为什么Spring @Autowired不能与java泛型一起使用 [英] Why doesn't the spring @Autowired work with java generics

问题描述

受到spring数据的启发，我想创建一个抽象的RESTController，我可以为很多控制器扩展它。我创建了以下类：

Inspired by spring data awesomeness I wanted to create a abstract RESTController that I could extend for a lot of my controllers. I created the following class:

@Controller
public abstract class RESTController<E, PK extends Serializable, R extends PagingAndSortingRepository<E, PK>>
{
    @Autowired
    private R repository;

    @RequestMapping(method=RequestMethod.GET, params={"id"})
    @ResponseBody 
    public E getEntity(@RequestParam PK id)
    {
        return repository.findOne(id);
    }

    ...
}

我希望泛型允许我在存储库中@Autowired，但是我收到以下错误：

I was hoping that the generics would allow me to @Autowired in the repository but I get the following error:

SEVERE: Allocate exception for servlet appServlet
org.springframework.beans.factory.NoSuchBeanDefinitionException: No unique bean of type [org.springframework.data.repository.PagingAndSortingRepository] is defined: expected single matching bean but found 3: [groupRepository, externalCourseRepository, managedCourseRepository]
    at org.springframework.beans.factory.support.DefaultListableBeanFactory.doResolveDependency(DefaultListableBeanFactory.java:800)
    at org.springframework.beans.factory.support.DefaultListableBeanFactory.resolveDependency(DefaultListableBeanFactory.java:707)
    at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor$AutowiredFieldElement.inject(AutowiredAnnotationBeanPostProcessor.java:478)
    at org.springframework.beans.factory.annotation.InjectionMetadata.inject(InjectionMetadata.java:87)
    at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor.postProcessPropertyValues(AutowiredAnnotationBeanPostProcessor.java:284)
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.populateBean(AbstractAutowireCapableBeanFactory.java:1106)
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:517)
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:456)
    at org.springframework.beans.factory.support.AbstractBeanFactory$1.getObject(AbstractBeanFactory.java:294)

我理解错误告诉我的内容，@ Autowired有多个匹配。我很困惑因为我想通过创建以下控制器它可以工作：

I understand what the error is telling me, there is more than one match for the @Autowired. I am confused because I thought by creating the following controller it would work:

@Controller
@RequestMapping(value="/managedCourse")
public class ManagedCourseController extends RESTController<ManagedCourse, Long, ManagedCourseRepository>
{
    ...
}

这很容易通过在RESTController中使用这样的方法来解决这个问题：

This is easy enough to work around by doing having a method like this in the RESTController:

protected abstract R getRepository();

然后在您的实施类中执行此操作：

and then doing this in your implementing class:

@Autowired
private ManagedCourseRepository repository;

@Override
protected ManagedCourseRepository getRepository() 
{
    return repository;
}

我只是想知道是否有人想到我怎么能得到这个工作。

I was just wondering if someone had any thoughts of how I could get this to work.

我还发现这个有趣的aritcle。

Also I found this interesting aritcle.

推荐答案

在这种情况下，我实际上会建议基于XML的注释。在我看来，你试图在控制器实例中避免大量不必要的重复。如果您有一个REST控制器类，则容器可以根据需要实例化任意数量的实例，将每个实例映射到不同的URI。

I would actually recommend XML-based wiring over annotations in this case. It seems to me you're trying to avoid a lot of needless duplication in your controller instances. If you had a single REST controller class, the container can then instantiate as many instances as you need, mapping each to a different URI.

注释更多的是计划创建单个实例的情况。这恰好覆盖了大约90％的基于表单的J2EE会话bean，但它不是灵丹妙药。

Annotations are more of a shorthand for cases where you plan to create a single instance. This happens to cover about 90% of form-based J2EE session beans, but it's not a panacea.

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