Question

I’m just getting acquainted with implementing REST web services in Java using JAX-RS and I ran into the following problem. One of my resource classes requires access to a storage backend, which is abstracted away behind a StorageEngine interface. I would like to inject the current StorageEngine instance into the resource class serving the REST requests and I thought a nice way of doing this would be by using the @Context annotation and an appropriate ContextResolver class. This is what I have so far:

In MyResource.java:

class MyResource {
    @Context StorageEngine storage;
    [...]
}

In StorageEngineProvider.java:

@Provider
class StorageEngineProvider implements ContextResolver<StorageEngine> {
    private StorageEngine storage = new InMemoryStorageEngine();

    public StorageEngine getContext(Class<?> type) {
        if (type.equals(StorageEngine.class))
            return storage;
        return null;
    }
}

I’m using com.sun.jersey.api.core.PackagesResourceConfig to discover the providers and the resource classes automatically, and according to the logs, it picks up the StorageEngineProvider class nicely (timestamps and unnecessary stuff left out intentionally):

INFO: Root resource classes found:
    class MyResource
INFO: Provider classes found:
    class StorageEngineProvider

However, the value of storage in my resource class is always null – neither the constructor of StorageEngineProvider nor its getContext method is called by Jersey, ever. What am I doing wrong here?

Answer 1

I don’t think there’s a JAX-RS specific way to do what you want. The closest would be to do:

@Path("/something/")
class MyResource {
    @Context
    javax.ws.rs.ext.Providers providers;

    @GET
    public Response get() {
        ContextResolver<StorageEngine> resolver = providers.getContextResolver(StorageEngine.class, MediaType.WILDCARD_TYPE);
        StorageEngine engine = resolver.get(StorageEngine.class);
        ...
    }
}

However, I think the @javax.ws.rs.core.Context annotation and javax.ws.rs.ext.ContextResolver is really for types related to JAX-RS and supporting JAX-RS providers.

You may want to look for Java Context and Dependency Injection (JSR-299) implementations (which should be available in Java EE 6) or other dependency injection frameworks such as Google Guice to help you here.