Using @Context, @Provider and ContextResolver in JAX-RS
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 resou开发者_StackOverflow社区rce 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?
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.
Implement a InjectableProvider. Most likely by extending PerRequestTypeInjectableProvider or SingletonTypeInjectableProvider.
@Provider
public class StorageEngineResolver extends SingletonTypeInjectableProvider<Context, StorageEngine>{
public MyContextResolver() {
super(StorageEngine.class, new InMemoryStorageEngine());
}
}
Would let you have:
@Context StorageEngine storage;
I found another way. In my case i want to provide the user currently logged in as a User entity from my persitence layer. This is the class:
@RequestScoped
@Provider
public class CurrentUserProducer implements Serializable, ContextResolver<User> {
/**
* Default
*/
private static final long serialVersionUID = 1L;
@Context
private SecurityContext secContext;
@Inject
private UserUtil userUtil;
/**
* Tries to find logged in user in user db (by name) and returns it. If not
* found a new user with role {@link UserRole#USER} is created.
*
* @return found user or a new user with role user
*/
@Produces
@CurrentUser
public User getCurrentUser() {
if (secContext == null) {
throw new IllegalStateException("Can't inject security context - security context is null.");
}
return userUtil.getCreateUser(secContext.getUserPrincipal().getName(),
secContext.isUserInRole(UserRole.ADMIN.name()));
}
@Override
public User getContext(Class<?> type) {
if (type.equals(User.class)) {
return getCurrentUser();
}
return null;
}
}
I only used implements ContextResolver<User>
and @Provider
to get this class discovered by Jax-Rs and get SecurityContext
injected.
To get the current user i use CDI with my Qualifier @CurrentUser
. So on every place where i need the current user i type:
@Inject
@CurrentUser
private User user;
And indeed
@Context
private User user;
does not work (user is null).
If anyone is using Resteasy this is what worked for me.
If you add something like this:
ResteasyContext.pushContext(StorageEngine.class, new StorageEngine());
into something like a jaxrs filter, it allows you to do something like this:
@GET
@Path("/some/path")
public Response someMethod(@Context StorageEngine myStorageEngine) {
...
}
This is specific to Resteasy, which doesn't have something like SingletonTypeInjectableProvider
.
A pattern that works for me: Add some fields on your Application subclass that provide the objects you need to inject. Then use an abstract base class to do the "injection":
public abstract class ServiceBase {
protected Database database;
@Context
public void setApplication(Application app) {
YourApplication application = (YourApplication) app;
database = application.getDatabase();
}
}
All your services that need to access the database may now extend ServiceBase and have the database available automatically via the protected field (or a getter, if you prefer that).
This works for me with Undertow and Resteasy. In theory this should work across all JAX-RS implementations since injection of the Application is supported by the standard AFAICS, but I haven't tested it in other settings.
For me, the advantage over Bryant's solution was that I don't have to write some resolver class just so I can get at my application-scoped singletons like the database.
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