Problem: no way to intercept GraphQL operations

For example, if one wants to log GraphQL operations and/or
failures associated with them, there isn't really a good way to do this.

Solution: introduce GraphQLOperationListener interface

Author: yrashk

src/main/java/graphql/servlet/GraphQLOperationListener.java

@@ -0,0 +1,27 @@
+/**
+ * Copyright 2016 Yurii Rashkovskii
+ *
+ * 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
+ */
+package graphql.servlet;
+
+import graphql.GraphQLError;
+
+import java.util.List;
+import java.util.Map;
+
+public interface GraphQLOperationListener {
+    void onGraphQLOperation(GraphQLContext context, String operationName, String query, Map<String, Object> variables,
+                            Object data);
+    void onFailedGraphQLOperation(GraphQLContext context, String operationName, String query,
+                                  Map<String, Object> variables, List<GraphQLError> errors);
+}

src/main/java/graphql/servlet/GraphQLServlet.java

@@ -160,6 +160,17 @@ protected GraphQLContext createContext(Optional<HttpServletRequest> req, Optiona
         return contextBuilder.build(req, resp);
     }
 
+    private List<GraphQLOperationListener> operationListeners = new ArrayList<>();
+
+    @Reference(cardinality = ReferenceCardinality.MULTIPLE, policyOption = ReferencePolicyOption.GREEDY)
+    public void bindOperationListener(GraphQLOperationListener listener) {
+        operationListeners.add(listener);
+    }
+
+    public void unbindOperationListener(GraphQLOperationListener listener) {
+        operationListeners.remove(listener);
+    }
+
     @Override @SneakyThrows
     public String executeQuery(String query) {
         try {
@@ -274,6 +285,8 @@ public Void run() {
                 Map<String, Object> dict = new HashMap<>();
                 dict.put("data", result.getData());
                 resp.getWriter().write(new ObjectMapper().writeValueAsString(dict));
+                operationListeners.forEach(l -> l.onGraphQLOperation(context, operationName, query, variables,
+                                                                     result.getData()));
             } else {
                 result.getErrors().stream().
                         filter(error -> (error instanceof ExceptionWhileDataFetching)).
@@ -285,6 +298,8 @@ public Void run() {
                 dict.put("errors", errors);
 
                 resp.getWriter().write(new ObjectMapper().writeValueAsString(dict));
+                operationListeners.forEach(l -> l.onFailedGraphQLOperation(context, operationName, query, variables,
+                                                                           result.getErrors()));
             }
         }
     }