Explain @Qualifier Annotation in Spring

@Qualifier Annotation in Spring

@Qualifier which helps fine-tune annotation-based autowiring. In the previous post we saw how we could use @Autowired annotation on setter methods, fields and constructors. There may be scenarios when we create more than one bean of the same type and want to wire only one of them with a property. This can be controlled using @Qualifier annotation along with the @Autowired annotation
.

Note : This example written using the XML configurations. If you are starting new spring framework project, please use spring boot for packaging and deploying your spring applications without any burden of writing XML configurations.

@Qualifier Annotation Example
Let us have working Eclipse IDE in place and follow the following steps to create a Spring application:

Create a project: Create a project with a name SpringAnnotationExamples and create a package com.aid.qualifierexample under the src directory in the created project.

Add Libraries: Add required Spring libraries using Add External JARs option as explained in the article Customizing callback methods.

Create source files: Create Java classes Product,Type and MainApp under the com.aid.qualifierexample package.

Create configuration file: Create XML based configuration file BeansQualifierAnnotation.xml under src directory.

The class Product.java, is simple POJO class having name,price and an object of Type class. Contents of Product.java are:

 package com.aid.qualifierexample;



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

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



 public class Product {

  private Integer price;

  private String name;



  @Autowired

  @Qualifier("typeB")

  private Type type;



  public Integer getPrice() {

   return price;

  }



  public void setPrice(Integer price) {

   this.price = price;

  }



  public Type getType() {

   return type;

  }



  public String getName() {

   return name;

  }



  public void setName(String name) {

   this.name = name;

  }

 }

 

Here you can see, I’ve used the @Qualifier annotation alongwith the @Autowired annotation.

The Type.java class is also a POJO class having a string object called “type”. Contents of Type.java are:

package com.aid.autowireexample;



 public class Type {



  private String type;



  public String getType() {

   return type;

  }



  public void setType(String type) {

   this.type = type;

  }

 }

  

Contents of MainApp are:

 package com.aid.qualifierexample;



 import org.springframework.context.ApplicationContext;

 import org.springframework.context.support.ClassPathXmlApplicationContext;



 public class MainApp {

  public static void main(String[] args) {

        ApplicationContext context =

               new ClassPathXmlApplicationContext(
                            "BeansQualifierAnnotation.xml");



        Product product = (Product) context.getBean("product");



        System.out.println("Product Name : " + product.getName() );

        System.out.println("Price : " + product.getPrice() );



        Type productType = product.getType();



        System.out.println(product.getName()+" is of type:" + 
                                      productType.getType() );

     }

 }

Contents of BeansQualifierAnnotation.xml are:

xml version="1.0" encoding="UTF-8"?>

<beans xmlns="http://www.springframework.org/schema/beans"

 xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:context="

 http://www.springframework.org/schema/context"

 xsi:schemaLocation="http://www.springframework.org/schema/beans

    http://www.springframework.org/schema/beans/spring-beans-3.2.xsd

    http://www.springframework.org/schema/context

    http://www.springframework.org/schema/context/spring-context-3.2.xsd">



 <context:annotation-config />



 

 <bean id="product" class="com.aid.qualifierexample.Product">

  <property name="name" value="ProductA" />

  <property name="price" value="400" />

 </bean>

 <bean id="typeA" class="com.aid.qualifierexample.Type">

   <property name="type" value="Export" />

 </bean>

 <bean id="typeB" class="com.aid.qualifierexample.Type">

   <property name="type" value="Import" />

 </bean>



</beans>

As you can note here we are having two beans of same type. In Product.java I’ve used @Qualifier(“typeB”) it means we want to autowire Type property of Product with bean id=”typeB” in XML configuration file.

Execute the code
The final step is run the application. Once all the above code is ready execute and the below output appears on the console:

Product Name : ProductA
Price : 400
ProductA is of type:Import