Inject custom logic in Guice binding process

Question

Is there a way to implement kind of before and after hook in Guice binding? E.g. before Guice calling a constructor to get the instance to be injected into a method, can I provide a logic check if the instance is already existed somewhere, if I can find the instance then I return it directly without calling the constructor; On the flip side is once an instance is constructed in Guice binding process, can I inject the logic to process that instance before it returned to the original caller?

Answer 1

Using a custom Typelistener should do the trick. From what I understand your problem is similar to the "postConstruct" problem, executing code of an instance while guice is creating it. Maybe this (german) blog article wrote a while ago pushes you in the right direction.

1. Using a Matcher to define on which instances the listener should react.

2. using the afterInjection hook to work with the instance

   @Override public void configure(final Binder binder) { binder.bindListener(Matchers.any(), this); }

   @Override public void hear(final TypeLiteral type, final TypeEncounter encounter) { encounter.register(new InjectionListener() {

       @Override
       public void afterInjection(final I injectee) {
           // alle postconstruct Methoden (nie null) ausführen.
           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);
               }
           }
       }
   });
   

   }