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simple barrier synchronisation in java

im trying to understand the barrier problem. im still really new to programming but was presented with this problem in class to solve.
"I have to solve the barrier problem using counting semaphores. You can assume that there is a shared variable N which indicates the number of concurrent threads in the system. When the first N −1 threads arrive at the barrier, they should block until the Nth thread arrives, at which point all threads might proceed.
The shared counter variable can be used to keep track of the number of threads that have arrived and the semaphores mutex and barrier can be used to solve the synchronization problem."
import java.util.concurrent.Semaphore;
public class BarrierSynchronization extends Thread {
int N;
int count;
Semaphore mutex;
Semaphore barrier;
public BarrierSynchronization ()
{
this.N = 5;
this.count = 0;
this.mutex = new Semaphore(1);
this.barrier = new Semaphore(0);
}
public void run()
{
try {
mutex.acquire();
count = count + 1;
System.out.println(Thread.currentThread().getName() + ": " + count);
mutex.release();
if (count == N)
{
barrier.release();
System.out.println("All " + count + " threads have reached the barrier. The barrier is now open" );
} // unblock one thread
barrier.acquire();
barrier.release();
System.out.println(Thread.currentThread().getName() + " has passed the barrier");
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
i tried to implement the pseudo code from the little book of semaphores. i called on this thread in main class and ran it but it gives an error about mutex.wait() for some reason. when i remove that bit of the code it runs but shows nothing. what exactly am i supposed to do for this problem?
public class Main {
public static void main(String[] args) throws InterruptedException
{
BarrierSynchronization barrier = new BarrierSynchronization();
Thread bs1 = new Thread(barrier);
Thread bs2 = new Thread(barrier);
Thread bs3 = new Thread(barrier);
Thread bs4 = new Thread(barrier);
Thread bs5 = new Thread(barrier);
bs1.start();
bs2.start();
bs3.start();
bs4.start();
bs5.start();
}
why does it output the rare one before the barrier is unlocked for all the threads? i think im close enough to solving this problem. is a race condition or something?
CLICK TO SEE IMAGE

Might be a bit late, but here is an implementation with a driver code that works. You have to garante mutual exclusion and keep track of the number of threads that arrive at the barrier.
public class Barrier {
private int capacity;
private Semaphore s, exclusao, counter;
public Barrier(int capacity) {
this.capacity = capacity;
counter = new Semaphore(0);
s = new Semaphore(0);
exclusao = new Semaphore(1);
}
public void espera() throws InterruptedException {
exclusao.acquire();
if (counter.availablePermits() < capacity - 1) {
counter.release();
exclusao.release();
s.acquire();
} else {
exclusao.release();
System.out.println("RELEASE ALL");
for (int i = 0; i < capacity; i++) {
s.release();
}
}
}
}
class TesteThread extends Thread {
private Barrier b;
private long waitPeriod;
public TesteThread(long wait, Barrier b) {
this.b = b;
this.waitPeriod = wait;
System.out.println("Thread started" + this.getName());
}
public void espera() throws InterruptedException {
b.espera();
}
#Override
public void run() {
try {
System.out.println("Thread a dormir " + this.getName());
sleep(waitPeriod);
System.out.println("Thread a esperar " + this.getName());
espera();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
class BarrierExample {
public static void main(String[] args) throws InterruptedException {
Barrier BR = new Barrier(5);
TesteThread[] teste = new TesteThread[5];
for (int i = 0; i < teste.length; i++) {
teste[i] = new TesteThread((long) (Math.random() * 1000), BR);
teste[i].start();
}
for (int i = 0; i < teste.length; i++) {
teste[i].join();
}
}
}`package examesFSO.exame2020_normal;
import java.util.concurrent.Semaphore;
public class Barrier {
private int capacity;
private Semaphore s, exclusao, counter;
public Barrier(int capacity) {
this.capacity = capacity;
counter = new Semaphore(0);
s = new Semaphore(0);
exclusao = new Semaphore(1);
}
public void espera() throws InterruptedException {
exclusao.acquire();
if (counter.availablePermits() < capacity - 1) {
counter.release();
exclusao.release();
s.acquire();
} else {
System.out.println("RELEASE ALL");
for (int i = 0; i < capacity; i++) {
s.release();
}
}
exclusao.release();
}
}
class TesteThread extends Thread {
private Barrier b;
private long waitPeriod;
public TesteThread(long wait, Barrier b) {
this.b = b;
this.waitPeriod = wait;
System.out.println("Thread instanciada " + this.getName());
}
public void espera() throws InterruptedException {
b.espera();
}
#Override
public void run() {
try {
System.out.println("Thread a dormir " + this.getName());
sleep(waitPeriod);
System.out.println("Thread a esperar " + this.getName());
espera();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
class BarrierExample {
public static void main(String[] args) throws InterruptedException {
Barrier BR = new Barrier(5);
TesteThread[] teste = new TesteThread[5];
for (int i = 0; i < teste.length; i++) {
teste[i] = new TesteThread((long) (Math.random() * 1000), BR);
teste[i].start();
}
for (int i = 0; i < teste.length; i++) {
teste[i].join();
}
}
}

Monitor in my java program gets into a deadlock

I'm trying to solve single consumer/producer problem using monitor in Java, and the code is as follows. When I run this code, it will finally get stucked. The most typical case is that the consumer calls wait(), and then the producer keeps producing but cannot notify the consumer (although it will call notify()). I don't know why it's happening. Java code:
import java.util.*;
class Monitor {
int length;
int size;
int begin, end;
int queue[];
private static Random randGenerator;
public Monitor() {}
public Monitor(int length) {
this.length = length;
this.size = 0;
begin = end = 0;
queue = new int[length];
randGenerator = new Random(10);
}
public synchronized void produce() throws InterruptedException {
while(size == length) {
System.out.println("Producer waiting");
wait();
}
int produced = randGenerator.nextInt();
size++;
queue[end] = produced;
end = (end + 1) % length;
System.out.println("Produce element " + produced + " size "+size);
// When size is not 1, no thread is blocked and therefore don't need to notify
if(size == 1) {
System.out.println("Notify consumer");
notify();
}
}
public synchronized void consume() throws InterruptedException {
while(size == 0) {
System.out.println("Consumer waiting, size " + size);
wait();
}
size--;
System.out.println("Consume element " + queue[begin] + " size " + size);
begin = (begin + 1) % length;
if(size == length - 1) {
System.out.println("Notify producer");
notify();
}
}
}
class Producer implements Runnable {
Monitor producer;
public Producer(Monitor m) {
producer = m;
}
#Override
public void run() {
producer = new Monitor();
System.out.println("Producer created");
try {
while(true) {
producer.produce();
}
}
catch (InterruptedException e) {
e.printStackTrace();
}
}
}
class Consumer implements Runnable {
Monitor consumer;
public Consumer(Monitor m) {
consumer = m;
}
#Override
public void run() {
System.out.println("Consumer created");
consumer = new Monitor();
try {
while(true) {
consumer.consume();
}
}
catch (InterruptedException e) {
e.printStackTrace();
}
}
}
public class monitorTest {
public static void main(String args[]) {
Monitor monitor = new Monitor(10);
Thread t1 = new Thread(new Producer(monitor));
Thread t2 = new Thread(new Consumer(monitor));
t1.start();
t2.start();
}
}

When the control of each thread enters the produce() or consume() methods, the size and length are both zero and hence both threads are waiting for the other to notify. Break this and your code will come out of the deadlock.
public synchronized void produce() throws InterruptedException {
while(size == length) { // size is 0 and length is 0; so wait
System.out.println("Producer waiting");
wait();
}
public synchronized void consume() throws InterruptedException {
while(size == 0) { // size is 0 so wait
System.out.println("Consumer waiting, size " + size);
wait();
}
This is happening because you have a default constructor which you are calling inside the run() method of your Producer and Consumer objects.
class Producer implements Runnable {
Monitor producer;
public Producer(Monitor m) {
producer = m;
}
#Override
public void run() {
producer = new Monitor(); // REMOVE THIS
class Consumer implements Runnable {
Monitor consumer;
public Consumer(Monitor m) {
consumer = m;
}
#Override
public void run() {
System.out.println("Consumer created");
consumer = new Monitor(); // AND REMOVE THIS
Hope this helps!

Java synchronized queue thread on producer and consumer

Java producer-consumer program using thread & synchronized queue, the program is separated into 3 classes but it couldn't be run.
Queue.java:
public class Queue {
static final int MAXQUEUE = 3;
int[] queue = new int[MAXQUEUE];
int front, rear;
public Queue(){ front = 0; rear = 0; }
public boolean isEmpty(){ return (front==rear); }
public boolean isFull(){
int index = rear+1 < MAXQUEUE ? rear+1 : 0;
return (index == front);
}
public void enqueue(int value) {
queue[rear] = value;
rear = rear+1 < MAXQUEUE ? rear+1 : 0;
}
public int dequeue(){
int data = queue[front];
front = front+1 < MAXQUEUE ? rear+1 : 0;
return data;
}
}
SynchronizedQueue.java:
import java.util.Queue;
public class SynchronizedQueue {
Queue queue;
public SynchronizedQueue() {queue = new Queue(); }
public synchronized void enqueue(int value) {
try {
while (queue.isFull()) {
System.out.println();
System.out.println("Queue is full, please wait....");
wait();
}
}
catch (InterruptedException e) { }
((SynchronizedQueue) queue).enqueue(value);
notify();
}
public synchronized int dequeue() {
try {
while (queue.isEmpty()) {
System.out.println();
System.out.println("Queue is empty, please wait....");
wait();
}
}
catch ( InterruptedException e ) { }
int data = ((SynchronizedQueue) queue).dequeue();
notify();
return data;
}
}
Main program Ch10_3.java:
class Producer extends Thread {
public int count = 0;
public void run() {
int value;
while ( Ch10_3.isRunning ) {
value = (int)(Math.random()*100);
Ch10_3.squeue.enqueue(value);
System.out.print(">" + value + "]");
count++;
try {
Thread.sleep((int)(Math.random()*100));
}
catch( InterruptedException e) { }
}
System.out.println("\n" + Thread.currentThread() + "Producer thread end.");
}
}
class Consumer extends Thread {
public int count = 0;
public void run() {
int data;
while (Ch10_3.isRunning) {
data = Ch10_3.squeue.dequeue();
System.out.println("[" + data + ">");
count++;
try {
Thread.sleep((int)(Math.random()*100));
}
catch( InterruptedException e) { }
}
System.out.println("\n" + Thread.currentThread() + "Consumer thread end.");
}
}
public class Ch10_3 {
static final int MAXITEMS = 10;
static SynchonizedQueue squeue = new SynchronizedQueue();
static boolean isRunning = true;
public static void main(String[] args) {
Producer producer = new Producer();
Consumer consumer = new Consumer();
producer.start(); consumer.start();
while (true)
if (producer.count >= MAXITEMS && producer.count == consumer.count)
{ isRunning = false; break; }
}
}
Error message:
Exception in thread "main" java.lang.Error: Unresolved compilation
problem: at Ch10_3.main(Ch10_3.java:41)

In the catch blocks from enqueue and dequeue methods form class SynchronizedQueue you are trying to cast the queue member attribute which is of type Queue, to SynchronizedQueue.
In SynchronizedQueue.enqueue() we have:
((SynchronizedQueue) queue).enqueue(value);
Since there is no relation between Queue and SynchronizedQueue the compiler gives a compilation error. You should remove the cast.
But the best solution is to just use a java.util.concurrent.BlockingQueue implementation available in JAVA SDK, which will handle all the synchronisation part for you.

Deadlock in Producer - consumer thread

I have a problem with possible deadlock in "producer - consumer task".
Everything should be working in following way:
Producer should generate and add int[] arrays to collection
Consumer should take those arrays, put them to second collection and print in output
In debbug mode I have noticed that after a while both taks are suspended on this.wait(); method.
Could you help me and explain what is wrong with this code? :)
Thanks!
Producer task class
public class ProducerTask extends Thread{
private static final Object bufforLock = new Object();
private static LinkedList<Integer[]> buffor;
public ProducerTask(){
if(buffor == null)
buffor = new LinkedList<>();
this.setName("#ProducerTask");
}
#Override
public void run() {
synchronized (this) {
try {
for (int i = 0; i < 100; i++) {
while (isBufforFull()) {
System.err.println("ProducerTask is waiting");
this.wait();
}
Integer[] randomIntArray = getRandomIntArray();
addToBuffor(randomIntArray);
}
}
catch (InterruptedException ex) {
}
}
}
public static void removeLast(){
synchronized(bufforLock){
buffor.removeLast();
bufforLock.notifyAll();
}
}
public static Integer[] getLast(){
synchronized(bufforLock){
return buffor.getLast();
}
}
public static boolean isBufforFull(){
synchronized(bufforLock){
return buffor.size() == 10;
}
}
public static boolean isBufforEmpty(){
synchronized(bufforLock){
return buffor.isEmpty();
}
}
public static void addToBuffor(Integer[] array){
synchronized(bufforLock){
buffor.addFirst(array);
bufforLock.notifyAll();
}
}
public static LinkedList<Integer[]> getBuffor(){
synchronized(bufforLock){
return buffor;
}
}
private Integer[] getRandomIntArray(){
int maxSize = 10;
Integer[] array = new Integer[maxSize];
for(int i = 0 ; i < maxSize ; i++){
int value = (int) (Math.random() * 100);
array[i] = Integer.valueOf(value);
}
return array;
}
}
Consumer task class
public class ConsumerTask extends Thread {
private static LinkedList<Integer[]> buffor;
public ConsumerTask() {
if (buffor == null) {
buffor = new LinkedList<>();
}
this.setName("#ConsumerTask");
}
#Override
public void run() {
synchronized (this) {
try {
while (true) {
while (ProducerTask.isBufforEmpty()) {
System.err.println("ConsumerTask is waiting");
this.wait();
}
Integer[] array = ProducerTask.getLast();
this.arraySortByInserting(array);
this.buffor.addFirst(array);
ProducerTask.removeLast();
}
}
catch (InterruptedException ex) {}
}
}
private Integer[] arraySortByInserting(Integer[] aArrayToSort) {
if(aArrayToSort == null || aArrayToSort.length == 0)
return null;
this.printArray(aArrayToSort, "Array before sorting");
for (int i = 1; i < aArrayToSort.length; i++) {
int intValue = aArrayToSort[i];
int j = i;
while ((j > 0) && (aArrayToSort[j - 1] > intValue)) {
aArrayToSort[j] = aArrayToSort[j - 1];
j--;
}
aArrayToSort[j] = intValue;
}
this.printArray(aArrayToSort, "Array after sorting");
return aArrayToSort;
}
private void printArray(Integer[] aArray, String aMessage) {
System.out.print(aMessage + " [");
for (int intElement : aArray) {
System.out.print(intElement + " ");
}
System.out.print("]");
System.out.println();
}
}

You need a common object which would be used for inter thread communication.
Right now you are using this as object on which you get lock on and you notify on bufferLock in producer thread and same applies for consumer thread.
Remember both are two different instances and both successfully obtain lock on individual objects and then both enters wait state.

notifyAll() method is not working in my code

I am trying to implement Bully Algorithm in Java using threads.
Here is the code which I have written.
package newbully;
public class NewBully {
public static void main(String[] args) {
int total_processes = 4;
Thread1[] t = new Thread1[total_processes];
for (int i = 0; i < total_processes; i++) {
t[i] = new Thread1(new Process(i+1, i+1), total_processes);
}
try {
Election.initialElection(t);
} catch (Exception e) {
System.out.println("Possibly you are using null references in array");
}
for (int i = 0; i < total_processes; i++) {
new Thread(t[i]).start();
}
}
}
package newbully;
public class Election {
private static boolean pingFlag = false;
private static boolean electionFlag = false;
private static boolean messageFlag = false;
public static boolean isMessageFlag() {
return messageFlag;
}
public static void setMessageFlag(boolean messageFlag) {
Election.messageFlag = messageFlag;
}
public static boolean isPingFlag() {
return pingFlag;
}
public static void setPingFlag(boolean pingFlag) {
Election.pingFlag = pingFlag;
}
public static boolean isElectionFlag() {
return electionFlag;
}
public static void setElectionFlag(boolean electionFlag) {
Election.electionFlag = electionFlag;
}
public static void initialElection(Thread1[] t) {
Process temp = new Process(-1, -1);
for (int i = 0; i < t.length; i++) {
if (temp.getPriority() < t[i].getProcess().getPriority()) {
temp = t[i].getProcess();
}
}
t[temp.pid - 1].getProcess().CoOrdinatorFlag = true;
}
}
package newbully;
public class Process {
int pid;
boolean downflag,CoOrdinatorFlag;
public boolean isCoOrdinatorFlag() {
return CoOrdinatorFlag;
}
public void setCoOrdinatorFlag(boolean isCoOrdinator) {
this.CoOrdinatorFlag = isCoOrdinator;
}
int priority;
public boolean isDownflag() {
return downflag;
}
public void setDownflag(boolean downflag) {
this.downflag = downflag;
}
public int getPid() {
return pid;
}
public void setPid(int pid) {
this.pid = pid;
}
public int getPriority() {
return priority;
}
public void setPriority(int priority) {
this.priority = priority;
}
public Process() {
}
public Process(int pid, int priority) {
this.pid = pid;
this.downflag = false;
this.priority = priority;
this.CoOrdinatorFlag = false;
}
}
package newbully;
import java.util.*;
import java.io.*;
import java.net.*;
public class Thread1 implements Runnable {
private Process process;
private int total_processes;
ServerSocket[] sock;
Random r;
public Process getProcess() {
return process;
}
public void setProcess(Process process) {
this.process = process;
}
public Thread1(Process process, int total_processes) {
this.process = process;
this.total_processes = total_processes;
this.r = new Random();
this.sock = new ServerSocket[total_processes];
}
private void recovery() {
}
synchronized private void pingCoOrdinator() {
try {
if (Election.isPingFlag()) {
wait();
}
if (!Election.isElectionFlag()) {
Election.setPingFlag(true);
System.out.println("Process[" + this.process.getPid() + "]: Are you alive?");
Socket outgoing = new Socket(InetAddress.getLocalHost(), 12345);
outgoing.close();
Election.setPingFlag(false);
notifyAll();
}
} catch (Exception ex) {
//Initiate Election
System.out.println("process[" + this.process.getPid() + "]: -> Co-Ordinator is down\nInitiating Election");
Election.setElectionFlag(true);
Election.setPingFlag(false);
notifyAll();
}
}
synchronized private void executeJob() {
int temp = r.nextInt(20);
for (int i = 0; i <= temp; i++) {
try {
Thread.sleep(100);
} catch (InterruptedException e) {
System.out.println("Error Executing Thread:" + process.getPid());
System.out.println(e.getMessage());
}
}
}
synchronized private boolean sendMessage() {
boolean response = false;
int i = 0;
try {
if (Election.isMessageFlag()) {
wait();
}
Election.setMessageFlag(true);
for (i = this.process.getPid() + 1; i <= this.total_processes; i++) {
try {
Socket electionMessage = new Socket(InetAddress.getLocalHost(), 10000 + i);
System.out.println("Process[" + this.process.getPid() + "] -> Process[" + i + "] responded to election message successfully");
electionMessage.close();
response = true;
} catch (Exception ex) {
System.out.println("Process[" + this.process.getPid() + "] -> Process[" + i + "] did not respond to election message");
}
}
Election.setMessageFlag(false);
notifyAll();
} catch (Exception ex1) {
System.out.println(ex1.getMessage());
}
return response;
}
synchronized private void serve() {
try {
//service counter
ServerSocket s = new ServerSocket(12345);
for (int counter = 0; counter <= 10; counter++) {
Socket incoming = s.accept();
System.out.println("Process[" + this.process.getPid() + "]:Yes");
Scanner scan = new Scanner(incoming.getInputStream());
PrintWriter out = new PrintWriter(incoming.getOutputStream(), true);
if (scan.hasNextLine()) {
if (scan.nextLine().equals("Who is the co-ordinator?")) {
System.out.print("Process[" + this.process.getPid() + "]:");
out.println(this.process);
}
}
if (counter == 10) {//after serving 10 requests go down
this.process.setCoOrdinatorFlag(false);
this.process.setDownflag(true);
try {
incoming.close();
s.close();
sock[this.process.getPid() - 1].close();
Thread.sleep((this.r.nextInt(10) + 1) * 50000);//going down
recovery();
} catch (InterruptedException e) {
System.out.println(e.getMessage());
}
}
}
} catch (IOException ex) {
System.out.println(ex.getMessage());
}
}
#Override
public void run() {
try {
sock[this.process.getPid() - 1] = new ServerSocket(10000 + this.process.getPid());
} catch (IOException ex) {
System.out.println(ex.getMessage());
}
while (true) {
if (process.isCoOrdinatorFlag()) {
//serve other processes
serve();
} else {
while (true) {
//Execute some task
executeJob();
//Ping the co-ordinator
pingCoOrdinator();
if (Election.isElectionFlag()) {
if (!sendMessage()) {//elect self as co-ordinator
System.out.println("New Co-Ordinator: Process[" + this.process.getPid() + "]");
this.process.setCoOrdinatorFlag(true);
Election.setElectionFlag(false);
break;
}
}
}
}
}
}
}
When I am trying to execute the code out of the 4 threads which I have created some threads are waiting premanently using wait() call. They are not being notified by notifyAll(). Can anyone suggest why this is happening?

Each thread is calling wait() on itself (on its own Thread1 instance). That means that when you call notifyAll() on that same Thread1 instance, only the single Thread1 that is waiting it will be notified, and not all the other threads.
What you have to do is make all your Thread1 objects call wait() on a single, common object, and also call notifyAll() on that same object.
Ofcourse you have to synchronize on the common object when you call wait() or notifyAll() on it; if you don't do that, you'll get an IllegalMonitorStateException.
// Object to be used as a lock; pass this to all Thread1 instances
Object lock = new Object();
// Somewhere else in your code
synchronized (lock) {
lock.wait();
}
// Where you want to notify
synchronized (lock) {
lock.notifyAll();
}

Both notify() (or notifyAll()) and wait() must be written into synchronized block on the same monitor.
For example:
synchronized(myLock) {
wait();
}
..................
synchronized(myLock) {
notifyAll();
}