If this graph contains a cycle, the system is deadlocked. Another way to illustrate a deadlock is to build a directed graph whose vertices are threads or processes and whose edges represent the 'is-waiting-for' relation. We say that a set of processes or threads is deadlocked when each thread is waiting for an event that only another process in the set can cause. However, another risk is associated with excessive synchronization: deadlock. So it seems that there is now less reason than ever to avoid synchronizing. Additionally, while synchronization carries a performance penalty, the penalty associated with uncontended synchronization is not as great as some sources have suggested, and has reduced steadily with each successive JVM implementation.

In my earlier article ' Double-Checked Locking: Clever, but Broken' ( JavaWorld, February 2001), I described how several common techniques for avoiding synchronization are in fact unsafe, and recommended a strategy of 'When in doubt, synchronize.' In general, you should synchronize whenever you are reading any variable that might have been previously written by a different thread, or whenever you are writing any variable that might be subsequently read by another thread.

Java deadlock