java 无法在Spring中继承@Component？
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Can't get @Component to be inherited in Spring?
提问by TheLQ
In my project there's a common base class that all client classes extend. This has an @Autowired field that needs to be injected by Hibernate. These are all grouped together in another class that has an @Autowired collection of the base class.
在我的项目中，有一个所有客户端类都扩展的公共基类。这有一个@Autowired 字段，需要由 Hibernate 注入。这些都在另一个类中组合在一起，该类具有基类的 @Autowired 集合。
In order to reduce boilerplate for client code I'm trying to get @Component inherited. With @Component not doing this by default (apparently it used to though), I created this workaround annotation
为了减少客户端代码的样板，我试图继承@Component。默认情况下@Component 不这样做（显然它曾经这样做过），我创建了这个解决方法注释
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Component
@Inherited
public @interface InheritedComponent {
}
... and annotated the base class with it. Its not pretty but I hoped it would work. Unfortunately it didn't, which really confuses me as @Inherited should make it work
...并用它注释基类。它不漂亮，但我希望它会起作用。不幸的是它没有，这真的让我感到困惑，因为@Inherited 应该让它工作
Is there any other way to get @Component inherited? Or do I just have to say that any class that extends the base class needs this boilerplate?
有没有其他方法可以继承@Component？或者我必须说任何扩展基类的类都需要这个样板？
采纳答案by matt b
The problem is that the Componentannotation type itself needs to be marked with @Inherited.
问题是Component注解类型本身需要用@Inherited.
Your @InheritedComponentannotation type is correctly inherited by any classes that extend a superclass which is marked with @InheritedComponent- but it does not inherit @Component. This is because you have @Componenton the annotation, not the parent type.
您的@InheritedComponent注释类型被任何扩展超类的类正确继承，该超类标记为@InheritedComponent- 但它不继承@Component。这是因为您@Component在annotation 上，而不是在父类型上。
An example:
一个例子：
public class InheritedAnnotationTest {
@InheritedComponent
public static class BaseComponent {
}
public static class SubClass extends BaseComponent {
}
public static void main(String[] args) {
SubClass s = new SubClass();
for (Annotation a : s.getClass().getAnnotations()) {
System.out.printf("%s has annotation %s\n", s.getClass(), a);
}
}
}
Output:
输出：
class brown.annotations.InheritedAnnotationTest$SubClass has annotation @brown.annotations.InheritedComponent()
类 brown.annotations.InheritedAnnotationTest$SubClass 有注解 @brown.annotations.InheritedComponent()
In other words, when resolving what annotations a class has, the annotations of the annotations are not resolved - they do not apply to the class, only the annotation (if that makes sense).
换句话说，当解析一个类有什么注解时，注解的注解并没有得到解析——它们不适用于类，只应用于注解（如果有意义的话）。
回答by noleto
I've dealt with this issue by creating my own annotation (heritable) and then customizing classpath scanning:
我通过创建自己的注释（可继承）然后自定义类路径扫描来处理这个问题：
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Component
@Inherited
public @interface BusinessService {
}
Spring configuration look likes this:
Spring 配置看起来像这样：
<context:component-scan base-package="io.bar">
<context:include-filter type="annotation"
expression="io.bar.core.annotations.BusinessService" />
</context:component-scan>
