SpringBoot Container - What is diffrence with @Autowired and no @Autowired?

Question

Maybe the best way to give a question will be to show code below :

public class DBConnection {


    private String Host;
    private String username;
    private String password;

    public DBConnection(String Host, String username, String password)
    {
        this.Host = Host;
        this.username = username;
        this.password = password;
    }

    public void addUser(String name)
    {
        System.out.println("add to db");
    }
}

And class below :

public class UserService {

    public DBConnection dbConnection;

    public UserService(DBConnection dbConnection){

        this.dbConnection = dbConnection;
    }

    public void register()
    {
        dbConnection.addUser("xd");
    }

}

And now, why i have to use spring for DI ? I can't see difference between above code and code below (of course in below case i know that in DBConnection i have to use @Component):

public class UserService {

    public DBConnection dbConnection;

    @Autowired
    public UserService(DBConnection dbConnection){

        this.dbConnection = dbConnection;
    }

    public void register()
    {
        dbConnection.addUser("xd");
    }

}

I can do DI without Spring. What is advantage of using spring Boot?

The difference is in picking up a book/googling/watching a tutorial - marking down for no research effort — asgarov1
– asgarov1, Commented May 10, 2021 at 7:08

Vladimir Shefer · Accepted Answer · 2021-05-09 23:53:36Z

0

Single constructor is concidered as @Autowired by Spring. Then there is no difference.

If there are multiple constructors, then you must mark one of them as @Autowired to let Spring know which ctor should be used to instantiate the bean.

answered May 9, 2021 at 23:53

Vladimir Shefer

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4 Comments

Mateusz Kaleta Over a year ago

But there is no difference between DI in Spring and no Spring? I use container without spring?

Vladimir Shefer Over a year ago

Im not sure that I got your question.

Vladimir Shefer Over a year ago

I mean that your code does not give us any information if you are using spring or not

Mateusz Kaleta Over a year ago

I want to know, why I have to use Autowired adnotation. Because I can't see difrence between code with Autowired and code without Autowired... What gives me, when I write Autowired above constructor?

chrylis -cautiouslyoptimistic- · Accepted Answer · 2021-05-10 01:11:52Z

Traditionally, Spring always needed @Autowired (or the newer @Inject) to tell it where to supply dependencies. As of Spring 4.3 (several years ago), if there is exactly one constructor on a class, the Spring container will understand that of course it has to use that constructor and will call it with the necessary dependencies. @Autowired is still needed if you have more than one constructor.

You can certainly do DI without a container, and one of the advantages of using constructor injection is that unit testing is simpler because you can inject mock dependencies. However, in actual applications the number of dependencies gets very large, and an advantage of a DI container is that it handles the bookkeeping for you. Spring also provides a number of other features, including Web APIs (MVC or WebFlux), AOP services (transactions, retry, tracing), convenient data access (Spring Data projects), automatic configuration (Boot), and more.

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