Inject into private, package or public field or provide a setter?

Question

I see many Java examples using dependency injection with private fields without a public setter like this:

public SomeClass {
  @Inject
  private SomeResource resource;
}

But that is a bad idea when the injection should be performed manually for example in unit tests.

There are several possibilities to solve this:

• add a public setter: setSomeResource(SomeResource r)
• make the field public
• make the field package protected

I'd like to avoid the setter, since nothing really happens in it. So I'd prefer public or package protected. What do you recommend?

Answer 1

One way to avoid creating a setter for the field is using constructor injection. This even allows you to declare the field as final.

It goes like this:

public class SomeClass {
    private final SomeResource resource;

    @Inject
    public SomeClass(SomeResource resource) {
        this.resource = resource;
    }
}