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Friday, 6 July 2018

Google Guice - Constructor Bindings



Google Guice - Constructor Bindings

Guice provides a way to create bindings with specific constructor of an object using toConstructor() method.
@Override
protected void configure() {
   try {
      bind(SpellChecker.class)
         .toConstructor(SpellCheckerImpl.class.getConstructor(String.class));
   } catch (NoSuchMethodException | SecurityException e) {
      System.out.println("Required constructor missing");
   } 
} 
See the complete example below.

Complete Example

Create a java class named GuiceTester.
GuiceTester.java
import com.google.inject.AbstractModule;
import com.google.inject.Guice;
import com.google.inject.Inject;
import com.google.inject.Injector;
import com.google.inject.name.Named;
import com.google.inject.name.Names;

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() {
      try {
         bind(SpellChecker.class)
            .toConstructor(SpellCheckerImpl.class.getConstructor(String.class));
      } catch (NoSuchMethodException | SecurityException e) {
         System.out.println("Required constructor missing");
      } 
      bind(String.class)
         .annotatedWith(Names.named("JDBC"))
         .toInstance("jdbc:mysql://localhost:5326/emp");
   } 
}

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

//spell checker implementation
class SpellCheckerImpl implements SpellChecker {
   private String dbUrl;
   public SpellCheckerImpl(){}

   public SpellCheckerImpl(@Named("JDBC") String dbUrl) {
      this.dbUrl = dbUrl;
   } 
   @Override
   public void checkSpelling() { 
      System.out.println("Inside checkSpelling." );
      System.out.println(dbUrl); 
   }
}

Output

Compile and run the file, you will see the following output.
Inside checkSpelling.
jdbc:mysql://localhost:5326/emp

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