[Spring] @Primary vs @Qualifier

@Primary annotation은 어디다가 쓰는거지

Resolving issue with Multiple Coach implementations

• In the case of Multiple Coach implementations
  • We resolved it usung @Qualifier
  • We specified a coach by name
• Alternate solution available ..
  • Instead of specifying a coach by name using @Qualifier
  • I simply need a coach .. I don’t care which coach
    • If there are multiple coaches
    • Then you coaches figure it out … and tell me who’s the primary coach
  • But when using @Primary, can have only one for multiple implementations
  • If you mark multiple classes with @Primary… umm, we have a problem

package com.luv2code.demo;  
  
import org.springframework.context.annotation.Primary;  
import org.springframework.stereotype.Component;  
  
@Component  
@Primary  // 기존의 Coach 클래스에서 Primary annotation 추가해줌.
public class CricketCoach implements Coach{  
    @Override  
    public String getDailyWorkout() {  
        return "Practice fast bowling for 15 min!!@!";  
    }  
}

package com.luv2code.springcoredemo.rest;  
  
import com.luv2code.demo.Coach;  
import org.springframework.beans.factory.annotation.Autowired;  
import org.springframework.beans.factory.annotation.Qualifier;  
import org.springframework.web.bind.annotation.GetMapping;  
import org.springframework.web.bind.annotation.RestController;  
  
@RestController  
public class DemoController {  
  
    //define a private field for the dependency  
    private Coach myCoach;  
  
    //define a constructor for dependency injection  
    @Autowired  
    public DemoController(//@Qualifier("trackCoach") <- @Primary 때문에 더 이상 필요 없음
						    Coach theCoach){  
        myCoach = theCoach;  
    }  
  
    @GetMapping("/dailyworkout")  
    public String getDailyWorkout(){  
        return myCoach.getDailyWorkout();  
    }  
  
}

Mixing @Primary and @Qualifier

• If you mix @Primary and @Qualifier
  • @Qualifier has higher priority
• @Primary leaves it up to the implementation classes
  • Could have the issue of multiple @Primary classes leading to an error
• @Qualifier allows to you be very specific on which bean you want
• In genenral, recommend using @Qualifier
  • More specific
  • Higher priority

@Primary vs @Qualifier - Which one to use?

@Component @Primary  
class QuickSort implements SortingAlgorithm {}  
  
@Component  
class BubbleSort implements SortingAlgorithm {}  
  
@Component @Qualifier("RadixSortQualifier")  
class RadixSort implements SortingAlgorithm {}  
  
@Component @Primary  
class ComplexAlgorithm  
    @Autowired  
    private SortingAlgorithm algorithm;  
  
@Component  
class AnotherComplexAlgorithm  
    @Autowired @Qualifier("RadixSortQualifier")  
    private SortingAlgorithm iWantToUseRadixSortOnly;

• @Primary - A bean should given preference when multiple candidates are qualified
• @Qualifier - A specific bean should be auto-wired (name of the bean can be used as qualifier)
• ALWAYS think from the perspective of the class using the SortingAlgorithm:
  • 1. Just @Autowired : Give me (preferred) SortingAlgorithm
  • 1. @Autowired + @Qualifier : I only want to use specific SortingAlogorithm - RadixSort
  • (REMEMBER) @Qualifier has higher priority then @Primary as text above