Refactor BackgroundExecutor add submit() methods returning Future (#2121)

- Adds submit() methods that return Future
- Refactor internals to use DaemonScheduleThreadPool
- Delete the now unused DaemonExecutorService
- Tidy internals using wrapMDC() methods

ebean-api/src/main/java/io/ebean/BackgroundExecutor.java

@@ -1,31 +1,40 @@
 package io.ebean;
 
 import java.util.concurrent.Callable;
+import java.util.concurrent.Future;
 import java.util.concurrent.ScheduledExecutorService;
 import java.util.concurrent.ScheduledFuture;
 import java.util.concurrent.TimeUnit;
 
 /**
- * Background thread pool service for executing of tasks asynchronously.
+ * Background executor service for executing of tasks asynchronously.
  * <p>
- * This service is used internally by Ebean for executing background tasks such
- * as the {@link Query#findFutureList()} and also for executing background tasks
- * periodically.
- * </p>
+ * This service can be used to execute tasks in the background.
  * <p>
- * This service has been made available so you can use it for your application
- * code if you want. It can be useful for some server caching implementations
- * (background population and trimming of the cache etc).
- * </p>
- *
- * @author rbygrave
+ * This service is managed by Ebean and will perform a clean shutdown
+ * waiting for background tasks to complete with a default 30 second
+ * timeout. Shutdown occurs prior to DataSource shutdown.
+ * <p>
+ * This also propagates MDC context from the current thread to the
+ * background task if defined.
  */
 public interface BackgroundExecutor {
 
   /**
-   * Execute a task in the background.
+   * Execute a callable task in the background returning the Future.
    */
-  void execute(Runnable r);
+  <T> Future<T> submit(Callable<T> task);
+
+  /**
+   * Execute a runnable task in the background returning the Future.
+   */
+  Future<?> submit(Runnable task);
+
+  /**
+   * Execute a task in the background. Effectively the same as
+   * {@link BackgroundExecutor#submit(Runnable)} but returns void.
+   */
+  void execute(Runnable task);
 
   /**
    * Execute a task periodically with a fixed delay between each execution.
@@ -36,12 +45,12 @@ public interface BackgroundExecutor {
    * That is, this method has the same behaviour characteristics as
    * {@link ScheduledExecutorService#scheduleWithFixedDelay(Runnable, long, long, TimeUnit)}
    */
-  void executePeriodically(Runnable r, long delay, TimeUnit unit);
+  void executePeriodically(Runnable task, long delay, TimeUnit unit);
 
   /**
    * Execute a task periodically additionally with an initial delay different from delay.
    */
-  void executePeriodically(Runnable r, long initialDelay, long delay, TimeUnit unit);
+  void executePeriodically(Runnable task, long initialDelay, long delay, TimeUnit unit);
 
   /**
    * Schedules a Runnable for one-shot action that becomes enabled after the given delay.
@@ -49,14 +58,13 @@ public interface BackgroundExecutor {
    * @return a ScheduledFuture representing pending completion of the task and
    *         whose get() method will return null upon completion
    */
-  ScheduledFuture<?> schedule(Runnable r, long delay, TimeUnit unit);
+  ScheduledFuture<?> schedule(Runnable task, long delay, TimeUnit unit);
 
   /**
    * Schedules a Callable for one-shot action that becomes enabled after the given delay.
    *
    * @return a ScheduledFuture that can be used to extract result or cancel
    */
-  <V> ScheduledFuture<V> schedule(Callable<V> c, long delay, TimeUnit unit);
-
+  <V> ScheduledFuture<V> schedule(Callable<V> task, long delay, TimeUnit unit);
 
 }