# Java blocking queue

public class BQueue<T> {

private Queue<T> q = new LinkedList<T>();
private int limit;

public BQueue(int limit) {
this.limit = limit;
}

public synchronized void put (T t) throws InterruptedException {
while (isFull()) {
wait();
}
boolean e = isEmpty();
if (e)
notifyAll();
}

public synchronized T get () throws InterruptedException {
while (isEmpty()) {
wait();
}
boolean f = isFull();
T t = q.poll();
if (f)
notifyAll();
return t;
}

private boolean isEmpty() {
return q.size() == 0;
}
private boolean isFull() {
return q.size() == limit;
}
}


• I think it probably is, although I occasionally get tripped up by some threading stuff. Is there a reason you're not implementing any of the interfaces (Queue, etc.)? Or even just using an existing Blocking Queue? Dec 19, 2011 at 19:44

Yes, it's thread-safe. (Or at least I haven't found any issue.) Please note, that the Java Collections Framework already has a BlockingQueue interface which has some implementations, for example a LinkedBlockingQueue (source). It's probably well tested add has better performance, so try not to reinvent the wheel if it's not necessary.

Some other notes:

1. Try using longer variable names:

boolean f = isFull();


It could be isFull which results more readable code. The same is true for q and t. (I'd rename it to queue and item.)

2. Check your input: What happens when limit is 0 or less than zero? (You should throw an IllegalArgumentException.)

3. The limit and q fields could be marked final. It would improve code readability since readers don't have to check whether their values have changed somewhere in the class or not. It also would prevent accidental value modifications.

4. The used Queue has an isEmpty method (by implementing Collection.isEmpty()), you could use that instead of your own.

• Thread-safety is a bona fide Hard Problem. Whenever possible, use someone else's tested code, especially code from the JDK. Jan 30, 2012 at 3:19
• 1. Should'nt isEmpty() and isFull() need to be synchronized too ? 2. Should'nt limit be volatile ? Jul 16, 2015 at 0:13
• @sreeprasad: 1. isEmpty() and isFull() are private methods and are called from only synchronized methods, so they're safe now. (As far as there isn't any public non-synchronized method which calls them, for example.) Anyway, synchronization keywords on them would not hurt, I would use that. (Write it as an answer, I would upvote it.) I usually use the @GuardedBy annotation too in my code for better clarity. Jul 16, 2015 at 6:04
• @sreeprasad: 2. Since limit is read and written in only synchronized methods it's safe without volatile, the synchronized block guarantees that changes are visible to other threads too. Jul 16, 2015 at 6:04

This looks thread-safe to me. However, I'm puzzled by the logic driving the notifications.

In put(), you wait until the queue is non-full (i.e., at least one slot open), then you add element t to the end of the queue. Cool, but then you only notify other threads if the queue was empty prior to adding t. Similarly, in get(), you wait until the queue is non-empty (i.e., has at least one item), then you fetch element t from the head of the queue. Again, cool, but then you only notify other threads if the queue was full prior to fetching t. This will have the effect of requiring the queue to be completely filled before it can be emptied, and conversely for it to be completely emptied before it can be refilled.

Maybe this was your intention, but this is not documented in the behavior, nor is it typical for a blocking queue.

I think what you might want is this instead:

public synchronized void put (T t) throws InterruptedException {
while (isFull())
wait();
notifyAll();
}


and:

public synchronized T get () throws InterruptedException {
while (isEmpty())
wait();
T t = q.poll();
notifyAll();
return t;
}


This version of put() will notify waiting threads immediately after a new element is added. This will wake up any waiting get() calls and immediately fetch the value you just added. This is usually what you want. If any other waiting put() call wakes up and finds that the queue is full (since you just added an element after possibly having waited for it to be non-full), it will simply cycle around the while loop and begin waiting again; no harm done.

Similarly, this version of get() will notify waiting threads immediately after an element is removed. This will wake up any waiting put() calls immediately and allow them to store a new value. This is also usually what you want. And if any other waiting get() call wakes up and finds that the queue is empty (since you may have just removed the last element), it will simply cycle around the while loop and begin waiting again until someone else adds another element.

Other than that, it looks good to me!

p.s. — Since get() and put() are synchronized, there's no harm in calling notifyAll() before you actually remove the item from the queue, because other waiting threads can't actually run again until you exit from get() (because you're not waiting). So, it can be simplified even further:

public synchronized T get () throws InterruptedException {
while (isEmpty())
wait();
notifyAll();
return q.poll();
}

• I'm not sure that I understand the first part completely. If the queue is not empty get won't call wait(), sou don't have to fill it completely to get something. (A small example code might help.) Anyway, the second points is a good catch, +1. Apr 13, 2014 at 0:22
• I don't really understand what you meant by "In put(), you wait until the queue is non-full (i.e., at least one slot open)". In the code it the put() method it only went to wait() if the queue was full. otherwise, it put the element in the queue right away. Sep 30, 2018 at 6:08

It looks to me like it is thread-safe. But don't take my word for it; you better consider each answer as a vote, and go with what the majority says. (Multi-threading is tricky!) Note: comments should not be considered as answers.

I think its thread safe.. Also there can be multiple consumers as the method get is synchronized so only 1 consumer would be able to acquire lock on get.

When the first consumer releases the lock, then the second consumer would check the while loop that if its empty and would again enter the wait state.

So it seems fine to me.

This is thread safe if there are only one producer and one consumer. The get() may return null or throw exceptions if there are two consumers. For instance if queue is empty and 2 consumers calling get(), then both consumers will be suspended at the wait(). When the producer pushes an object into queue and notify both consumers, both consumers will break the while loop and call get(). Unfortunately one consumer will get the object but the other consumer will receive null object. The fix should extend the while loop to cover this case

• I believe you are mistaken in this case. With the entire methods being synchronized, only one of the waiting threads will exit the while() loop Apr 2, 2014 at 11:14