Spring - Beans Autowiring By Constructor

This mode is very similar to byType, but it applies to constructor arguments. Spring container looks at the beans on which autowire attribute is set constructor in the XML configuration file. It then tries to match and wire its constructor's argument with exactly one of the beans name in the configuration file. If matches are found, it will inject those beans. Otherwise, bean(s) will not be wired.

For example, if a bean definition is set to autowire by constructor in configuration file, and it has a constructor with one of the arguments of SpellChecker type, Spring looks for a bean definition named SpellChecker, and uses it to set the constructor's argument. Still you can wire remaining arguments using <constructor-arg> tags. The Following example will illustrate the concept.

Example - Autowiring Beans By Constructor

Let us have a working Eclipse IDE in place and take the following steps to create a Spring application −

TextEditor.java

package com.tutorialspoint;

public class TextEditor {
   private SpellChecker spellChecker;
   private String name;

   public TextEditor( SpellChecker spellChecker ) {
      this.spellChecker = spellChecker;
   }
   public SpellChecker getSpellChecker() {
      return spellChecker;
   }
   public String getName() {
      return name;
   }
   public void setName(String name) {
      this.name = name;
   }
   public void spellCheck() {
      spellChecker.checkSpelling();
   }
}

SpellChecker.java

package com.tutorialspoint;

public class SpellChecker {
   public SpellChecker() {
      System.out.println("Inside SpellChecker constructor." );
   }
   public void checkSpelling() {
      System.out.println("Inside checkSpelling." );
   }
}

MainApp.java

package com.tutorialspoint;

import org.springframework.context.ApplicationContext;
import org.springframework.context.support.ClassPathXmlApplicationContext;

public class MainApp {
   public static void main(String[] args) {
      ApplicationContext context = new ClassPathXmlApplicationContext("Beans.xml");
      TextEditor te = (TextEditor) context.getBean("textEditor");
      te.spellCheck();
   }
}

Beans.xml

Following is the configuration file Beans.xml in normal condition

<?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">
      <property name = "spellChecker" ref = "spellChecker" />
      <property name = "name" value = "Generic Text Editor" />
   </bean>

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

</beans>

Beans.xml

But if you are going to use autowiring by constructor, then your XML configuration file will become as follows −

<?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" autowire = "constructor">
      <property name = "name" value = "Generic Text Editor" />
   </bean>

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

</beans>

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

Inside SpellChecker constructor.
Inside checkSpelling.