Spring DI - Constructor-Based

Constructor-Based DI is accomppshed when the container invokes a class constructor with a number of arguments, each representing a dependency on the other class.

Example

The following example shows a class TextEditor that can only be dependency-injected with constructor 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;
   }
   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 constructor-based injection −

<?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 -->
   <bean id = "textEditor" class = "com.tutorialspoint.TextEditor">
      <constructor-arg ref = "spellChecker"/>
   </bean>

   <!-- Definition for spellChecker bean -->
   <bean id = "spellChecker" class = "com.tutorialspoint.SpellChecker"></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.