Automatically register plugin bindings

If a plugin has no modules declared in the manifest, automatically
generate the modules for the plugin based on the class files that
appear in the plugin and the @Export annotations that appear on these
concrete classes.

For any non-abstract command that extends SshCommand (or really the
internal MINA SSHD Command interface that Gerrit uses), plugins may
declare the command with @Export("name") to bind the implementation
as that SSH command, e.g.:

  @Export("print-hello")
  class Hello extend SshCommand {

Likewise HTTP servlets can also be bound to URLs, but this only works
for standard servlet mappings like "/foo" or "/foo/*". Regex style
bindings must use the Guice ServletModule declared in the manifest:

  @Export("/print-hello")
  class Hello extends HttpServlet {

Change-Id: Iae4cffcd62d7d2911d3f2705e226fbe21434be68

gerrit-httpd/src/main/java/com/google/gerrit/httpd/plugins/HttpAutoRegisterModuleGenerator.java

@@ -0,0 +1,69 @@
+// Copyright (C) 2012 The Android Open Source Project
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package com.google.gerrit.httpd.plugins;
+
+import com.google.common.collect.Maps;
+import com.google.gerrit.extensions.annotations.Export;
+import com.google.gerrit.server.plugins.InvalidPluginException;
+import com.google.gerrit.server.plugins.ModuleGenerator;
+import com.google.inject.Module;
+import com.google.inject.Scopes;
+import com.google.inject.servlet.ServletModule;
+
+import java.util.Map;
+
+import javax.servlet.http.HttpServlet;
+
+class HttpAutoRegisterModuleGenerator extends ServletModule
+    implements ModuleGenerator {
+  private final Map<String, Class<HttpServlet>> serve = Maps.newHashMap();
+
+  @Override
+  protected void configureServlets() {
+    for (Map.Entry<String, Class<HttpServlet>> e : serve.entrySet()) {
+      bind(e.getValue()).in(Scopes.SINGLETON);
+      serve(e.getKey()).with(e.getValue());
+    }
+  }
+
+  @Override
+  public void setPluginName(String name) {
+  }
+
+  @SuppressWarnings("unchecked")
+  @Override
+  public void export(Export export, Class<?> type)
+      throws InvalidPluginException {
+    if (HttpServlet.class.isAssignableFrom(type)) {
+      Class<HttpServlet> old = serve.get(export.value());
+      if (old != null) {
+        throw new InvalidPluginException(String.format(
+            "@Export(\"%s\") has duplicate bindings:\n  %s\n  %s",
+            export.value(), old.getName(), type.getName()));
+      }
+      serve.put(export.value(), (Class<HttpServlet>) type);
+    } else {
+      throw new InvalidPluginException(String.format(
+          "Class %s with @Export(\"%s\") must extend %s",
+          type.getName(), export.value(),
+          HttpServlet.class.getName()));
+    }
+  }
+
+  @Override
+  public Module create() throws InvalidPluginException {
+    return this;
+  }
+}