Added keep-alive mechanism that detects disconnects (Fixes #166)

src/main/java/net/schmizz/keepalive/KeepAliveRunner.java

@@ -0,0 +1,60 @@
+package net.schmizz.keepalive;
+
+import net.schmizz.concurrent.Promise;
+import net.schmizz.sshj.common.SSHPacket;
+import net.schmizz.sshj.connection.ConnectionException;
+import net.schmizz.sshj.connection.ConnectionImpl;
+import net.schmizz.sshj.transport.TransportException;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+import java.util.LinkedList;
+import java.util.Queue;
+
+import static java.lang.String.format;
+import static net.schmizz.sshj.common.DisconnectReason.CONNECTION_LOST;
+
+public class KeepAliveRunner extends KeepAlive {
+
+    /** The max number of keep-alives that should be unanswered before killing the connection. */
+    private int maxAliveCount = 5;
+
+    /** The queue of promises. */
+    private final Queue<Promise<SSHPacket, ConnectionException>> queue =
+            new LinkedList<Promise<SSHPacket, ConnectionException>>();
+
+    KeepAliveRunner(ConnectionImpl conn) {
+        super(conn, "keep-alive");
+    }
+
+    synchronized public int getMaxAliveCount() {
+        return maxAliveCount;
+    }
+
+    synchronized public void setMaxAliveCount(int maxAliveCount) {
+        this.maxAliveCount = maxAliveCount;
+    }
+
+    @Override
+    protected void doKeepAlive() throws TransportException, ConnectionException {
+        emptyQueue(queue);
+        checkMaxReached(queue);
+        queue.add(conn.sendGlobalRequest("keepalive@openssh.com", true, new byte[0]));
+    }
+
+    private void checkMaxReached(Queue<Promise<SSHPacket, ConnectionException>> queue) throws ConnectionException {
+        if (queue.size() >= maxAliveCount) {
+            throw new ConnectionException(CONNECTION_LOST,
+                    format("Did not receive any keep-alive response for %s seconds", maxAliveCount * keepAliveInterval));
+        }
+    }
+
+    private void emptyQueue(Queue<Promise<SSHPacket, ConnectionException>> queue) {
+        Promise<SSHPacket, ConnectionException> peek = queue.peek();
+        while (peek != null && peek.isFulfilled()) {
+            log.debug("Received response from server to our keep-alive.");
+            queue.remove();
+            peek = queue.peek();
+        }
+    }
+}