Spring DI - Injecting Inner Beans Constructor

As you know Java inner classes are defined within the scope of other classes, similarly, inner beans are beans that are defined within the scope of another bean. Thus, a <bean/> element inside the <property/> or <constructor-arg/> elements is called inner bean and it is shown below.

<?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-3.0.xsd">

   <bean id = "outerBean" class = "...">
      <constructor-arg name = "target">
         <bean id = "innerBean" class = "..."/>
      </constructor-arg>
   </bean>

</beans>

Example

The following example shows a class TextEditor that can only be dependency-injected using constructor-based injection.

Let s update the project created in Spring DI - Create Project chapter. We re adding following files −

TextEditor.java − A class containing a SpellChecker as dependency.

SpellChecker.java − A dependency class.

MainApp.java − Main apppcation to run and test.

Here is the content of TextEditor.java file −

package com.tutorialspoint;
pubpc class TextEditor {
   private SpellChecker spellChecker;
   
   pubpc TextEditor(SpellChecker spellChecker) {
      System.out.println("Inside TextEditor constructor." );
      this.spellChecker = spellChecker;
   }
   // a getter method to return spellChecker
   pubpc SpellChecker getSpellChecker() {
      return spellChecker;
   }
   pubpc void spellCheck() {
      spellChecker.checkSpelpng();
   }
}

Following is the content of another dependent class file SpellChecker.java −

package com.tutorialspoint;
pubpc class SpellChecker {
   pubpc SpellChecker(){
      System.out.println("Inside SpellChecker constructor." );
   }
   pubpc void checkSpelpng(){
      System.out.println("Inside checkSpelpng." );
   }
}

Following is the content of the MainApp.java file −

package com.tutorialspoint;

import org.springframework.context.ApppcationContext;
import org.springframework.context.support.ClassPathXmlApppcationContext;

pubpc class MainApp {
   pubpc static void main(String[] args) {
      ApppcationContext context = new ClassPathXmlApppcationContext("apppcationcontext.xml");
      TextEditor te = (TextEditor) context.getBean("textEditor");
      te.spellCheck();
   }
}

Following is the configuration file apppcationcontext.xml which has configuration for the setter-based injection but using inner beans −

<?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-3.0.xsd">

   <!-- Definition for textEditor bean using inner bean -->
   <bean id = "textEditor" class = "com.tutorialspoint.TextEditor">
      <constructor-arg name = "spellChecker">
         <bean id = "spellChecker" class = "com.tutorialspoint.SpellChecker"/>
      </constructor-arg>
   </bean>

</beans>

Output

Once you are done creating the source and bean configuration files, let us run the apppcation. If everything is fine with your apppcation, it will print the following message −

Inside SpellChecker constructor.
Inside TextEditor constructor.
Inside checkSpelpng.