CODEWITHSUNDEEP
javaandroidwhile-loopinfinite-loop
daisura99
daisura99

Reputation: 1110

while(true) loop without break

I am quite new to Java programming. For now I am studying source code of an android app called Evercam. However, I have a problem understanding a part of the code which involves while(true) loop.

Here is the snippet of the code: 

while (true)
{
    while (true)
    {
        byte[] responseMessageByteArray = new byte[4000];
        DatagramPacket datagramPacketRecieve = new DatagramPacket(responseMessageByteArray, responseMessageByteArray.length);
        datagramSocket.receive(datagramPacketRecieve);
        String responseMessage = new String(datagramPacketRecieve.getData());
        EvercamDiscover.printLogMessage("\nResponse Message:\n" + responseMessage);
        StringReader stringReader = new StringReader(responseMessage);
        InputNode localInputNode = NodeBuilder.read(stringReader);
        EnvelopeProbeMatches localEnvelopeProbeMatches = (EnvelopeProbeMatches)(new Persister()).read(EnvelopeProbeMatches.class, localInputNode);
        if (localEnvelopeProbeMatches.BodyProbeMatches.ProbeMatches.listProbeMatches.size() > 0)
        {
            ProbeMatch localProbeMatch = (ProbeMatch) localEnvelopeProbeMatches.BodyProbeMatches.ProbeMatches.listProbeMatches.get(0);
            if (uuidArrayList.contains(localProbeMatch.EndpointReference.Address))
            {
                EvercamDiscover.printLogMessage("ONVIFDiscovery: Address " + localProbeMatch.EndpointReference.Address + " already added");
            }
            else
            {
                uuidArrayList.add(localProbeMatch.EndpointReference.Address);
                DiscoveredCamera discoveredCamera = getCameraFromProbeMatch(localProbeMatch);
                if (discoveredCamera.hasValidIpv4Address())
                {
                    this.onActiveOnvifDevice(discoveredCamera);
                    cameraList.add(discoveredCamera);
                }
            }
        }
    }
}

Doesn't this create an infinite loop? My fundamentals in Java isn't strong, so I would be so grateful if anyone can tell in in what instances will a while(true){//codes} actually exits without any break or does it ever exit??

EDIT

My bad for actually extracting this snippet from decompiling directly from the android project files. I did not know that it would be different, and then again, I know very little. Here is the original code:

    public ArrayList<DiscoveredCamera> probe() {
ArrayList<DiscoveredCamera> cameraList = new ArrayList<DiscoveredCamera>();

try {
    DatagramSocket datagramSocket = new DatagramSocket();
    datagramSocket.setSoTimeout(SOCKET_TIMEOUT);
    InetAddress multicastAddress = InetAddress.getByName(PROBE_IP);

    if (multicastAddress == null) {
    // System.out.println("InetAddress.getByName() for multicast returns null");
    return cameraList;
    }

    // Send the UDP probe message
    String soapMessage = getProbeSoapMessage();
    // System.out.println(soapMessage);
    byte[] soapMessageByteArray = soapMessage.getBytes();
    DatagramPacket datagramPacketSend = new DatagramPacket(
        soapMessageByteArray, soapMessageByteArray.length,
        multicastAddress, PROBE_PORT);
    datagramSocket.send(datagramPacketSend);

    ArrayList<String> uuidArrayList = new ArrayList<String>();
    while (true) {
    // System.out.println("Receiving...");
    byte[] responseMessageByteArray = new byte[4000];
    DatagramPacket datagramPacketRecieve = new DatagramPacket(
        responseMessageByteArray,
        responseMessageByteArray.length);
    datagramSocket.receive(datagramPacketRecieve);

    String responseMessage = new String(
        datagramPacketRecieve.getData());

    EvercamDiscover.printLogMessage("\nResponse Message:\n"
        + responseMessage);

    StringReader stringReader = new StringReader(responseMessage);
    InputNode localInputNode = NodeBuilder.read(stringReader);
    EnvelopeProbeMatches localEnvelopeProbeMatches = new Persister()
        .read(EnvelopeProbeMatches.class, localInputNode);
    if (localEnvelopeProbeMatches.BodyProbeMatches.ProbeMatches.listProbeMatches
        .size() <= 0) {
        continue;
    }

    ProbeMatch localProbeMatch = localEnvelopeProbeMatches.BodyProbeMatches.ProbeMatches.listProbeMatches
        .get(0);
    // EvercamDiscover.printLogMessage("Probe matches with UUID:\n"
    // +
    // localProbeMatch.EndpointReference.Address + " URL: " +
    // localProbeMatch.XAddrs);
    if (uuidArrayList
        .contains(localProbeMatch.EndpointReference.Address)) {
        EvercamDiscover.printLogMessage("ONVIFDiscovery: Address "
            + localProbeMatch.EndpointReference.Address
            + " already added");
        continue;
    }
    uuidArrayList.add(localProbeMatch.EndpointReference.Address);
    DiscoveredCamera discoveredCamera = getCameraFromProbeMatch(localProbeMatch);

    if (discoveredCamera.hasValidIpv4Address()) {
        onActiveOnvifDevice(discoveredCamera);
        cameraList.add(discoveredCamera);
    }
    }
} catch (Exception e) {
    // ONVIF timeout. Don't print anything.
}

Turns out there is continue statement in the actual code. Thank you so much for the response, I will remember that de-compiled classes should not be depended on.

Upvotes: 5

Views: 1818

Answers (3)

L01c
L01c

Reputation: 1063

Infinite loops without any breaks could be useful for a Service running in background.

You create a new Thread doing the service infinitely thanks to a while(true) and when you stop your application you simply kill the process corresponding to the service.

Upvotes: 0

Ganesh Karewad
Ganesh Karewad

Reputation: 1218

the only possible way to exit from while loop is if one of the methods being called in loop throws exception. check code of these methods for exception or share it here

Upvotes: 0

M A
M A

Reputation: 72844

This looks like an infinite loop. To be absolutely sure, you would have to statically read every statement and follow invoked methods to see if any possible invocations like Activity#finish() or Service#stopSelf() exists which would finish the currently running activity, effectively breaking the loop.

Another possibility is that the code is intended to be running in an infinite loop as a background thread service, and some other component would have an option to kill that service when it reaches a certain condition. For example, it could be part of a Runnable class and executed in a thread pool, and when a timeout exists, the pool is shut down.

Upvotes: 4

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