Spring acquisition component by @Autowired

@Autowired annotation can be added to the constructors, methods, parameters, properties, types of annotations. If there is argument constructor or annotation @Bean only one way into the reference, can not write @Autowired omitted.

1, property

@Autowired
MyService bbb;

2, the constructor

@Autowired
public MyAuto(MyService bbb) {
    this.bbb = bbb;
}
// 或者去掉 @Autowired 也可以
public MyAuto(MyService bbb) {
    this.bbb = bbb;
}

3, method

@Autowired
public void setBbb(MyService bbb) {
  this.bbb = bbb;
}

@Autowired automatically find the order of Bean

1. Find @Qualifier according to specified Bean name
2. If not specified @Qualifier, then find the notes according to @Primary
3. If no @Primary, put the property name as Bean name lookup
4. If you have not found, according to the type of search, if there are multiple types of the same category, can not be found, an error

Designated @Qualifier

@Autowired
@Qualifier("bbb")
MyService xx;

Specify @Primary

public interface MyService {}
@Service
@Primary
class Aaa implements MyService {}
@Service
class Bbb implements MyService {}
@Service
class Ccc implements MyService {}

How to choose injection method

Spring officially recommended to use constructor injection method, citing the original words:

The Spring team generally advocates constructor injection, as it lets you implement application components as immutable objects and ensures that required dependencies are not null. Furthermore, constructor-injected components are always returned to the client (calling) code in a fully initialized state. As a side note, a large number of constructor arguments is a bad code smell, implying that the class likely has too many responsibilities and should be refactored to better address proper separation of concerns.

Roughly translated means:

Use constructor injection, you can declare a variable as final, and to ensure that dependent object is not null, moreover, constructor injection to ensure the object initialization is complete. And if there are a lot of constructor parameters, it may require that the code reconstructed.