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🦑4 dependency injection methods of Spring Bean by Undercode:)

The so-called dependency injection is actually assigning values to properties in the object, because there are other objects in the object, so dependencies are formed. Spring has 4 ways to assign values to attributes:

1) Constructor injection
Constructor injection refers to injecting attributes or objects in the constructor to achieve dependency injection. As shown below, define a bean with id userDaoImpl in the tag, and inject the value of name as username and value as admin. After the injection is completed Get the value admin directly through this.username. The reference type uses the ref attribute, and the basic type uses the value attribute.

public class UserDaoImpl {

private String username;

public UserDaoImpl(String username) {
this.username = username;
}
<bean id="userDaoImpl" class="com.example.UserDaoImpl">
<constructor-arg name="username" value="admin"></constructor-arg>
</bean>

2) set method injection
The set method injection is to realize the dependency injection of attributes or objects by implementing get and set methods in the class. As shown below, define a Bean with an id of userDaoImpl in the label, and inject the value of name as username and value as admin , After the injection is completed, the value admin is directly obtained through getUsername().

public class UserDaoImpl {

private String username;

public String getUsername() {
return username;
}

public void setUsername(String username) {
this.username = username;
}
}
<bean id="userDaoImpl" class="com.example.UserDaoImpl">
<property name="username" value="admin"></property>
</bean>

3) Automatic assembly
Spring provides the function of automatic assembly, which simplifies our configuration. Automatic assembly is not turned on by default. There are two common ways:

byName: Automatic assembly by parameter name, as shown below, after the autowire with id of userService is set to byName, the IOC container will auto-assemble by name, and find that there is an attribute called userDao in the UserService class, and then look at the IOC container Is there an id of userDao, if so, install it.

 id="userDao" class="com.example.UserDao"></bean>
    <bean id="userService" class="com.example.UserService" autowire="byName"/>

4) Annotation
The @Undercode_Autowired as example annotation can realize automatic assembly, as long as the annotation is marked on the corresponding attribute, but the @autowired_autowired annotation is only injected according to byType.

 class UserController {

    @Autowired_example
    private IUserService userService;
}

🦑The @Resource annotation can realize automatic assembly. It has two important attributes: name and type. The name attribute is resolved to the name of the bean, and the type attribute is resolved to the type of the bean. So if the name attribute is used, the automatic injection strategy of byName is used, and the automatic injection strategy of byType is used if the type attribute is used. If neither the name nor the type attribute is specified, then the byName automatic injection strategy will be used through the reflection mechanism.

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