Homework 4

Name: Multi-threaded application
Points: 3
Type: Solitaire assignment. Everybody solves it alone!

Assignment

Write a simple program which spawns 3 threads. These threads cooperate on generating numbers from 0 to 50 and printing them on standard output in the following form:

Thread 1: 0
Thread 1: 1
Thread 3: 2
Thread 2: 3
...
Thread 3: 49
Thread 1: 50

Additional instructions and useful hints

Please keep in mind the following facts when dealing with the solution:

• Do not extend class Thread but implement the Runnable interface instead in your solution.
• Keep the thread behavior in a single class, i.e. do not introduce a separate class for each thread but only a new instance of the same class.
• Use Thread.currentThread().getName() for obtaining the thread name.
• Beware of what you really need to synchronize! Mistakes in synchronization will be subject to penalization. Not only insufficient synchronization is a mistake but also redundant synchronization applies here.
• Every generated number must be present in the resulting output just once.
• Very common mistake is to exceed the maximum number at the end. Please keep this in mind and do not repeat it.
• In cases when all the numbers are being generated by a single thread (because the context switch is too slow), put so called active wait loop to an appropriate place. For waiting 50 ms use waitActively(50L, TimeUnit.MILLISECONDS), where the method body looks like this:

  private static void waitActively(long duration, TimeUnit timeUnit) {
      long durationNanos = timeUnit.toNanos(duration);
      long start = System.nanoTime();
      while ((System.nanoTime() - start) < durationNanos);
  }

  NOTE: Active wait loop is an exceptional instrument and should never be used under normal circumstances as it wastes CPU time and therefore has negative impact on the overall performance. However, in this particular case it is satisfactory as the standard way of waiting for a specified time e.g. Thread.sleep() would have caused context switch and therefore masked off many possible synchronization mistakes. No matter how much actively you wait in your solution, there is no guarantee that you avoided all synchronization mistakes. Active wait loop only increases the chance that synchronization mistakes manifest themselves early.
• It is very likely that numbers will not be in the correct order in the first version of your implementation. Please, save this version before you start correcting it with respect to the order of the numbers. Then show us both versions so that we see the evolution (the first one should be simpler, the final one a bit more complicated).