Starting work on I/O loop.
diff --git a/utils/nio/src/main/java/org/onlab/nio/SelectorLoop.java b/utils/nio/src/main/java/org/onlab/nio/SelectorLoop.java
new file mode 100644
index 0000000..fc88e16
--- /dev/null
+++ b/utils/nio/src/main/java/org/onlab/nio/SelectorLoop.java
@@ -0,0 +1,121 @@
+package org.onlab.nio;
+
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+import java.io.IOException;
+import java.nio.channels.Selector;
+
+import static com.google.common.base.Preconditions.checkArgument;
+
+/**
+ * Abstraction of an I/O processing loop based on an NIO selector.
+ */
+public abstract class SelectorLoop implements Runnable {
+
+ protected final Logger log = LoggerFactory.getLogger(getClass());
+
+ /**
+ * Selector used by this loop to pace the I/O operations.
+ */
+ protected final Selector selector;
+
+ /**
+ * Selection operations timeout; specified in millis.
+ */
+ protected long selectTimeout;
+
+ /**
+ * Retains the error that caused the loop to exit prematurely.
+ */
+ private Throwable error;
+
+ // State indicator
+ private enum State { STARTING, STARTED, STOPPING, STOPPED };
+ private volatile State state = State.STOPPED;
+
+ /**
+ * Creates a new selector loop with the given selection timeout.
+ *
+ * @param selectTimeout selection timeout; specified in millis
+ * @throws IOException if the backing selector cannot be opened
+ */
+ public SelectorLoop(long selectTimeout) throws IOException {
+ checkArgument(selectTimeout > 0, "Timeout must be positive");
+ this.selectTimeout = selectTimeout;
+ this.selector = openSelector();
+ }
+
+ /**
+ * Opens a new selector for the use by the loop.
+ *
+ * @return newly open selector
+ * @throws IOException if the backing selector cannot be opened
+ */
+ protected Selector openSelector() throws IOException {
+ return Selector.open();
+ }
+
+ /**
+ * Indicates that the loop is marked to run.
+ */
+ protected boolean isRunning() {
+ return state == State.STARTED || state == State.STARTING;
+ }
+
+ /**
+ * Returns the error, if there was one, that caused the loop to terminate
+ * prematurely.
+ *
+ * @return error or null if there was none
+ */
+ public Throwable getError() {
+ return error;
+ }
+
+ /**
+ * Contains the body of the I/O selector loop.
+ *
+ * @throws IOException if an error is encountered while selecting I/O
+ */
+ protected abstract void loop() throws IOException;
+
+ @Override
+ public void run() {
+ error = null;
+ state = State.STARTING;
+ try {
+ loop();
+ } catch (Throwable e) {
+ error = e;
+ log.error("Loop aborted", e);
+ }
+ notifyDone();
+ }
+
+ /**
+ * Notifies observers waiting for loop to become ready.
+ */
+ protected synchronized void notifyReady() {
+ state = State.STARTED;
+ notifyAll();
+ }
+
+ /**
+ * Triggers loop shutdown.
+ */
+ public void shutdown() {
+ // Mark the loop as no longer running and wake up the selector.
+ state = State.STOPPING;
+ selector.wakeup();
+ }
+
+ /**
+ * Notifies observers waiting for loop to fully stop.
+ */
+ private synchronized void notifyDone() {
+ state = State.STOPPED;
+ notifyAll();
+ }
+
+}