如何在Spring MVC中实现DI？

Question

I have a confusion in Dependency Injection in Spring MVC.

First Code:-

@Service
@Transactional
public class UserServiceImpl implements UserService {

    @Autowired
    private Userdao udo;



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

It is working Fine. I would like to implement Dependency Injection in this class.Am doing right or wrong?

second code:-

`@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();
    }
}

Its not working. 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>()

I am a beginner .please help me to solve this.How to implement DI in Spring MVC.

Answer 1

You can do this two ways.

1. You can use field based autowiring. But in this case you will need a default constructor.

    @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. You can use constructor based dependency injection. To do this simply move your @Autowire annotation to your constructor. And remove it from the field.

    @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(); } } 

Answer 2

First setup is alright, and should work.

In the second setup you're getting following exception

class [com.xxx.ServiceImp.UserServiceImpl]: No default constructor found;

Which means what it says, since you've defined a constructor public UserServiceImpl(Userdao udo) Spring can't find an no-argument constructor.

You can either remove this constructor and use the first one or you can define the no argument constructor yourself.

You shouldn't actually need to define a constructor in a bean as you're not going to create bean object yourself. You would only need it if you're autorwiring constructor arguments.

If you're trying to autowire constructor then you can do it like below.

@Service
@Transactional
public class UserServiceImpl implements UserService {

    private Userdao udo;

    @Autowired //autowired constructor, instead of the field
    public UserServiceImpl(Userdao udo) {
        this.udo = udo;
    }

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

Answer 3

In simple words: If you define a constructor (overloaded) then you must use the @Autowired annotation on the constructor, if you do not define a constructor, then you must use the @Autowired annotation for each Object you need to add as dependency injection. For example:

With constructor overloaded:

@Service
@Transactional
public class UserServiceImpl implements UserService {

    private final Userdao userDao;
    private final RoleDao roleDao;

    @Autowired
    public UserServiceImpl(Userdao userDao, RoleDao roleDao) {
        this.userDao = userDao;
        this.roleDao = roleDao;
    }

}

Without constructor overloaded

@Service
@Transactional
public class UserServiceImpl implements UserService {

    @Autowired
    private UserDao userDao;

    @Autowired
    private RoleDao roleDao;

    // default constructor
    public UserServiceImpl() {}

}

Defining a constructor with a single @Autowired is better than having many objects @Autowired