Dependency Injection with the Spring Framework - Tutorial

Java components / classes should be as independent as possible of other Java classes. This increases the possibility to reuse these classes and to test them independently of other classes(Unit Testing). To decouple Java components from other Java components the dependency to a certain other class should get injected into them rather that the class itself creates / finds this object. A class A has a dependency to class B if class uses class B as a variable.

If dependency injection is used then the class B is given to class A via

• the constructor of the class A - this is then called construction injection

• a setter - this is then called setter injection

The general concept between dependency injection is called Inversion of Control. A class should not configure itself but should be configured from outside. A design based on independent classes / components increases the re-usability and possibility to test the software. For example, if a class A expects a Dao (Data Access object) for receiving the data from a database you can easily create another test object which mocks the database connection and inject this object into A to test A without having an actual database connection. A software design based on dependency injection is possible with standard Java. Spring just simplifies the use of dependency injection by providing a standard way of providing the configuration and by managing the reference to the created objects.

As of Spring 2.5 it is possible to configure the dependency injection via annotations. I recommend to use this way of configuring your Spring beans. The next chapter will also describe the way to configure this via XML. Create a new Java project "de.vogella.spring.di.annotations.first" and include the minimal required spring jars into your classpath. Copy your model class from the de.vogella.spring.di.model project into this project. You need now to add annotations to your model to tell Spring which beans should be managed by Spring and how they should be connected. Add the @Service annotation the MySpringBeanWithDependency.java and NiceWriter.java. Also define with @Autowired on the setWriter method that the property "writer" will be autowired by Spring.

@Autowired will tell Spring to search for a Spring bean which implements the required interface and place it automatically into the setter.

package testbean;

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Service;

import writer.IWriter;

@Service
public class MySpringBeanWithDependency {
    private IWriter writer;

    @Autowired
    public void setWriter(IWriter writer) {
        this.writer = writer;
    }

    public void run() {
        String s = "This is my test";
        writer.writer(s);
    }
}

package writer;

import org.springframework.stereotype.Service;

@Service
public class NiceWriter implements IWriter {
    public void writer(String s) {
        System.out.println("The string is " + s);
    }
}

Under the src folder create a folder META-INF and create the following file in this folder. This is the Spring configuration file.

<beans xmlns="http://www.springframework.org/schema/beans"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xmlns:aop="http://www.springframework.org/schema/aop"
    xmlns:context="http://www.springframework.org/schema/context"
    xsi:schemaLocation="http://www.springframework.org/schema/beans
           http://www.springframework.org/schema/beans/spring-beans-2.5.xsd
           http://www.springframework.org/schema/aop
           http://www.springframework.org/schema/aop/spring-aop-2.5.xsd
           http://www.springframework.org/schema/context
           http://www.springframework.org/schema/context/spring-context-2.5.xsd">

    <context:component-scan base-package="testbean" />
    <context:component-scan base-package="writer" />

</beans>

You can also configure the log4j logger (this is optional) by copying the following file into the source folder.

log4j.rootLogger=FATAL, first
log4j.appender.first=org.apache.log4j.ConsoleAppender
log4j.appender.first.layout=org.apache.log4j.PatternLayout
log4j.appender.first.layout.ConversionPattern=%-4r [%t] %-5p %c %x - %m%n

Afer this setup you can wire the application together. Create a main class which reads the configuration file and starts the application.

package main;

import org.springframework.beans.factory.BeanFactory;
import org.springframework.context.ApplicationContext;
import org.springframework.context.support.ClassPathXmlApplicationContext;

import testbean.MySpringBeanWithDependency;

public class Main {
    public static void main(String[] args) {
        ApplicationContext context = new ClassPathXmlApplicationContext(
                "META-INF/beans.xml");
        BeanFactory factory = context;
        MySpringBeanWithDependency test = (MySpringBeanWithDependency) factory
                .getBean("mySpringBeanWithDependency");
        test.run();
    }
}

If you run the application then the class for the IWriterInterface will be inserted into the Test class. By applying the dependency injecting I can later replace this writer with a more sophisticated writer. As a result the class Test does not depend on the concrete Writer class, is extensible and can be easily tested.

The following example will demonstrate the usage of the dependency injection via xml. The example will inject a writer into another class.

I think annotations rock in general, therefore I recommend not to use the XML configuration but the annotation one. If you have good reason to use the XML configuration please feel free to do so.

Create a new Java project "de.vogella.spring.di.xml.first" and include the minimal required spring jars into your classpath. Copy your model class from the de.vogella.spring.di.model project into this project. Under the src folder create a folder META-INF and create the following file in this folder. This is the Spring configuration file.

<beans xmlns="http://www.springframework.org/schema/beans"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xmlns:aop="http://www.springframework.org/schema/aop"
    xmlns:context="http://www.springframework.org/schema/context"
    xsi:schemaLocation="http://www.springframework.org/schema/beans
           http://www.springframework.org/schema/beans/spring-beans-2.5.xsd
           http://www.springframework.org/schema/aop
           http://www.springframework.org/schema/aop/spring-aop-2.5.xsd
           http://www.springframework.org/schema/context
           http://www.springframework.org/schema/context/spring-context-2.5.xsd">


<bean id="writer" class="writer.NiceWriter" />

<bean id="mySpringBeanWithDependency" class="testbean.MySpringBeanWithDependency">
<property name="writer" ref="writer" />
</bean>

</beans>

Again, you can now wire the application together. Create a main class which reads the configuration file and starts the application.

package main;

import org.springframework.beans.factory.BeanFactory;
import org.springframework.context.ApplicationContext;
import org.springframework.context.support.ClassPathXmlApplicationContext;

import testbean.MySpringBeanWithDependency;

public class Main {
    public static void main(String[] args) {
        ApplicationContext context = new ClassPathXmlApplicationContext(
                "META-INF/beans.xml");
        BeanFactory factory = context;
        MySpringBeanWithDependency test = (MySpringBeanWithDependency) factory
                .getBean("mySpringBeanWithDependency");
        test.run();
    }
}