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/*-
* ============LICENSE_START=======================================================
* ONAP
* ================================================================================
* Copyright (C) 2020 AT&T Intellectual Property. All rights reserved.
* ================================================================================
* Licensed 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.
* ============LICENSE_END=========================================================
*/
package org.onap.policy.controlloop.tdjam;
import java.util.LinkedList;
import lombok.Getter;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
/**
* This class provides a way to handle synchronization, with minimal blocking. Requests
* are queued until {@link #start()} is invoked.
*/
public class SerialWorkQueue {
private static Logger logger = LoggerFactory.getLogger(SerialWorkQueue.class);
// current work list
private LinkedList<Runnable> workQueue;
@Getter
private boolean running = false;
/**
* Constructor - no initial Runnable.
*/
public SerialWorkQueue() {
workQueue = new LinkedList<>();
}
/**
* Constructor - initial 'Runnable' is specified.
*
* @param runnable an initial 'Runnnable' to run
*/
public SerialWorkQueue(Runnable runnable) {
workQueue = new LinkedList<>();
workQueue.add(runnable);
}
/**
* Starts the queue. If the current thread is the first to start it, then the current
* thread will process any requests in the queue before returning.
*/
public void start() {
Runnable item;
synchronized (this) {
if (running) {
// already running
return;
}
running = true;
item = workQueue.peekFirst();
}
if (item != null) {
processQueue(item);
}
}
/**
* Called to add a 'Runnable' to the work queue. If the queue was empty, the current
* thread is used to process the queue.
*
* @param work the Runnable to be queued, and eventually run
*/
public void queueAndRun(Runnable work) {
synchronized (this) {
workQueue.add(work);
if (!running || workQueue.size() > 1) {
// there was already work in the queue, so presumably there is
// already an associated thread running
return;
}
// if we reach this point, the queue was empty when this method was
// called, so this thread will process the queue
}
processQueue(work);
}
/**
* Internal method to process the work queue until it is empty. Note that entries
* could be added by this thread or another one while we are working.
*
* @param firstItem the first item in the queue
*/
private void processQueue(Runnable firstItem) {
Runnable next = firstItem;
while (next != null) {
try {
next.run();
} catch (Exception e) {
logger.error("SerialWorkQueue.processQueue exception", e);
}
synchronized (this) {
// remove the job we just ran
workQueue.removeFirst();
next = workQueue.peekFirst();
}
}
}
}
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