JAVA EE: Apply @Resource Annotation in setter method in Spring Autowiring

Friday, 21 September 2018

Apply @Resource Annotation in setter method in Spring Autowiring | Spring - @Resource annotation

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Overview: Setter-Based Injection with @Resource

The standard Java @Resource annotation (part of the JSR-250 specification) provides a clean, standardized approach to dependency injection in the Spring Framework. While commonly used for field injection, applying @Resource on setter methods enables setter-based dependency injection, promoting proper encapsulation, easier testing, and flexibility during bean configuration.

In this guide, you will learn how to configure and use @Resource on setter methods to inject dependencies cleanly into your Spring beans.

Understanding Setter-Based @Resource Injection

Using @Resource at the setter method level follows intuitive resolution rules:

Injection by Name by Default: When placed on a setter (such as setAddress(Address address)), Spring derives the default bean name from the property name (e.g., address).
Explicit Name Matching: You can explicitly specify the target bean name using @Resource(name = "customBeanName") directly on the setter method.
Fallback to Type: If no bean is found matching the derived name and no explicit name is specified, Spring resolves the dependency by type.
Encapsulation & Reusability: Setter injection keeps member variables private while still allowing dependencies to be provided or mocked during unit tests.

Step-by-Step Implementation

Define the Dependency Bean: Create the component to be injected and register it with the Spring container.
Add Setter Method with @Resource: Create a standard setter method in your dependent class and annotate it with @Resource:
@Resource public void setService(OrderService service) { this.service = service; }
Initialize the Spring ApplicationContext: When the container starts, Spring invokes the annotated setter method and injects the matching bean.

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Apply @Resource Annotation in setter method in Spring Autowiring | Spring - @Resource 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 javax.annotation.Resource;

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;

    }

    @Resource

    public void setAddress(Address address)

    {

        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" 

    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 22, 2018 10:00:33 AM org.springframework.context.support.AbstractApplicationContext prepareRefresh

INFO: Refreshing org.springframework.context.support.ClassPathXmlApplicationContext@5a2e4553: startup date [Wed Aug 22 10:00:33 IST 2018]; root of context hierarchy

Aug 22, 2018 10:00:34 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]]