JAVA EE: NoUniqueBeanDefinitionException

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Monday, 16 July 2018

What will happen in byType Auto wiring mode, multiple bean of one type is there?

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Introduction

In the Spring Framework, byType autowiring allows the container to automatically inject a dependency into a setter method if exactly one bean matches the property's data type. But what happens if multiple beans of that same data type are defined in your XML configuration? Let us explore how the Spring container responds and how to resolve the resulting ambiguity.

What Happens When Multiple Beans of the Same Type Exist?

Fatal Exception Thrown: If more than one candidate bean of the expected type is present in the ApplicationContext, Spring cannot choose which one to inject.
NoUniqueBeanDefinitionException: Spring throws a fatal NoUniqueBeanDefinitionException (or UnsatisfiedDependencyException), preventing application context initialization.
Resolution Techniques: You can resolve this issue by marking one bean as primary (primary="true"), excluding beans with autowire-candidate="false", switching to byName autowiring, or using explicit wiring.

The Ambiguity Scenario Illustrated

Consider two Address beans configured in applicationContext.xml, while an Employee bean uses autowire="byType":

1. Problematic Configuration

<!-- Multiple beans of the same type (Address) -->
<bean id="address1" class="com.ram.model.Address">
    <property name="city" value="Bangalore" />
</bean>

<bean id="address2" class="com.ram.model.Address">
    <property name="city" value="Hyderabad" />
</bean>

<!-- byType autowiring encounters ambiguity here -->
<bean id="employee" 
      class="com.ram.model.Employee" 
      autowire="byType" />

2. Container Exception Result

org.springframework.beans.factory.NoUniqueBeanDefinitionException: 
No qualifying bean of type 'com.ram.model.Address' available: 
expected single matching bean but found 2: address1,address2

Key Takeaway

When using autowire="byType", Spring guarantees type safety only when exactly one qualifying bean is present. To avoid fatal runtime exceptions when multiple beans share the same type, make sure to designate a primary candidate, use qualifier annotations, or switch to explicit bean references.

Recommended Auto-Wiring Tutorials from Ram N Java

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What will happen in byType Auto wiring mode, multiple bean of one type is there?

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;

public class Employee

{

    private int id;

    private String name;

    private Address address;

    public int getId()

    {

        return id;

    }

    public void setId(int id)

    {

        this.id = id;

    }

    public String getName()

    {

        return name;

    }

    public void setName(String name)

    {

        this.name = name;

    }

    public Address getAddress()

    {

        return address;

    }

    public void setAddress(Address address)

    {

        this.address = address;

    }

    public void displayEmployeeDetails()

    {

        System.out.println("Employee [id=" + id + ", name=" + name

                + ", address=" + address + "]");

    }

}

applicationContext.xml

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

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

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

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

    <bean id="address1" 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="address2" 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" autowire="byType">

        <property name="id" value="100"></property>

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

    </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

Jun 30, 2018 8:41:53 AM org.springframework.context.support.AbstractApplicationContext prepareRefresh

INFO: Refreshing org.springframework.context.support.ClassPathXmlApplicationContext@5b37e0d2: startup date [Sat Jun 30 08:41:53 IST 2018]; root of context hierarchy

Jun 30, 2018 8:41:53 AM org.springframework.beans.factory.xml.XmlBeanDefinitionReader loadBeanDefinitions

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

Jun 30, 2018 8:41:53 AM org.springframework.context.support.AbstractApplicationContext refresh

WARNING: Exception encountered during context initialization - cancelling refresh attempt: org.springframework.beans.factory.UnsatisfiedDependencyException: Error creating bean with name 'employee' defined in class path resource [applicationContext.xml]: Unsatisfied dependency expressed through bean property 'address'; nested exception is org.springframework.beans.factory.NoUniqueBeanDefinitionException: No qualifying bean of type 'com.ram.model.Address' available: expected single matching bean but found 2: address1,address2

Exception in thread "main" org.springframework.beans.factory.UnsatisfiedDependencyException: Error creating bean with name 'employee' defined in class path resource [applicationContext.xml]: Unsatisfied dependency expressed through bean property 'address'; nested exception is org.springframework.beans.factory.NoUniqueBeanDefinitionException: No qualifying bean of type 'com.ram.model.Address' available: expected single matching bean but found 2: address1,address2

    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.autowireByType(AbstractAutowireCapableBeanFactory.java:1439)

    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.populateBean(AbstractAutowireCapableBeanFactory.java:1326)

    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:582)

    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:502)

    at org.springframework.beans.factory.support.AbstractBeanFactory.lambda$doGetBean$0(AbstractBeanFactory.java:312)

    at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:228)

    at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:310)

    at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:200)

    at org.springframework.beans.factory.support.DefaultListableBeanFactory.preInstantiateSingletons(DefaultListableBeanFactory.java:758)

    at org.springframework.context.support.AbstractApplicationContext.finishBeanFactoryInitialization(AbstractApplicationContext.java:868)

    at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:549)

    at org.springframework.context.support.ClassPathXmlApplicationContext.<init>(ClassPathXmlApplicationContext.java:144)

    at org.springframework.context.support.ClassPathXmlApplicationContext.<init>(ClassPathXmlApplicationContext.java:85)

    at com.ram.core.App.main(App.java:12)

Caused by: org.springframework.beans.factory.NoUniqueBeanDefinitionException: No qualifying bean of type 'com.ram.model.Address' available: expected single matching bean but found 2: address1,address2

    at org.springframework.beans.factory.config.DependencyDescriptor.resolveNotUnique(DependencyDescriptor.java:215)

    at org.springframework.beans.factory.support.DefaultListableBeanFactory.doResolveDependency(DefaultListableBeanFactory.java:1113)

    at org.springframework.beans.factory.support.DefaultListableBeanFactory.resolveDependency(DefaultListableBeanFactory.java:1062)

    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.autowireByType(AbstractAutowireCapableBeanFactory.java:1424)

    ... 13 more