Effective Java 72 Don't depend on the thread scheduler

Principle

• Any program that relies on the thread scheduler for correctness or performance is likely to be nonportable.

  The best way to write a robust, responsive, portable program is to ensure that the average number of runnable threads is not significantly greater than the number of processors. This leaves the thread scheduler with little choice: it simply runs the runnable threads till they're no longer runnable.

  Note

  The number of runnable threads isn't the same as the total number of threads, which can be much higher. Threads that are waiting are not runnable.

• Threads should not run if they aren't doing useful work.

  // Awful CountDownLatch implementation - busy-waits incessantly!

  public class SlowCountDownLatch {

  private int count;

  public SlowCountDownLatch(int count) {

  if (count < 0)

  throw new IllegalArgumentException(count + " < 0");

  this.count = count;

  }

  public void await() {

  while (true) {

  synchronized(this) {

  if (count == 0) return;

  }

  }

  }

  public synchronized void countDown() {

  if (count != 0)

  count--;

  }

  }

• When faced with a program that barely works because some threads aren't getting enough CPU time relative to others, resist the temptation to "fix" the program by putting in calls to Thread.yield.
• Thread priorities are among the least portable features of the Java platform.

  Summary

  Do not depend on the thread scheduler for the correctness of your program. The resulting program will be neither robust nor portable. As a corollary, do not rely on Thread.yield or thread priorities. These facilities are merely hints to the scheduler. Thread priorities may be used sparingly to improve the quality of service of an already working program, but they should never be used to "fix" a program that barely works.