Resolving Multiple Bean Implementations in Spring Boot with @Qualifier and @Primary

1. Multiple Implementations

When the application defines several Coach beans (e.g., BaseballCoach, CricketCoach, TennisCoach, TrackCoach), Spring cannot decide which one to inject and throws a NoSuchBeanDefinitionException indicating that four beans were found but only one was expected.

Description:
Parameter 0 of constructor in com.greenbook.springbootdemo.rest.FunRestController required a single bean, but 4 were found:
    - baseballCoach: defined in file [/Users/xyz/local/springbootdemo/target/classes/com/greenbook/springbootdemo/service/impl/BaseballCoach.class]
    - cricketCoach: defined in file [/Users/xyz/local/springbootdemo/target/classes/com/greenbook/springbootdemo/service/impl/CricketCoach.class]
    - tennisCoach: defined in file [/Users/xyz/local/springbootdemo/target/classes/com/greenbook/springbootdemo/service/impl/TennisCoach.class]
    - trackCoach: defined in file [/Users/xyz/local/springbootdemo/target/classes/com/greenbook/springbootdemo/service/impl/TrackCoach.class]

This may be due to missing parameter name information

Action:
Consider marking one of the beans as @Primary, updating the consumer to accept multiple beans, or using @Qualifier to identify the bean that should be consumed

To resolve the ambiguity you can either designate a primary bean, inject a collection of beans, or explicitly qualify the desired bean.

2. Using @Qualifier

The @Qualifier annotation lets you specify the exact bean name to inject, eliminating the ambiguity.

package com.greenbook.springbootdemo.rest;

import com.greenbook.springbootdemo.service.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 FunRestController {
    private Coach coach;

    @Autowired
    public FunRestController(@Qualifier("cricketCoach") Coach coach) {
        this.coach = coach;
    }

    @GetMapping("/workout")
    public String getDailyWorkout() {
        return coach.getDailyWorkout();
    }
}

The same approach works for setter injection:

package com.greenbook.springbootdemo.rest;

import com.greenbook.springbootdemo.service.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 FunRestController {
    private Coach coach;

    @Autowired
    public void setCoach(@Qualifier("cricketCoach") Coach coach) {
        this.coach = coach;
    }

    @GetMapping("/workout")
    public String getDailyWorkout() {
        return coach.getDailyWorkout();
    }
}

3. Using @Primary

When you want a default bean without qualifying each injection point, annotate one implementation with @Primary. Spring will prefer this bean whenever multiple candidates exist.

package com.greenbook.springbootdemo.service.impl;

import com.greenbook.springbootdemo.service.Coach;
import org.springframework.context.annotation.Primary;
import org.springframework.stereotype.Component;

@Primary
@Component
public class TrackCoach implements Coach {
    @Override
    public String getDailyWorkout() {
        return "Run a hard 5K!";
    }
}

After marking TrackCoach as primary, the @Qualifier can be removed from the controller, and the application starts successfully.

If more than one bean is annotated with @Primary, Spring throws an error because it cannot determine a unique default:

No qualifying bean of type 'com.greenbook.springbootdemo.service.Coach' available: more than one 'primary' bean found among candidates: [baseballCoach, cricketCoach, tennisCoach, trackCoach]

Therefore, only a single bean should be marked @Primary. In most cases, using @Qualifier is recommended because it has higher precedence than @Primary and avoids accidental conflicts in large projects.