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Single Producer Multiple Consumer Java

I am new to Java Concurrency and trying to achieve/implement Single Producer[P1] and Multiple Consumer [C1,C2,C3].
The idea is producer [P1] puts in the value and consumers C1,C2,C3 all runs their task to read the value individually as put in by P1. Once C1,C2,C3 reads the values, P1 again puts a new data. Then C1,C2,C3 reads data and this loop goes on.
Wait Notify works fine for Single Producer Single Consumer, but in this case of Single Producer Multiple Consumer wait notify concept doesn't look to be good strategy. How should I approach this problem.

Thanks to #Ivan and #Andreas.
#Ivan - In his comment made me understand how Producer Consumer pattern behaves.
#Andreas - In his Comment suggested the usage of Phaser. (I used Cyclic Barrier instead since my number of registered threads does not vary dynamically)
With both their comments sharing the below sample code.
Please do suggest improvisation if there any or a better way to handle this.
Main Class
public static void main(String[] args)
{
SharedSpace sharedSpace = new SharedSpace(new LinkedBlockingQueue<Integer>(1));
new Thread(new Producer(sharedSpace)).start();
Consumer consumerRunnable = new Consumer(sharedSpace);
new Thread(consumerRunnable).start();
CyclicBarrier barrier = new CyclicBarrier(3,consumerRunnable);
new Thread(new EndUser(barrier,consumerRunnable)).start();
new Thread(new EndUser(barrier,consumerRunnable)).start();
new Thread(new EndUser(barrier,consumerRunnable)).start();
}
Producer
private SharedSpace sharedSpace;
public Producer(SharedSpace sharedSpace) {
super();
this.sharedSpace = sharedSpace;
}
public SharedSpace getSharedSpace() {
return sharedSpace;
}
public void setSharedSpace(SharedSpace sharedSpace) {
this.sharedSpace = sharedSpace;
}
#Override
public void run() {
for(int i=0;i<3;i++)
{
int value = (int) (Math.random()*30);
sharedSpace.addValue(value);
}
}
Queue Shared by Producer and Consumer
private BlockingQueue<Integer> queue;
public SharedSpace(BlockingQueue<Integer> queue) {
super();
this.queue = queue;
}
public BlockingQueue<Integer> getQueue() {
return queue;
}
public void setQueue(BlockingQueue<Integer> queue) {
this.queue = queue;
}
public void addValue(int value)
{
try {
queue.put(value);
System.out.println(System.nanoTime()+" Producer added value "+value);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
public int getValue() throws InterruptedException
{
return queue.take();
}
Consumer
private SharedSpace sharedSpace;
private Integer value;
public Consumer(SharedSpace sharedSpace) {
super();
this.sharedSpace = sharedSpace;
}
public SharedSpace getSharedSpace() {
return sharedSpace;
}
public void setSharedSpace(SharedSpace sharedSpace) {
this.sharedSpace = sharedSpace;
}
public Integer getValue() {
return value;
}
public void setValue(Integer value) {
this.value = value;
}
#Override
public void run()
{
try {
setValue(sharedSpace.getValue());
} catch (InterruptedException e) {
e.printStackTrace();
}
}
EndUser
CyclicBarrier barrier;
Consumer consumer;
public EndUser(CyclicBarrier barrier) {
super();
this.barrier = barrier;
}
public EndUser(CyclicBarrier barrier, Consumer consumer) {
super();
this.barrier = barrier;
this.consumer = consumer;
}
public Consumer getConsumer() {
return consumer;
}
public void setConsumer(Consumer consumer) {
this.consumer = consumer;
}
public CyclicBarrier getBarrier() {
return barrier;
}
public void setBarrier(CyclicBarrier barrier) {
this.barrier = barrier;
}
#Override
public void run() {
try
{
while(true)
{
System.out.println(consumer.getValue());
barrier.await();
}
}
catch (InterruptedException e) {
e.printStackTrace();
} catch (BrokenBarrierException e) {
e.printStackTrace();
}
}
Output [Consumer doesn't read from Producer unless all EndUser has taken their Data]
Producer added value 24
Producer added value 10
24
24
24
10
10
Producer added value 0
10
0
0
0

Stop the whole producer and consumer threads and yield the control to main thread

DefaultRunners are producers
and OrderTaker is a consumer
They both share a OrderQueue.
Currently, I use the variable isDone to indicate if a game is finished.
Once each round is done, I want to make it repeat again and again.
However, in my current implementation it will only run once.
How could I solve it?
public class OrderQueue {
public synchronized void pushOrder(Order order) throws InterruptedException {
if (isDone) {
wait();
} else {
runnersQueue.addLast(order);
notifyAll();
}
}
public void pullOrder() {
try {
if (runnersQueue.size() == 0) {
} else if (isDone) {
wait();
} else {
handleOrder(runnersQueue.pop());
}
} catch (InterruptedException e) {
}
}
In my main class
while(true){
enterYesToStart();
DefaultRunners dfltRunner = new DefaultRunners(queue);
OrderTaker taker = new OrderTaker(queue);
taker.run();
System.out.println("This round is finished"); # never reach to this line
}
Here's the full source code for the example
https://gist.github.com/poc7667/d98e3bf5b3b470fcb51e00d9a0d80931

I've taken a look at your code snippets and the problem is fairly obvious.
The main thread runs the OrderTaker runnable. The main thread is stuck in an eternal loop as the while statement cannot complete unless it throws an exception. (Note that the same is true for your ThreadRunner runnable.)
This means that the main thread i still pulling orders while the race is already done.
The OrderTaker should exit it's while loop while once the race is done. I guess that there are multiple ways achieve this, but one way is use a shared variable.
I took your code and adapted it into a working example.
import java.util.*;
import java.util.concurrent.ConcurrentLinkedDeque;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReadWriteLock;
import java.util.concurrent.locks.ReentrantReadWriteLock;
public class RaceApp {
public static void main(String[] args) throws InterruptedException {
final RaceUpdateManager queue = new RaceUpdateManager();
for (int i = 0; i < 3; i++) {
queue.reset();
List<Thread> threads = Arrays.asList(
new Thread(new Runner("Tortoise", 0, 10, queue)),
new Thread(new Runner("Hare", 90, 100, queue))
);
for (Thread thread : threads) {
thread.start();
}
RaceUpdatesProcessor processor = new RaceUpdatesProcessor(queue);
processor.run();
System.out.println("Game finished");
}
}
private static class RaceUpdateManager {
private static final int TOTAL_DISTANCE = 300;
//thread-safe implementation for queue so no external syncrhonization is required when adding/removing updates
private final Deque<RaceUpdate> runnersQueue = new ConcurrentLinkedDeque<>();
//lock used to sync changes to runnersRecords and done variables
private final ReadWriteLock raceStatusLock = new ReentrantReadWriteLock();
private final Map<String, Integer> runnersRecords = new HashMap<>();
private volatile boolean raceDone = false;//volatile keyword guarantees visibility of changes to variables across threads
public boolean isRaceDone() {
return raceDone;
}
//updates can by added simultaneously (read lock)
public void register(RaceUpdate raceUpdate) throws InterruptedException {
Lock readLock = raceStatusLock.readLock();
readLock.lock();
try {
if (!raceDone) {
runnersQueue.addLast(raceUpdate);
}//ignore updates when the race is done
} finally {
readLock.unlock();
}
}
//but they need to be processed in order (exclusive write lock)
public void processOldestUpdate() {
Lock writeLock = raceStatusLock.writeLock();
writeLock.lock();
try {
RaceUpdate raceUpdate = runnersQueue.poll();
if (raceUpdate != null) {
handleUpdate(raceUpdate);
}
} finally {
writeLock.unlock();
}
}
private void handleUpdate(RaceUpdate raceUpdate) {
Integer distanceRun = runnersRecords.merge(
raceUpdate.runner, raceUpdate.distanceRunSinceLastUpdate, (total, increment) -> total + increment
);
System.out.printf("%s: %d\n", raceUpdate.runner, distanceRun);
if (distanceRun >= TOTAL_DISTANCE) {
raceDone = true;
System.out.printf("Winner %s\n", raceUpdate.runner);
}
}
public void reset() {
Lock writeLock = raceStatusLock.writeLock();
writeLock.lock();
try {
runnersQueue.clear();
runnersRecords.clear();
raceDone = false;
} finally {
writeLock.unlock();
}
}
}
public static class Runner implements Runnable {
private final String name;
private final int rest;
private final int speed;
private final RaceUpdateManager queue;
private final Random rand = new Random();
public Runner(String name, int rest, int speed, RaceUpdateManager queue) {
this.name = name;
this.rest = rest;
this.speed = speed;
this.queue = queue;
}
#Override
public void run() {
while (!queue.isRaceDone()) {
try {
if (!takeRest()) {
queue.register(new RaceUpdate(this.name, this.speed));
}
Thread.sleep(100);
} catch (InterruptedException e) {
//signal that thread was interrupted and exit method
Thread.currentThread().interrupt();
return;
}
}
}
private boolean takeRest() {
return rand.nextInt(100) < rest;
}
}
public static class RaceUpdatesProcessor implements Runnable {
private final RaceUpdateManager queue;
public RaceUpdatesProcessor(RaceUpdateManager queue) {
this.queue = queue;
}
#Override
public void run() {
while (!queue.isRaceDone()) {
try {
queue.processOldestUpdate();
Thread.sleep(50);
} catch (InterruptedException e) {
//signal that thread was interrupted and exit method
Thread.currentThread().interrupt();
return;
}
}
}
}
public static class RaceUpdate {
public final String runner;
public final int distanceRunSinceLastUpdate;
public RaceUpdate(String runner, int distanceRunSinceLastUpdate) {
this.runner = runner;
this.distanceRunSinceLastUpdate = distanceRunSinceLastUpdate;
}
}
}

static volatile boolean - thread not getting terminated

I wrote simple multithreaded application, just to play around with concurrency but I have a problem with boolean variable which controles the loop in thread. One of the functions should stop the thread if there's noelements left in queue and I guess that is my problem because If I add something in between braces to:
while (!queue.isEmpty()) {
}
isRunning = false;
So it becomes :
while (!queue.isEmpty()) {
System.out.println("ASD");
}
isRunning = false;
It is working a bit better - the program terminates after executing turnOff method
Any Ideas?
Here is full code of my app:
package test;
public class xxx {
public static void main(String[] args) {
Foo instance = Foo.getInstance();
Thread x = new Thread(instance);
x.start();
for (int count = 1; count < 100000; count++)
instance.addToQueue(count + "");
instance.turnOff();
}
}
And:
package test;
import java.util.LinkedList;
import java.util.List;
public class Foo implements Runnable {
private static Foo inner = null;
private static List<String> queue = new LinkedList<String>();
private volatile static boolean isRunning = false;
private Foo() { }
public static Foo getInstance() {
if (inner == null) {
inner = new Foo();
}
return inner;
}
public void addToQueue(String toPrint) {
synchronized (queue) {
queue.add(toPrint);
}
}
public void removeFromQueue(String toRemove) {
synchronized (queue) {
queue.remove(toRemove);
}
}
public void turnOff() {
while (!queue.isEmpty()) {
}
System.out.println("end");
isRunning = false;
}
#Override
public void run() {
isRunning = true;
while (isRunning) {
if (!queue.isEmpty()) {
String string = queue.get(0);
System.out.println(string);
removeFromQueue(string);
}
}
}
}

It is a race condition problem. Possibly the run method (the other thread) is executed after the turnOff in in the main thread so the flag isRunning is set as true again and the loop never ends.
That would explain why with a simple System.out.println("ASD") becomes better: the isRunning=false is delayed.

You have lots of problems in your code.
Busy loops in turnOff and wait
Unsynchronized access to queue in turnOff and run
Non-volatile, non-final access to inner
Needlessly static isRunning and queue variables
Race condition between turnOff and start invocations
Some of these are harmless in this specific instance (e.g. instance is always accessed from the main thread), but depending on your hardware configuration you are going to get bitten by some combination of the rest of them. The reason that adding the System.out "fixes" the problem is that it renders one of the busy loops less busy (fixes 1) and has an internal synchronization mechanism (fixes 2), but the others are still there.
I suggest getting rid of the isRunning variable and the test for queue.isEmpty() and replacing with a CountDownLatch.
package test;
import java.util.LinkedList;
import java.util.List;
import java.util.concurrent.CountDownLatch;
public class Foo implements Runnable {
private static final Foo inner = new Foo();
private final List<String> queue = new LinkedList<String>();
private final CountDownLatch latch = new CountDownLatch(1);
private Foo() { }
public static Foo getInstance() {
return inner;
}
public void addToQueue(String toPrint) {
synchronized (queue) {
queue.add(toPrint);
}
}
public void removeFromQueue(String toRemove) {
synchronized (queue) {
queue.remove(toRemove);
}
}
public boolean isEmpty() {
synchronized (queue) {
return queue.isEmpty();
}
}
public String getHead() {
synchronized (queue) {
return queue.get(0);
}
}
public void turnOff() throws InterruptedException {
latch.await();
System.out.println("end");
}
#Override
public void run() {
while (!isEmpty()) {
String string = getHead();
System.out.println(string);
removeFromQueue(string);
}
latch.countDown();
}
}
And the runner
package test;
public class XXX {
public static void main(String[] args) throws InterruptedException {
Foo instance = Foo.getInstance();
Thread x = new Thread(instance);
for (int count = 1; count < 100000; count++)
instance.addToQueue(count + "");
x.start();
instance.turnOff();
}
}

The main problem is the race condition between adding/removing elements and checking whether the queue is empty. In more words:
Wrapping add and remove calls in synchronized block provides you guarantees that all invocations of these methods will be performed sequentially. But, there is one more access to queue variable outside of synchronized block - it is queue.isEmpty(). It means there is a chance that some thread will get the result of this call and while it performs actions inside if block, other thread may add or remove elements.
This code also has some more concurrency problems, please let me know if you want them to be discussed (they are a little bit offtopic).

As Germann Arlington point, the value of queue.isEmpty() seems to be cached in the main thread. Try synchronize it:
while (true) {
synchronized(queue) {
if(queue.isEmpty())
break;
}
}
Or just make the queue to be volatile:
private volatile static List<String> queue = new LinkedList<String>();

This will solve your problem.
Use volatile variable isRunning in turnOff() method's while loop also.
public void turnOff() {
while (isRunning && !queue.isEmpty()) {
}
System.out.println("end");
isRunning = false;
}

Java ThreadPoolExecutor strategy, 'Direct Handoff' with queue?

I'm looking to have a ThreadPoolExecutor where I can set a corePoolSize and a maximumPoolSize and what happens is the queue would hand off task immediately to the thread pool and thus create new threads until it reaches the maximumPoolSize then start adding to a queue.
Is there such a thing? If not, are there any good reason it doesn't have such a strategy?
What I want essentially is for tasks to be submitted for execution and when it reaches a point where it is essentially going to get 'worst' performance from having too many threads (by setting maximumPoolSize), it would stop adding new threads and work with that thread pool and start queuing, then if the queue is full it rejects.
And when load comes back down, it can start dismantling threads that are unused back to the corePoolSize.
This makes more sense to me in my application than the 'three general strategies' listed in http://docs.oracle.com/javase/6/docs/api/java/util/concurrent/ThreadPoolExecutor.html

Note: these implementations are somewhat flawed and non-deterministic. Please read the entire answer and the comments before using this code.
How about creating a work queue that rejects items while the executor is below the maximum pool size, and starts accepting them once the maximum has been reached?
This relies on the documented behavior:
"If a request cannot be queued, a new thread is created unless this
would exceed maximumPoolSize, in which case, the task will be
rejected."
public class ExecutorTest
{
private static final int CORE_POOL_SIZE = 2;
private static final int MAXIMUM_POOL_SIZE = 4;
private static final int KEEP_ALIVE_TIME_MS = 5000;
public static void main(String[] args)
{
final SaturateExecutorBlockingQueue workQueue =
new SaturateExecutorBlockingQueue();
final ThreadPoolExecutor executor =
new ThreadPoolExecutor(CORE_POOL_SIZE,
MAXIMUM_POOL_SIZE,
KEEP_ALIVE_TIME_MS,
TimeUnit.MILLISECONDS,
workQueue);
workQueue.setExecutor(executor);
for (int i = 0; i < 6; i++)
{
final int index = i;
executor.submit(new Runnable()
{
public void run()
{
try
{
Thread.sleep(1000);
}
catch (InterruptedException e)
{
e.printStackTrace();
}
System.out.println("Runnable " + index
+ " on thread: " + Thread.currentThread());
}
});
}
}
public static class SaturateExecutorBlockingQueue
extends LinkedBlockingQueue<Runnable>
{
private ThreadPoolExecutor executor;
public void setExecutor(ThreadPoolExecutor executor)
{
this.executor = executor;
}
public boolean offer(Runnable e)
{
if (executor.getPoolSize() < executor.getMaximumPoolSize())
{
return false;
}
return super.offer(e);
}
}
}
Note: Your question surprised me because I expected your desired behavior to be the default behavior of a ThreadPoolExecutor configured with a corePoolSize < maximumPoolSize. But as you point out, the JavaDoc for ThreadPoolExecutor clearly states otherwise.
Idea #2
I think I have what is probably a slightly better approach. It relies on the side-effect behavior coded into the setCorePoolSize method in ThreadPoolExecutor. The idea is to temporarily and conditionally increase the core pool size when a work item is enqueued. When increasing the core pool size, the ThreadPoolExecutor will immediately spawn enough new threads to execute all the queued (queue.size()) tasks. Then we immediately decrease the core pool size, which allows the thread pool to shrink naturally during future periods of low activity. This approach is still not fully deterministic (it is possible for the pool size to grow above max pool size, for example), but I think it is in almost all cases it is better than the first strategy.
Specifically, I believe this approach is better than the first because:
It will reuse threads more often
It will not reject execution as a result of a race
I would like to mention again that the first approach causes the thread pool to grow to its maximum size even under very light use. This approach should be much more efficient in that regard.
-
public class ExecutorTest2
{
private static final int KEEP_ALIVE_TIME_MS = 5000;
private static final int CORE_POOL_SIZE = 2;
private static final int MAXIMUM_POOL_SIZE = 4;
public static void main(String[] args) throws InterruptedException
{
final SaturateExecutorBlockingQueue workQueue =
new SaturateExecutorBlockingQueue(CORE_POOL_SIZE,
MAXIMUM_POOL_SIZE);
final ThreadPoolExecutor executor =
new ThreadPoolExecutor(CORE_POOL_SIZE,
MAXIMUM_POOL_SIZE,
KEEP_ALIVE_TIME_MS,
TimeUnit.MILLISECONDS,
workQueue);
workQueue.setExecutor(executor);
for (int i = 0; i < 60; i++)
{
final int index = i;
executor.submit(new Runnable()
{
public void run()
{
try
{
Thread.sleep(1000);
}
catch (InterruptedException e)
{
e.printStackTrace();
}
System.out.println("Runnable " + index
+ " on thread: " + Thread.currentThread()
+ " poolSize: " + executor.getPoolSize());
}
});
}
executor.shutdown();
executor.awaitTermination(Long.MAX_VALUE, TimeUnit.MILLISECONDS);
}
public static class SaturateExecutorBlockingQueue
extends LinkedBlockingQueue<Runnable>
{
private final int corePoolSize;
private final int maximumPoolSize;
private ThreadPoolExecutor executor;
public SaturateExecutorBlockingQueue(int corePoolSize,
int maximumPoolSize)
{
this.corePoolSize = corePoolSize;
this.maximumPoolSize = maximumPoolSize;
}
public void setExecutor(ThreadPoolExecutor executor)
{
this.executor = executor;
}
public boolean offer(Runnable e)
{
if (super.offer(e) == false)
{
return false;
}
// Uncomment one or both of the below lines to increase
// the likelyhood of the threadpool reusing an existing thread
// vs. spawning a new one.
//Thread.yield();
//Thread.sleep(0);
int currentPoolSize = executor.getPoolSize();
if (currentPoolSize < maximumPoolSize
&& currentPoolSize >= corePoolSize)
{
executor.setCorePoolSize(currentPoolSize + 1);
executor.setCorePoolSize(corePoolSize);
}
return true;
}
}
}

We found a solution to that problem with the following code :
This queue is a hybrid SynchronousQueue / LinkedBlockingQueue.
public class OverflowingSynchronousQueue<E> extends LinkedBlockingQueue<E> {
private static final long serialVersionUID = 1L;
private SynchronousQueue<E> synchronousQueue = new SynchronousQueue<E>();
public OverflowingSynchronousQueue() {
super();
}
public OverflowingSynchronousQueue(int capacity) {
super(capacity);
}
#Override
public boolean offer(E e) {
// Create a new thread or wake an idled thread
return synchronousQueue.offer(e);
}
public boolean offerToOverflowingQueue(E e) {
// Add to queue
return super.offer(e);
}
#Override
public E take() throws InterruptedException {
// Return tasks from queue, if any, without blocking
E task = super.poll();
if (task != null) {
return task;
} else {
// Block on the SynchronousQueue take
return synchronousQueue.take();
}
}
#Override
public E poll(long timeout, TimeUnit unit) throws InterruptedException {
// Return tasks from queue, if any, without blocking
E task = super.poll();
if (task != null) {
return task;
} else {
// Block on the SynchronousQueue poll
return synchronousQueue.poll(timeout, unit);
}
}
}
For it to work, we need to wrap the RejectedExecutionHandler to call "offerToOverflowingQueue" when a task is rejected.
public class OverflowingRejectionPolicyAdapter implements RejectedExecutionHandler {
private OverflowingSynchronousQueue<Runnable> queue;
private RejectedExecutionHandler adaptedRejectedExecutionHandler;
public OverflowingRejectionPolicyAdapter(OverflowingSynchronousQueue<Runnable> queue,
RejectedExecutionHandler adaptedRejectedExecutionHandler)
{
super();
this.queue = queue;
this.adaptedRejectedExecutionHandler = adaptedRejectedExecutionHandler;
}
#Override
public void rejectedExecution(Runnable r, ThreadPoolExecutor executor) {
if (!queue.offerToOverflowingQueue(r)) {
adaptedRejectedExecutionHandler.rejectedExecution(r, executor);
}
}
}
Here's how we create the ThreadPoolExecutor
public static ExecutorService newSaturatingThreadPool(int corePoolSize,
int maxPoolSize,
int maxQueueSize,
long keepAliveTime,
TimeUnit timeUnit,
String threadNamePrefix,
RejectedExecutionHandler rejectedExecutionHandler)
{
OverflowingSynchronousQueue<Runnable> queue = new OverflowingSynchronousQueue<Runnable>(maxQueueSize);
OverflowingRejectionPolicyAdapter rejectionPolicyAdapter = new OverflowingRejectionPolicyAdapter(queue,
rejectedExecutionHandler);
ThreadPoolExecutor executor = new ThreadPoolExecutor(corePoolSize,
maxPoolSize,
keepAliveTime,
timeUnit,
queue,
new NamedThreadFactory(threadNamePrefix),
rejectionPolicyAdapter);
return executor;
}

Producer/Consumer threads using a Queue

I'd like to create some sort of Producer/Consumer threading app. But I'm not sure what the best way to implement a queue between the two.
So I've some up with two ideas (both of which could be entirely wrong). I would like to know which would be better and if they both suck then what would be the best way to implement the queue. It's mainly my implementation of the queue in these examples that I'm concerned about. I'm extending a Queue class that is an in house class and is thread safe. Below are two examples with 4 classes each.
Main class-
public class SomeApp
{
private Consumer consumer;
private Producer producer;
public static void main (String args[])
{
consumer = new Consumer();
producer = new Producer();
}
}
Consumer class-
public class Consumer implements Runnable
{
public Consumer()
{
Thread consumer = new Thread(this);
consumer.start();
}
public void run()
{
while(true)
{
//get an object off the queue
Object object = QueueHandler.dequeue();
//do some stuff with the object
}
}
}
Producer class-
public class Producer implements Runnable
{
public Producer()
{
Thread producer = new Thread(this);
producer.start();
}
public void run()
{
while(true)
{
//add to the queue some sort of unique object
QueueHandler.enqueue(new Object());
}
}
}
Queue class-
public class QueueHandler
{
//This Queue class is a thread safe (written in house) class
public static Queue<Object> readQ = new Queue<Object>(100);
public static void enqueue(Object object)
{
//do some stuff
readQ.add(object);
}
public static Object dequeue()
{
//do some stuff
return readQ.get();
}
}
OR
Main class-
public class SomeApp
{
Queue<Object> readQ;
private Consumer consumer;
private Producer producer;
public static void main (String args[])
{
readQ = new Queue<Object>(100);
consumer = new Consumer(readQ);
producer = new Producer(readQ);
}
}
Consumer class-
public class Consumer implements Runnable
{
Queue<Object> queue;
public Consumer(Queue<Object> readQ)
{
queue = readQ;
Thread consumer = new Thread(this);
consumer.start();
}
public void run()
{
while(true)
{
//get an object off the queue
Object object = queue.dequeue();
//do some stuff with the object
}
}
}
Producer class-
public class Producer implements Runnable
{
Queue<Object> queue;
public Producer(Queue<Object> readQ)
{
queue = readQ;
Thread producer = new Thread(this);
producer.start();
}
public void run()
{
while(true)
{
//add to the queue some sort of unique object
queue.enqueue(new Object());
}
}
}
Queue class-
//the extended Queue class is a thread safe (written in house) class
public class QueueHandler extends Queue<Object>
{
public QueueHandler(int size)
{
super(size); //All I'm thinking about now is McDonalds.
}
public void enqueue(Object object)
{
//do some stuff
readQ.add();
}
public Object dequeue()
{
//do some stuff
return readQ.get();
}
}
And go!

Java 5+ has all the tools you need for this kind of thing. You will want to:
Put all your Producers in one ExecutorService;
Put all your Consumers in another ExecutorService;
If necessary, communicate between the two using a BlockingQueue.
I say "if necessary" for (3) because from my experience it's an unnecessary step. All you do is submit new tasks to the consumer executor service. So:
final ExecutorService producers = Executors.newFixedThreadPool(100);
final ExecutorService consumers = Executors.newFixedThreadPool(100);
while (/* has more work */) {
producers.submit(...);
}
producers.shutdown();
producers.awaitTermination(Long.MAX_VALUE, TimeUnit.NANOSECONDS);
consumers.shutdown();
consumers.awaitTermination(Long.MAX_VALUE, TimeUnit.NANOSECONDS);
So the producers submit directly to consumers.

OK, as others note, the best thing to do is to use java.util.concurrent package. I highly recommend "Java Concurrency in Practice". It's a great book that covers almost everything you need to know.
As for your particular implementation, as I noted in the comments, don't start Threads from Constructors -- it can be unsafe.
Leaving that aside, the second implementation seem better. You don't want to put queues in static fields. You are probably just loosing flexibility for nothing.
If you want to go ahead with your own implementation (for learning purpose I guess?), supply a start() method at least. You should construct the object (you can instantiate the Thread object), and then call start() to start the thread.
Edit: ExecutorService have their own queue so this can be confusing.. Here's something to get you started.
public class Main {
public static void main(String[] args) {
//The numbers are just silly tune parameters. Refer to the API.
//The important thing is, we are passing a bounded queue.
ExecutorService consumer = new ThreadPoolExecutor(1,4,30,TimeUnit.SECONDS,new LinkedBlockingQueue<Runnable>(100));
//No need to bound the queue for this executor.
//Use utility method instead of the complicated Constructor.
ExecutorService producer = Executors.newSingleThreadExecutor();
Runnable produce = new Produce(consumer);
producer.submit(produce);
}
}
class Produce implements Runnable {
private final ExecutorService consumer;
public Produce(ExecutorService consumer) {
this.consumer = consumer;
}
#Override
public void run() {
Pancake cake = Pan.cook();
Runnable consume = new Consume(cake);
consumer.submit(consume);
}
}
class Consume implements Runnable {
private final Pancake cake;
public Consume(Pancake cake){
this.cake = cake;
}
#Override
public void run() {
cake.eat();
}
}
Further EDIT:
For producer, instead of while(true), you can do something like:
#Override
public void run(){
while(!Thread.currentThread().isInterrupted()){
//do stuff
}
}
This way you can shutdown the executor by calling .shutdownNow(). If you'd use while(true), it won't shutdown.
Also note that the Producer is still vulnerable to RuntimeExceptions (i.e. one RuntimeException will halt the processing)

I have extended cletus proposed answer to working code example.
One ExecutorService (pes) accepts Producer tasks.
One ExecutorService (ces) accepts Consumer tasks.
Both Producer and Consumer shares BlockingQueue.
Multiple Producer tasks generates different numbers.
Any of Consumer tasks can consume number generated by Producer
Code:
import java.util.concurrent.*;
public class ProducerConsumerWithES {
public static void main(String args[]){
BlockingQueue<Integer> sharedQueue = new LinkedBlockingQueue<Integer>();
ExecutorService pes = Executors.newFixedThreadPool(2);
ExecutorService ces = Executors.newFixedThreadPool(2);
pes.submit(new Producer(sharedQueue,1));
pes.submit(new Producer(sharedQueue,2));
ces.submit(new Consumer(sharedQueue,1));
ces.submit(new Consumer(sharedQueue,2));
// shutdown should happen somewhere along with awaitTermination
/ * https://stackoverflow.com/questions/36644043/how-to-properly-shutdown-java-executorservice/36644320#36644320 */
pes.shutdown();
ces.shutdown();
}
}
class Producer implements Runnable {
private final BlockingQueue<Integer> sharedQueue;
private int threadNo;
public Producer(BlockingQueue<Integer> sharedQueue,int threadNo) {
this.threadNo = threadNo;
this.sharedQueue = sharedQueue;
}
#Override
public void run() {
for(int i=1; i<= 5; i++){
try {
int number = i+(10*threadNo);
System.out.println("Produced:" + number + ":by thread:"+ threadNo);
sharedQueue.put(number);
} catch (Exception err) {
err.printStackTrace();
}
}
}
}
class Consumer implements Runnable{
private final BlockingQueue<Integer> sharedQueue;
private int threadNo;
public Consumer (BlockingQueue<Integer> sharedQueue,int threadNo) {
this.sharedQueue = sharedQueue;
this.threadNo = threadNo;
}
#Override
public void run() {
while(true){
try {
int num = sharedQueue.take();
System.out.println("Consumed: "+ num + ":by thread:"+threadNo);
} catch (Exception err) {
err.printStackTrace();
}
}
}
}
output:
Produced:11:by thread:1
Produced:21:by thread:2
Produced:22:by thread:2
Consumed: 11:by thread:1
Produced:12:by thread:1
Consumed: 22:by thread:1
Consumed: 21:by thread:2
Produced:23:by thread:2
Consumed: 12:by thread:1
Produced:13:by thread:1
Consumed: 23:by thread:2
Produced:24:by thread:2
Consumed: 13:by thread:1
Produced:14:by thread:1
Consumed: 24:by thread:2
Produced:25:by thread:2
Consumed: 14:by thread:1
Produced:15:by thread:1
Consumed: 25:by thread:2
Consumed: 15:by thread:1
Note. If you don't need multiple Producers and Consumers, keep single Producer and Consumer. I have added multiple Producers and Consumers to showcase capabilities of BlockingQueue among multiple Producers and Consumers.

You are reinventing the wheel.
If you need persistence and other enterprise features use JMS (I'd suggest ActiveMq).
If you need fast in-memory queues use one of the impementations of java's Queue.
If you need to support java 1.4 or earlier, use Doug Lea's excellent concurrent package.

This is a very simple code.
import java.util.*;
// #author : rootTraveller, June 2017
class ProducerConsumer {
public static void main(String[] args) throws Exception {
Queue<Integer> queue = new LinkedList<>();
Integer buffer = new Integer(10); //Important buffer or queue size, change as per need.
Producer producerThread = new Producer(queue, buffer, "PRODUCER");
Consumer consumerThread = new Consumer(queue, buffer, "CONSUMER");
producerThread.start();
consumerThread.start();
}
}
class Producer extends Thread {
private Queue<Integer> queue;
private int queueSize ;
public Producer (Queue<Integer> queueIn, int queueSizeIn, String ThreadName){
super(ThreadName);
this.queue = queueIn;
this.queueSize = queueSizeIn;
}
public void run() {
while(true){
synchronized (queue) {
while(queue.size() == queueSize){
System.out.println(Thread.currentThread().getName() + " FULL : waiting...\n");
try{
queue.wait(); //Important
} catch (Exception ex) {
ex.printStackTrace();
}
}
//queue empty then produce one, add and notify
int randomInt = new Random().nextInt();
System.out.println(Thread.currentThread().getName() + " producing... : " + randomInt);
queue.add(randomInt);
queue.notifyAll(); //Important
} //synchronized ends here : NOTE
}
}
}
class Consumer extends Thread {
private Queue<Integer> queue;
private int queueSize;
public Consumer(Queue<Integer> queueIn, int queueSizeIn, String ThreadName){
super (ThreadName);
this.queue = queueIn;
this.queueSize = queueSizeIn;
}
public void run() {
while(true){
synchronized (queue) {
while(queue.isEmpty()){
System.out.println(Thread.currentThread().getName() + " Empty : waiting...\n");
try {
queue.wait(); //Important
} catch (Exception ex) {
ex.printStackTrace();
}
}
//queue not empty then consume one and notify
System.out.println(Thread.currentThread().getName() + " consuming... : " + queue.remove());
queue.notifyAll();
} //synchronized ends here : NOTE
}
}
}

Java code "BlockingQueue" which has synchronized put and get method.
Java code "Producer" , producer thread to produce data.
Java code "Consumer" , consumer thread to consume the data produced.
Java code "ProducerConsumer_Main", main function to start the producer and consumer thread.
BlockingQueue.java
public class BlockingQueue
{
int item;
boolean available = false;
public synchronized void put(int value)
{
while (available == true)
{
try
{
wait();
} catch (InterruptedException e) {
}
}
item = value;
available = true;
notifyAll();
}
public synchronized int get()
{
while(available == false)
{
try
{
wait();
}
catch(InterruptedException e){
}
}
available = false;
notifyAll();
return item;
}
}
Consumer.java
package com.sukanya.producer_Consumer;
public class Consumer extends Thread
{
blockingQueue queue;
private int number;
Consumer(BlockingQueue queue,int number)
{
this.queue = queue;
this.number = number;
}
public void run()
{
int value = 0;
for (int i = 0; i < 10; i++)
{
value = queue.get();
System.out.println("Consumer #" + this.number+ " got: " + value);
}
}
}
ProducerConsumer_Main.java
package com.sukanya.producer_Consumer;
public class ProducerConsumer_Main
{
public static void main(String args[])
{
BlockingQueue queue = new BlockingQueue();
Producer producer1 = new Producer(queue,1);
Consumer consumer1 = new Consumer(queue,1);
producer1.start();
consumer1.start();
}
}

Use this typesafe pattern with poison pills:
public sealed interface BaseMessage {
final class ValidMessage<T> implements BaseMessage {
#Nonnull
private final T value;
public ValidMessage(#Nonnull T value) {
this.value = value;
}
#Nonnull
public T getValue() {
return value;
}
#Override
public boolean equals(Object o) {
if (this == o) return true;
if (o == null || getClass() != o.getClass()) return false;
ValidMessage<?> that = (ValidMessage<?>) o;
return value.equals(that.value);
}
#Override
public int hashCode() {
return Objects.hash(value);
}
#Override
public String toString() {
return "ValidMessage{value=%s}".formatted(value);
}
}
final class PoisonedMessage implements BaseMessage {
public static final PoisonedMessage INSTANCE = new PoisonedMessage();
private PoisonedMessage() {
}
#Override
public String toString() {
return "PoisonedMessage{}";
}
}
}
public class Producer implements Callable<Void> {
#Nonnull
private final BlockingQueue<BaseMessage> messages;
Producer(#Nonnull BlockingQueue<BaseMessage> messages) {
this.messages = messages;
}
#Override
public Void call() throws Exception {
messages.put(new BaseMessage.ValidMessage<>(1));
messages.put(new BaseMessage.ValidMessage<>(2));
messages.put(new BaseMessage.ValidMessage<>(3));
messages.put(BaseMessage.PoisonedMessage.INSTANCE);
return null;
}
}
public class Consumer implements Callable<Void> {
#Nonnull
private final BlockingQueue<BaseMessage> messages;
private final int maxPoisons;
public Consumer(#Nonnull BlockingQueue<BaseMessage> messages, int maxPoisons) {
this.messages = messages;
this.maxPoisons = maxPoisons;
}
#Override
public Void call() throws Exception {
int poisonsReceived = 0;
while (poisonsReceived < maxPoisons && !Thread.currentThread().isInterrupted()) {
BaseMessage message = messages.take();
if (message instanceof BaseMessage.ValidMessage<?> vm) {
Integer value = (Integer) vm.getValue();
System.out.println(value);
} else if (message instanceof BaseMessage.PoisonedMessage) {
++poisonsReceived;
} else {
throw new IllegalArgumentException("Invalid BaseMessage type: " + message);
}
}
return null;
}
}

public class QueueHandler
{
//winstead of Queue<Object> will replace BlockingQueue <String> queue = new LinkedBlockingQueue <> ();
public static Queue<Object> readQ = new Queue<Object>(100);
public static void enqueue(Object object)
{
readQ.add(object);
}
public static Object dequeue()
{
return readQ.get();
}
}
When
public static BlockingQueue <String> queue = new LinkedBlockingQueue <> ();
it is static it works, but when it is non-static it doesn't work properly.
How to fix it?