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java/org/apache/tomcat/util/collections/SynchronizedQueue.java

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+/*
+ * 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.tomcat.util.collections;
+
+/**
+ * This is intended as a (mostly) GC-free alternative to
+ * {@link java.util.concurrent.ConcurrentLinkedQueue} when the requirement is to
+ * create an unbounded queue with no requirement to shrink the queue. The aim is
+ * to provide the bare minimum of required functionality as quickly as possible
+ * with minimum garbage.
+ *
+ * @param <T> The type of object managed by this queue
+ */
+public class SynchronizedQueue<T> {
+
+    public static final int DEFAULT_SIZE = 128;
+
+    private Object[] queue;
+    private int size;
+    private int insert = 0;
+    private int remove = 0;
+
+    public SynchronizedQueue() {
+        this(DEFAULT_SIZE);
+    }
+
+    public SynchronizedQueue(int initialSize) {
+        queue = new Object[initialSize];
+        size = initialSize;
+    }
+
+    public synchronized boolean offer(T t) {
+        queue[insert++] = t;
+
+        // Wrap
+        if (insert == size) {
+            insert = 0;
+        }
+
+        if (insert == remove) {
+            expand();
+        }
+        return true;
+    }
+
+    public synchronized T poll() {
+        if (insert == remove) {
+            // empty
+            return null;
+        }
+
+        @SuppressWarnings("unchecked")
+        T result = (T) queue[remove];
+        queue[remove] = null;
+        remove++;
+
+        // Wrap
+        if (remove == size) {
+            remove = 0;
+        }
+
+        return result;
+    }
+
+    private void expand() {
+        int newSize = size * 2;
+        Object[] newQueue = new Object[newSize];
+
+        System.arraycopy(queue, insert, newQueue, 0, size - insert);
+        System.arraycopy(queue, 0, newQueue, size - insert, insert);
+
+        insert = size;
+        remove = 0;
+        queue = newQueue;
+        size = newSize;
+    }
+
+    public synchronized int size() {
+        int result = insert - remove;
+        if (result < 0) {
+            result += size;
+        }
+        return result;
+    }
+
+    public synchronized void clear() {
+        queue = new Object[size];
+        insert = 0;
+        remove = 0;
+    }
+}