1 files changed, 190 insertions, 0 deletions
diff --git a/utils/src/main/java/org/onap/policy/common/utils/services/ServiceManager.java b/utils/src/main/java/org/onap/policy/common/utils/services/ServiceManager.java
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+++ b/utils/src/main/java/org/onap/policy/common/utils/services/ServiceManager.java
@@ -0,0 +1,190 @@
+/*
+ * ============LICENSE_START=======================================================
+ * ONAP PAP
+ * ================================================================================
+ * Copyright (C) 2019 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.common.utils.services;
+
+import java.util.Deque;
+import java.util.Iterator;
+import java.util.LinkedList;
+import org.onap.policy.common.capabilities.Startable;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+/**
+ * Manages a series of services. The services are started in order, and stopped in reverse
+ * order.
+ */
+public class ServiceManager {
+    private static final Logger logger = LoggerFactory.getLogger(ServiceManager.class);
+
+    /**
+     * Services to be started/stopped.
+     */
+    private final Deque<Service> items = new LinkedList<>();
+
+    /**
+     * {@code True} if the services are currently running, {@code false} otherwise.
+     */
+    private boolean running;
+
+    /**
+     * Adds a pair of service actions to the manager.
+     *
+     * @param stepName name to be logged when the service is started/stopped
+     * @param starter function to start the service
+     * @param stopper function to stop the service
+     * @return this manager
+     */
+    public synchronized ServiceManager addAction(String stepName, RunnableWithEx starter, RunnableWithEx stopper) {
+        if (running) {
+            throw new IllegalStateException("services are already running; cannot add " + stepName);
+        }
+
+        items.add(new Service(stepName, starter, stopper));
+        return this;
+    }
+
+    /**
+     * Adds a service to the manager. The manager will invoke the service's
+     * {@link Startable#start()} and {@link Startable#stop()} methods.
+     *
+     * @param stepName name to be logged when the service is started/stopped
+     * @param service object to be started/stopped
+     * @return this manager
+     */
+    public synchronized ServiceManager addService(String stepName, Startable service) {
+        if (running) {
+            throw new IllegalStateException("services are already running; cannot add " + stepName);
+        }
+
+        items.add(new Service(stepName, () -> service.start(), () -> service.stop()));
+        return this;
+    }
+
+    /**
+     * Starts each service, in order. If a service throws an exception, then the
+     * previously started services are stopped, in reverse order.
+     *
+     * @throws ServiceManagerException if a service fails to start
+     */
+    public synchronized void start() throws ServiceManagerException {
+        if (running) {
+            throw new IllegalStateException("services are already running");
+        }
+
+        // tracks the services that have been started so far
+        Deque<Service> started = new LinkedList<>();
+        Exception ex = null;
+
+        for (Service item : items) {
+            try {
+                logger.info("starting {}", item.stepName);
+                item.starter.run();
+                started.add(item);
+
+            } catch (Exception e) {
+                logger.error("failed to start {}; rewinding steps", item.stepName);
+                ex = e;
+                break;
+            }
+        }
+
+        if (ex == null) {
+            running = true;
+            return;
+        }
+
+        // one of the services failed to start - rewind those we've previously started
+        try {
+            rewind(started);
+
+        } catch (ServiceManagerException e) {
+            logger.error("rewind failed", e);
+        }
+
+        throw new ServiceManagerException(ex);
+    }
+
+    /**
+     * Stops the services, in reverse order from which they were started. Stops all of the
+     * services, even if one of the "stop" functions throws an exception. Assumes that
+     * {@link #start()} has completed successfully.
+     *
+     * @throws ServiceManagerException if a service fails to stop
+     */
+    public synchronized void stop() throws ServiceManagerException {
+        if (!running) {
+            throw new IllegalStateException("services are not running");
+        }
+
+        running = false;
+        rewind(items);
+    }
+
+    /**
+     * Rewinds a list of services, stopping them in reverse order. Stops all of the
+     * services, even if one of the "stop" functions throws an exception.
+     *
+     * @param running services that are running, in the order they were started
+     * @throws ServiceManagerException if a service fails to stop
+     */
+    private void rewind(Deque<Service> running) throws ServiceManagerException {
+        Exception ex = null;
+
+        // stop everything, in reverse order
+        Iterator<Service> it = running.descendingIterator();
+        while (it.hasNext()) {
+            Service item = it.next();
+            try {
+                logger.info("stopping {}", item.stepName);
+                item.stopper.run();
+            } catch (Exception e) {
+                logger.error("failed to stop {}", item.stepName);
+                ex = e;
+
+                // do NOT break or re-throw, as we must stop ALL remaining items
+            }
+        }
+
+        if (ex != null) {
+            throw new ServiceManagerException(ex);
+        }
+    }
+
+    /**
+     * Service information.
+     */
+    private static class Service {
+        private String stepName;
+        private RunnableWithEx starter;
+        private RunnableWithEx stopper;
+
+        public Service(String stepName, RunnableWithEx starter, RunnableWithEx stopper) {
+            this.stepName = stepName;
+            this.starter = starter;
+            this.stopper = stopper;
+        }
+    }
+
+    @FunctionalInterface
+    public static interface RunnableWithEx {
+        public void run() throws Exception;
+    }
+}