Post creation initialization of guice singleton

Question

Is there a way to have guice call a init() method after it has instantiated a singleton? Calling init() inside the constructor is not an option since init() could be overriden by a subclass.

Answer 1

You can annotate a method in your module with @Inject and then request injection on the module:

class MyModule extends AbstractModule {
  @Override public void configure() {
    requestInjection(this);
  }

  @Inject void initMyClass(MyClass instance) {
     instance.init();
  }
}

See also https://stackoverflow.com/a/24480630/3788176.

Answer 2

You can use `@PostConstruct' in guice when you use the mycila/jsr250 extension. This will cause your init() method to be called right after instantiation.

@PostConstruct
void init() {
     // ...
}

If you do not can/want to add 3rd party libs for this, I wrote a simple postconstruct module for this as well a while ago:

public enum PostConstructModule implements Module, TypeListener {

INSTANCE;

@Override
public void configure(final Binder binder) {
    // all instantiations will run through this typeListener
    binder.bindListener(Matchers.any(), this);
}

/**
 * Call postconstruct method (if annotation exists).
 */
@Override
public <I> void hear(final TypeLiteral<I> type, final TypeEncounter<I> encounter) {
    encounter.register(new InjectionListener<I>() {

        @Override
        public void afterInjection(final I injectee) {
            for (final Method postConstructMethod : filter(asList(injectee.getClass().getMethods()), MethodPredicate.VALID_POSTCONSTRUCT)) {
                try {
                    postConstructMethod.invoke(injectee);
                } catch (final Exception e) {
                    throw new RuntimeException(format("@PostConstruct %s", postConstructMethod), e);
                }
            }
        }
    });
   }
  }