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Do not share same socket between two threads at the same time

I have around 60 sockets and 20 threads and I want to make sure each thread works on different socket everytime so I don't want to share same socket between two threads at all.
In my SocketManager class, I have a background thread which runs every 60 seconds and calls updateLiveSockets() method. In the updateLiveSockets() method, I iterate all the sockets I have and then start pinging them one by one by calling send method of SendToQueue class and basis on the response I mark them as live or dead. In the updateLiveSockets() method, I always need to iterate all the sockets and ping them to check whether they are live or dead.
Now all the reader threads will call getNextSocket() method of SocketManager class concurrently to get the next live available socket to send the business message on that socket. So I have two types of messages which I am sending on a socket:
One is ping message on a socket. This is only sent from timer thread calling updateLiveSockets() method in SocketManager class.
Other is business message on a socket. This is done in SendToQueue class.
So if pinger thread is pinging a socket to check whether they are live or not then no other business thread should use that socket. Similarly if business thread is using a socket to send data on it, then pinger thread should not ping that socket. And this applies to all the socket. But I need to make sure that in updateLiveSockets method, we are pinging all the available sockets whenever my background thread starts so that we can figure out which socket is live or dead.
Below is my SocketManager class:
public class SocketManager {
private static final Random random = new Random();
private final ScheduledExecutorService scheduler = Executors.newSingleThreadScheduledExecutor();
private final Map<Datacenters, List<SocketHolder>> liveSocketsByDatacenter =
new ConcurrentHashMap<>();
private final ZContext ctx = new ZContext();
// ...
private SocketManager() {
connectToZMQSockets();
scheduler.scheduleAtFixedRate(new Runnable() {
public void run() {
updateLiveSockets();
}
}, 60, 60, TimeUnit.SECONDS);
}
// during startup, making a connection and populate once
private void connectToZMQSockets() {
Map<Datacenters, List<String>> socketsByDatacenter = Utils.SERVERS;
for (Map.Entry<Datacenters, List<String>> entry : socketsByDatacenter.entrySet()) {
List<SocketHolder> addedColoSockets = connect(entry.getValue(), ZMQ.PUSH);
liveSocketsByDatacenter.put(entry.getKey(), addedColoSockets);
}
}
private List<SocketHolder> connect(List<String> paddes, int socketType) {
List<SocketHolder> socketList = new ArrayList<>();
// ....
return socketList;
}
// this method will be called by multiple threads concurrently to get the next live socket
// is there any concurrency or thread safety issue or race condition here?
public Optional<SocketHolder> getNextSocket() {
for (Datacenters dc : Datacenters.getOrderedDatacenters()) {
Optional<SocketHolder> liveSocket = getLiveSocket(liveSocketsByDatacenter.get(dc));
if (liveSocket.isPresent()) {
return liveSocket;
}
}
return Optional.absent();
}
private Optional<SocketHolder> getLiveSocket(final List<SocketHolder> listOfEndPoints) {
if (!listOfEndPoints.isEmpty()) {
// The list of live sockets
List<SocketHolder> liveOnly = new ArrayList<>(listOfEndPoints.size());
for (SocketHolder obj : listOfEndPoints) {
if (obj.isLive()) {
liveOnly.add(obj);
}
}
if (!liveOnly.isEmpty()) {
// The list is not empty so we shuffle it an return the first element
return Optional.of(liveOnly.get(random.nextInt(liveOnly.size()))); // just pick one
}
}
return Optional.absent();
}
// runs every 60 seconds to ping all the available socket to make sure whether they are alive or not
private void updateLiveSockets() {
Map<Datacenters, List<String>> socketsByDatacenter = Utils.SERVERS;
for (Map.Entry<Datacenters, List<String>> entry : socketsByDatacenter.entrySet()) {
List<SocketHolder> liveSockets = liveSocketsByDatacenter.get(entry.getKey());
List<SocketHolder> liveUpdatedSockets = new ArrayList<>();
for (SocketHolder liveSocket : liveSockets) {
Socket socket = liveSocket.getSocket();
String endpoint = liveSocket.getEndpoint();
Map<byte[], byte[]> holder = populateMap();
Message message = new Message(holder, Partition.COMMAND);
// pinging to see whether a socket is live or not
boolean isLive = SendToQueue.getInstance().send(message.getAddress(), message.getEncodedRecords(), socket);
SocketHolder zmq = new SocketHolder(socket, liveSocket.getContext(), endpoint, isLive);
liveUpdatedSockets.add(zmq);
}
liveSocketsByDatacenter.put(entry.getKey(), Collections.unmodifiableList(liveUpdatedSockets));
}
}
}
And here is my SendToQueue class:
// this method will be called by multiple reader threads (around 20) concurrently to send the data
public boolean sendAsync(final long address, final byte[] encodedRecords) {
PendingMessage m = new PendingMessage(address, encodedRecords, true);
cache.put(address, m);
return doSendAsync(m);
}
private boolean doSendAsync(final PendingMessage pendingMessage) {
Optional<SocketHolder> liveSocket = SocketManager.getInstance().getNextSocket();
if (!liveSocket.isPresent()) {
// log error
return false;
}
ZMsg msg = new ZMsg();
msg.add(pendingMessage.getEncodedRecords());
try {
// send data on a socket LINE A
return msg.send(liveSocket.get().getSocket());
} finally {
msg.destroy();
}
}
public boolean send(final long address, final byte[] encodedRecords, final Socket socket) {
PendingMessage m = new PendingMessage(address, encodedRecords, socket, false);
cache.put(address, m);
try {
if (doSendAsync(m, socket)) {
return m.waitForAck();
}
return false;
} finally {
cache.invalidate(address);
}
}
Problem Statement
Now as you can see above that I am sharing same socket between two threads. It seems getNextSocket() in SocketManager class could return a 0MQ socket to Thread A. Concurrently, the timer thread may access the same 0MQ socket to ping it. In this case Thread A and the timer thread are mutating the same 0MQ socket, which can lead to problems. So I am trying to find a way so that I can prevent different threads from sending data to the same socket at the same time and mucking up my data.
One solution I can think of is using synchronization on a socket while sending the data but if many threads uses the same socket, resources aren't well utilized. Moreover If msg.send(socket); is blocked (technically it shouldn't) all threads waiting for this socket are blocked. So I guess there might be a better way to ensure that every thread uses a different single live socket at the same time instead of synchronization on a particular socket.

So I am trying to find a way so that I can prevent different threads from sending data to the same socket at the same time and mucking up my data.
There are certainly a number of different ways to do this. For me this seems like a BlockingQueue is the right thing to use. The business threads would take a socket from the queue and would be guaranteed that no one else would be using that socket.
private final BlockingQueue<SocketHolder> socketHolderQueue = new LinkedBlockingQueue<>();
...
public Optional<SocketHolder> getNextSocket() {
SocketHolder holder = socketHolderQueue.poll();
return holder;
}
...
public void finishedWithSocket(SocketHolder holder) {
socketHolderQueue.put(holder);
}
I think that synchronizing on the socket is not a good idea for the reasons that you mention – the ping thread will be blocking the business thread.
There are a number of ways of handling the ping thread logic. I would store your Socket with a last use time and then your ping thread could every so often take each of the sockets from the same BlockingQueue, test it, and put each back onto the end of the queue after testing.
public void testSockets() {
// one run this for as many sockets as are in the queue
int numTests = socketHolderQueue.size();
for (int i = 0; i < numTests; i++) {
SocketHolder holder = socketHolderQueue.poll();
if (holder == null) {
break;
}
if (socketIsOk(socketHolder)) {
socketHolderQueue.put(socketHolder);
} else {
// close it here or something
}
}
}
You could also have the getNextSocket() code that dequeues the threads from the queue check the timer and put them on a test queue for the ping thread to use and then take the next one from the queue. The business threads would never be using the same socket at the same time as the ping thread.
Depending on when you want to test the sockets, you can also reset the timer after the business thread returns it to the queue so the ping thread would test the socket after X seconds of no use.

It looks like you should consider using the try-with-resource feature here. You have the SocketHolder or Option class implement the AutoCloseable interface. For instance, let us assume that Option implements this interface. The Option close method will then add back the instance to the container. I created a simple example that shows what I mean. It is not complete but it gives you an idea on how to implement this in your code.
public class ObjectManager implements AutoCloseable {
private static class ObjectManagerFactory {
private static ObjectManager objMgr = new ObjectManager();
}
private ObjectManager() {}
public static ObjectManager getInstance() { return ObjectManagerFactory.objMgr; }
private static final int SIZE = 10;
private static BlockingQueue<AutoCloseable> objects = new LinkedBlockingQueue<AutoCloseable>();
private static ScheduledExecutorService sch;
static {
for(int cnt = 0 ; cnt < SIZE ; cnt++) {
objects.add(new AutoCloseable() {
#Override
public void close() throws Exception {
System.out.println(Thread.currentThread() + " - Adding object back to pool:" + this + " size: " + objects.size());
objects.put(this);
System.out.println(Thread.currentThread() + " - Added object back to pool:" + this);
}
});
}
sch = Executors.newSingleThreadScheduledExecutor();
sch.scheduleAtFixedRate(new Runnable() {
#Override
public void run() {
// TODO Auto-generated method stub
updateObjects();
}
}, 10, 10, TimeUnit.MICROSECONDS);
}
static void updateObjects() {
for(int cnt = 0 ; ! objects.isEmpty() && cnt < SIZE ; cnt++ ) {
try(AutoCloseable object = objects.take()) {
System.out.println(Thread.currentThread() + " - updateObjects - updated object: " + object + " size: " + objects.size());
} catch (Throwable t) {
// TODO Auto-generated catch block
t.printStackTrace();
}
}
}
public AutoCloseable getNext() {
try {
return objects.take();
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
return null;
}
}
public static void main(String[] args) {
try (ObjectManager mgr = ObjectManager.getInstance()) {
for (int cnt = 0; cnt < 5; cnt++) {
try (AutoCloseable o = mgr.getNext()) {
System.out.println(Thread.currentThread() + " - Working with " + o);
Thread.sleep(1000);
} catch (Throwable t) {
t.printStackTrace();
}
}
} catch (Throwable tt) {
tt.printStackTrace();
}
}
#Override
public void close() throws Exception {
// TODO Auto-generated method stub
ObjectManager.sch.shutdownNow();
}
}

I will make some points here. In the getNextSocket method getOrderedDatacenters method will always return the same ordered list, so you will always pick from the same datacenters from start to end (it's not a problem).
How do you guarantee that two threads wont get the same liveSocket from getNextSocket?
What you are saying here it is true:
Concurrently, the timer thread may access the same 0MQ socket to ping
it.
I think the main problem here is that you don't distinguish between free sockets and reserved sockets.
One option as you said is to synchronize up on each socket. An other option is to keep a list of reserved sockets and when you want to get a next socket or to update sockets, to pick only from the free sockets. You don't want to update a socket which is already reserved.
Also you can take a look at here if it fits your needs.

There's a concept in operating systems software engineering called the critical section. A critical section occurs when 2 or more processes have shared data and they are concurrently executed, in this case, no process should modify or even read this shared data if there's another process accessing these data. So as a process enters the critical section it should notify all other concurrently executed processes that it's currently modifying the critical section, so all other processes should be blocked-waiting-to enter this critical section. you would ask who organize what process enters, this is another problem called process scheduling that controls what process should enter this critical section and the operating system do that for you.
so the best solution to you is using a semaphore where the value of the semaphore is the number of sockets, in your case, I think you have one socket so you will use a semaphore-Binary Semaphore- initialized with a semaphore value = 1, then your code should be divided into four main sections: critical section entry, the critical section, critical section exiting and remainder section.
Critical section entry: where a process enters the critical section and block all other processes. The semaphore will allow one Process-Thread-to enter the critical section-use a socket- and the value of the semaphore will be decremented-equal to zero-.
The critical section: the critical section code that the process should do.
Critical section exiting: the process releasing the critical section for another process to enter. The semaphore value will be incremented-equal to 1-allowing another process to enter
Remainder section: the rest of all your code excluding the previous 3 sections.
Now all you need is to open any Java tutorials about semaphores to know how to apply a semaphore in Java, it's really easy.

Mouhammed Elshaaer is right, but in additional you can also use any concurrent collection, for example ConcurrentHashMap where you can track that each thread works on different socket (for example ConcurrentHashMap key: socket hash code, value: thread hash code or smth else).
As for me it's a little bit stupid solution, but it can be used to.

For the problem of threads (Thread A and timer thread) accessing the same socket, I would keep 2 socket list for each datacenter:
list A: The sockets that are not in use
list B: The sockets that are in use
i.e.,
call synchronisation getNextSocket() to find an not-in-use socket from list A, remove it from list A and add it to list B;
call synchronisation returnSocket(Socket) upon receiving the reponse/ACK for a sent message (either business or ping), to move the socket from list B back to list A. Put a try {} finally {} block around "sending message" to make sure that the socket will be put back to list A even if there is an exception.

I have a simple solution maybe help you. I don't know if in Java you can add a custom attribute to each socket. In Socket.io you can. So I wanna considerate this (I will delete this answer if not).
You will add a boolean attribute called locked to each socket. So, when your thread check the first socket, locked attribute will be True. Any other thread, when ping THIS socket, will check if locked attribute is False. If not, getNextSocket.
So, in this stretch below...
...
for (SocketHolder liveSocket : liveSockets) {
Socket socket = liveSocket.getSocket();
...
You will check if socket is locked or not. If yes, kill this thread or interrupt this or go to next socket. (I don't know how you call it in Java).
So the process is...
Thread get an unlocked socket
Thread set this socket.locked to True.
Thread ping socket and do any stuff you want
Thread set this socket.locked to False.
Thread go to next.
Sorry my bad english :)

Access Queue from two separate Threads in parallel

So my goal is to measure the performance of a Streaming Engine. It's basically a library to which i can send data-packages. The idea to measure this is to generate data, put it into a Queue and let the Streaming Engine grab the data and process it.
I thought of implementing it like this: The Data Generator runs in a thread and generates data packages in an endless loop with a certain Thread.sleep(X) at the end. When doing the tests the idea is to minimize tis Thread.sleep(X) to see if this has an impact on the Streaming Engine's performance. The Data Generator writes the created packages into a queue, that is, a ConcurrentLinkedQueue, which at the same time is a Singleton.
In another thread I instantiate the Streaming Engine which continuously removes the packages from the queue by doing queue.remove(). This is done in an endlees loop without any sleeping, because it should just be done as fast as possible.
In a first try to implement this I ran into a problem. It seems as if the Data Generator is not able to put the packages into the Queue as it should be. It is doing that too slow. My suspicion is that the endless loop of the Streaming Engine thread is eating up all the resources and therefore slows down everything else.
I would be happy about how to approach this issue or other design patterns, which could solve this issue elegantly.
the requirements are: 2 threads which run in parallel basically. one is putting data into a queue. the other one is reading/removing from the queue. and i want to measure the size of the queue regularly in order to know if the engine which is reading/removing from the queue is fast enough to process the generated packages.

You can use a BlockingQueue, for example ArrayBlockingQueue, you can initialize these to a certain size, so the number of items queued will never exceed a certain number, as per this example:
// create queue, max size 100
final ArrayBlockingQueue<String> strings = new ArrayBlockingQueue<>(100);
final String stop = "STOP";
// start producing
Runnable producer = new Runnable() {
#Override
public void run() {
try {
for(int i = 0; i < 1000; i++) {
strings.put(Integer.toHexString(i));
}
strings.put(stop);
} catch(InterruptedException ignore) {
}
}
};
Thread producerThread = new Thread(producer);
producerThread.start();
// start monitoring
Runnable monitor = new Runnable() {
#Override
public void run() {
try {
while (true){
System.out.println("Queue size: " + strings.size());
Thread.sleep(5);
}
} catch(InterruptedException ignore) {
}
}
};
Thread monitorThread = new Thread(monitor);
monitorThread.start();
// start consuming
Runnable consumer = new Runnable() {
#Override
public void run() {
// infinite look, will interrupt thread when complete
try {
while(true) {
String value = strings.take();
if(value.equals(stop)){
return;
}
System.out.println(value);
}
} catch(InterruptedException ignore) {
}
}
};
Thread consumerThread = new Thread(consumer);
consumerThread.start();
// wait for producer to finish
producerThread.join();
consumerThread.join();
// interrupt consumer and monitor
monitorThread.interrupt();
You could also have third thread monitoring the size of the queue, to give you an idea of which thread is outpacing the other.
Also, you can used the timed put method and the timed or untimed offer methods, which will give you more control of what to do if the queue if full or empty. In the above example execution will stop until there is space for the next element or if there are no further elements in the queue.

Queue Worker Thread stops working, thread safety issue?

i want to introduce my problem first.
I have several WorkingThreads that are receiving a string, processing the string and afterwards sending the processed string to a global Queue like this:
class Main {
public static Queue<String> Q;
public static void main(String[] args) {
//start working threads
}
}
WorkingThread.java:
class WorkingThread extends Thread {
public void run() {
String input;
//do something with input
Main.q.append(processedString);
}
So now every 800ms another Thread called Inserter dequeues all the entries to formulate some sql, but thats not important.
class Inserter extends Thread {
public void run() {
while(!Main.Q.isEmpty()) {
System.out.print(".");
// dequeue and formulate some SQL
}
}
}
Everything works for about 5 to 10 minutes but then suddenly, i cannot see any dots printed (what is basically a heartbeat for the Inserter). The Queue is not empty i can assure that but the inserter just wont work even though it get started regulary.
I have a suspision that there is a problem when a worker wants to insert something while the Inserter dequeues the Queue, could this possibly be some kind of "deadlock"?
I really hope somebody has an explanation for this behaviour. I am looking forward to learn ;).
EDIT: I am using
Queue<String> Q = new LinkedList<String>();

You are not using a synchronized or thread safe Queue therefore you have a race hazard. Your use of a LinkedList shows a (slightly scary) lack of knowledge of this fact. You may want to read more about threading and thread safety before you try and tackle any more threaded code.
You must either synchronize manually or use one of the existing implementations provided by the JDK. Producer/consumer patterns are usually implemented using one of the BlockingQueue implementations.
A BlockingQueue of a bounded size will block producers trying to put if the queue is full. A BlockingQueue will always block consumers if the queue is empty.
This allows you to remove all of your custom logic that spins on the queue and waits for items.
A simple example using Java 8 lambdas would look like:
public static void main(String[] args) throws Exception {
final BlockingQueue<String> q = new LinkedBlockingQueue<>();
final ExecutorService executorService = Executors.newFixedThreadPool(4);
final Runnable consumer = () -> {
while (true) {
try {
System.out.println(q.take());
} catch (InterruptedException e) {
return;
}
}
};
executorService.submit(consumer);
final Stream<Runnable> producers = IntStream.range(0, 5).mapToObj(i -> () -> {
final Random random = ThreadLocalRandom.current();
while (true) {
q.add("Consumer " + i + " putting " + random.nextDouble());
try {
TimeUnit.MILLISECONDS.sleep(random.nextInt(2000));
} catch (InterruptedException e) {
//ignore
}
}
});
producers.forEach(executorService::submit);
}
The consumer blocks on the BlockingQueue.take method and immediately there is an item available, it will be woken and will print the item. If there are no items, the thread will be suspended - allowing the physical CPU to do something else.
The producers each push a String onto the queue using add. As the queue is unbounded, add will always return true. In the case where there is likely to be a backlog of work the for consumer you can bound the queue and use the put method (that throws an InterruptedException so requires a try..catch which is why it's easier to use add) - this will automatically create flow control.

Seems more like synchronization issue.. You are trying to do a simulation of - Producer - Consumer problem. You need to synchronize your Queue or use a BlockingQueue. You probably have a race condition.

You are going to need to synchronize access to your Queue or
use ConcurrentLinkedQueue see http://docs.oracle.com/javase/7/docs/api/java/util/concurrent/ConcurrentLinkedQueue.html
or as also suggested using a BlockingQueue (depending on your requirements) http://docs.oracle.com/javase/7/docs/api/java/util/concurrent/BlockingQueue.html
For a more detailed explanation of the BlockingQueue see
http://tutorials.jenkov.com/java-util-concurrent/blockingqueue.html

Java / Prioritization of threads

I have an application that starts four threads to listen on incoming packets. Every thread opens a socket on a different port. Normally packets are received only on one port at the time, but in some cases messages can be received on two ports for some seconds. Every of these threads processes the messages and updates a bunch of listeners (all of them are doing some Swing painting stuff). As the messages are sent with a frequency of 10 Hz and the painting actions on the Swing components take some time, my first approach was to process only one messages out of 20 (2 seconds time to finish the paint on the components). Works well...
But when receiving two messages at the time, I need to tell my application just to process one of them (the one received only for short time). In summary 10 messages are received on the 2nd port, also with a frequency of 10 Hz. Means, using the first approach sometimes I miss all 10 of them, because only one out of 20 is processed.
Whenever a messages on the 2nd port is received I want my application to process that one, doesn't matter what is received on the 1st port or if something is painted at that time.
The following code shows the implementation of my threads, four of these are started at the same time depending on the ports given through the constructor.
private class IncomingRunner implements Runnable {
private int listenPort;
private DatagramSocket localSocket;
private DatagramPacket packet;
private int counter = 0;
public IncomingRunner(int port) {
this.listenPort = port;
}
#Override
public void run() {
try {
localSocket = new DatagramSocket(listenPort);
byte[] buffer = new byte[1024];
packet = new DatagramPacket(buffer, buffer.length);
while(isRunning)
recvIncomingMsg();
} catch (SocketException e) {
e.printStackTrace();
}
}
private void recvIncomingMsg() {
try {
localSocket.receive(packet);
port = localSocket.getLocalPort();
ReceivedMsg eventMsg;
if(port == Config.PORT_1) {
eventMsg = new ReceivedMsg(Config.PORT_1, Config.SOMETHING_1);
System.out.println(HexWriter.getHex(packet.getData()));
} else if (port == Config.PORT_2) {
eventMsg = new ReceivedMsg(Config.PORT_2, Config.SOMETHING_2);
System.out.println(HexWriter.getHex(packet.getData()));
} else if (port == Config.PORT_3) {
eventMsg = new ReceivedMsg(Config.PORT_3, Config.SOMETHING_3);
System.out.println(HexWriter.getHex(packet.getData()));
} else {
eventMsg = new ReceivedMsg(Config.PORT_4, Config.SOMETHING_4);
System.out.println(HexWriter.getHex(packet.getData()));
}
counter++;
if(counter%20 == 0) {
forward2PacketPanel(eventMsg);
counter = 0;
}
} catch (IOException e) {
e.printStackTrace();
}
}
private void forward2PacketPanel(final ReceivedMsg t) {
for(final IPacketListener c : listeners) {
if(c instanceof IPacketListener) {
new Thread(new Runnable() {
#Override
public void run() {
((IPacketListener)c).update(t);
}
}).start();
}
}
}
}
UPDATE:
The reason why I am starting new Threads to update the listeners is, because all of them should update the GUI at the same time. Every updated calls a paintComponent() method on a different JPanel. So all of them should run together.
UPDATE2:
I cannot use the first approach as this causes messages loss of maybe important messages (received on 2nd port). What I need is, when a normal Msg is received just process it and do the painting, doesn't matter how many new normal messages (on 1st port) come in. But even if only one Msg on 2nd port is received, the application needs to process that one, regardless what is going on in the normal receiver thread.
I guess I am facing two problems here:
I need to make each thread waiting until the painting is finished, as that is UDP I can process a normal packet, and forget about all following normal packets, during the painting actions. When done, process the next normal packet.
If a packet on 2nd port is received, break all normal packet processing actions and do the things needed to process the special packet.
Problem (1) is solved using a BitSet in the MainIncomingClass. Every Listener uses some kind of callback function to indicate that its done with painting and sets a specific Bit in the BitSet. If not all are true, I do not process any new Packet, just let them go.

They talk about the event dispatch thread here. You have to use it to update your GUI. Fortunately, you can also use it to post your updates in whatever order you want. The EDT takes care of the start() for you. You'll still have to synchronize access to t.
EventQueue.invokeLater(new Runnable() {
#Override
public void run() {
((IPacketListener)c).update(t);
}
});

Java concurrent queue for broadcast?

I have a bunch of producer threads adding to a BlockingQueue and one worker thread taking objects. Now I want to extend this so two worker threads are taking objects, but doing different work on the objects. Here's the twist: I want an object that has been put on the queue to be worked on by both of the receiving threads.
If I keep using BlockingQueue, the two threads will compete for the objects, and only one of the worker threads will get the object.
So I'm looking for something similar to BlockingQueue, but with broadcast behaviour.
The application: The producer threads are actually creating performance measurements, and one of the workers is writing the measurements to a file, while the other worker is aggregating statistics.
I'm using Java 6.
So does such a mechanism exist? In Java SE? Elsewhere? Or do I need to code my own?
I'm looking for solutions with a small footprint - I'd prefer not to install some framework to do this.

One option: have three blocking queues. Your main producer puts items into a "broadcast" queue. You then have a consumer of that queue which consumes each item, putting it into both of the other queues, each of which is serviced by a single consumer:
Q2
---- Real Consumer 1
Q1 /
Producer ---- Broadcast Consumer
\
---- Real Consumer 2
Q3
Alternatively, you could give two blocking queues to the producer, and just get it to put the items it produces into both. That's less elegant, but slightly simpler overall :)

Jon Skeet idea is beautiful in its simplicity. Other than that you can use the disruptor pattern, which is faster and addresses exactly this problem. I can give you a code example with CoralQueue, which is an implementation of the disruptor pattern done by Coral Blocks, with which I am affiliated. It provides a data structure called Splitter that accepts a single producer offering messages and accepts multiple consumers polling messages, in a way that all messages are delivered to each and every consumer.
package com.coralblocks.coralqueue.sample.splitter;
import com.coralblocks.coralqueue.splitter.AtomicSplitter;
import com.coralblocks.coralqueue.splitter.Splitter;
import com.coralblocks.coralqueue.util.Builder;
public class Basics {
private static final int NUMBER_OF_CONSUMERS = 4;
public static void main(String[] args) {
Builder<StringBuilder> builder = new Builder<StringBuilder>() {
#Override
public StringBuilder newInstance() {
return new StringBuilder(1024);
}
};
final Splitter<StringBuilder> splitter = new AtomicSplitter<StringBuilder>(1024, builder, NUMBER_OF_CONSUMERS);
Thread producer = new Thread(new Runnable() {
private final StringBuilder getStringBuilder() {
StringBuilder sb;
while((sb = splitter.nextToDispatch()) == null) {
// splitter can be full if the size of the splitter
// is small and/or the consumer is too slow
// busy spin (you can also use a wait strategy instead)
}
return sb;
}
#Override
public void run() {
StringBuilder sb;
while(true) { // the main loop of the thread
// (...) do whatever you have to do here...
// and whenever you want to send a message to
// the other thread you can just do:
sb = getStringBuilder();
sb.setLength(0);
sb.append("Hello!");
splitter.flush();
// you can also send in batches to increase throughput:
sb = getStringBuilder();
sb.setLength(0);
sb.append("Hi!");
sb = getStringBuilder();
sb.setLength(0);
sb.append("Hi again!");
splitter.flush(); // dispatch the two messages above...
}
}
}, "Producer");
final Thread[] consumers = new Thread[NUMBER_OF_CONSUMERS];
for(int i = 0; i < consumers.length; i++) {
final int index = i;
consumers[i] = new Thread(new Runnable() {
#SuppressWarnings("unused")
#Override
public void run() {
while (true) { // the main loop of the thread
// (...) do whatever you have to do here...
// and whenever you want to check if the producer
// has sent a message you just do:
long avail;
while((avail = splitter.availableToPoll(index)) == 0) {
// splitter can be empty!
// busy spin (you can also use a wait strategy instead)
}
for(int i = 0; i < avail; i++) {
StringBuilder sb = splitter.poll(index);
// (...) do whatever you want to do with the data
// just don't call toString() to create garbage...
// copy byte-by-byte instead...
}
splitter.donePolling(index);
}
}
}, "Consumer" + index);
}
for(int i = 0; i < consumers.length; i++) {
consumers[i].start();
}
producer.start();
}
}
Disclaimer: I am one of the developers of CoralQueue.