Spring MVC-静态上下文中自动连接的存储库NullPointerException [英] Spring MVC - Autowired Repository NullPointerException in static context

问题描述

在我的模型中，我有一个名为UserRepository的存储库.另外，我有一个UserFacade，它基本上将用户添加到存储库中，并且可以由Controller访问.存储库是立面中的@Autowired.当我想添加一个新用户时，我会为存储库得到一个nullPointerException.

In my model I have a repository called UserRepository. Additionally I have a UserFacade that basically adds the user to the repository and is accessed by the Controller. The repo is @Autowired in the Facade. When I want to add a new user I get a nullPointerException for the repository.

我的spring-servlet.xml包含必需的

<jpa:repositories base-package="project.user.repositories" />

而存储库是包含UserRepository.java的文件夹.它扩展了CrudRepository:

while repositories is the folder containing the UserRepository.java. It extends the CrudRepository:

@Repository
public interface UserRepository extends CrudRepository<User, Long> {
    User findByUsername(String username);
}

Facade.java是包含存储库的对象:

Facade.java is the object containing repository:

public class UserFacade {
    @Autowired
    private static UserRepository userRepo;

    private static Logger logger = LoggerFactory.getLogger(UserFacade.class);

    public UserFacade(){} //Thought it might work if I add a constructor and call it?

    public static User findByUsername(String username) {
        logger.debug(username+userRepo);  // prints SomeStringnull
        return userRepo.findByUsername(username); //NullPointerException
    }
}

在我的controller中，我有一个类似的方法:

And from my controller I have a method like:

@RequestMapping(value = CONTEXT)
public String test(){
    User user = UserFacade.findByUsername("get"); 
    //obviously user will be null if there is no such user
    return "success";
}

当我使用Android Studio时，导入应该不会成为问题.我想念什么?

Imports will should not be a problem as I am using Android Studio. What did I miss?

注意:相关问题有很多很好的答案(例如这个)，但是每个都有它自己的，不同的上下文并且对我没有帮助.

Note: There are many great answers to related questions (like this one), but each has it's own, different context and didn't help me.

推荐答案

Spring不会自动关联static字段.这就是为什么userRepo字段为null的原因.一种方法是使UserFacade本身成为bean，然后可以使userRepo为非静态字段.我希望这样. UserFacade不应真正是实用程序类，因为它正在与存储库bean交互.将其制成一个bean会更有意义.

Spring doesn't autowire static fields. That is why userRepo field is null. One way is to make the UserFacade a bean itself, and then you can make userRepo a non-static field. I would prefer this way. UserFacade shouldn't really be a utility class, since it is interacting with the repository bean. It would make much more sense to make it a bean.

另一种选择是提供设置器，并在其上使用@Autowired:

Another option is to provide a setter, and use @Autowired on that:

@Autowired
public void setUserRepo(UserRepository userRepo) {
    UserFacade.userRepo = userRepo;
}

或者甚至可以在参数化的构造函数上使用它.

Or using it even on a parameterized constructor would work.

这篇关于Spring MVC-静态上下文中自动连接的存储库NullPointerException的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！