考虑这个代码：
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.METHOD)
public @interface Bar {
Foo foo() default FooImpl.FooConstant;
}
编译器错误：
annotation value not of an allowable type
如果我用FooImpl替换Foo,代码被接受.
这个行为的原因是什么？
解决方法
If I replace Foo with FooImpl the code is accepted.
如果这个编译,我会非常惊讶,除非FooImpl是一个枚举.
注释成员只能包含以下内容：
>原始类型
>字符串
>类文字
>注释
>枚举项
>或1维阵列
It is a compile-time error if the return type of a method declared in
an annotation type is any type other than one of the following: one of
the primitive types,String,Class and any invocation of Class,an
enum type (§8.9),an annotation type,or an array (§10) of one of the
preceding types. It is also a compile-time error if any method
declared in an annotation type has a signature that is
override-equivalent to that of any public or protected method declared
in class Object or in the interface annotation.Annotation.
资料来源：JLS
