Dependency injection is the most basic thing that Spring does. But how dependency injection works?
We will see how Spring achieve dependency injection with a simple example. In this example will demonstrate the basics of Spring. Here the first class in the example is a service class whose purpose is to print “Welcome to j2eereference.com”. Below code is the GreetingService interface, which defines the service class.
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public interface GreetingService { void sayWelcome(); } |
And GreetingServiceImpl implements the GreetingService interface. Although it’s not necessary to hide the implementation behind an interface, it’s highly recommended as a way to separate the implementation from its contract.
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public class GreetingServiceImpl implements GreetingService { private String greeting; public GreetingServiceImpl() {} public GreetingServiceImpl(String greeting) { this.greeting = greeting; } public void sayWelcome() { System.out.println(greeting); } public void setGreeting(String greeting) { this.greeting = greeting; } |
You may have noticed the below points in the above class
1) The GreetingServiceImpl class has a single property: greeting. This property is simply a String that holds the message that will be printed when the sayWelcome () method is called.
2) You may have noticed that greeting can be set in two different ways: by the constructor or by the property’s setter method.
Who will make the call to either the constructor or the setGreeting() method to set the property ? We are going to let the Spring container to set the greeting property.
Spring configuration file
The Spring configuration file (In our example, conf.xml) tells the container how to configure the greeting service.
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<?xml version="1.0" encoding="UTF-8"?> <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.xsd"> <bean id="greetingService" class="GreetingServiceImpl"> <property name="greeting" value="Welcome to j2eereference.com" /> </bean> </beans> |
The above XML declares an instance of a GreetingServiceImpl in the Spring container and configures its greeting property with a value of “Welcome to j2eereference.com”
Let’s dig into the details of this XML file a bit to understand how it works.
The <bean> element is used to tell the Spring container about a class and how it should be configured. Here, the id attribute is used to name the bean greetingService and the class attribute specifies the bean’s fully qualified class name.
Within the <bean> element, the <property> element is used to set a property, in this case the greeting property. As shown here, the <property> element tells the Spring container to call setGreeting(), passing in “Welcome to j2eereference.com” when instantiating the bean.
The following snippet of code illustrates roughly what the container does when instantiating the greeting service based on the above XML definition.
GreetingServiceImpl greetingService = new GreetingServiceImpl();
greetingService.setGreeting(“Welcome to j2eereference.com”);
Setting the constructor argument value
You can let the Spring to set the greeting property through GreetingServiceImpl’s single argument constructor. For example:
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<bean id="greetingService" |
class=”GreetingServiceImpl”>
<constructor-arg value=” Welcome to j2eereference.com” />
</bean>
The following code illustrates how the container will instantiate the greeting service when using the <constructor-arg> element:
GreetingServiceImpl greetingService =new GreetingServiceImpl(“Welcome to j2eereference.com”);
The below class- MyFirstSpringApp that loads the Spring container and uses it to retrieve the greeting service.
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import org.springframework.beans.factory.BeanFactory; import org.springframework.beans.factory.xml.XmlBeanFactory; import org.springframework.core.io.FileSystemResource; public class MyFirstSpringApp { public static void main(String[] args) throws Exception { BeanFactory factory = new XmlBeanFactory(new FileSystemResource("conf.xml")); GreetingService greetingService = (GreetingService) factory.getBean("greetingService"); greetingService.sayWelcome (); } } |
The BeanFactory class used here is the Spring container. After loading the conf.xml file into the container, the main() method calls the getBean() method on the BeanFactory to retrieve a reference to the greeting service. With this reference in hand, it finally calls the sayWelcome() method. When you run the MyFirstSpringApp application, it prints
Welcome to j2eereference.com
satyansu says
Nice work indeed!
Satyansu