What is Constructor Auto-wiring mode in Spring?

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Introduction

Auto-wiring in Spring allows the container to automatically resolve and inject collaborating beans without requiring manual <constructor-arg ref="..."/> declarations. When using constructor autowiring mode (autowire="constructor"), the Spring IoC container inspects the bean's constructor parameter types and injects matching bean instances configured in the context.

How Does Constructor Auto-Wiring Work?

• Type Matching: Similar to byType autowiring, Spring searches the application context for beans whose data types match each argument of the constructor.
• Constructor Selection: If a class contains multiple overloaded constructors, Spring selects the greediest constructor whose parameters can all be satisfied by existing beans in the context.
• Guaranteed Immutability: Because dependencies are passed through the constructor, the bean can be instantiated as fully initialized and immutable.
• No Multiple Matches Allowed: If more than one candidate bean of the exact same type exists in the container, Spring throws an UnsatisfiedDependencyException due to ambiguity.

Step-by-Step Configuration Example

Consider an Employee class that receives an Address dependency strictly via its constructor:

1. The Java Model Class

package com.ram.model;

public class Employee {
    private Address address;

    // Constructor for autowiring
    public Employee(Address address) {
        this.address = address;
    }

    public void display() {
        System.out.println("Employee address: " + address);
    }
}

2. Configuring autowire="constructor" in XML

Notice that the employee bean does not need explicit <constructor-arg> tags:

<!-- Dependent Bean -->
<bean id="address" class="com.ram.model.Address">
    <property name="city" value="Bangalore" />
    <property name="state" value="Karnataka" />
</bean>

<!-- Employee Bean with Constructor Auto-Wiring -->
<bean id="employee" 
      class="com.ram.model.Employee" 
      autowire="constructor" />

3. Loading the Application Context

ApplicationContext context = new ClassPathXmlApplicationContext("applicationContext.xml");

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

Key Takeaway

Constructor autowiring combines the benefits of constructor-based dependency injection (immutability and mandatory dependency enforcement) with automated container wiring, keeping your XML configuration clean and concise.

Recommended Auto-Wiring Tutorials from Ram N Java

• Understanding No Auto-Wiring Mode in Spring | Spring Framework Tutorial | Spring Tutorial
• byName Auto-Wiring in Spring: How Does It Work? | Spring Framework Tutorial | Spring Tutorial
• byType Auto-Wiring in Spring: How Does It Work? | Spring Framework Tutorial | Spring Tutorial

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What is Constructor Auto-wiring mode in Spring?

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 String name;
    private Address address;

    public Employee(String name)
    {
        System.out.println("Employee(String name) constructor "
                + "is called by the Spring container");
        this.name = name;
    }

    public Employee(String name, Address address)
    {
        System.out.println("Employee(String name, Address address) "
                + "constructor is called by the Spring container");
        this.name = name;
        this.address = address;
    }

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

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

Jul 01, 2018 10:27:21 AM org.springframework.context.support.AbstractApplicationContext prepareRefresh
INFO: Refreshing org.springframework.context.support.ClassPathXmlApplicationContext@5b37e0d2: startup date [Sun Jul 01 10:27:21 IST 2018]; root of context hierarchy
Jul 01, 2018 10:27:21 AM org.springframework.beans.factory.xml.XmlBeanDefinitionReader loadBeanDefinitions
INFO: Loading XML bean definitions from class path resource [applicationContext.xml]
Employee(String name, Address address) constructor is called by the Spring container
---------------------------------------
Got employee object from the ApplicationContext[Spring Container]
Employee [name=Peter, address=Address [city=Chennai, state=Tamil Nadu, country=India]