How to make sure Java won't switch threads when in certain block of code?

Question

I have 6 threads accessing one common queue container, they have this in their run() method:

if (queue.size() != 0) {
    Element e = queue.remove();
    //code

It works as intended almost every time, however, once in a while thread switches just between the if check and remove() method, so some other thread gets the element and this thread gets a NoSuchElementException. How do i make sure that between those two lines of code threads won't switch?

You need to syncronise your threads docs.oracle.com/javase/tutorial/essential/concurrency/sync.html — litelite
– litelite, Commented Aug 3, 2017 at 14:52
Or you can use a thread safe queue docs.oracle.com/javase/7/docs/api/java/util/concurrent/… — litelite
– litelite, Commented Aug 3, 2017 at 14:53
@litelite, Using a thread-safe queue is not a substitute for synchronizing higher-level code that uses the queue. Making a thread-safe program is not as simple as just building it from thread-safe objects. — Solomon Slow
– Solomon Slow, Commented Aug 3, 2017 at 18:18
@jameslarge Since the queue seems to be the only resource shared between threads it will be enough for his case. However, it will need some small tweeks to adapt the code to the new queue. And it is also what the two other answers are saying... — litelite
– litelite, Commented Aug 3, 2017 at 18:23

assylias · Accepted Answer · 2017-08-03 14:54:01Z

3

The two main options are:

making the two statements atomic, for example by using a synchronized block
using a concurrent collection.

In your case, you could use a BlockingQueue which has several methods that could solve your problem (see the table in the javadoc). BlockinQueue::take seems to be a good candidate based on the information you have provided.

answered Aug 3, 2017 at 14:54

assylias

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OldCurmudgeon · Accepted Answer · 2017-08-03 14:55:02Z

0

Assuming it's a BlockingQueue (which it should be to be properly thread safe) you should probably use poll(long,TimeUnit). This should atomically check if there is an element in the queue withing the time period and return it if there is one (or null if there isn't).

answered Aug 3, 2017 at 14:55

OldCurmudgeon

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