Temporarily copied SerialExecutor.java from dependencymanager/core to dependencymanager/runtime, until we decide to store this class in utils. Also slightly modified the scheduleNext() method in order to avoid unecessary NoSuchElementException.
git-svn-id: https://svn.apache.org/repos/asf/felix/trunk@942913 13f79535-47bb-0310-9956-ffa450edef68
diff --git a/dependencymanager/runtime/src/main/java/org/apache/felix/dm/runtime/SerialExecutor.java b/dependencymanager/runtime/src/main/java/org/apache/felix/dm/runtime/SerialExecutor.java
new file mode 100644
index 0000000..f985da4
--- /dev/null
+++ b/dependencymanager/runtime/src/main/java/org/apache/felix/dm/runtime/SerialExecutor.java
@@ -0,0 +1,93 @@
+/*
+ * 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.felix.dm.runtime;
+
+import java.util.LinkedList;
+
+/**
+ * Allows you to enqueue tasks from multiple threads and then execute
+ * them on one thread sequentially. It assumes more than one thread will
+ * try to execute the tasks and it will make an effort to pick the first
+ * task that comes along whilst making sure subsequent tasks return
+ * without waiting.
+ *
+ * @author <a href="mailto:dev@felix.apache.org">Felix Project Team</a>
+ */
+public final class SerialExecutor
+{
+ private final LinkedList m_workQueue = new LinkedList();
+ private Runnable m_active;
+
+ /**
+ * Enqueue a new task for later execution. This method is
+ * thread-safe, so multiple threads can contribute tasks.
+ *
+ * @param runnable the runnable containing the actual task
+ */
+ public synchronized void enqueue(final Runnable runnable)
+ {
+ m_workQueue.addLast(new Runnable()
+ {
+ public void run()
+ {
+ try
+ {
+ runnable.run();
+ }
+ finally
+ {
+ scheduleNext();
+ }
+ }
+ });
+ }
+
+ /**
+ * Execute any pending tasks. This method is thread safe,
+ * so multiple threads can try to execute the pending
+ * tasks, but only the first will be used to actually do
+ * so. Other threads will return immediately.
+ */
+ public void execute()
+ {
+ Runnable active;
+ synchronized (this)
+ {
+ active = m_active;
+ }
+ if (active == null)
+ {
+ scheduleNext();
+ }
+ }
+
+ private void scheduleNext()
+ {
+ Runnable active;
+ synchronized (this)
+ {
+ m_active = (m_workQueue.size() == 0) ? null : (Runnable) m_workQueue.removeFirst();
+ active = m_active;
+ }
+ if (active != null)
+ {
+ active.run();
+ }
+ }
+}