Does method injection with Guice work if constructor is not injected?

Question

I have a class where I need to inject a variable but the constructor is not guice injected.

public class Foo{
  private boolean x;
  public Foo(){
     //sets some variables
  }

  @Inject
  public void setX(boolean x){
     this.x=x;
  }
}

The value of x is not getting injected. Is it necessary to use injection in constructor for guice to recognize method injection? Do I need to call this method from some other class for this to work? Thanx in advance

Answer 1

If you're going to use Guice to @Inject Foo then Guice needs to create the object - either from the injector or from another @Inject point in separate class. Guice can't help you inject objects that it didn't create.

In this case it's not necessary to add @Inject to the constructor because Guice can automatically inject a no argument constructor. For example:

public class MethodInjectionTest {

  static class Foo {
    private boolean x;
    public Foo(){
      //sets some variables
    }

    @Inject
    public void setX(boolean x){
      this.x=x;
    }
  }

  Injector i = Guice.createInjector(new AbstractModule() {
    protected void configure() {
      bind(Boolean.class).toInstance(Boolean.TRUE);
    }
  });

  @Test
  public void methodInjection() {
    Foo foo = i.getInstance(Foo.class);
    assertThat(foo.x, is(true));
  }

  // EDIT: An example based on the comment by @JeffBowman
  @Test
  public void memberInjection() {
    Foo foo = new Foo();
    i.injectMembers(foo);
    assertThat(foo.x, is(true));
  }

}

If your real world class has a need to mix Guice controlled dependencies with client provided dependencies during construction have a look at assisted inject