Add DelayedCommand base class

The class is suitable for creating annotated commands that can
execute on another thread.

Target: trunk
Require-notes: no
Require-book: no
Acked-by: Paul Millar <paul.millar@desy.de>
Patch: http://rb.dcache.org/r/5598/

modules/cells/src/main/java/dmg/util/command/DelayedCommand.java

@@ -0,0 +1,72 @@
+package dmg.util.command;
+
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+import java.io.Serializable;
+import java.util.concurrent.Callable;
+import java.util.concurrent.Executor;
+
+import dmg.cells.nucleus.DelayedReply;
+import dmg.cells.nucleus.NoRouteToCellException;
+import dmg.cells.nucleus.Reply;
+
+/**
+ * Abstract base class for annotated commands for executing the command
+ * asynchronously using an executor.
+ *
+ * By default a new thread is spawned for each command.
+ */
+public abstract class DelayedCommand<T extends Serializable>
+        extends DelayedReply
+        implements Callable<Reply>, Runnable
+{
+    private static final Logger LOGGER = LoggerFactory.getLogger(DelayedCommand.class);
+    private static final Executor NEW_THREAD_EXECUTOR = new Executor()
+    {
+        @Override
+        public void execute(Runnable command)
+        {
+            new Thread(command).start();
+        }
+    };
+
+    private final Executor executor;
+
+    protected DelayedCommand()
+    {
+        this(NEW_THREAD_EXECUTOR);
+    }
+
+    protected DelayedCommand(Executor executor)
+    {
+        this.executor = executor;
+    }
+
+    @Override
+    public Reply call() throws Exception
+    {
+        executor.execute(this);
+        return this;
+    }
+
+    protected abstract T execute() throws Exception;
+
+    @Override
+    public void run()
+    {
+        Serializable result;
+        try {
+            result = execute();
+        } catch (Exception e) {
+            result = e;
+        }
+        try {
+            send(result);
+        } catch (NoRouteToCellException e) {
+            LOGGER.warn("Failed to send reply: {}", e.getMessage());
+        } catch (InterruptedException e) {
+            LOGGER.warn("Failed to send reply as request was interrupted");
+        }
+    }
+}