Initialized static variable becomes null

Question

I have a simple class like so:

class MyClass {
  private static SomeClass object = null;

  public void init(Someclass injectedObject) {
    object = injectedObject;
  }
}

I init this class in my Guice module:

public class MyModule extends AbstractModule {

  @Provides @Singleton
  public SomeClass getStuff() {
    SomeClass injectObject = new SomeClass();
    MyClass.init(injectObject);
  }

}

But when I package the my jar and submit it as a job to Flink runtime, I get an error saying NullPointerException: object in MyClass in null

I have even tried adding this to my method:

public static void main(String[] args) {
  // Guice stuff
}

public void doStuff() {
  MyClass.init(injectedObject);
}

Am I missing something? Why is an initialized variable getting reset to null in Flink runtime?

Answer 1

There's a better way to inject into static fields using Guice:

public class MyModule extends AbstractModule {
  @Override
  protected void configure() {
    bind(SomeClass.class).in(Scopes.SINGLETON);
    requestStaticInjection(MyClass.class);
  }
}

class MyClass {
  @Inject private static SomeClass object;
  // no init method needed
}

However, it would be even better if you can omit the static field altogether:

public class MyModule extends AbstractModule {
  @Override
  protected void configure() {
    bind(SomeClass.class).in(Scopes.SINGLETON);
    bind(MyClass.class);
  }
}

class MyClass {
  private final SomeClass object;
  
  @Inject 
  MyClass(SomeClass object) {
    this.object = object;
  }
}

Now you can request MyClass instances either by injecting them or requesting them directly from the injector, and they will all have access to the same SomeClass instance.