如何从Java 8中的getAnnotatedParameterTypes()获取泛型类型信息? [英] How to get generic type information from getAnnotatedParameterTypes() in Java 8?
问题描述
看起来像 getAnnotatedParameterTypes()
返回一个包含 AnnotatedType
的数组,它保存原始类型,而不是通用类型。例如:
public< T> void genericMethod(T t){
}
@Test
public void testAnnotatedTypes()throws ReflectiveOperationException {
方法method = getClass()。getMethod(genericMethod ,Object.class);
类型type = method.getGenericParameterTypes()[0];
assertTrue(类型instanceof TypeVariable);
AnnotatedType annotatedType = method.getAnnotatedParameterTypes()[0];
//失败; annotatedType仅实现AnnotatedType
assertTrue(annotatedType instanceof AnnotatedTypeVariable);
//这也失败了; type是TypeVariable,而annotatedType.getType()是
// Object.class
assertEquals(type,annotatedType.getType());
$ b 与不一致的原因是什么getGenericParameterTypes()
?
解决方案有一个关于这个bug的错误报告,并且它已经被修复。 p>
方法#getGenericParameterTypes()
和方法#getAnnotatedParameterTypes()
$ b
前者保证返回的类型
如果形式参数类型是一个参数化类型,返回的Type对象
必须准确反映源代码中使用的实际类型参数
。
如果一个形式参数类型是一个类型变量或一个参数化类型,则创建它
。
而后者没有,至少不清楚:
返回一个 AnnotatedType
对象的数组,这些对象表示使用
类型指定方法的形式参数类型/构造函数
代表这个可执行文件
。
假设 getAnnotatedParameterTypes()
返回擦除类型(尽管它可能没有这种意图)。无界类型变量 T
被擦除为 Object
。如果你有< T extends Foo>
,它将被清除为 Foo
。
至于注释,关于从方法参数中的类型参数获取注释,上面没有办法给出。人们会认为它的工作原理与字段一样。
public static void main(String [] args)throws Exception {
Field field = Example.class.getField(field);
AnnotatedParameterizedType annotatedParameterizedType =(AnnotatedParameterizedType)字段
.getAnnotatedType();
System.out.println(annotatedParameterizedType
.getAnnotatedActualTypeArguments()[0] .getType());
System.out.println(Arrays.toString(annotatedParameterizedType
.getAnnotatedActualTypeArguments()[0] .getAnnotations()));
}
@Retention(RetentionPolicy.RUNTIME)
@Target(value = {ElementType.TYPE_USE})
@interface Bar {
}
公共列表< @Bar String>领域;
可以打印
class java.lang.String
[@ com.example.Example $ Bar()]
我非常认为这是一个需要修复的错误,并且会跟随上面链接的错误报告。
It seems like getAnnotatedParameterTypes()
returns an array of AnnotatedType
s holding raw, rather than generic, types. For example:
public <T> void genericMethod(T t) {
}
@Test
public void testAnnotatedTypes() throws ReflectiveOperationException {
Method method = getClass().getMethod("genericMethod", Object.class);
Type type = method.getGenericParameterTypes()[0];
assertTrue(type instanceof TypeVariable);
AnnotatedType annotatedType = method.getAnnotatedParameterTypes()[0];
// This fails; annotatedType implements only AnnotatedType
assertTrue(annotatedType instanceof AnnotatedTypeVariable);
// This fails too; type is a TypeVariable while annotatedType.getType() is
// Object.class
assertEquals(type, annotatedType.getType());
}
What's the reason for the disagreement with getGenericParameterTypes()
?
解决方案 There's a bug report about this, and it has since been fixed.
There's a difference between Method#getGenericParameterTypes()
and Method#getAnnotatedParameterTypes()
.
The former makes guarantees about the types it returns
If a formal parameter type is a parameterized type, the Type object
returned for it must accurately reflect the actual type parameters
used in the source code.
If a formal parameter type is a type variable or a parameterized type,
it is created. Otherwise, it is resolved.
while the latter doesn't, not clearly at least:
Returns an array of AnnotatedType
objects that represent the use of
types to specify formal parameter types of the method/constructor
represented by this Executable
.
We have to assume that getAnnotatedParameterTypes()
returns the erased types (although it may not have been intended that way). The unbounded type variable T
is erased to Object
. If you had <T extends Foo>
, it would be erased to Foo
.
As for the comments, about getting the annotation from a type argument in a method parameter, there's no way given the above. One would think it works as it does for fields.
public static void main(String[] args) throws Exception {
Field field = Example.class.getField("field");
AnnotatedParameterizedType annotatedParameterizedType = (AnnotatedParameterizedType) field
.getAnnotatedType();
System.out.println(annotatedParameterizedType
.getAnnotatedActualTypeArguments()[0].getType());
System.out.println(Arrays.toString(annotatedParameterizedType
.getAnnotatedActualTypeArguments()[0].getAnnotations()));
}
@Retention(RetentionPolicy.RUNTIME)
@Target(value = { ElementType.TYPE_USE })
@interface Bar {
}
public List<@Bar String> field;
which prints
class java.lang.String
[@com.example.Example$Bar()]
I very much think it's a bug that needs fixing and will be following the bug report linked above.
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