JAVA EE: What is byType Auto-wiring mode in Spring?

Monday, 16 July 2018

What is byType Auto-wiring mode in Spring?

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Introduction

Spring's dependency injection container can automatically wire collaborating beans through autowiring modes. One of the most commonly used approaches is byType autowiring (autowire="byType"). In this mode, Spring inspects the property types of a bean and searches the application context for another bean matching that exact class or interface type to inject.

How Does byType Auto-Wiring Work?

Type-Based Inspection: Spring inspects each setter method in the bean. If a setter accepts an object of type Address, Spring searches the XML context for a bean of type Address.
Bean Name Independence: Unlike byName autowiring, the ID or name of the candidate bean does not need to match the property name—only the data type matters.
Uniqueness Requirement: Exactly one bean of the expected type must exist in the context. If multiple matching beans are found, Spring throws a fatal NoUniqueBeanDefinitionException.
Optional Injection: If no bean matches the type, Spring leaves the property un-set (null) without throwing an error.

Step-by-Step Configuration Example

Consider an Employee bean that has an Address property with setter methods:

1. The Java Model Classes

package com.ram.model;

public class Employee {
    private Address address;

    public void setAddress(Address address) {
        this.address = address;
    }

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

2. Configuring autowire="byType" in applicationContext.xml

Notice that the bean ID userAddress does not match the property name address, yet Spring wires it successfully because of matching types:

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

<!-- Employee Bean configured with byType autowiring -->
<bean id="employee" 
      class="com.ram.model.Employee" 
      autowire="byType" />

3. Loading the Application Context

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

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

Key Takeaway

autowire="byType" provides seamless dependency wiring decoupled from bean naming conventions. As long as only one unique bean matches the property's type, Spring takes care of dependency injection automatically.

Recommended Auto-Wiring Tutorials from Ram N Java

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What is byType 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 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="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 29, 2018 9:15:43 AM org.springframework.context.support.AbstractApplicationContext prepareRefresh

INFO: Refreshing org.springframework.context.support.ClassPathXmlApplicationContext@5b37e0d2: startup date [Fri Jun 29 09:15:43 IST 2018]; root of context hierarchy

Jun 29, 2018 9:15:43 AM org.springframework.beans.factory.xml.XmlBeanDefinitionReader loadBeanDefinitions

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

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

Got employee object from the ApplicationContext[Spring Container]

Employee [id=100, name=Peter, address=Address [city=Chennai, state=Tamil Nadu, country=India]]