CODEWITHSUNDEEP
javadependency-injectionguice
Roman
Roman

Reputation: 66166

What's the correct way of setting properties with Guice?

Suppose, we have 2 classes - Main and MainDependency. The second class is used only by Main, and the purpose of using IoC is constructing an instance of Main class.

MainDependency class has a field of integer type. This field is not required to be set (or, let's assume it should always have a default value if nothing else is specified).

The problem: what's the most correct way to set the integer field? One way I see is creating similar field inside my Module and then using that value inside configure module. But I feel it's a wrong way.

Please, share your experience. Thx in advance.

Upvotes: 1

Views: 1243

Answers (1)

rodion
rodion

Reputation: 15029

I think you mainly have two options:

1) Inject it using constant binding. The value of MY_CONSTANT could be passed to Module at instantiation time; could be taken from a system property, or maybe some other way.

class MainDependency{
  @Inject
  public MainDependency(@Named("myConst") int myConst){
    //...
  }
}

class Module extends AbstractModule{
  public void configure(){
    bindConstant().annotatedWith(Names.named("myConst").to(MY_CONSTANT);
  }
}

2) Use assisted inject to create a factory which will take your value as a parameter and return an instance of MainDependency:

interface MainDependencyFactory{
  MainDependency create(int myConst);
}

class MainDependency{
  @Inject
  public MainDependency(@Assisted int myConst){
    //..
  }
}

class Module extends AbstractModule{
  public void configure(){
    bind(MainDependencyFactory.class).toProvider(
      FactoryProvider.newFactory(MainDependencyFactory.class, MainDependency.class));
  }
}

//to use the above, instantiate your factory (or inject it somewhere)
MainDependencyFactory f = injector.getInstance(MainDependencyFactory.class);
//Then you can create MainDependency with any value
MainDependency md = f.create(MY_CONSTANT);

Note that with assisted inject you don't need to implement MainDependencyFactory. Guice will create it for you.

Upvotes: 2

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