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javamultithreadingconcurrency

What is the most frequent concurrency issue you've encountered in Java?

This is a poll of sorts about common concurrency problems in Java. An example might be the classic deadlock or race condition or perhaps EDT threading bugs in Swing. I'm interested both in a breadth of possible issues but also in what issues are most common. So, please leave one specific answer of a Java concurrency bug per comment and vote up if you see one you've encountered.

Solution

  • The most common concurrency problem I've seen, is not realizing that a field written by one thread is not guaranteed to be seen by a different thread. A common application of this:

    class MyThread extends Thread {
  private boolean stop = false;
  
  public void run() {
    while(!stop) {
      doSomeWork();
    }
  }
  
  public void setStop() {
    this.stop = true;
  }
}

    As long as stop is not volatile or setStop and run are not synchronized this is not guaranteed to work. This mistake is especially devilish as in 99.999% it won't matter in practice as the reader thread will eventually see the change - but we don't know how soon he saw it.