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Why is my boolean not being changed?

So I'm trying to create a client/server program. I want to know when my client disconnects of his own accord, so I've setup a heartbeat system. Every 6 seconds my client sends a ping to my server, if the client doesn't send a ping for a total of 30 seconds the client is considered disconnected and removed from the current connections list (for which I plan to implement a GUI). Or at least, that's the plan.
ConnectionManager.java
public class ConnectionManager implements Runnable{
static Socket connection;
private ArrayList<Thread> allConnections;
private ArrayList<Connection> allConnectionList;
private ServerSocket server;
private int id = 0;
public ConnectionManager() {
allConnections = new ArrayList<Thread>();
allConnectionList = new ArrayList<Connection>();
}
#Override
public void run() {
try {
server = new ServerSocket(5555);
System.out.println("Server is running!");
while(true) {
connection = server.accept();
Connection a = new Connection(connection, id);
Runnable runnable = a;
allConnectionList.add(a);
allConnections.add(new Thread(runnable));
allConnections.get(allConnections.size() - 1).start();
id++;
}
} catch (IOException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
public void removeConnection(int id) {
allConnections.remove(id);
allConnectionList.remove(id);
}
Connection.java
public class Connection implements Runnable {
private Socket a;
public boolean amIActive;
private int id;
public Connection(Socket a, int id) {
amIActive = true;
this.a = a;
this.id = id;
}
public void onConnect() {
try {
String TimeStamp = new java.util.Date().toString();
String formattedAddress = a.getInetAddress().toString().replace("/", "");
System.out.println("Received connection from: " + formattedAddress + " at " + TimeStamp);
Runnable runnable = new ConnectionListener(this);
Thread connectionThread = new Thread(runnable);
connectionThread.start();
String returnCode = "Server repsonded to " + a.getInetAddress().toString().replace("/", "") + " at "+ TimeStamp + (char) 13;
BufferedOutputStream os = new BufferedOutputStream(a.getOutputStream());
OutputStreamWriter osw = new OutputStreamWriter(os, "US-ASCII");
osw.write(returnCode);
osw.flush();
} catch (IOException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
#Override
public void run() {
onConnect();
System.out.println("We got this far!");
while(amIActive) {
whileTrue();
}
System.out.println("This code never gets run because we get stuck in the while loop above");
Main.b.removeConnection(id);
System.out.println("Connection was closed from " + a.getInetAddress());
}
public void setOffline(boolean state) {
this.amIActive = state;
}
public void whileTrue() {
}
public Socket getSocket() {
return a;
}
ConnectionListener.java
public class ConnectionListener implements Runnable{
public Connection myConnection;
public boolean receivedHeartbeat;
public int missedHeartbeats = 0;
public ConnectionListener(Connection a) {
this.myConnection = a;
}
#Override
public void run() {
Runnable runnable = new Heartbeat(this);
Thread thread = new Thread(runnable);
thread.start();
while(myConnection.amIActive) {
try {
BufferedInputStream is;
is = new BufferedInputStream(myConnection.getSocket().getInputStream());
InputStreamReader isr = new InputStreamReader(is);
StringBuffer process = new StringBuffer();
int character;
while((character = isr.read()) != 13) { //GETTING STUCK HERE BECAUSE STUPID.
if(character == -1) {
myConnection.setOffline(true);
} else {
process.append((char)character);
}
}
handleInput(process);
} catch (Exception e) {
e.printStackTrace();
}
}
}
public void handleInput(StringBuffer process) {
String messageSent = process.toString();
if(messageSent.equals("Ping!")) {
receivedHeartbeat = true;
}
}
Heartbeat.java
public class Heartbeat implements Runnable{
private ConnectionListener b;
public Heartbeat(ConnectionListener a) {
b = a;
}
#Override
public void run() {
while(true) {
try {
Thread.sleep(1000);
if(b.missedHeartbeats > 5) {
b.myConnection.amIActive = false;
System.out.println("Setting amIActiveToFalse!");
}
if(b.receivedHeartbeat) {
b.receivedHeartbeat = false;
} else {
b.missedHeartbeats++;
}
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
}
My console is spammed with System.out.println("Setting amIActiveToFalse!"); from Heartbeat.java. But the while loop in Connection.java keeps running. I believe this might be something to do with my threading, but I can't figure it out.

When you have a non-volatile variable, there is no guarentee of visability of a change in one thread to another. In particular, if the JVM detects that a thread doesn't alter a boolean it can inline it, meaning you will never see the value change.
The simple solution is to make the boolean volatile and it will not be inlined and one thread will see when another changes it.
For more details http://vanillajava.blogspot.com/2012/01/demonstrating-when-volatile-is-required.html

The trivial answer to this is: make the variable volatile.
Without this, it is allowed for the thread changing the value to basically keep its updates in cache, committing them to main memory some time later.
This allows threaded code to run much faster, since it can keep its variables in cache rather than having to fetch from main memory. However, the consequence of this is that other threads don't see the update.
Making the variable volatile prevents this from happening: a thread always reads the value from main memory, and writes are immediately committed.
I say that this is the trivial answer because it doesn't necessarily fix all of your problems. There may also be an atomicity issue: in between one thread reading the variable and writing it again, another thread might sneak in and change its value, which may or may not put the first thread into an undefined state from the perspective of its invariants.
Specifically:
if(b.receivedHeartbeat) { b.receivedHeartbeat = false;
It is possible that some other thread can change b.receivedHeartbeat to false after this thread evaluates it to true, so this iteration is erroneously counted as a "non-missed" heartbeat.
This can be fixed by making the variable a (non-volatile) AtomicBoolean, on which there is an atomic compare-and-set method, which avoids such race conditions.
Java Concurrency In Practice is a great reference on these issues, I wholeheartedly recommend it. Look for the topics "visibility" and "atomicity".
Also read the advanced chapter on the Java Memory Model. That made me doubt myself at first, but made me a much stronger programmer after I digested it.

There are a couple issues I saw while debugging the code you posted, but I was able to successfully get the heartbeat functionality working.
In the Connection Listener class I don't think the if statement with .equals("Ping!") will match, because of the newline character at the end of each line.
In the Connection Listener class I would probably put the socket's Input Stream at the top of the loop not inside the loop. (I don't think this will break it but it's probably nicer this way)
ConnectionListener Updates:
public void run() {
Runnable runnable = new Heartbeat(this);
Thread thread = new Thread(runnable);
thread.start();
BufferedReader br = null;
try {
//is = new BufferedInputStream(myConnection.getSocket().getInputStream());
br = new BufferedReader(new InputStreamReader(myConnection.getSocket().getInputStream()));
} catch (IOException e1) {
// TODO Auto-generated catch block
e1.printStackTrace();
}
while(myConnection.amIActive) {
try {
String processLine = br.readLine();
System.out.println("handleInput:" + processLine);
handleInput(processLine);
} catch (Exception e) {
System.out.println("Exception!");
e.printStackTrace();
}
}
}
public void handleInput(String messageSent) {
if(messageSent.startsWith("Ping!")) { //Need to use startsWith, or add newline character
receivedHeartbeat = true;
System.out.println("receivedHeartbeat!");
}
}
Also, in your Heartbeat class make sure you reset the missedHeartbeats counter to 0 on true:
if(b.receivedHeartbeat) {
b.receivedHeartbeat = false;
b.missedHeartbeats = 0;
} else {
b.missedHeartbeats++;
}

How to get value from new Thread(new Runnable() { public void run() {} in Android

Here is my code and in my class I have declared ansResultinThread varible as follows.
protected static String ansResultinThread = "";
Whne I try to access to ansResultinThread value after the callServerForResult method is completed. I am not getting the Updated value.
public void callServerForResult(String ans, String casCmd) {
// String ParsedEquation = parseQuestionForSymPy(ans);
String ParsedEquation = ans;
final String stringUrl = "http://localhost:40001/" + casCmd + "/"
+ ParsedEquation;
new Thread(new Runnable() {
public void run() {
String Result = "";
try {
Result = GetServerResult(stringUrl);
} catch (IOException e) {
System.out.println(e);
}
ansResultinThread = Result;
}
}).start();
}
How to fix this Issue.

Well, since it happens asynchronously, you need to use the Future interface (see here: http://www.vogella.com/tutorials/JavaConcurrency/article.html , 8. Futures and Callables)

Your solution do not work because callServerForResult returns immediately after the thread is started.
Also keep in mind that using a static variable with multiple threads is generally not a good idea.
The simplest solution in my opinion is to use an AsyncTask instead of a Thread.
If you want to keep a Thread, another solution is to use a callback.
private final Handler handler = new Handler();
// Definition of the callback interface
public interface Callback {
void onResultReceived(String result);
}
public void callServerForResult(String ans, String casCmd, final Callback callback) {
// String ParsedEquation = parseQuestionForSymPy(ans);
String ParsedEquation = ans;
final String stringUrl = "http://localhost:40001/" + casCmd + "/"
+ ParsedEquation;
new Thread(new Runnable() {
public void run() {
try {
final String result = GetServerResult(stringUrl);
// Use a handler to invoke the callback on the main thread
handler.post(new Runnable() {
public void run() {
callback.onResultReceived(result);
}
});
} catch (IOException e) {
System.out.println(e);
}
}
}).start();
}
// Usage
callServerForResult("...", "...", new Callback() {
public void onResultReceived(String result) {
// Do something with the result
}
});
The Handler is necessary if the callback code needs to run on the same thread as the caller (usually the main thread)

Java looping Threads using CyclicBarrier

I have a program with this general structure:
init
create CyclicBarrier
initialise all threads, attaching to barrier
*start all threads*
wait for join
display stats
*start all threads*
perform calculation
await barrier
My problem is I need the threads' run() method to keep looping until a certain condition is met, but pausing after every iteration to let all threads synchronise.
I've already tried attaching a Runnable method to the barrier, but this ends up requiring the recreation and restarting of each thread, which isn't a very good solution.
I've also tried using the CyclicBarrier's reset() method, but this just seems to cause errors on the existing threads, even when executed after all threads have completed.
My question is:
-Is it possible to 'reset' a barrier and have all the barrier's threads follow the same conditions as they did before the first invocations of await()?
-Or is there another method I should be using to achieve this?
Thanks in advance

The barrier.wait() will suspend the threads. The barrier is already in the main thread, it does not need another. In your algorithm above you show the threads being restarted after displaying stats. You should not need to do this. If the recently awakened threads are in a loop they will go back into the barrier.wait() again.

Following #Totoro's answer, below is a little bit of example code which also incorporates the requirement "I need the threads' run() method to keep looping until a certain condition is met, pausing after every iteration to let all threads synchronise". That makes it complex pretty quick, but hopefully the program output will clarify the example code (or I should just make better examples).
import java.util.concurrent.CyclicBarrier;
import java.util.concurrent.atomic.AtomicBoolean;
import java.util.concurrent.atomic.AtomicInteger;
public class BarrierCalc implements Runnable {
public static final int CALC_THREADS = 3;
private static final AtomicBoolean runCondition = new AtomicBoolean();
private static final AtomicBoolean stopRunning = new AtomicBoolean();
public static void main(String[] args) {
CyclicBarrier barrier = new CyclicBarrier(CALC_THREADS + 1);
for (int i = 0; i < CALC_THREADS; i++) {
new Thread(new BarrierCalc(barrier)).start();
}
try {
runCondition.set(true);
barrier.await();
showln(0, "STATS!");
barrier.await();
showln(0, "start looping 1");
Thread.sleep(200);
runCondition.set(false);
showln(0, "stop looping 1");
barrier.await();
runCondition.set(true);
barrier.await();
showln(0, "start looping 2");
Thread.sleep(100);
runCondition.set(false);
showln(0, "stop looping 2");
barrier.await();
stopRunning.set(true);
showln(0, "finishing");
barrier.await();
} catch (Exception e) {
e.printStackTrace();
}
}
private static final AtomicInteger calcId = new AtomicInteger();
private CyclicBarrier barrier;
private int id;
public BarrierCalc(CyclicBarrier barrier) {
this.barrier = barrier;
id = calcId.incrementAndGet();
}
public void run() {
showln(id, "waiting for start");
try {
barrier.await(); // display stats
barrier.await(); // start running
int loopNumber = 0;
while (!stopRunning.get()) {
showln(id, "looping " + (++loopNumber));
while (runCondition.get()) {
Thread.sleep(10); // simulate looping
}
showln(id, "synchronizing " + loopNumber);
barrier.await();
showln(id, "synchronized " + loopNumber);
// give main thread a chance to set stopCondition and runCondition
barrier.await();
}
showln(id, "finished");
} catch (Exception e) {
e.printStackTrace();
}
}
private static final long START_TIME = System.currentTimeMillis();
public static void showln(int id, String msg) {
System.out.println((System.currentTimeMillis() - START_TIME) + "\t ID " + id + ": " + msg);
}
}
Keep in mind that program output might not be in the order expected: threads that are writing at the same time to one synchronized output (System.out) are given write-access in random order.

You can take a look at my example where I played with CyclicBarrier.Here each worker makes some calculation and at the barrier the condition is checked. If it meets the condition than all workers stop calculations, otherwise they continue:
class Solver {
private static final int REQUIRED_AMOUNT = 100;
private static final int NUMBER_OF_THREADS = 4;
AtomicInteger atomicInteger = new AtomicInteger();
AtomicBoolean continueCalculation = new AtomicBoolean(true);
final CyclicBarrier barrier;
public static void main(String[] args) {
new Solver();
}
class Worker implements Runnable {
int workerId;
Worker(int workerId) {
this.workerId = workerId;
}
public void run() {
try {
while(continueCalculation.get()) {
calculate(workerId);
barrier.await();
}
} catch (Exception ex) {
System.out.println("Finishing " + workerId);
}
}
}
public Solver() {
Runnable barrierAction = () -> {
if (done()) {
continueCalculation.set(false);
}
};
barrier = new CyclicBarrier(NUMBER_OF_THREADS, barrierAction);
List<Thread> threads = new ArrayList(NUMBER_OF_THREADS);
for (int i = 0; i < NUMBER_OF_THREADS; i++) {
Thread thread = new Thread(new Worker(i));
threads.add(thread);
thread.start();
}
}
private void calculate(int workerId) throws InterruptedException {
// Some long-running calculation
Thread.sleep(2000L);
int r = new Random().nextInt(12);
System.out.println("Worker #" + workerId + " added " + r +" = " + atomicInteger.addAndGet(r));
}
private boolean done() {
int currentResult = atomicInteger.get();
boolean collected = currentResult >= REQUIRED_AMOUNT;
System.out.println("=======================================================");
System.out.println("Checking state at the barrier: " + currentResult);
if (collected) {
System.out.println("Required result is reached");
}
System.out.println("=======================================================");
return collected;
}
}

How can I make the thread sleep for a while and then process all the messages?

I'm writing an Android app that uses two threads. One is UI thread and the other handles server communication. Is it possible for the other thread to wait for a specified amount of time and then process all the messages that have arrived and then wait again?
I need this so that I can collect different data and send it to server in one session.
I've build my thread with HandlerThread but now I'm stuck. Can anyone point me to the right direction?
This is the code I'm using inside the second thread:
public synchronized void waitUntilReady() {
serverHandler = new Handler(getLooper()){
public void handleMessage(Message msg) { // msg queue
switch(msg.what) {
case TEST_MESSAGE:
testMessage(msg);
break;
case UI_MESSAGE:
break;
case SERVER_MESSAGE:
break;
default:
System.out.println(msg.obj != null ? msg.obj.getClass().getName() : "is null");
break;
}
}
};
}
EDIT:
I resolved my issue by going with Thread instead of HandlerThread and using queue.
I'm new to programming so I apologize for any horrenous errors but here's the code I ended up using.
public class ServiceThread extends Thread {
// TODO maybe set the thread priority to background?
static ServiceThread sThread = new ServiceThread(); // service thread instance
private volatile Handler mainHandler;
//
public Thread mainThread;
private boolean OK = true;
public Queue<MessageService> msgQueue;
private ThreadPoolExecutor exec;
private ServiceThread() { }
#Override
public void run() {
synchronized (this){
msgQueue = new ConcurrentLinkedQueue<MessageService>();
notifyAll();
}
mainHandler = new Handler(Looper.getMainLooper());
ThreadPoolExecutor exPool = (ThreadPoolExecutor) Executors.newFixedThreadPool(2);
exec = exPool;
// MAIN LOOP
try {
while(OK) {
getMessagesFromQueue();
Thread.sleep(3000);
}
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
//end of loop
}
public void ProcessMessage(MessageService message) {
System.err.println("ProcessMessage with command: " + message.command);
}
/** Called from the Main thread. Waits until msgQueue is instantiated and then passes the reference
* #return Message Queue
*/
public Queue<MessageService> sendQueue() {
synchronized (this){
while(msgQueue == null) {
try {
wait();
} catch (InterruptedException e) {
// TODO Auto-generated catch block -- move the try block!
e.printStackTrace();
}
}
}
return msgQueue;
}
public void setOkFalse () {
if (OK == true)
OK = false;
}
// Message handling methods
/** Priority message from UI thread, processed in another thread ASAP.
* Should be used on commands like getBigPicture or getPics when cached pics are running out
* or upload picture etc.
* #param message - Message should always be MessageService class
* TODO check that it really is.
*/
public void prioTask (MessageService message) {
final MessageService taskMsg = message;
Runnable task = new Runnable() {
#Override
public void run(){
ProcessMessage(taskMsg);
}
};
exec.execute(task);
}
/**
* Gets messages from queue, puts them in the list, saves the number of messages retrieved
* and sends them to MessageService.handler(int commands, list messageList)
* (method parameters may change and probably will =) )
*/
public void getMessagesFromQueue() {
int commands = 0;
ArrayList <MessageService> msgList = new ArrayList <MessageService>();
while(!msgQueue.isEmpty()) {
if(msgQueue.peek() instanceof MessageService) {
//put into list?
msgList.add(msgQueue.remove());
commands++;
} else {
//Wrong type of message
msgQueue.remove();
System.err.println("getMessagesFromQueue: Message not" +
" instanceof MessageService, this shouldn't happen!");
}
}
if (commands > 0) {
HTTPConnection conn;
try {
conn = new HTTPConnection();
MessageService.handleSend(commands, msgList, conn);
conn.disconnect();
} catch (IOException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
}
}
P.S. This is also my first post here. Should I mark it solved or something? How?

How to achieve multi threading while one thread is at sleep mode

I have a problem where my class is performing the first run method after which it is not proceeding into a second, overidden run method.
The program execution beings in a controller class which has a main method and a thread pool:
public class RunnableController {
// Main method
public static void main(String[] args) throws InterruptedException {
try {
RunnableController controller = new RunnableController();
controller.initializeDb();
controller.initialiseThreads();
System.out.println("Polling");
} catch (Exception e) {
e.printStackTrace();
}
}
private void initialiseThreads() {
try {
threadExecutorRead = Executors.newFixedThreadPool(10);
PollingSynchronizer read = new PollingSynchronizer(incomingQueue, dbConncetion);
threadExecutorRead.submit(read);
} catch (Exception e){
e.printStackTrace();
}
}
}
My poller class which fetches new data and should do updating simulateously:
public class PollingSynchronizer implements Runnable {
public PollingSynchronizer(Collection<KamMessage> incomingQueue,
Connection dbConnection) {
super();
this.incomingQueue = incomingQueue;
this.dbConnection = dbConnection;
}
private int seqId;
public int getSeqId() {
return seqId;
}
public void setSeqId(int seqId) {
this.seqId = seqId;
}
// The method which runs Polling action and record the time at which it is done
public void run() {
int seqId = 0;
while (true) {
List<KamMessage> list = null;
try {
list = fullPoll(seqId);
if (!list.isEmpty()) {
seqId = list.get(0).getSequence();
incomingQueue.addAll(list);
this.outgoingQueue = incomingQueue;
System.out.println("waiting 3 seconds");
System.out.println("new incoming message");
Thread.sleep(3000);//at this wait I should execute run()
//when I debug my execution stops here and throws " Class not found Exception "
// its does not enters the message processor class
MessageProcessor processor = new MessageProcessor() {
//the run method which should fetch the message processor class.
final public void run() {
MessageProcessor(outgoingQueue).generate(outgoingQueue);
}
};
new Thread(processor).start();
}
} catch (Exception e1) {
e1.printStackTrace();
}
}
}
}
My message processor class:
public abstract class MessageProcessor implements Runnable {
private Connection dbConnection;
Statement st = null;
ResultSet rs = null;
PreparedStatement pstmt = null;
private Collection<KamMessage> outgoingQueue;
public KamMsg804 MessageProcessor(Collection<KamMessage> outgoingQueue,
Connection dbConnection) {
this.outgoingQueue = outgoingQueue;
this.dbConnection = dbConnection;
return (KpiMsg804) fetchedMessages;
}
public Collection<KamMessage> generate(Collection<KamMessage> outgoingQueue) {
while (true) {
try {
while (rs.next()) {
KamMessage filedClass = convertRecordsetToPojo(rs);
outgoingQueue.add(filedClass);
}
for (KamMessage pojoClass : outgoingQueue) {
KamMsg804 updatedValue = createKamMsg804(pojoClass);
System.out.print(" " + pojoClass.getSequence());
System.out.print(" " + pojoClass.getTableName());
System.out.print(" " + pojoClass.getAction());
System.out.print(" " + updatedValue.getKeyInfo1());
System.out.print(" " + updatedValue.getKeyInfo2());
System.out.println(" " + pojoClass.getEntryTime());
}
return outgoingQueue;
} catch (SQLException e) {
e.printStackTrace();
}
}
}
}
My problem is exactly at the second run(9 method where I am getting exception in MessageProcessor class and it loops back to Polling.
How do I implement multithreading here, as when the thread sleeps for 3 seocnds in polling it should simultaneously update the database.
After which, how can the data be fed and updated back into the db.
My program flow - I have three classes:
1.Controller
2.PollerSynchro
3.Msgprocessor
I have database records, which are converted into POJO form and stored in a Collection. With these POJOs my classes try to do multiprocessing and updating in a single stretch.
Controller - has the thread pool, initiates poller class with poll method - done
Poller - should poll for new incoming messages and stores it in incoming queue - done
MsgProcessor - should look for new incoming messages and pass them from outgoing queue to incoming queue - also done
Problem:
Now my problem is
I have to implement this update while the poll thread sleeps for 3 sec,
In my code for the second void run() method in the Poller class, the outgoing queue is not passed and fed to the messageprocessor class for updating. My flow of execution only just loops back to first run method and am getting Class exception.
Please help me to solve these problems.

I can't sugar coat this, your code is a mess. However, as far as why your message processor code is not being executed, you never actually start the thread you created with this code:
MessageProcessor processor = new MessageProcessor() {
// the run method which should fetch the message processor class.
final public void run() {
MessageProcessor(outgoingQueue).generate(outgoingQueue);
}
};
Ignoring the confusingly named method being called, your code should look more like this:
Message processor = new MessageProcessor() {
// the run method which should fetch the message processor class.
final public void run() {
MessageProcessor(outgoingQueue).generate(outgoingQueue);
}
};
new Thread(processor).start();