java - 如何在Spring MVC中实现DI？

Question

我对 Spring MVC 中的依赖注入(inject)感到困惑。

第一个代码:-

@Service
@Transactional
public class UserServiceImpl implements UserService {

    @Autowired
    private Userdao udo;



    @Override
    @Transactional
    public List<Positions> plist() {
        return udo.plist();
    }
}

工作正常。 我想在这个类中实现依赖注入(inject)。我这样做是对还是错？

第二个代码:-

`@Service
@Transactional
public class UserServiceImpl implements UserService {

    @Autowired
    private Userdao udo;

    public UserServiceImpl(Userdao udo) {
        this.udo = udo;
    }

    @Override
    @Transactional
    public List<Positions> plist() {
        return udo.plist();
    }
}

它不工作。 ERROR: org.springframework.web.context.ContextLoader - Context initialization failed org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'homeController': Injection of autowired dependencies failed; nested exception is org.springframework.beans.factory.BeanCreationException: Could not autowire field: com.xxx.Services.UserService com.xxx.java.HomeController.uservice; nested exception is org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'userServiceImpl' defined in file [D:\xxxWorkspace\.metadata\.plugins\org.eclipse.wst.server.core\tmp1\wtpwebapps\abcd\WEB-INF\classes\com\xxx\ServiceImp\UserServiceImpl.class]: Instantiation of bean failed; nested exception is org.springframework.beans.BeanInstantiationException: Could not instantiate bean class [com.xxx.ServiceImp.UserServiceImpl]: No default constructor found; nested exception is java.lang.NoSuchMethodException: com.xxx.ServiceImp.UserServiceImpl.<init>()

我是初学者。请帮我解决这个问题。如何在 Spring MVC 中实现 DI。

Answer 1

您可以通过两种方式执行此操作。

1. 您可以使用基于字段的 Autowiring 。但在这种情况下，您将需要一个默认构造函数。

   @Service
   @Transactional
   public class UserServiceImpl implements UserService {
   
       @Autowired // field based DI
       private Userdao udo;
   
       // default constructor
       public UserServiceImpl() {}
   
       @Override
       @Transactional
       public List<Positions> plist() {
           return udo.plist();
       }
   }
   

2. 您可以使用基于构造函数的依赖注入(inject)。为此，只需将 @Autowire 注释移至构造函数即可。并将其从字段中删除。

   @Service
   @Transactional
   public class UserServiceImpl implements UserService {
   
       private Userdao udo;
   
       @Autowired // constructor based DI
       public UserServiceImpl(Userdao udo) {
           this.udo = udo;
       }
   
       @Override
       @Transactional
       public List<Positions> plist() {
           return udo.plist();
       }
   
   }