Refactor RequestHandler to handle request component construction

This refactors RequestHandler so that it handles the construction of the request
component itself, rather than being handed a pre-built request component
instance constructed by the invoking servlet.

The motivation for this change is so that RequestHandler can be extended in
future CLs to compute authentication results, and can provide those results as
an available binding in the constructed request component.  An alternative
approach could have been to compute the authentication results within
RequestModule itself, but I think it's clearer to keep business logic like
that outside of Dagger providers.

This CL makes the following individual changes:

- Adds request component builders, which implement a RequestComponentBuilder
  interface so they can all be manipulated by RequestHandler

- Instead of obtaining request components via factory methods on the global
  components, one now can have global-scoped bindings just inject the request
  component builders (which requires adding a module to each global component
  declaring the subcomponent).  This follows the recommended approach here:
  http://google.github.io/dagger/subcomponents.html

- Instead of exposing request components on the global component interface,
  we now expose module-specific subclasses of RequestHandler that @Inject the
  appropriate request component builder's provider and pass it to the superclass
  (note that inheritance isn't strictly necessary here but saves boilerplate)

- RequestHandler now takes the Provider<RequestComponentBuilder> and builds
  the component itself using its own fresh RequestModule instance. This provides
  some nice encapsulation but is mainly needed for adding a RequestAuthModule
  in future work.

- RequestHandler also takes UserService now, which can be provided via Dagger
  by the subclass.  Longer-term that will go away in favor of instead providing
  AuthStrategy instances, some of which will use UserService internally.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=138815648

java/google/registry/request/RequestHandler.java

@@ -26,17 +26,18 @@ import static javax.servlet.http.HttpServletResponse.SC_MOVED_TEMPORARILY;
 import static javax.servlet.http.HttpServletResponse.SC_NOT_FOUND;
 
 import com.google.appengine.api.users.UserService;
-import com.google.appengine.api.users.UserServiceFactory;
 import com.google.common.base.Optional;
 import google.registry.util.FormattingLogger;
-import google.registry.util.NonFinalForTesting;
+import google.registry.util.TypeUtils.TypeInstantiator;
 import java.io.IOException;
+import javax.annotation.Nullable;
+import javax.inject.Provider;
 import javax.servlet.http.HttpServletRequest;
 import javax.servlet.http.HttpServletResponse;
 import org.joda.time.Duration;
 
 /**
- * Dagger request processor for Nomulus.
+ * Dagger-based request processor.
  *
  * <p>This class creates an HTTP request processor from a Dagger component. It routes requests from
  * your servlet to an {@link Action @Action} annotated handler class.
@@ -64,37 +65,58 @@ import org.joda.time.Duration;
  *
  * <p>This class also enforces the {@link Action#requireLogin() requireLogin} setting.
  *
- * @param <C> component type
+ * @param <C> request component type
+ * @param <B> builder for the request component
  */
-public final class RequestHandler<C> {
+public class RequestHandler<C, B extends RequestComponentBuilder<C, B>> {
 
   private static final FormattingLogger logger = FormattingLogger.getLoggerForCallerClass();
 
   private static final Duration XSRF_VALIDITY = Duration.standardDays(1);
 
-  @NonFinalForTesting
-  private static UserService userService = UserServiceFactory.getUserService();
-
-  /** Creates a new request processor based off your component methods. */
-  public static <C> RequestHandler<C> create(Class<C> component) {
-    return new RequestHandler<>(component, Router.create(component));
-  }
-
   private final Router router;
-
-  private RequestHandler(Class<C> component, Router router) {
-    checkNotNull(component);
-    this.router = router;
-  }
+  private final Provider<B> requestComponentBuilderProvider;
+  private final UserService userService;
 
   /**
-   * Runs the appropriate action for a servlet request.
+   * Constructor for subclasses to create a new request handler for a specific request component.
    *
-   * @param component is an instance of the component type passed to {@link #create(Class)}
+   * <p>This operation will generate a routing map for the component's {@code @Action}-returning
+   * methods using reflection, which is moderately expensive, so a given servlet should construct a
+   * single {@code RequestHandler} and re-use it across requests.
+   *
+   * @param requestComponentBuilderProvider a Dagger {@code Provider} of builder instances that can
+   *     be used to construct new instances of the request component (with the required
+   *     request-derived modules provided by this class)
+   * @param userService an instance of the App Engine UserService API
    */
-  public void handleRequest(HttpServletRequest req, HttpServletResponse rsp, C component)
-      throws IOException {
-    checkNotNull(component);
+  protected RequestHandler(Provider<B> requestComponentBuilderProvider, UserService userService) {
+    this(null, requestComponentBuilderProvider, userService);
+  }
+
+  /** Creates a new RequestHandler with an explicit component class for test purposes. */
+  public static <C, B extends RequestComponentBuilder<C, B>> RequestHandler<C, B> createForTest(
+      Class<C> component, Provider<B> requestComponentBuilderProvider, UserService userService) {
+    return new RequestHandler<>(
+        checkNotNull(component), requestComponentBuilderProvider, userService);
+  }
+
+  private RequestHandler(
+      @Nullable Class<C> component,
+      Provider<B> requestComponentBuilderProvider,
+      UserService userService) {
+    // If the component class isn't explicitly provided, infer it from the class's own typing.
+    // This is safe only for use by subclasses of RequestHandler where the generic parameter is
+    // preserved at runtime, so only expose that option via the protected constructor.
+    this.router = Router.create(
+        component != null ? component : new TypeInstantiator<C>(getClass()){}.getExactType());
+    this.requestComponentBuilderProvider = checkNotNull(requestComponentBuilderProvider);
+    this.userService = checkNotNull(userService);
+  }
+
+  /** Runs the appropriate action for a servlet request. */
+  public void handleRequest(HttpServletRequest req, HttpServletResponse rsp) throws IOException {
+    checkNotNull(req);
     checkNotNull(rsp);
     Action.Method method;
     try {
@@ -130,6 +152,11 @@ public final class RequestHandler<C> {
       rsp.sendError(SC_FORBIDDEN, "Invalid " + X_CSRF_TOKEN);
       return;
     }
+    // Build a new request component using any modules we've constructed by this point.
+    C component = requestComponentBuilderProvider.get()
+        .requestModule(new RequestModule(req, rsp))
+        .build();
+    // Apply the selected Route to the component to produce an Action instance, and run it.
     try {
       route.get().instantiator().apply(component).run();
       if (route.get().action().automaticallyPrintOk()) {