JAVA EE: Spring Auto-Wiring Beans with @Autowired annotation in field

Monday, 10 September 2018

Spring Auto-Wiring Beans with @Autowired annotation in field | Spring @Autowired Annotation

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Overview: Field Injection with @Autowired

In the Spring Framework, Dependency Injection (DI) allows you to wire components together effortlessly. Among the various injection techniques, Field Injection using the @Autowired annotation is one of the most widely used and concise approaches. By placing @Autowired directly on class fields, Spring automatically assigns dependencies behind the scenes using reflection.

In this guide, you will learn how field injection works in Spring, how to configure it cleanly, and key considerations to keep in mind when using it in your Java projects.

How Field Injection Works

Applying @Autowired directly on class attributes provides several distinct characteristics:

Zero Boilerplate Code: Eliminates the need to write constructors or getter and setter methods solely for dependency injection.
Reflection-Based Wiring: Spring accesses private fields directly using Java reflection, injecting the matching bean discovered in the container.
Injection by Type: By default, Spring identifies the collaborator bean matching the declared data type of the annotated field.
Clean and Compact Syntax: Keeps your entity and service classes concise and readable.

Step-by-Step Implementation

Declare the Collaborator Bean: Create and register the dependency component in your Spring container using @Component, @Service, or XML.
Annotate the Private Field: Add @Autowired directly above the member field in your target bean:
@Autowired private OrderService orderService;
Run the Application Context: Upon container startup, Spring instantiates the bean and injects the corresponding instance directly into the field.

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Spring Auto-Wiring Beans with @Autowired annotation in field | Spring @Autowired Annotation

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;

public class Employee

{

    private String name;

    @Autowired

    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="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">

        <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 18, 2018 9:51:49 AM org.springframework.context.support.AbstractApplicationContext prepareRefresh

INFO: Refreshing org.springframework.context.support.ClassPathXmlApplicationContext@5b37e0d2: startup date [Sat Aug 18 09:51:49 IST 2018]; root of context hierarchy

Aug 18, 2018 9:51:50 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=Chennai, state=Tamil Nadu, country=India]]