Code bloat! The functionality of this class could be implemented fairly

easily in java 1.5 by using a ThreadPoolExecutor with 1 thread in the
pool and a special queue implementation that merged like entries,
but we want this special functionality now.


git-svn-id: https://samskivert.googlecode.com/svn/trunk@1538 6335cc39-0255-0410-8fd6-9bcaacd3b74c

projects/samskivert/src/java/com/samskivert/util/SerialExecutor.java

@@ -0,0 +1,133 @@
+//
+// $Id$
+
+package com.samskivert.util;
+
+import java.util.ArrayList;
+
+import com.samskivert.Log;
+
+/**
+ * A simple serial executor, similar to what can be done in java 1.5, only
+ * using 1.4 and samskivert-style tasks. This is like RunQueue only slightly
+ * more sophisticated. Bloat!
+ */
+public class SerialExecutor
+{
+    /**
+     * A task to run in serial on the executor.
+     */
+    public static interface ExecutorTask
+    {
+        /**
+         * This will be called when one task is on the queue to be processed
+         * and another task is going to be added to the queue.
+         *
+         * @return true if the tasks were merged and the other task does not
+         * need to be added to the queue.
+         */
+        public boolean merge (ExecutorTask other);
+
+        /**
+         * The portion of the task that will be executed on the executor's
+         * thread.
+         */
+        public void executeTask ();
+
+        /**
+         * After the task has been executed, this will be called
+         * on the ResultReceiver thread to post-process the results of the
+         * task.
+         */
+        public void resultReceived ();
+    }
+
+    /**
+     * Construct the SerialExecutor.
+     */
+    public SerialExecutor (Invoker.ResultReceiver receiver)
+    {
+        _receiver = receiver;
+    }
+
+    /**
+     * Add a task to the executor, it is expected that this method is
+     * called on the ResultReceiver thread.
+     */
+    public void addTask (ExecutorTask task)
+    {
+        for (int ii=0, nn=_queue.size(); ii < nn; ii++) {
+            ExecutorTask taskOnQueue = (ExecutorTask) _queue.get(ii);
+            if (taskOnQueue.merge(task)) {
+                return;
+            }
+        }
+
+        // otherwise, add it on
+        _queue.add(task);
+
+        // and perhaps start it going now
+        if (!_executingNow) {
+            checkNext();
+        }
+    }
+
+    /**
+     * Execute the next task, if applicable.
+     */
+    protected void checkNext ()
+    {
+        _executingNow = !_queue.isEmpty();
+        if (_executingNow) {
+            ExecutorTask task = (ExecutorTask) _queue.remove(0);
+            ExecutorThread thread = new ExecutorThread(task);
+            thread.start();
+        }
+    }
+
+    /**
+     * The basic processing unit of the Executor.
+     */
+    protected class ExecutorThread extends Thread
+    {
+        public ExecutorThread (ExecutorTask task)
+        {
+            _task = task;
+        }
+
+        // documentation inherited
+        public void run ()
+        {
+            try {
+                _task.executeTask();
+            } catch (Throwable t) {
+                Log.warning("ExecutorTask choked: " + t);
+                Log.logStackTrace(t);
+            }
+
+            _receiver.postUnit(new Runnable() {
+                public void run () {
+                    try {
+                        _task.resultReceived();
+                    } catch (Throwable t) {
+                        Log.warning("ExecutorTask choked: " + t);
+                        Log.logStackTrace(t);
+                    }
+                    checkNext();
+                }
+            });
+        }
+
+        /** The task to execute. */
+        protected ExecutorTask _task;
+    }
+
+    /** The receiver to which we post a unit to process results. */
+    protected Invoker.ResultReceiver _receiver;
+
+    /** True if there is a thread currently executing a task. */
+    protected boolean _executingNow = false;
+
+    /** The queue of tasks to execute. */
+    protected ArrayList _queue = new ArrayList();
+}