JAVA EE: Spring auto-wiring using @Qualifier annotation | Spring @Qualifier Annotation

Thursday, 13 September 2018

Spring auto-wiring using @Qualifier annotation | Spring @Qualifier Annotation | Spring Tutorial

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Overview: Resolving Ambiguity with @Qualifier

In the Spring Framework, dependency injection with @Autowired matches beans primarily by type. However, when multiple beans implement the exact same interface or share a common class type, Spring encounters ambiguity and throws a NoUniqueBeanDefinitionException. The @Qualifier annotation solves this problem by giving you fine-grained control to specify precisely which bean should be autowired.

In this guide, you will learn why bean ambiguity occurs, how the @Qualifier annotation works, and how to use it alongside @Autowired in your Spring projects.

Why Use the @Qualifier Annotation?

The @Qualifier annotation eliminates autowiring conflicts through explicit identification:

Eliminates Ambiguity: When multiple candidate beans of the same type exist in the ApplicationContext, @Qualifier points directly to the target bean name.
Works with Multiple Injection Points: You can apply @Qualifier on fields, constructor parameters, or setter methods.
Pairs Seamlessly with @Autowired: While @Autowired targets dependencies by type, @Qualifier narrows the selection down to a specific bean identifier.

Step-by-Step Implementation

Define Multiple Beans: Register multiple implementations of the same interface (e.g., @Component("emailService") and @Component("smsService")).
Inject with @Autowired and @Qualifier: In your consuming class, qualify the exact bean required:
@Autowired @Qualifier("emailService") private MessageService messageService;
Run the Application: Spring resolves the injection cleanly by choosing the specifically qualified bean without conflicting with other implementations.

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Spring auto-wiring using @Qualifier annotation | Spring @Qualifier Annotation | Spring Tutorial

pom.xml

<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"

    xsi:schemaLocation="http://maven.apache.org/POM/4.0.0%20http://maven.apache.org/maven-v4_0_0.xsd">

    <modelVersion>4.0.0</modelVersion>

    <groupId>com.ram.core</groupId>

    <artifactId>SpringDemo</artifactId>

    <packaging>jar</packaging>

    <version>1.0-SNAPSHOT</version>

    <name>SpringDemo</name>

    <url>http://maven.apache.org</url>

    <properties>

        <spring.version>5.0.2.RELEASE</spring.version>

    </properties>

    <dependencies>

        <dependency>

            <groupId>junit</groupId>

            <artifactId>junit</artifactId>

            <version>3.8.1</version>

            <scope>test</scope>

        </dependency>

        <!-- Spring 5 dependencies -->

        <dependency>

            <groupId>org.springframework</groupId>

            <artifactId>spring-core</artifactId>

            <version>${spring.version}</version>

        </dependency>

        <dependency>

            <groupId>org.springframework</groupId>

            <artifactId>spring-context</artifactId>

            <version>${spring.version}</version>

        </dependency>

    </dependencies>

</project>

Address.java

package com.ram.model;

public class Address

{

    private String city;

    private String state;

    private String country;

    public Address(String city, String state, String country)

    {

        this.city = city;

        this.state = state;

        this.country = country;

    }

    @Override

    public String toString()

    {

        return "Address [city=" + city + ", state=" + state + ", country="

                + country + "]";

    }

}

Employee.java

package com.ram.model;

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

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

public class Employee

{

    private String name;

    @Autowired

    @Qualifier("bangaloreAddress")

    private Address address;

    public Employee(String name)

    {

        System.out.println("Employee(String name) constructor "

                + "is called by the Spring container");

        this.name = name;

    }

    public void displayEmployeeDetails()

    {

        System.out.println(

                "Employee [name=" + name + ", address=" + address + "]");

    }

}

applicationContext.xml

<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-4.3.xsd

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

    http://www.springframework.org/schema/context/spring-context-4.3.xsd">

    <context:annotation-config />

    <bean id="chennaiAddress" class="com.ram.model.Address">

        <constructor-arg value="Chennai"></constructor-arg>

        <constructor-arg value="Tamil Nadu"></constructor-arg>

        <constructor-arg value="India"></constructor-arg>

    </bean>

    <bean id="bangaloreAddress" class="com.ram.model.Address">

        <constructor-arg value="Bangalore"></constructor-arg>

        <constructor-arg value="Karnataka"></constructor-arg>

        <constructor-arg value="India"></constructor-arg>

    </bean>

    <bean id="employee" class="com.ram.model.Employee">

        <constructor-arg value="Peter"></constructor-arg>

    </bean>

</beans>

App.java

package com.ram.core;

import org.springframework.context.ApplicationContext;

import org.springframework.context.support.ClassPathXmlApplicationContext;

import com.ram.model.Employee;

public class App

{

    public static void main(String[] args)

    {

        ApplicationContext context = new ClassPathXmlApplicationContext(

                "applicationContext.xml");

        System.out.println("---------------------------------------");

        Employee employee = (Employee) context.getBean("employee");

        System.out.println(

                "Got employee object from the ApplicationContext[Spring Container]");

        employee.displayEmployeeDetails();

    }

}

Output

Aug 21, 2018 10:02:00 AM org.springframework.context.support.AbstractApplicationContext prepareRefresh

INFO: Refreshing org.springframework.context.support.ClassPathXmlApplicationContext@5a2e4553: startup date [Tue Aug 21 10:02:00 IST 2018]; root of context hierarchy

Aug 21, 2018 10:02:00 AM org.springframework.beans.factory.xml.XmlBeanDefinitionReader loadBeanDefinitions

INFO: Loading XML bean definitions from class path resource [applicationContext.xml]

Employee(String name) constructor is called by the Spring container

---------------------------------------

Got employee object from the ApplicationContext[Spring Container]

Employee [name=Peter, address=Address [city=Bangalore, state=Karnataka, country=India]]