1 files changed, 123 insertions, 0 deletions
diff --git a/controlloop/common/controller-tdjam/src/main/java/org/onap/policy/controlloop/tdjam/SerialWorkQueue.java b/controlloop/common/controller-tdjam/src/main/java/org/onap/policy/controlloop/tdjam/SerialWorkQueue.java
new file mode 100644
index 000000000..7d83765a3
--- /dev/null
+++ b/controlloop/common/controller-tdjam/src/main/java/org/onap/policy/controlloop/tdjam/SerialWorkQueue.java
@@ -0,0 +1,123 @@
+/*-
+ * ============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();
+            }
+        }
+    }
+}