ZOOKEEPER-3605: connThrottle needs to be assigned in alternate consructor

`connThrottle` needs to be assigned in alternate consructor to avoid NPEs

Author: randgalt <jordan@jordanzimmerman.com>

Reviewers: Enrico Olivelli, Andor Molnár

Closes #1132 from Randgalt/ZOOKEEPER-3605

zookeeper-server/src/main/java/org/apache/zookeeper/server/ZooKeeperServer.java

@@ -236,7 +236,7 @@ public class ZooKeeperServer implements SessionExpirer, ServerStats.Provider {
     }
 
     // Connection throttling
-    private BlueThrottle connThrottle;
+    private BlueThrottle connThrottle = new BlueThrottle();
 
     @SuppressFBWarnings(value = "IS2_INCONSISTENT_SYNC", justification =
         "Internally the throttler has a BlockingQueue so "
@@ -308,8 +308,6 @@ public class ZooKeeperServer implements SessionExpirer, ServerStats.Provider {
 
         readResponseCache = new ResponseCache();
 
-        connThrottle = new BlueThrottle();
-
         this.initialConfig = initialConfig;
 
         this.requestPathMetricsCollector = new RequestPathMetricsCollector();

zookeeper-server/src/test/java/org/apache/zookeeper/server/ZooKeeperServerCreationTest.java

@@ -0,0 +1,61 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you 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 org.apache.zookeeper.server;
+
+import java.io.ByteArrayOutputStream;
+import java.io.File;
+import java.nio.ByteBuffer;
+import org.apache.jute.BinaryOutputArchive;
+import org.apache.zookeeper.proto.ConnectRequest;
+import org.apache.zookeeper.server.persistence.FileTxnSnapLog;
+import org.apache.zookeeper.test.ClientBase;
+import org.junit.Test;
+
+public class ZooKeeperServerCreationTest {
+
+    /**
+     * Test the default ZooKeeperServer and call processConnectRequest() to make sure
+     * that all needed fields are initialized properly, etc.
+     */
+    @Test
+    public void testDefaultConstructor() throws Exception {
+        File tmpDir = ClientBase.createEmptyTestDir();
+        FileTxnSnapLog fileTxnSnapLog = new FileTxnSnapLog(new File(tmpDir, "data"), new File(tmpDir, "data_txnlog"));
+
+        ZooKeeperServer zks = new ZooKeeperServer() {
+            @Override
+            public void submitRequest(Request si) {
+                // NOP
+            }
+        };
+        zks.setTxnLogFactory(fileTxnSnapLog);
+        zks.setZKDatabase(new ZKDatabase(fileTxnSnapLog));
+        zks.createSessionTracker();
+
+        ServerCnxnFactory cnxnFactory = ServerCnxnFactory.createFactory();
+        ServerCnxn cnxn = new MockServerCnxn();
+
+        ConnectRequest connReq = new ConnectRequest();
+        ByteArrayOutputStream baos = new ByteArrayOutputStream();
+        BinaryOutputArchive boa = BinaryOutputArchive.getArchive(baos);
+        connReq.serialize(boa, "connect");
+        zks.processConnectRequest(cnxn, ByteBuffer.wrap(baos.toByteArray()));
+    }
+
+}