Develop Reference

How to avoid Stack Overflow errors in infinitely recursive method?

I know there are tons of posts about stack overflow errors and i understand why my specific one is happening, my question is basically how to move away from recursion in this specific case. I have a class which establishes and maintains a client connection (for HL7 messaging specifically but it's essentially a glorified client connection) to another system which hosts corresponding server connections. This class' constructor starts a new thread and runs the following method :
#Override
public void connect()
{
try
{
setStatus("Connecting");
connection = context.newClient(intfc.getIp(), port, false);
connected = true;
setStatus("Connected");
logEntryService.logInfo(LogEntry.CONNECTIVITY, "Successfully connected " + connectionType + " client connection to "
+ intfc.getName() + "(" + intfc.getIp() + ") on port " + port);
monitor();
}
catch (HL7Exception ex)
{
connected = false;
setStatus("Disconnected");
try
{
TimeUnit.SECONDS.sleep(connectionRetryIntervalInSeconds);
connect();
}
catch (InterruptedException ex2)
{}
}
}
Upon successfully connecting with the server, the monitor method simply checks, in yet another thread, if the connection is still up at a given interval. If it goes down, the monitoring thread is interrupted and the connect() method is called again.
I did not anticipate this at first but you can quickly see why the connect() method is causing stack overflow errors after several days running. I'm struggling to think of a way to get the same functionality to work without the connect method calling itself again every time the connection fails.
Any suggestions would be appreciated.
Thanks!

Typically you'd use a Stack object to emulate recursion when required.
However, in your case, why are you using recursion at all? A while loop fits the purpose.
while(true /**or some relevant condition**/){
try{ //try to connect
....
catch(HL7Exception ex){
//sleep
}
}
I'm not sure of the purpose of your application, but there are may be better methods than sleeping. You could use a ScheduledExecutorService, but if it's a single threaded program with one purpose it's probably unnecessary.

When I had to deal with this issue in c# I used a Stack, and added new classes to it, instead of using recursion. Then a second loop would check to see if there were any objects in the stack that needed dealing with. That avoided stack overflow when I would have had huge amounts of recursion otherwise. Is there a similar Stack collection in Java?

Why are you calling the monitor() method in the first place? You mention that it is launched in a separate thread, then can't you just launch it in a new thread when the application comes up? Then there won't be a recursive call.

I changed my code to an iterative approach as suggested, works beautifully!
#Override
public void initThread()
{
initConnectionEntity();
mainThread = new Thread()
{
#Override
public void run()
{
while (running)
{
if (!connected)
{
try
{
connect();
}
catch (HL7Exception ex)
{
connected = false;
setStatus("Disconnected");
try
{
TimeUnit.SECONDS.sleep(connectionRetryIntervalInSeconds);
}
catch (InterruptedException ex2)
{}
}
}
try
{
TimeUnit.MILLISECONDS.sleep(500);
}
catch (InterruptedException ex2)
{}
}
}
};
mainThread.setName(intfc.getName() + " " + connectionType + " Main Thread");
mainThread.start();
}
#Override
public void connect() throws HL7Exception
{
setStatus("Connecting");
connection = context.newClient(intfc.getIp(), port, false);
connected = true;
setStatus("Connected");
logEntryService.logInfo(LogEntry.CONNECTIVITY, "Successfully connected " + connectionType + " client connection to "
+ intfc.getName() + "(" + intfc.getIp() + ") on port " + port);
monitor();
}
private void monitor()
{
monitoringThread = new Thread()
{
#Override
public void run()
{
try
{
while (running)
{
if (!connection.isOpen())
{
if (connected == true)
{
logEntryService.logWarning(LogEntry.CONNECTIVITY, "Lost " + connectionType + " connection to "
+ intfc.getName() + "(" + intfc.getIp() + ") on port " + port);
}
connected = false;
setStatus("Disconnected");
monitoringThread.interrupt();
}
else
{
connected = true;
}
TimeUnit.SECONDS.sleep(connectionMonitorIntervalInSeconds);
}
}
catch (InterruptedException ex)
{
logEntryService.logDebug(LogEntry.CONNECTIVITY, "Monitoring thread for " + connectionType
+ " connection to " + intfc.getName() + " interrupted");
}
}
};
monitoringThread.setName(intfc.getName() + " " + connectionType + " Monitoring Thread");
monitoringThread.start();
}

Building a Fing like network scanning to our android application. It works right now but takes painstakingly long to process

we were wanting to build a functionality where if a user is connected to a WiFi network, we can display the details of other devices connected to the same WiFi. How do we go about it?
Right now, we are able to achieve it by:
pinging all the IP addresses on the current network(looping from 1-255) - this is the most time consuming step (code snippet below)
for the IP addresses that responded, fetching their MAC addresses and finally
fetching the manufacturer for the MAC addresses using an external API
we have success in this but the issue is that it takes way too long - around 4-5 minutes to do this. I have a feeling that someone can point us towards a better, faster solution.
There was a similar question(although it was about iOS) but didn't find any answer, hence posting this again. Please pardon if that was against the rules. Any help in this would be highly appreciated.
Here's the snippet of code which is taking too long to give the results back(step 1)
for (int i = 0; i < 255; i++) {
String host = "";
try {
String subnet = "192.168.2";
host = subnet + "." + i;
Process exec = Runtime.getRuntime().exec(String.format(CMD, host));
int i1 = exec.waitFor();
if (i1 == 0) {
InetAddress a = InetAddress.getByName(host);
Log.i("TAG", "run: " + a.getHostAddress());
} else {
throw new IOException("Unable to get ping from runtime");
}
} catch (IOException | InterruptedException e) {
try {
InetAddress a = InetAddress.getByName(host);
if (a.isReachable(200)) {
Log.i("TAG", "run: " + a.getHostAddress());
}
} catch (IOException ioe) {
ioe.printStackTrace();
}
} catch (Exception e) {
e.printStackTrace();
}

Solr Trigger Optimize And Check Progress From Java Code

From this topic there are two ways to trigger solr optimize from Java code. Either sending an http request, or using solrj api.
But how to check the progress of it?
Say, an api which returns the progress of optimize in percentage
or strings like RUNNING/COMPLETED/FAILED.
Is there such an api?

Yes, optimize() in solrj api is a sync method. Here is what I used to monitor the optimization progress.
CloudSolrClient client = null;
try {
client = new CloudSolrClient(zkClientUrl);
client.setDefaultCollection(collectionName);
m_logger.info("Explicit optimize of collection " + collectionName);
long optimizeStart = System.currentTimeMillis();
UpdateResponse optimizeResponse = client.optimize();
for (Object object : optimizeResponse.getResponse()) {
m_logger.info("Solr optimizeResponse" + object.toString());
}
if (optimizeResponse != null) {
m_logger.info(String.format(
" Elapsed Time(in ms) - %d, QTime (in ms) - %d",
optimizeResponse.getElapsedTime(),
optimizeResponse.getQTime()));
}
m_logger.info(String.format(
"Time spent on Optimizing a collection %s :"
+ (System.currentTimeMillis() - optimizeStart)
/ 1000 + " seconds", collectionName));
} catch (Exception e) {
m_logger.error("Failed during explicit optimize on collection "
+ collectionName, e);
} finally {
if (client != null) {
try {
client.close();
} catch (IOException e) {
throw new RuntimeException(
"Failed to close CloudSolrClient connection.", e);
}
client = null;
}
}

Java - TCP exception

I have a TCP Server and Client both written in Java and running on separate machines on Rhel 5.3 with jdk1.6. I have handled pretty much all the methods i could find to detect a disconnection on the "Server".
Following is a snippet of the Server code
private void listenforConnection() {
try {
socket = serverSocket.accept();
socket.setTcpNoDelay(true);
socket.setKeepAlive(true);
socket.setSoTimeout(5);
bosTcpOutStream = new BufferedOutputStream(socket.getOutputStream());
bisTcpInStream = new BufferedInputStream(socket.getInputStream());
log("New connection accepted from " + socket.getRemoteSocketAddress().toString());
sendHeartBeatsToClient();
} catch (IOException ie) {
log("Listener IOException : " + ie.getMessage());
}
}
private void sendHeartBeatsToClient() {
try {
while (true) {
long lngCurrentMillis=System.currentTimeMillis() ;
if ((lngCurrentMillis - lngLastSentMessageTime) >= 5000) {
byte[] bHeartBeat = getHeartBeatMessage();
bosTcpOutStream.write(bHeartBeat);
bosTcpOutStream.flush();
lngLastSentMessageTime = System.currentTimeMillis();
log("Heartbeat sent.");
} else {
try {
if (bisTcpInStream.read() == -1) {
log("Read Input Stream returned -1");
break;
}
} catch (SocketTimeoutException se) {
//Do nothing as i am not expecting the client to send anything.
} catch (IOException e) {
log("Read Input Stream error - " + e.getMessage());
break;
}
}
Thread.sleep(1);
}
} catch (IOException e) {
disconnectClientAndCloseSocket();
log("IO Exception" +e.getMessage());
} catch (InterruptedException e) {
disconnectClientAndCloseSocket();
log("Thread interrupted terminating." + e.getMessage());
}
}
I have also modified the tcp-keepalive kernel parameters on the "Server" machine as below:
net.ipv4.tcp_keepalive_time=2
net.ipv4.tcp_keepalive_probes=1
net.ipv4.tcp_keepalive_intvl=2
Now when i am simulating a disconnection by unplugging the network cable of the Client machine(after it has established the connection and received the initial data from the Server), I am seeing two different outcomes which i am unable to understand:-
If i unplug the cable after 10 to 15 seconds of successful client connection. On the "Server" I receive an IO Exception with "no route to host" after 10 minutes of unplugging the cable.
If i unplug the cable after 60 or so seconds of successful client connection. On the "Server" an IO exception is thrown with "Connection timed out" within 10 seconds. This is valid behavior keeping in mind the keep alive settings.
I have tried this a couple of times and i always get the same result.
What i don't understand is why the first outcome takes 10 minutes and it doesn't behave like the second outcome. Am i missing something?

Blackberry Connection Nightmare

I'm busy writing a Program that Transmits GPS Coordinates to a Server from a mobile phone where the coordinates are then used for calculations. But I'm constantly hitting a wall with blackberry. I have built the Android App and it works great but can't seem to contact the server on a real blackberry device. I have tested the application in a simulator and it works perfectly but when I install it on a real phone I get no request the phone.
I have read quite a bit about the secret strings to append at the end of the url so I adapted some demo code to get me the first available transport but still nothing ...
The Application is Signed and I normally then either install it by debugging through eclipse or directly on the device from the .jad file and allow the application the required permissions.
The current code was adapted from the HTTP Connection Demo in the Blackberry SDK.
Could you have a look and give me some direction. I'm losing too much hair here ...
The Backend Service that keeps running:
public void run() {
System.out.println("Starting Loop");
Criteria cr = new Criteria();
cr.setHorizontalAccuracy(Criteria.NO_REQUIREMENT);
cr.setVerticalAccuracy(Criteria.NO_REQUIREMENT);
cr.setCostAllowed(false);
cr.setPreferredPowerConsumption(Criteria.NO_REQUIREMENT);
cr.setPreferredResponseTime(1000);
LocationProvider lp = null;
try {
lp = LocationProvider.getInstance(cr);
} catch (LocationException e) {
System.out.println("*****************Exception" + e);
}
if (lp == null) {
UiApplication.getUiApplication().invokeLater(new Runnable() {
public void run() {
Dialog.alert("GPS not supported!");
return;
}
});
} else {
System.out
.println(lp.getState() + "-" + LocationProvider.AVAILABLE);
switch (lp.getState()) {
case LocationProvider.AVAILABLE:
// System.out.println("Provider is AVAILABLE");
while (true) {
Location l = null;
int timeout = 120;
try {
l = lp.getLocation(timeout);
final Location fi = l;
System.out.println("Got a Coordinate "
+ l.getQualifiedCoordinates().getLatitude()
+ ", "
+ l.getQualifiedCoordinates().getLongitude());
System.out.println("http://" + Constants.website_base
+ "/apis/location?device_uid=" + Constants.uid
+ "&lat="
+ l.getQualifiedCoordinates().getLatitude()
+ "&lng="
+ l.getQualifiedCoordinates().getLongitude());
if (!_connectionThread.isStarted()) {
fetchPage("http://"
+ Constants.website_base
+ "/apis/location?device_uid="
+ Constants.uid
+ "&lat="
+ l.getQualifiedCoordinates().getLatitude()
+ "&lng="
+ l.getQualifiedCoordinates()
.getLongitude());
} else {
createNewFetch("http://"
+ Constants.website_base
+ "/apis/location?device_uid="
+ Constants.uid
+ "&lat="
+ l.getQualifiedCoordinates().getLatitude()
+ "&lng="
+ l.getQualifiedCoordinates()
.getLongitude());
}
Thread.sleep(1000 * 60);
} catch (LocationException e) {
System.out.println("Location timeout");
} catch (InterruptedException e) {
System.out.println("InterruptedException"
+ e.getMessage());
} catch (Exception ex) {
System.err.println(ex.getMessage());
ex.printStackTrace();
}
}
}
}
My Connection is Made with:
ConnectionFactory connFact = new ConnectionFactory();
ConnectionDescriptor connDesc = connFact.getConnection(getUrl());
// Open the connection and extract the data.
try {
// StreamConnection s = null;
// s = (StreamConnection) Connector.open(getUrl());
HttpConnection httpConn = (HttpConnection) connDesc.getConnection();
/* Data is Read here with a Input Stream */
Any Ideas ?

Figured it out!
Using a function I found online to determine which ; extension to use when connecting by using numerous Try / Catch. Then had to set the Internet APN settings. I'm in South-Africa using Vodacom so the APN is "Internet" with no Password.
Barely have hair left ....