Guice - Constructor Injection

Introduction

Injection is a process of injecting dependency into an object. Constructor injection is quite common. In this process, dependency is injected as argument to the constructor. See the example below −

class TextEditor {
   private SpellChecker spellChecker;

   @Inject
   public TextEditor(SpellChecker spellChecker) {
      this.spellChecker = spellChecker;
   }
   ...
}

Example - Usage of Constructor Injection

GuiceTester.java

package com.tutorialspoint;

import com.google.inject.AbstractModule;
import com.google.inject.Guice;
import com.google.inject.Inject;
import com.google.inject.Injector;

public class GuiceTester {
   public static void main(String[] args) {
      Injector injector = Guice.createInjector(new TextEditorModule());
      TextEditor editor = injector.getInstance(TextEditor.class);
      editor.makeSpellCheck(); 
   } 
}

class TextEditor {
   private SpellChecker spellChecker;

   @Inject
   public TextEditor(SpellChecker spellChecker) {
      this.spellChecker = spellChecker;
   }

   public void makeSpellCheck(){
      spellChecker.checkSpelling();
   }
}

//Binding Module
class TextEditorModule extends AbstractModule {

   @Override
   protected void configure() {
      bind(SpellChecker.class).to(SpellCheckerImpl.class);
   } 
}

//spell checker interface
interface SpellChecker {
   public void checkSpelling();
}


//spell checker implementation
class SpellCheckerImpl implements SpellChecker {

   @Override
   public void checkSpelling() {
      System.out.println("Inside checkSpelling." );
   } 
}

Output

Compile and run the file, you will see the following output −

Inside checkSpelling.