Using Spring @Autowired with @Qualifier

Background

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This article demonstrates how to qualify beans of the same reference type in Spring. @Qualifier enables you to select and use a particular bean from a set of beans of the same type existing in the Spring container.

Software Environment

Windows 7 Professional SP1
Eclipse – Kepler Release
Java 1.7 (1.7.0_67 – Windows x86)
Spring Framework
- Spring Context 4.1.2

Create a Maven Project

If you have not yet created a maven project, you may want to follow the steps on How to create a Maven project in Eclipse. Put the following dependency in your pom.xml file:

<dependencies>
    <dependency>
        <groupId>org.springframework</groupId>
        <artifactId>spring-context</artifactId>
        <version>4.1.2.RELEASE</version>
    </dependency>
</dependencies>

<groupId>org.springframework</groupId>

<artifactId>spring-context</artifactId>

<version>4.1.2.RELEASE</version>

</dependency>

</dependencies>

The Codes

All in all, there are only three (3) source code files for this article.

The main class – Application.java

Here, two (2) instances of anynomous class of type JdbcDao is created. They are declared as beans within the Spring container and each is labeled with a name. Bean “mockdao” is a mock-up of the real DAO; while bean “dao” is the real DAO that retrieves data directly from the database.

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Just imagine we are doing a test-driven development where we are at the initial stage of a new development project and persistence storage is still unavailable.

package com.turreta.sample;

import java.util.HashMap;
import java.util.Map;

import org.springframework.context.ApplicationContext;
import org.springframework.context.annotation.*;

@Configuration
@ComponentScan
public class Application {

    @Bean(name="mockdao")
    public JdbcDao mockDao() {        
        // Use this for now while DB is down or still being designed
        return new JdbcDao() {
            public Map<String, String> getCountries() {
                Map<String, String> countries = new HashMap<String, String>();
                
                // Mock data
                countries.put("PH", "Philippines");
                countries.put("SG", "Singapore");
                countries.put("TH", "Thailand");
                countries.put("MY", "Malaysia");
                return countries;
            }
        };
    }
    
    @Bean(name="dao")
    public JdbcDao dao() {
        return new JdbcDao() {
            public Map<String, String> getCountries() {                
                Map<String, String> countries = new HashMap<String, String>();                
                // Fetch from database                
                return countries;
            }
        };
    }

  public static void main(String[] args) {
      ApplicationContext context = 
          new AnnotationConfigApplicationContext(Application.class);
      CommonService service = context.getBean(CommonService.class);     
      
      for (Map.Entry<String, String> entry : service.getCountries().entrySet()) {
          System.out.println("Country Code : " + entry.getKey() + " | Country Name : "
              + entry.getValue());
      }
  }
}

package com.turreta.sample;

import java.util.HashMap;

import java.util.Map;

import org.springframework.context.ApplicationContext;

import org.springframework.context.annotation.*;

@Configuration

@ComponentScan

public class Application {

@Bean(name="mockdao")

public JdbcDao mockDao() {

// Use this for now while DB is down or still being designed

return new JdbcDao() {

public Map<String, String> getCountries() {

Map<String, String> countries = new HashMap<String, String>();

// Mock data

countries.put("PH", "Philippines");

countries.put("SG", "Singapore");

countries.put("TH", "Thailand");

countries.put("MY", "Malaysia");

return countries;

}

};

}

@Bean(name="dao")

public JdbcDao dao() {

return new JdbcDao() {

public Map<String, String> getCountries() {

Map<String, String> countries = new HashMap<String, String>();

// Fetch from database

return countries;

}

};

}

public static void main(String[] args) {

ApplicationContext context =

new AnnotationConfigApplicationContext(Application.class);

CommonService service = context.getBean(CommonService.class);

for (Map.Entry<String, String> entry : service.getCountries().entrySet()) {

System.out.println("Country Code : " + entry.getKey() + " | Country Name : "

+ entry.getValue());

}

The Interface – JdbcDao.java

package com.turreta.sample;
import java.util.Map;

public interface JdbcDao {
    // Country Code -> Country Name
    Map<String, String> getCountries();
}

package com.turreta.sample;

import java.util.Map;

public interface JdbcDao {

// Country Code -> Country Name

Map<String, String> getCountries();

}

The Service – CommonService.java

This is where you specify which instance of JdbcDao will be used. Bean “mockdao” or “dao”? The codes below uuse “mockdao”.

package com.turreta.sample;
import java.util.Map;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.beans.factory.annotation.Qualifier;
import org.springframework.stereotype.Component;

@Component
public class CommonService {

    @Autowired
    @Qualifier("mockdao")  // TODO: change this later to use "dao" once database is available
    private JdbcDao service;

    public Map<String, String> getCountries() {
        return this.service.getCountries();
    }
}

package com.turreta.sample;

import java.util.Map;

import org.springframework.beans.factory.annotation.Autowired;

import org.springframework.beans.factory.annotation.Qualifier;

import org.springframework.stereotype.Component;

@Component

public class CommonService {

@Autowired

@Qualifier("mockdao") // TODO: change this later to use "dao" once database is available

private JdbcDao service;

public Map<String, String> getCountries() {

return this.service.getCountries();

}

Sample Output

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Nov 23, 2014 5:53:02 PM org.springframework.context.annotation.AnnotationConfigApplicationContext prepareRefresh
INFO: Refreshing org.springframework.context.annotation.AnnotationConfigApplicationContext@131c89c: startup date [Sun Nov 23 17:53:02 SGT 2014]; root of context hierarchy
Country Code : PH | Country Name : Philippines
Country Code : MY | Country Name : Malaysia
Country Code : SG | Country Name : Singapore
Country Code : TH | Country Name : Thailand

Nov 23, 2014 5:53:02 PM org.springframework.context.annotation.AnnotationConfigApplicationContext prepareRefresh

INFO: Refreshing org.springframework.context.annotation.AnnotationConfigApplicationContext@131c89c: startup date [Sun Nov 23 17:53:02 SGT 2014]; root of context hierarchy

Country Code : PH | Country Name : Philippines

Country Code : MY | Country Name : Malaysia

Country Code : SG | Country Name : Singapore

Country Code : TH | Country Name : Thailand

What happens if @Qualifier is not used?

If you comment out line 11 in CommonService.java and run Application.java again, the following exception is thrown:

Nov 23, 2014 5:47:57 PM org.springframework.context.annotation.AnnotationConfigApplicationContext prepareRefresh
INFO: Refreshing org.springframework.context.annotation.AnnotationConfigApplicationContext@1697b67: startup date [Sun Nov 23 17:47:57 SGT 2014]; root of context hierarchy
Nov 23, 2014 5:47:57 PM org.springframework.context.annotation.AnnotationConfigApplicationContext refresh
WARNING: Exception encountered during context initialization - cancelling refresh attempt
org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'commonService': Injection of autowired dependencies failed; nested exception is org.springframework.beans.factory.BeanCreationException: Could not autowire field: private com.turreta.sample.JdbcDao com.turreta.sample.CommonService.service; nested exception is org.springframework.beans.factory.NoUniqueBeanDefinitionException: No qualifying bean of type [com.turreta.sample.JdbcDao] is defined: expected single matching bean but found 2: dao,mockdao
	at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor.postProcessPropertyValues(AutowiredAnnotationBeanPostProcessor.java:334)
	at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.populateBean(AbstractAutowireCapableBeanFactory.java:1204)
	at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:538)
	at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:476)
	at org.springframework.beans.factory.support.AbstractBeanFactory$1.getObject(AbstractBeanFactory.java:302)
	at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:229)
	at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:298)
	at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:193)
	at org.springframework.beans.factory.support.DefaultListableBeanFactory.preInstantiateSingletons(DefaultListableBeanFactory.java:762)
	at org.springframework.context.support.AbstractApplicationContext.finishBeanFactoryInitialization(AbstractApplicationContext.java:757)
	at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:480)
	at org.springframework.context.annotation.AnnotationConfigApplicationContext.<init>(AnnotationConfigApplicationContext.java:84)
	at com.turreta.sample.Application.main(Application.java:47)
Caused by: org.springframework.beans.factory.BeanCreationException: Could not autowire field: private com.turreta.sample.JdbcDao com.turreta.sample.CommonService.service; nested exception is org.springframework.beans.factory.NoUniqueBeanDefinitionException: No qualifying bean of type [com.turreta.sample.JdbcDao] is defined: expected single matching bean but found 2: dao,mockdao
	at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor$AutowiredFieldElement.inject(AutowiredAnnotationBeanPostProcessor.java:555)
	at org.springframework.beans.factory.annotation.InjectionMetadata.inject(InjectionMetadata.java:87)
	at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor.postProcessPropertyValues(AutowiredAnnotationBeanPostProcessor.java:331)
	... 12 more
Caused by: org.springframework.beans.factory.NoUniqueBeanDefinitionException: No qualifying bean of type [com.turreta.sample.JdbcDao] is defined: expected single matching bean but found 2: dao,mockdao
	at org.springframework.beans.factory.support.DefaultListableBeanFactory.doResolveDependency(DefaultListableBeanFactory.java:1061)
	at org.springframework.beans.factory.support.DefaultListableBeanFactory.resolveDependency(DefaultListableBeanFactory.java:949)
	at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor$AutowiredFieldElement.inject(AutowiredAnnotationBeanPostProcessor.java:527)
	... 14 more

Exception in thread "main" org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'commonService': Injection of autowired dependencies failed; nested exception is org.springframework.beans.factory.BeanCreationException: Could not autowire field: private com.turreta.sample.JdbcDao com.turreta.sample.CommonService.service; nested exception is org.springframework.beans.factory.NoUniqueBeanDefinitionException: No qualifying bean of type [com.turreta.sample.JdbcDao] is defined: expected single matching bean but found 2: dao,mockdao
	at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor.postProcessPropertyValues(AutowiredAnnotationBeanPostProcessor.java:334)
	at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.populateBean(AbstractAutowireCapableBeanFactory.java:1204)
	at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:538)
	at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:476)
	at org.springframework.beans.factory.support.AbstractBeanFactory$1.getObject(AbstractBeanFactory.java:302)
	at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:229)
	at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:298)
	at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:193)
	at org.springframework.beans.factory.support.DefaultListableBeanFactory.preInstantiateSingletons(DefaultListableBeanFactory.java:762)
	at org.springframework.context.support.AbstractApplicationContext.finishBeanFactoryInitialization(AbstractApplicationContext.java:757)
	at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:480)
	at org.springframework.context.annotation.AnnotationConfigApplicationContext.<init>(AnnotationConfigApplicationContext.java:84)
	at com.turreta.sample.Application.main(Application.java:47)
Caused by: org.springframework.beans.factory.BeanCreationException: Could not autowire field: private com.turreta.sample.JdbcDao com.turreta.sample.CommonService.service; nested exception is org.springframework.beans.factory.NoUniqueBeanDefinitionException: No qualifying bean of type [com.turreta.sample.JdbcDao] is defined: expected single matching bean but found 2: dao,mockdao
	at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor$AutowiredFieldElement.inject(AutowiredAnnotationBeanPostProcessor.java:555)
	at org.springframework.beans.factory.annotation.InjectionMetadata.inject(InjectionMetadata.java:87)
	at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor.postProcessPropertyValues(AutowiredAnnotationBeanPostProcessor.java:331)
	... 12 more
Caused by: org.springframework.beans.factory.NoUniqueBeanDefinitionException: No qualifying bean of type [com.turreta.sample.JdbcDao] is defined: expected single matching bean but found 2: dao,mockdao
	at org.springframework.beans.factory.support.DefaultListableBeanFactory.doResolveDependency(DefaultListableBeanFactory.java:1061)
	at org.springframework.beans.factory.support.DefaultListableBeanFactory.resolveDependency(DefaultListableBeanFactory.java:949)
	at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor$AutowiredFieldElement.inject(AutowiredAnnotationBeanPostProcessor.java:527)
	... 14 more

Nov 23, 2014 5:47:57 PM org.springframework.context.annotation.AnnotationConfigApplicationContext prepareRefresh

INFO: Refreshing org.springframework.context.annotation.AnnotationConfigApplicationContext@1697b67: startup date [Sun Nov 23 17:47:57 SGT 2014]; root of context hierarchy

Nov 23, 2014 5:47:57 PM org.springframework.context.annotation.AnnotationConfigApplicationContext refresh

WARNING: Exception encountered during context initialization - cancelling refresh attempt

org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'commonService': Injection of autowired dependencies failed; nested exception is org.springframework.beans.factory.BeanCreationException: Could not autowire field: private com.turreta.sample.JdbcDao com.turreta.sample.CommonService.service; nested exception is org.springframework.beans.factory.NoUniqueBeanDefinitionException: No qualifying bean of type [com.turreta.sample.JdbcDao] is defined: expected single matching bean but found 2: dao,mockdao

at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor.postProcessPropertyValues(AutowiredAnnotationBeanPostProcessor.java:334)

at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.populateBean(AbstractAutowireCapableBeanFactory.java:1204)

at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:538)

at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:476)

at org.springframework.beans.factory.support.AbstractBeanFactory$1.getObject(AbstractBeanFactory.java:302)

at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:229)

at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:298)

at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:193)

at org.springframework.beans.factory.support.DefaultListableBeanFactory.preInstantiateSingletons(DefaultListableBeanFactory.java:762)

at org.springframework.context.support.AbstractApplicationContext.finishBeanFactoryInitialization(AbstractApplicationContext.java:757)

at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:480)

at org.springframework.context.annotation.AnnotationConfigApplicationContext.<init>(AnnotationConfigApplicationContext.java:84)

at com.turreta.sample.Application.main(Application.java:47)

Caused by: org.springframework.beans.factory.BeanCreationException: Could not autowire field: private com.turreta.sample.JdbcDao com.turreta.sample.CommonService.service; nested exception is org.springframework.beans.factory.NoUniqueBeanDefinitionException: No qualifying bean of type [com.turreta.sample.JdbcDao] is defined: expected single matching bean but found 2: dao,mockdao

at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor$AutowiredFieldElement.inject(AutowiredAnnotationBeanPostProcessor.java:555)

at org.springframework.beans.factory.annotation.InjectionMetadata.inject(InjectionMetadata.java:87)

at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor.postProcessPropertyValues(AutowiredAnnotationBeanPostProcessor.java:331)

... 12 more

Caused by: org.springframework.beans.factory.NoUniqueBeanDefinitionException: No qualifying bean of type [com.turreta.sample.JdbcDao] is defined: expected single matching bean but found 2: dao,mockdao

at org.springframework.beans.factory.support.DefaultListableBeanFactory.doResolveDependency(DefaultListableBeanFactory.java:1061)

at org.springframework.beans.factory.support.DefaultListableBeanFactory.resolveDependency(DefaultListableBeanFactory.java:949)

at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor$AutowiredFieldElement.inject(AutowiredAnnotationBeanPostProcessor.java:527)

... 14 more

Exception in thread "main" org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'commonService': Injection of autowired dependencies failed; nested exception is org.springframework.beans.factory.BeanCreationException: Could not autowire field: private com.turreta.sample.JdbcDao com.turreta.sample.CommonService.service; nested exception is org.springframework.beans.factory.NoUniqueBeanDefinitionException: No qualifying bean of type [com.turreta.sample.JdbcDao] is defined: expected single matching bean but found 2: dao,mockdao