我怎么可以指定一个注释应该在的Java应用到字段的类型？ [英] How can I specify the type of the field that an Annotation should be applied to in Java?

问题描述

我要创建一个简单的注解，帮我膨胀在我的Andr​​oid应用程序的设置。注释是这样的：

I'm creating a simple annotation, to help me inflating settings inside my Android application. The annotation is this:

@Target(ElementType.FIELD)
@Retention(RetentionPolicy.RUNTIME)
public @interface CoreSettings {
    String name() default "";
}

我要确保它只会在哪种类型的扩展名为 BaseSettings自定义类领域。在下面的例子中，MainSettings扩展抽象类BaseSettings

I want to ensure it will only be used in fields which type extends a custom class named BaseSettings. In the example below, MainSettings extends the abstract class BaseSettings.

public class MainActivity extends BaseActivity {

    @CoreSettings("prefs")
    MainSettings settings;


    (...)

}

我该怎么办呢？

How can I do it?

推荐答案

是的，你可以让编译器，如果你写在错误类型的字段注释发出错误信息。

Yes, you can cause the compiler to issue an error message if you write the annotation on a field of the wrong type.

您不能只使用了 @Target 元注解做到这一点。相反，你需要编写一个注解处理器。注解处理器会检查你的源$ C ​​$ c中的注释 @CoreSettings 的每一次出现，如果<$ C则发出错误（就像任何其他的编译器错误） $ C> @CoreSettings 适用于一个类型，没有延伸 BaseSettings 。这是一个简短的注释处理器来写。

You cannot do this using just the @Target meta-annotation. Instead, you need to write an annotation processor. The annotation processor will examine each occurrence of the annotation @CoreSettings in your source code, and it issues an error (just like any other compiler error) if @CoreSettings is applied to a type that does not extend BaseSettings. This is a short, simple annotation processor to write.

运行批注处理器时，你将只能得到编译时警告，但你可以添加 -processor ​​... 命令行参数在您的项目的给javac构建文件。

You will only get the compile-time warning when running the annotation processor, but you can add the -processor ... command-line argument to javac in your project's buildfile.

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