Using @Context, @Provider and ContextResolver in JAX-RS

java, jax-rs, jersey

Solution

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.

Problem

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?

Original source