Overview
Spring bean injection is also popularly known as ‘Dependency Injection (DI)’. The heart of Spring framework is ‘Inversion Of Control (IoC)’ and DI is the implementation part of IoC. We will start our discussion with the assumption that the concept and purpose of IoC and DI is already know.
In this article, we will focus on different types of Spring bean injections with some practical coding examples.
Dependency Injection
Spring bean injections can be performed in two major ways, one is known as Constructor-based DI and the other one is known as Setter-based DI. In the following sub-sections we will check how these can be achieved.
Constructor Based DI
In a constructor based DI, dependencies are injected through constructors. Each argument of the constructor is a dependency object/bean. The Spring container invokes the constructors with the arguments.
Let us check one example. There are two classes, one is the Country class and the other one is the State class. The Country class has a dependency on the State class. So, we will pass the dependency to the Country class through its constructor (as shown below).
Listing 1: Country class
package com.techalpine.spring;
public class Country {
private State state;
public Country(State state) {
System.out.println(“Inside Country constructor.” );
this.state = state;
}
public void stateName() {
state.stateName();
}
}
Now, this dependency can be used to call different methods.
Following is the State class code snippets.
Listing 2: State class
package com.techalpine.spring;
public class State {
public State(){
System.out.println(“Inside State constructor.” );
}
public void stateName() {
System.out.println(“Inside State name: West Bengal”);
}
}
Following is the XML configuration file to wire those beans and their dependencies. The bean id (‘stateBean’) of the State class is used as a constructor argument reference in the Country class bean definition. Now, the beans are defined and wired together.
Listing 3: XML configuration file
<?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-3.2.xsd”>
<!– Definition for Country bean –>
<bean id=”countryBean” class=”com.techalpine.spring.Country”>
<constructor-arg ref=”stateBean”/>
</bean>
<!– Definition for State bean –>
<bean id=”stateBean” class=”com.techalpine.spring.State”>
</bean>
</beans>
Following is the main class to test the constructor-based bean injection.
Listing 4: Main application file
package com.techalpine.spring;
import org.springframework.context.ApplicationContext;
import org.springframework.context.support.FileSystemXmlApplicationContext;
public class TestBeanInjection {
public static void main(String[] args) {
ApplicationContext context = new FileSystemXmlApplicationContext(
“WebContent/WEB-INF/beanConfig.xml”);
Country countryobj = (Country)context.getBean(“countryBean”);
countryobj.stateName();
}
}
Now, run the class as a stand-alone application and it will show the following output on the console.
Inside State constructor
Inside Country constructor
Inside State name: West Bengal
Setter Bases DI
In a Setter based DI, dependencies are passed through setter methods. Spring container invokes those setter methods to instantiate the beans. Here, the mechanism and the components are almost similar to the constructor based DI. The only difference is, in the passing of dependencies and the XML configuration. In a setter based injection, <property> tags are used instead of <constructor-arg> tag.
Conclusion
In this article we have explained the ways how spring beans can be injected. There is no clear-cut rule when to use setter based or constructor based DI. We generally go for constructor based DI when the dependencies are mandatory, and select setter-based DI when the dependencies are optional. But combinations of both are also allowed.
