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Friday, 6 July 2018

Google Guice - Binding Annotations



Google Guice - Binding Annotations

As we can bind a type with its implementation. In case we want to map a type with multiple implementations, 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 applicability of annotation.
  • @Retention − Marks availability of annotation as runtime.

Mapping using binding annotation

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

Inject using binding annotation

@Inject
public 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.RetentionPolicy.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 {}

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(@WinWord SpellChecker spellChecker) {
      this.spellChecker = spellChecker;
   }
   public void makeSpellCheck() {
      spellChecker.checkSpelling(); 
   } 
}

//Binding Module
class TextEditorModule extends AbstractModule {
   @Override
   
   protected void configure() {
      bind(SpellChecker.class).annotatedWith(WinWord.class)
         .to(WinWordSpellCheckerImpl.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." );
   } 
}

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

Output

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

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