type?

问题描述

我没有得到以下代码的行为:https://gist.github.com/tomaszalusky/3e3777b4fd0c6096f3f70c6096f3f707bb19b50b52"

I don't get the behaviour of following code: https://gist.github.com/tomaszalusky/3e3777b4fd0c6096f3f707bb19b50b52 - see embedded:

import java.lang.reflect.*;
import java.util.*;
import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;


public class AnnotationOnTypeArgument {

    @Target({ElementType.FIELD,ElementType.PARAMETER,ElementType.METHOD,ElementType.TYPE_USE})
    @Retention(RetentionPolicy.RUNTIME)
    public @interface Anno {

    }

    interface Nested<T> {

    }

    Toplevel<@Anno Integer> toplevel;

    Nested<@Anno Integer> nested;

    public static void main(String[] args) throws Exception {
        print(AnnotationOnTypeArgument.class.getDeclaredField("toplevel"));
        print(AnnotationOnTypeArgument.class.getDeclaredField("nested"));
    }

    private static void print(Field field) {
        AnnotatedType annotatedType = field.getAnnotatedType();
        AnnotatedParameterizedType annotatedParameterizedType = (AnnotatedParameterizedType)annotatedType;
        ParameterizedType parameterizedType = (ParameterizedType)annotatedParameterizedType.getType();
        AnnotatedType argType = annotatedParameterizedType.getAnnotatedActualTypeArguments()[0];
        System.out.printf("field %s%ntype=%s%nannotatedType=%s%nannotations=%s%ntype=%s%n%n",
                field.getName(), parameterizedType, argType, Arrays.asList(argType.getDeclaredAnnotations()), argType.getType());
    }

}

interface Toplevel<T> {

}

实际结果是:

field toplevel
type=Toplevel<java.lang.Integer>
annotatedType=sun.reflect.annotation.AnnotatedTypeFactory$AnnotatedTypeBaseImpl@1540e19d
annotations=[@AnnotationOnTypeArgument$Anno()]
type=class java.lang.Integer

field nested
type=AnnotationOnTypeArgument.AnnotationOnTypeArgument$Nested<java.lang.Integer>
annotatedType=sun.reflect.annotation.AnnotatedTypeFactory$AnnotatedTypeBaseImpl@677327b6
annotations=[]
type=class java.lang.Integer

当周围类型嵌套时，为什么类型参数上声明的注释数组为空?我希望像顶级类型一样有一个元素.我将不胜感激任何基于 JLS 的解释.

Why the array of declared annotations on type argument is empty when surrounding type is nested? I expect to have one element just as for top level type. I would appreciate any explanation based on JLS.

在 JDK8u101 (http://compilejava.net)、较旧的 JDK8 和 Eclipse 上一致发生.

Happens consistently on JDK8u101 (http://compilejava.net), older JDK8 and Eclipse.

谢谢！

推荐答案

这是 OpenJDK 中的一个错误，我已经报告了一段时间，希望能够修复.类型注释在实践中并没有真正使用那么多，它似乎不是一个优先事项.正如 Holger 所提到的，这是 AnnotatedTypeFactory 实现中的一个混淆.

This is a bug in the OpenJDK, I have reported this a while ago and I hope it to be fixed. Type annotations are not really used that much in practice and it does not seem a priority. As Holger mentioned, this is a mixup in the implementation of the AnnotatedTypeFactory.

您可以使用 Byte Buddy 来正确解析类文件元数据:

You can use Byte Buddy which parses the class file meta data correctly:

public static void main(String[] args) throws Exception {

    TypeDescription type = TypePool.Default.ofClassPath()
       .describe(AnnotationOnTypeArgument.class.getName())
       .resolve();

    print(type.getDeclaredFields().filter(named("toplevel")).getOnly());
    print(type.getDeclaredFields().filter(named("nested")).getOnly());
}

private static void print(FieldDescription field) {
    System.out.printf("field %s%ntype=%s%nannotations=%s%ntype=%s%n%n",
            field.getName(),
            field.getType(),
            field.getType().getTypeArguments().get(0),
            field.getType().getTypeArguments().get(0).getDeclaredAnnotations());
}

这为您提供了预期的输出:

This gives you the expected output:

field toplevel
type=net.bytebuddy.Toplevel<java.lang.Integer>
annotations=class java.lang.Integer
type=[@net.bytebuddy.AnnotationOnTypeArgument$Anno()]

field nested
type=net.bytebuddy.AnnotationOnTypeArgument.net.bytebuddy.AnnotationOnTypeArgument$Nested<java.lang.Integer>
annotations=class java.lang.Integer
type=[@net.bytebuddy.AnnotationOnTypeArgument$Anno()]

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