Java Network Programming, Second Edition

Deadlock

Synchronization can lead to another possible problem with your code: deadlock. Deadlock occurs when two threads each need exclusive access to the same set of resources, but each thread possesses a different subset of those resources. If neither thread is willing to give up the resources it has, both threads will come to an indefinite halt. This can bring your program to a halt. This isn’t quite a hang in the classical sense, because the program is still active and behaving normally from the perspective of the OS, but to a user the difference is insignificant.

To return to the library example, deadlock is what occurs when Jack and Jill are each writing a term paper on Thomas Jefferson and each needs the two books Thomas Jefferson and Sally Hemings: An American Controversy and Sally Hemings and Thomas Jefferson: History, Memory and Civic Culture. If Jill has checked out the first book, while Jack has checked out the second, then neither can finish the paper. Eventually the deadline expires, and they both get an F. That’s the problem of deadlock.

Worse yet, deadlock can be a sporadic and hard-to-detect bug. Deadlock closely depends on unpredictable issues of timing. Most of the time, either Jack or Jill will get to the library first and get both books. In this case, the one who gets the books writes his paper and returns the books; then the other one gets the books and writes her paper. Only rarely will they arrive at the same time and each get one of the two books. Ninety-nine ...