为什么泛型类型参数的注释对于嵌套类型不可见？

Question

我没有得到以下代码的行为： https://gist.github.com/tomaszalusky/3e3777b4fd0c6096f3f707bb19b50b52 - 见嵌入：

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> {

}

编辑：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

为什么嵌套包围类型时，类型参数上声明的注释数组为空？我希望有一个元素，就像顶级类型一样。我将不胜感激任何基于 JLS 的解释。

在 JDK8u101 (http://compilejava.net)、旧版 JDK8 和 Eclipse 上始终如一。

谢谢！

Answer 1

这是 OpenJDK 中的一个错误，我前段时间已经报告了这个问题，希望能够修复它。类型注释在实践中并没有真正使用那么多，它似乎也不是优先事项。正如 Holger 所说，这是AnnotatedTypeFactory 实现中的混淆。

您可以使用Byte Buddy 正确解析类文件元数据：

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());
}

这会给你预期的输出：

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()]

Answer 2

我花了一些时间进行调试，annotatedParameterizedType 变量似乎仍然包含对嵌套案例的 allOnSameTargetTypeAnnotations 字段中的 Anno 注释的引用

annotatedParameterizedType = {AnnotatedTypeFactory$AnnotatedParameterizedTypeImpl@539} 
 type = {ParameterizedTypeImpl@540} "com.sample.Toplevel<java.lang.Integer>"
 decl = {Field@538} "com.sample.Toplevel com.sample.AnnotationOnTypeArgument.toplevel"
 location = {TypeAnnotation$LocationInfo@543} 
  depth = 0
  locations = {TypeAnnotation$LocationInfo$Location[0]@549} 
 allOnSameTargetTypeAnnotations = {TypeAnnotation[1]@544} 
  0 = {TypeAnnotation@551} "@com.sample.AnnotationOnTypeArgument$Anno() with Targetnfo: FIELD: -2, -2 on base declaration: com.sample.Toplevel com.sample.AnnotationOnTypeArgument.toplevel"
 annotations = {LinkedHashMap@546}  size = 0

对

annotatedParameterizedType = {AnnotatedTypeFactory$AnnotatedParameterizedTypeImpl@602} 
 type = {ParameterizedTypeImpl@603} "com.sample.AnnotationOnTypeArgument.com.sample.AnnotationOnTypeArgument$Nested<java.lang.Integer>"
 decl = {Field@601} "com.sample.AnnotationOnTypeArgument$Nested com.sample.AnnotationOnTypeArgument.nested"
 location = {TypeAnnotation$LocationInfo@606} 
  depth = 1
  locations = {TypeAnnotation$LocationInfo$Location[1]@611} 
 allOnSameTargetTypeAnnotations = {TypeAnnotation[1]@607} 
  0 = {TypeAnnotation@612} "@com.sample.AnnotationOnTypeArgument$Anno() with Targetnfo: FIELD: -2, -2 on base declaration: com.sample.AnnotationOnTypeArgument$Nested com.sample.AnnotationOnTypeArgument.nested"
 annotations = {LinkedHashMap@608}  size = 0 

但是位置深度有所不同，AnnotatedTypeFactory 即将推出的 getAnnotatedActualTypeArguments() 方法包含一个 TypeAnnotation.isSameLocationInfo() 比较作为向注释映射添加元素的前提条件，在嵌套情况下将变为 false，因此最终没有添加任何元素

我也没有找到任何文档。也许你在这里发现了一个问题