JAVA EE: How to do Dependency Injection using Spring Expression Language SpEL(Annotation)

Tuesday, 25 September 2018

How to do Dependency Injection using Spring Expression Language SpEL(Annotation) | Spring Tutorial

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Overview: Dependency Injection with SpEL Annotations

In Spring applications, Dependency Injection (DI) is traditionally performed using annotations like @Autowired or XML property wirings. However, the Spring Expression Language (SpEL) paired with the @Value annotation provides a dynamic, expressive alternative to inject bean instances, literal values, or computed configurations directly into your components.

In this guide, you will learn how to implement Dependency Injection using SpEL and annotation-based configuration in Spring.

How SpEL-Based Dependency Injection Works

SpEL expressions allow you to wire dependencies with additional flexibility:

Injecting Bean Dependencies: Wire an entire target bean directly into a reference field: @Value("#{itemBean}").
Dynamic Bean Resolution: Resolve dependencies dynamically by name or conditional logic during container startup.
Combining Values and Objects: Inject both primitive configuration values and complex object references using the unified @Value syntax.
Loose Coupling: Decouple dependency resolution logic from class constructors and setters.

Step-by-Step Configuration

Define the Dependency Bean: Create the component to be injected and annotate it with @Component("beanId").
Annotate the Target Field: In the dependent class, annotate the target field with @Value("#{beanId}").
Run the Container: When Spring starts up, it resolves the expression within the ApplicationContext and injects the dependency bean automatically.

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How to do Dependency Injection using Spring Expression Language SpEL(Annotation) | Spring Tutorial

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>5.0.5.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;

import org.springframework.beans.factory.annotation.Value;

import org.springframework.stereotype.Component;

@Component("address")

public class Address

{

    @Value("Chennai") //inject String directly

    private String city;

    @Value("TamilNadu")

    private String state;

    public String getCity()

    {

        return city;

    }

    public void setCity(String city)

    {

        this.city = city;

    }

    public String getState()

    {

        return state;

    }

    public void setState(String state)

    {

        this.state = state;

    }

    @Override

    public String toString()

    {

        return "Address [city=" + city + ", state=" + state + "]";

    }

}

Employee.java

package com.ram.model;

import org.springframework.beans.factory.annotation.Value;

import org.springframework.stereotype.Component;

@Component("employee")

public class Employee

{

    @Value("100") //inject int directly

    private int id;

    @Value("Peter")

    private String name;

    @Value("#{address.state}")

    private String employeeState;

    @Value("#{address}")

    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 String getEmployeeState()

    {

        return employeeState;

    }

    public void setEmployeeState(String employeeState)

    {

        this.employeeState = employeeState;

    }

    public Address getAddress()

    {

        return address;

    }

    public void setAddress(Address address)

    {

        this.address = address;

    }

    @Override

    public String toString()

    {

        return "Employee [id=" + id + ", name=" + name + ", employeeState="

                + employeeState + ", 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:component-scan base-package="com.ram" />

</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)");

        System.out.println(employee);   

    }

}

Output

Aug 31, 2018 10:42:19 AM org.springframework.context.support.AbstractApplicationContext prepareRefresh

INFO: Refreshing org.springframework.context.support.ClassPathXmlApplicationContext@736e9adb: startup date [Fri Aug 31 10:42:19 IST 2018]; root of context hierarchy

Aug 31, 2018 10:42:19 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, employeeState=TamilNadu, address=Address [city=Chennai, state=TamilNadu]]