Spring DI - Autowiring 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.

Example

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

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 application to run and test.

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("applicationcontext.xml");
      TextEditor te = (TextEditor) context.getBean("textEditor");
      te.spellCheck();
   }
}

Following is the configuration file applicationcontext.xml which has configuration for autowiring byName

<?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">
      <constructor-arg name = "name" value = "Generic Text Editor" />
   </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 application. If everything is fine with your application, it will print the following message −

Inside SpellChecker constructor.
Inside checkSpelling.
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