Add overloaded constructor for operationListeners.

Author: apottere

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

@@ -66,8 +66,15 @@ public abstract class GraphQLServlet extends HttpServlet implements Servlet, Gra
     protected abstract ExecutionStrategy getExecutionStrategy();
     protected abstract Map<String, Object> transformVariables(GraphQLSchema schema, String query, Map<String, Object> variables);
 
+    private List<GraphQLOperationListener> operationListeners;
 
-    private List<GraphQLOperationListener> operationListeners = new ArrayList<>();
+    public GraphQLServlet() {
+        this(new ArrayList<>());
+    }
+
+    public GraphQLServlet(List<GraphQLOperationListener> operationListeners) {
+        this.operationListeners = new ArrayList<>(operationListeners);
+    }
 
     public void addOperationListener(GraphQLOperationListener operationListener) {
         operationListeners.add(operationListener);

src/main/java/graphql/servlet/SimpleGraphQLServlet.java

@@ -19,6 +19,8 @@
 
 import javax.servlet.http.HttpServletRequest;
 import javax.servlet.http.HttpServletResponse;
+import java.util.ArrayList;
+import java.util.List;
 import java.util.Map;
 import java.util.Optional;
 
@@ -28,6 +30,12 @@
 public class SimpleGraphQLServlet extends GraphQLServlet {
 
     public SimpleGraphQLServlet(GraphQLSchema schema, ExecutionStrategy executionStrategy) {
+        this(schema, executionStrategy, new ArrayList<>());
+    }
+
+    public SimpleGraphQLServlet(GraphQLSchema schema, ExecutionStrategy executionStrategy, List<GraphQLOperationListener> operationListeners) {
+        super(operationListeners);
+
         this.schema = schema;
         this.readOnlySchema = new GraphQLSchema(schema.getQueryType(), null, schema.getDictionary());