Guice and RequestScoped behaviour in multiple threads

Question

I am using Guice\'s RequestScoped and Provider in order to get instances of some classes during a user request. This works fine currently. Now I want to do some job in a bac

Answer 1

I recently solved this exact problem. There are a few things you can do. First, read up on ServletScopes.continueRequest(), which wraps a callable so it will execute as if it is within the current request. However, that's not a complete solution because it won't forward @RequestScoped objects, only basic things like the HttpServletResponse. That's because @RequestScoped objects are not expected to be thread safe. You have some options:

• If your entire @RequestScoped hierarchy is computable from just the HTTP response, you're done! You will get new instances of these objects in the other thread though.

• You can use the code snippet below to explicitly forward all RequestScoped objects, with the caveat that they will all be eagerly instantiated.

• Some of my @RequestScoped objects couldn't handle being eagerly instantiated because they only work for certain requests. I extended the below solution with my own scope, @ThreadSafeRequestScoped, and only forwarded those ones.

Code sample:

public class RequestScopePropagator {
    private final Map, Provider> requestScopedValues = new HashMap<>();

    @Inject
    RequestScopePropagator(Injector injector) {
        for (Map.Entry, Binding> entry : injector.getAllBindings().entrySet()) {
            Key key = entry.getKey();
            Binding binding = entry.getValue();
            // This is like Scopes.isSingleton() but we don't have to follow linked bindings
            if (binding.acceptScopingVisitor(IS_REQUEST_SCOPED)) {
                requestScopedValues.put(key, binding.getProvider());
            }
        }
    }

    private final BindingScopingVisitor IS_REQUEST_SCOPED = new BindingScopingVisitor() {
        @Override
        public Boolean visitScopeAnnotation(Class scopeAnnotation) {
            return scopeAnnotation == RequestScoped.class;
        }

        @Override
        public Boolean visitScope(Scope scope) {
            return scope == ServletScopes.REQUEST;
        }

        @Override
        public Boolean visitNoScoping() {
            return false;
        }

        @Override
        public Boolean visitEagerSingleton() {
            return false;
        }
    };

    public  Callable continueRequest(Callable callable) {
        Map, Object> seedMap = new HashMap<>();
        for (Map.Entry, Provider> entry : requestScopedValues.entrySet()) {
            // This instantiates objects eagerly
            seedMap.put(entry.getKey(), entry.getValue().get());
        }

        return ServletScopes.continueRequest(callable, seedMap);
    }
}