Autowired in Spring Boot

                                 Autowired in Spring Boot

Keyword : @Autowired

1. byName
2. byType
3. constructor
4. setter
5. @Qualifier
6. Custom Qualifier annotation
7. @Primary
8. JSR-250 Annotation : @Resource
9. JSR-330 Annotation : @Inject

Dependancies : web , lombok , 

## byType :

It basically check the type . If the type is matched with service then it automatically create the bean.

CarController.java

package com.vrushant.AutowiredInSpringBoot.controller;

@RestController
@RequestMapping("/cars")
public class CarController {
    @Autowired
    private Car car; // By Type 
    @GetMapping
    public String drive(){
        return car.drive();
    }
}

Car.java

interface

package com.vrushant.AutowiredInSpringBoot;

public interface Car {
    String drive();
}

SedanCar.java

Service

package com.vrushant.AutowiredInSpringBoot;
import org.springframework.stereotype.Service;
@Service
public class SedanCar implements Car{
    @Override
    public String drive(){
        return "Driving Sedan Car";
    }
}

Only one service can be used in byType autowired , like if we create another service named "SuvCar.java" then spring will be confused which one to used . 

package com.vrushant.AutowiredInSpringBoot;
import org.springframework.stereotype.Service;
@Service
public class SuvCar implements Car{
    @Override
    public String drive(){
        return "Driving SUV Car";
    }
}

To Solve this problem we can use @Qualifier or ByName .

### byName

CarController.java

@RestController
@RequestMapping("/cars")
public class CarController {
    @Autowired
    private Car sedanCar;  // By Name
    @GetMapping
    public String drive(){
        return sedanCar.drive();
    }
}

Note : All other codes will same as above code in ByType.

In ByName , we have to specify the name of the service class (  sedanCar  in this case ) and spring will create the bean of that service class.

### Primary

CarController.java

@RestController
@RequestMapping("/cars")
public class CarController {
    @Autowired
    private Car car;
    @GetMapping
    public String drive(){
        return car.drive();
    }
}

we have 2 beans named sedanCar and SuvCar . So controller is confused which one to call . So solve this problem we can make one bean / service as primary so that controller will give priority to it.

SedanCar.java

@Primary
@Service
public class SedanCar implements Car{
    @Override
    public String drive(){
        return "Driving Sedan Car";
    }
}

SuvCar.java

@Service
public class SuvCar implements Car{
    @Override
    public String drive(){
        return "SUV Car Driving";
    }
}

Controller will call SedanCar .

###  @Qualifier

Using Qualifer we can specify the perticular service ( Car ) in Controller and spring will create the bean of that service.

So using Qualifier spring can call specific bean from multiple bean.

CarController.java

package com.vrushant.AutowiredInSpringBoot.controller;

@RestController
@RequestMapping("/cars")
public class CarController {
    @Qualifier("suvCar")
    @Autowired
    private Car car;
    @GetMapping
    public String drive(){
        return car.drive();
    }
}

Note : All other codes will same as above code in ByType.

### Contructor

CarController.java

@RestController
@RequestMapping("/cars")
public class CarController {
    private final Car car;
    @Autowired
    public CarController(@Qualifier("suvCar") Car car) {
        this.car = car;
    }
    @GetMapping
    public String drive(){
        return car.drive();
    }
}

In this , we have to specify the Qualifier in contructor and make the contructor autowired instead of variable.

Now using lombok we can automatically call the contructor by using @RequiredArgsContructor annotation. It is similar to the byName but it used when variable is declared as final.

@RestController
@RequestMapping("/cars")
@RequiredArgsConstructor
public class CarController {
    private final Car sedanCar;
//    @Autowired                                             // No Need of contructor
//    public CarController(@Qualifier("suvCar") Car car) {
//        this.car = car;
//    }
    @GetMapping
    public String drive(){
        return sedanCar.drive();
    }
}

### Setter

CarController.java

@RestController
@RequestMapping("/cars")
public class CarController {
    private Car car;
    @Autowired
    public void setCar(@Qualifier("suvCar") Car car) {
        this.car = car;
    }
    @GetMapping
    public String drive(){
        return car.drive();
    }
}

In this , we are making autowired to the setter and specifing the Qualifier in setter.

### Custom Qualifier Annotation

CarController,java

@RestController
@RequestMapping("/cars")
public class CarController {
    @CarQualifier("suvCar")
    @Autowired
    private Car car;
    @GetMapping
    public String drive(){
        return car.drive();
    }
}

To Create the custom Qualifier annotation, first create pacakage called annotation then create qualifier called CarQualifier in  it.

CarQualifier.java

Qualifier

package com.vrushant.AutowiredInSpringBoot.annotation;
import org.springframework.beans.factory.annotation.Qualifier;
import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;
@Qualifier
@Target({ElementType.FIELD,ElementType.PARAMETER,ElementType.TYPE})
@Retention(RetentionPolicy.RUNTIME)
public @interface CarQualifier {
    String value();
}

### Resource 

Resource is similier as Autowired.

@RestController
@RequestMapping("/cars")
public class CarController {
    @Qualifier("suvCar")
    @Resource
    private Car car;
    @GetMapping
    public String drive(){
        return car.drive();
    }
}

### Inject

Inject is similier as Autowired.

@RestController
@RequestMapping("/cars")
public class CarController {
    @Qualifier("suvCar")
    @Inject
    private Car car;
    @GetMapping
    public String drive(){
        return car.drive();
    }
}

Note:  Inject required javax.inject dependency