java - 如何制作保留泛型信息的@Documented 注释？

Question

我目前定义了这个注解:

@Documented
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.SOURCE)
@Beta
public @interface Needs
{
    Class<?>[] value();
}

问题是虽然我能做到:

@Needs({SomeClass.class, OtherClass.class})

我做不到，例如:

@Needs(Map<String, SomeClass>)

目的是将其记录下来并保留上述通用信息。可能吗？

Answer 1

Java 语言规范 writes :

It is a compile-time error if the return type of a method declared in an annotation type is not one of the following: a primitive type, String, Class, any parameterized invocation of Class, an enum type (§8.9), an annotation type, or an array type (§10) whose element type is one of the preceding types.

此限制的原因是注释值在编译时计算，并且只存储值。因此，这些值不能是任意对象，因为不清楚如何将它们存储在类文件中，并在运行时解码它们。

通常的解决方案是嵌套注释技巧:

public @interface Needs {
    Need[] value();
}

public @interface Need {
    String key();
    Class value();
}

然后你可以像这样使用

@Needs([
    @Need(key = "aKey", value = A.class)
    @Need(key = "anotherKey", value = Another.class)
])