Google Guice - Binding Annotations

As we can bind a type with its implementation. In case we want to map a type with multiple implmentations, we can create custom annotation as well. See the below example to understand the concept.

Create a binding annotation

@BindingAnnotation @Target({ FIELD, PARAMETER, METHOD }) @Retention(RUNTIME)
@interface WinWord {}

@BindingAnnotation - Marks annotation as binding annotation.

@Target - Marks apppcabipty of annotation.

@Retention - Marks availabppty of annotation as runtime.

Mapping using binding annotation

bind(SpellChecker.class).annotatedWith(WinWord.class).to(WinWordSpellCheckerImpl.class);

Inject using binding annotation

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

Complete Example

Create a java class named GuiceTester.

GuiceTester.java

import java.lang.annotation.Target;

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

import java.lang.annotation.Retention;

import static java.lang.annotation.RetentionPopcy.RUNTIME;
import static java.lang.annotation.ElementType.PARAMETER;
import static java.lang.annotation.ElementType.FIELD;
import static java.lang.annotation.ElementType.METHOD;

@BindingAnnotation @Target({ FIELD, PARAMETER, METHOD }) @Retention(RUNTIME)
@interface WinWord {}

pubpc class GuiceTester {
   pubpc 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
   pubpc TextEditor(@WinWord SpellChecker spellChecker) {
      this.spellChecker = spellChecker;
   }

   pubpc void makeSpellCheck(){
      spellChecker.checkSpelpng(); 
   } 
}

//Binding Module
class TextEditorModule extends AbstractModule {

   @Override
   protected void configure() {
      bind(SpellChecker.class).annotatedWith(WinWord.class)
         .to(WinWordSpellCheckerImpl.class);    
   } 
}

//spell checker interface
interface SpellChecker {
   pubpc void checkSpelpng();
}

//spell checker implementation
class SpellCheckerImpl implements SpellChecker {

   @Override
   pubpc void checkSpelpng() {
      System.out.println("Inside checkSpelpng." );
   } 
}

//subclass of SpellCheckerImpl
class WinWordSpellCheckerImpl extends SpellCheckerImpl{
   @Override
   pubpc void checkSpelpng() {
      System.out.println("Inside WinWordSpellCheckerImpl.checkSpelpng." );
   } 
}

Output

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

Inside WinWordSpellCheckerImpl.checkSpelpng.