對於嵌套類型，爲什麼對泛型類型參數的註釋不可見？

Question

我不明白的下面的代碼的行爲： https://gist.github.com/tomaszalusky/3e3777b4fd0c6096f3f707bb19b50b52 - 見嵌入式：

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> { 

} 

編輯：實際的結果是：

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 

爲什麼宣佈註釋對類型參數的數組周圍類型嵌套時爲空？我希望有一個元素，就像頂級類型一樣。我很感謝任何基於JLS的解釋。

在JDK8u101（http://compilejava.net），較老的JDK8和Eclipse中一致發生。

謝謝！

Answer 1

我給一些時間來調試和annotatedParameterizedType變量似乎仍包含嵌套的情況下，在其allOnSameTargetTypeAnnotations領域的參考安諾註解太

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 

VS

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 

然而，有是位置深度的差異，即將到來的AnnotatedTypeFactory的getAnnotatedActualTypeArguments（）方法包含TypeAnnotation.isSameLocationInfo（）比較，作爲將註釋添加到註釋的前提條件地圖，這將成爲在嵌套的情況下，因此最終沒有添加任何元素

我沒有找到它的任何文檔。也許你在這裏發現了一個問題

Answer 2

這是OpenJDK中的一個錯誤，我前面已經報道過，我希望它能夠修復。類型註釋在實踐中並沒有真正使用過，它似乎並不是一個優先事項。正如霍爾格所說，這是在執行AnnotatedTypeFactory時的混淆。

您可以使用Byte Buddy其正確解析類文件元數據：

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()); 
} 

這給了你所期望的輸出：

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()]