[GitHub] [helix] Marcosrico commented on a diff in pull request #2338: Implement a thread-safe listener container for zkclient

[GitBox <gi...@apache.org>]

Marcosrico commented on code in PR #2338:
URL: https://github.com/apache/helix/pull/2338#discussion_r1068357047


##########
zookeeper-api/src/main/java/org/apache/helix/zookeeper/zkclient/ListenerContainer.java:
##########
@@ -0,0 +1,186 @@
+package org.apache.helix.zookeeper.zkclient;
+
+/*
+ * 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.
+ */
+
+import com.google.common.annotations.VisibleForTesting;
+import java.util.Collection;
+import java.util.Collections;
+import java.util.HashSet;
+import java.util.Map;
+import java.util.Set;
+import java.util.concurrent.ConcurrentHashMap;
+import java.util.concurrent.CopyOnWriteArraySet;
+import java.util.concurrent.locks.ReentrantLock;
+import java.util.function.Consumer;
+
+
+/**
+ * A thread-safe container wrapper class for Listeners registered to ZkClient.
+ * It stores 2 types of listener separately, one-time listener and persistent listener. The former ones are removed
+ * right after its consumption, while the latter need to be explicitly removed. 
+ * @param <T> the type of listener
+ */
+public class ListenerContainer<T> {
+  private final ReentrantLock _lock = new ReentrantLock(true);
+  private final Map<String, Set<T>> _persistentListener = new ConcurrentHashMap<>();
+  private final Map<String, Set<T>> _onetimeListener = new ConcurrentHashMap<>();
+
+  /**
+   * Add listener to the container with specified key.
+   * @param key the key to register to
+   * @param listener the listener to register
+   * @param persistListener true if the listener is persistent
+   */
+  public void addListener(String key, T listener, boolean persistListener) {
+    addListener(key, listener, persistListener ? _persistentListener : _onetimeListener);
+  }
+
+  /**
+   * Remove listener from the container.
+   * This operation removes both one-time and persistent listener from the specified key.
+   * @param key the key to remove
+   * @param listener the listener to remove
+   */
+  public void removeListener(String key, T listener) {
+    _lock.lock();
+    try {
+      removeFromListenerMap(key, listener, _persistentListener);
+      removeFromListenerMap(key, listener, _onetimeListener);
+    } finally {
+      _lock.unlock();
+    }
+  }
+
+  /**
+   * Remove listener from the container.
+   * @param key the key to remove
+   * @param listener the listener to remove
+   * @param persistListener true if remove the persistent listener, otherwise remove the one-time listener
+   */
+  public void removeListener(String key, T listener, boolean persistListener) {
+    _lock.lock();
+    try {
+      if (persistListener) {
+        removeFromListenerMap(key, listener, _persistentListener);
+      } else {
+        removeFromListenerMap(key, listener, _onetimeListener);
+      }
+    } finally {
+      _lock.unlock();
+    }
+  }
+
+  /**
+   * Remove listeners from the container.
+   * @param key the key to remove
+   * @param listeners the listeners to remove
+   * @param persistListener true if remove the persistent listener, otherwise remove the one-time listener
+   */
+  public void removeListeners(String key, Collection<T> listeners, boolean persistListener) {
+    _lock.lock();
+    try {
+      if (persistListener) {
+        removeFromListenerMap(key, listeners, _persistentListener);
+      } else {
+        removeFromListenerMap(key, listeners, _onetimeListener);
+      }
+    } finally {
+      _lock.unlock();
+    }
+  }
+
+  /**
+   * Consume the listeners registered to the given key. The given consumer is applied to all one-time and persistent
+   * listeners with the given key.
+   * The one-time listeners are removed after this operation, as a result, this operation is NOT idempotent.
+   * @param key the key of the listeners
+   * @param consumer the consuming action for the listeners
+   */
+  public void consumeListeners(String key, Consumer<? super T> consumer) {
+    Set<T> onetimeListeners;
+    Set<T> persistentListeners;
+    _lock.lock();
+    try {
+      // remove one-time listeners
+      onetimeListeners = _onetimeListener.remove(key);

Review Comment:
   Please correct me if I'm wrong but I believe the remove method in ConcurrentHashMap are atomic in itself so not sure if it is necessary to lock beforehand.
   
   Also should we check if key is null? Given underlying data structure of a ConcurrentHashMap is a HashTable, attempting to remove a null key would throw a NullPointerException which might not be the desired effect. Just a thought



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