Spring Autowiring by Constructor

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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.

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

Steps	Description
1	Create a project with a name SpringExample and create a package com.tutorialspoint under the src folder in the created project.
2	Add required Spring libraries using Add External JARs option as explained in the Spring Hello World Example chapter.
3	Create Java classes TextEditor, SpellChecker and MainApp under the com.tutorialspoint package.
4	Create Beans configuration file Beans.xml under the src folder.
5	The final step is to create the content of all the Java files and Bean Configuration file and run the application as explained below.

Here is the content of TextEditor.java file −

package com.tutorialspoint;

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

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

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

package com.tutorialspoint;

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

Following is the content of the MainApp.java file −

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();
   }
}

Following is the configuration file Beans.xml in normal condition −

<?xml version = "1.0" encoding = "UTF-8"?>

<beans xmlns = "https://www.springframework.org/schema/beans"
   xmlns:xsi = "https://www.w3.org/2001/XMLSchema-instance"
   xsi:schemaLocation = "https://www.springframework.org/schema/beans
   https://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" />
      <constructor-arg  value = "Generic Text Editor"/>
   </bean>

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

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 = "https://www.springframework.org/schema/beans"
   xmlns:xsi = "https://www.w3.org/2001/XMLSchema-instance"
   xsi:schemaLocation = "https://www.springframework.org/schema/beans
   https://www.springframework.org/schema/beans/spring-beans-3.0.xsd">

   <!-- Definition for textEditor bean -->
   <bean id = "textEditor" class = "com.tutorialspoint.TextEditor" 
      autowire = "constructor">
      <constructor-arg 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.