Guice injection null pointer

Question

We try to refactoring a project with Guice. The idea is to bind all the Language interface to a concreate object like French or Polish .

We have a module for binding:

public class StandardModule extends AbstractModule {

    @Override
    protected void configure() {

       bind(Language.class).to(Polish.class);

    }
 }

And a classe (AboutDialog.java) that use this injected object :

@Inject Language language;

public AboutDialog(JFrame parent) {
    super(parent, "", true);
    this.language=language;
    this.setTitle(language.getLanguageInUse().getString("AboutDialog.title"));
    this.parent = parent;
    try {
        jbInit();
    } catch (Exception e) {
        e.printStackTrace();
    }
    pack();
}

And we have as result:

java.lang.NullPointerException at net.sf.jmoney.gui.AboutDialog.<init>(AboutDialog.java:67)

Line 67 is:

this.setTitle(language.getLanguageInUse().getString("AboutDialog.title"));

Our interface is:

public interface Language {

    public ResourceBundle getLanguageInUse();
}

And the Polish class is:

public class Polish implements Language {

    private ResourceBundle languageInUse;

    public Polish() {
        languageInUse = ResourceBundle.getBundle(Constants.LANGUAGE_PL);
    }

    public ResourceBundle getLanguageInUse() {
        return languageInUse;
    }


}

We are lost...

Answer 1

You're using "field injection". This will make it hard to use your injected values in a constructor; even if Guice were to create the object (which is not happening now) or you were to use injector.injectMembers(aboutDialog) , the constructor would run before the injector has a chance to inject the field you want.

It's a little more tricky to create a class that takes a varying parameter as well as an injected parameter. This leaves you with a few options:

• Inject the JFrame. If you know what JFrame you're going to use when the constructor is being created, then just use bind(JFrame.class).toInstance(myJFrame); in your Module. Then Guice can create the AboutDialog entirely.

• Create a Factory manually. That way you can inject AboutDialog.Factory and just call create to get your AboutDialog . It'll look something like this:

   public class AboutDialog extends JDialog { /** Injectable factory. */ public static class Factory { @Inject private Language language; public AboutDialog create(JFrame parent) { return new AboutDialog(parent, language); } } // no @Inject parameter; you're calling "new" yourself above! public AboutDialog(JFrame parent, Language language) { super(parent, "", true); this.language = language; // ... other initialization } } 

• Create a Factory and let Guice wire it up for you via assisted injection .

   public class AboutDialog extends JDialog { public interface Factory { public AboutDialog create(JFrame parent); } // you need the @Inject, and also the @Assisted to tell Guice to // use the parameter instead of Guice bindings @Inject public AboutDialog(@Assisted JFrame parent, Language language) { super(parent, "", true); this.language = language; // ... other initialization } } public class StandardModule extends AbstractModule { @Override protected void configure() { bind(Language.class).to(Polish.class); // here every method in AboutDialog.Factory will be implemented // to create the method's return type [AboutDialog] based on // the parameters (like JFrame) and the bindings (like Language) install(new FactoryModuleBuilder().build(AboutDialog.Factory.class)); } } 

As noted in the question comments, make sure you're getting your AboutDialog (or AboutDialog.Factory via an @Inject ed constructor/field or from the Injector itself, or else Guice will not know to inject the parameters.

Answer 2

I assume that your are not creating your AboutDialog with the help of Guice.

What you could do is use injector.injectMembers(this) where this is the AboutDialog .

The best way would be that the AboutDialog will be created by Guice, so all members will be injected.