Github user michaelandrepearce commented on a diff in the pull request:
https://github.com/apache/activemq-artemis/pull/2488#discussion_r245013090
--- Diff:
artemis-server/src/main/java/org/apache/activemq/artemis/core/server/impl/QueueConsumersImpl.java
---
@@ -0,0 +1,648 @@
+/*
+ * 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.activemq.artemis.core.server.impl;
+
+import org.apache.activemq.artemis.core.server.PriorityAware;
+
+import java.lang.reflect.Array;
+import java.util.AbstractCollection;
+import java.util.Arrays;
+import java.util.Collection;
+import java.util.Iterator;
+import java.util.Objects;
+import java.util.Set;
+import java.util.concurrent.locks.ReentrantLock;
+import java.util.function.Consumer;
+import java.util.stream.Collectors;
+
+/**
+ * This class's purpose is to hold the consumers, it models around multi
getPriority (getPriority) varient of
+ * java.util.concurrent.CopyOnWriteArrayList, so that reads are concurrent
safe and non blocking.
+ *
+ * N.b. we could have made Level extend CopyOnWriteArrayList but due to
the need to access the internal Array structure,
+ * which is privileged to package java.util.concurrent. As such much of
Level is is taken from here.
+ *
+ * Modifications like in CopyOnWriteArrayList are single threaded via a
single re-entrant lock.
+ *
+ * Iterators iterate over a snapshot of the internal array structure, so
will not see mutations.
+ *
+ * There can only be one resettable iterable view, this is exposed at the
top getPriority,
+ * and is intended for use in QueueImpl only.
+ * All other iterators are not reset-able and are created on calling
iterator().
+ *
+ * Methods getArray, setArray MUST never be exposed, and all array
modifications must go through these.
+ *
+ * @param <T> The type this class may hold, this is generic as can be
anything that extends PriorityAware,
+ * but intent is this is the QueueImpl:ConsumerHolder.
+ */
+public class QueueConsumersImpl<T extends PriorityAware> extends
AbstractCollection<T> implements QueueConsumers<T> {
+
+ private final QueueConsumersIterator<T> iterator = new
QueueConsumersIterator<>(this, true);
+
+ private volatile Level<T>[] levels;
+ private volatile int size;
+ private volatile T first;
+
+ private void setArray(Level<T>[] array) {
+ this.levels = array;
+ }
+
+ private Level<T>[] getArray() {
+ return levels;
+ }
+
+
+ public QueueConsumersImpl() {
+ levels = newLevelArrayInstance(0);
+ }
+
+ @SuppressWarnings("unchecked")
+ private static <T> Level<T>[] newLevelArrayInstance(int length) {
+ return (Level<T>[]) Array.newInstance(Level.class, length);
+ }
+
+ @Override
+ public int size() {
+ return size;
+ }
+
+ @Override
+ public boolean isEmpty() {
+ return size() == 0;
+ }
+
+ @Override
+ public Set<Integer> getPriorites() {
+ Level<T>[] levels = getArray();
+ return
Arrays.stream(levels).map(Level::level).collect(Collectors.toSet());
+ }
+
+ @Override
+ public Iterator<T> iterator() {
+ return new QueueConsumersIterator<>(this, false);
+ }
+
+ @Override
+ public boolean hasNext() {
+ return iterator.hasNext();
+ }
+
+ @Override
+ public T next() {
+ return iterator.next();
+ }
+
+ @Override
+ public QueueConsumers<T> reset() {
+ iterator.reset();
+ return this;
+ }
+
+ @Override
+ public void forEach(Consumer<? super T> action) {
+ Objects.requireNonNull(action);
+ Level<T>[] current = getArray();
+ int len = current.length;
+ for (int i = 0; i < len; ++i) {
+ current[i].forEach(action);
+ }
+ }
+
+ private Level<T> getLevel(int level, boolean createIfMissing) {
+ Level<T>[] current = getArray();
+ int low = 0;
+ int high = current.length - 1;
+
+ while (low <= high) {
+ int mid = (low + high) >>> 1;
+ Level<T> midVal = current[mid];
+
+ if (midVal.level() > level)
+ low = mid + 1;
+ else if (midVal.level() < level)
+ high = mid - 1;
+ else
+ return midVal; //key found
+ }
+
+ if (createIfMissing) {
+ Level<T>[] newLevels = newLevelArrayInstance(current.length + 1);
+ if (low > 0) {
+ System.arraycopy(current, 0, newLevels, 0, low);
+ }
+ if (current.length - low > 0) {
+ System.arraycopy(current, low, newLevels, low + 1,
current.length - low);
+ }
+ newLevels[low] = new Level<T>(level);
+ setArray(newLevels);
+ return newLevels[low];
+ }
+ return null;
+ }
+
+ @Override
+ public synchronized boolean add(T t) {
--- End diff --
All methods added we are using. Its not used in single write. This class
essentially replaces the CopyOnWriteArrayList we used to hold the consumers,
and supported only simple round robin before, and why most of its design is
based on that.
---
