@The difference between Autowired and @resource

List of articles

• One 、 Preface
• • Spring Bean Override configuration
• Two 、@Autowired
• • Annotation processor
  • Assembly method
  • Annotation Properties
  • Scope of action
  • • 1. Member variables
    • 2. Constructors
    • 3. Method
    • 4. Parameters
    • 5. annotation
  • Using skills
  • • Multiple of the same type bean
    • @Primary Use
    • @Qualifier Use
    • Assemble multiple instances
    • @Autowired Assembly not effective
• 3、 ... and 、@Resource
• • Annotation processor
  • Assembly method
  • Annotation Properties
  • Scope of action
  • • 1. Member variables
    • 2. Method
    • 3. class
• Four 、@Autowired And @Resource contrast
• • distinguish between
  • @Autowired Assembly process
  • @Resource Assembly process

---

One 、 Preface

@Autowired and @Resource Are used for automatic assembly bean Of .

• @Resource yes JSR-250 Provided , It is Java standard , Most frameworks support .
• @Autowired Very powerful , But only for Spring frame , If you change it to JFinal Etc , The function will fail .

Spring Bean Override configuration

spring:
  main:
    allow-bean-definition-overriding: true

allow-bean-definition-overriding Property is used to configure the occurrence of the same name bean How to deal with ：

• The value is false when （ The default value is false）, If the same name appears bean, Direct throw anomaly .
• The value is true when , Indicates that the same name is supported bean Cover , Later defined bean Will overwrite the previously defined with the same name bean.

Assembly by type... Will be mentioned below , So what is the same type ？

• Parent class And its Subclass , All belong to the type of parent class .
• Interface And its Implementation class , All belong to the type of interface .

Two 、@Autowired

@Autowired yes Spring Notes provided , For automatic assembly .

Annotation processor

AutowiredAnnotationBeanPostProcessor Class is Autowired Annotation processor for annotations .

For annotation processor, please refer to the article ：https://blog.csdn.net/JokerLJG/article/details/123548694

Assembly method

• Assemble by type （ Assembly method used by default ）.
• Assemble by name （ combination @Qualifier Annotations use ）.

Annotation Properties

• required： The default value is true. The value is true when , Indicates that... Must be injected , Such as bean If it doesn't exist, it will report an error ; The value is false when , Express bean Exist and inject , If it does not exist, it will not be injected .

Scope of action

@Autowired The scope of action of ： Member variables 、 Constructors 、 Method 、 Parameters 、 annotation .

1. Member variables

@Service
public class UserService {

    @Autowired
    private IUser user;
}

The most used way .

2. Constructors

@Service
public class UserService {

    private IUser user;

    @Autowired
    public UserService(IUser user) {
        this.user = user;
    }
}

Use... On constructors Autowired annotation , In fact, the method of member variable assembly is used , It's not a constructor assembly .

3. Method

@Service
public class UserService {

    @Autowired
    public void test(IUser user) {
       user.test();
    }
}

Spring During the start of the project , Automatic call once added @Autowired Method of annotation , We can do some initialization work in this method .

4. Parameters

Add... To the input parameters of the constructor Autowired annotation

@Service
public class UserService {

    private IUser user;

    public UserService(@Autowired IUser user) {
        this.user = user;
        System.out.println("user:" + user);
    }
}

Add... To the input parameters of non static methods Autowired annotation

@Service
public class UserService {

    public void test(@Autowired IUser user) {
       user.test();
    }
}

5. annotation

A little .

Using skills

Multiple of the same type bean

When assembling by type , If this type of bean More than one time , Will report an error directly . Illustrate with examples ：

Interface ：

public interface IUser {
    void test();
}

Implementation class 1：

@Service
public class User1 implements IUser{
    @Override
    public void test() {
    }
}

Implementation class 2：

@Service
public class User2 implements IUser{
    @Override
    public void test() {
    }
}

Automatic assembly ：

@Service
public class UserService {

    @Autowired
    private IUser user;
}

Error message at startup ：

Field userService in com.joker.controller.UserController required a single bean, but 2 were found:
	- userServiceImpl1: defined in file [D:workmyspringboot	argetclassescomjokercontrollerUserServiceImpl1.class]
	- userServiceImpl2: defined in file [D:workmyspringboot	argetclassescomjokercontrollerUserServiceImpl2.class]

@Primary Use

@Primary Annotations can solve the above problems （ When assembling by type , If this type of bean More than one time , Will report a mistake ）.

When we use auto configuration to assemble Bean when , If this Bean There are multiple candidates , If one of the candidates has @Primary To modify , The candidate will be chosen , As an automatic assembly bean.

With the above code unchanged , Just in User1 or User2 add @Primary annotation , here @Autowired Automatic assembly will succeed , And the automatic assembly is added @Primary The annotated class corresponds to bean.

User1 Analogous addition @Primary annotation

@Service
@Primary
public class User1 implements IUser{
    @Override
    public void test() {
    }
}

@Qualifier Use

adopt @Autowired and @Qualifier Can be assembled by name .

@Service
public class UserService {

    @Autowired
    @Qualifier("user1")
    private IUser user;
}

The name of the automatic assembly is user1 Of bean（ Be careful ：bean The type of must also be IUser type ）.

Assemble multiple instances

We usually use @Autowired Automatically assemble a single instance , But it can also be used to assemble multiple instances . Can pass List、Set、Map To assemble multiple instances , as follows ：

@Service
public class UserService {

    @Autowired
    private List<IUser> userList;

    @Autowired
    private Set<IUser> userSet;

    @Autowired
    private Map<String, IUser> userMap;
}

The above assembly method will IUser Multiple instances of type bean All assembled List、Set、Map in .

@Autowired Assembly not effective

Here are some common @Autowired The assembly is not effective ：

1. @Autowired The class does not add @Controller、@Service、@Component、@Repository Etc , Or some other situation （ Such as direct new Object to instance ）. These conditions can lead to this kind of bean I didn't give it to you spring Containers to manage ,spring Can't complete the function of automatic assembly .

   public class UserService {
   
       @Autowired
       private IUser user;
   
       public void test() {
           user.say();
       }
   }
   

2. The annotation was not modified @ComponentScan Scan to .

3、 ... and 、@Resource

@Resource yes JDK A self explanatory note , For automatic assembly .

Annotation processor

CommonAnnotationBeanPostProcessor Class is Resource The annotation processor .

Assembly method

@Resource Automatically inject by name by default .

• It's not specified name, Not specified type, Automatically assemble by name （ When comments are written on fields , The default is the field name , When the annotation is written in setter When it comes to methods , By default, the attribute name is used for assembly .）; If there is no match , Then retreat and assemble according to the type , Throw exception if not found .

  If not specified name attribute ,

• If you specify name, Find name from context （id） Matching bean Assembly , Throw exception if not found .

• If you specify type, Then find the unique similar match from the context bean Assembly , Can't find or multiple , Will throw an exception .

• If you also specify name and type, From Spring Unique match found in context bean Assembly , Throw exception if not found .

Annotation Properties

Resource The main attributes of annotations ：

• name： Specify the... To be injected bean The name of
• type： Specify the... To be injected bean The type of

Scope of action

@Resource The scope of action of ： class 、 Member variables 、 Method .

1. Member variables

@Service
public class UserService {

    @Resource
    private IUser user;
}

2. Method

@Service
public class UserService {

    @Resource
    public void test(IUser user) {
        user.test();
    }
}

3. class

A little .

Four 、@Autowired And @Resource contrast

distinguish between

@Autowired

@Resource

Spring Defined comments

JSR-250 Defined comments

Auto assemble by type by default

Auto assemble by name by default

One parameter ：required( Default true), Indicates whether it is necessary to inject

Seven parameters ： The two most important parameters are name、type

Auto assemble by type by default
If you want to assemble automatically by name , Need to use @Qualifier Work together

Auto assemble by name by default
If you specify name, Automatically assemble by name ; If you specify type, Automatically assemble by type

Scope of action ： Constructors 、 Method 、 Parameters 、 Member variables 、 annotation

Scope of action ： class 、 Member variables 、 Method

@Autowired Assembly process

@Resource Assembly process