Deadlocks are situations in which two or more actions are waiting for the others to finish, making all actions in a blocked state forever. They can be very hard to detect during development, and they usually require restart of the application in order to recover. To make things worse, deadlocks usually manifest in production under the heaviest load, and are very hard to spot during testing. The reason for this is it’s not practical to test all possible interleavings of a program’s threads. Although some statical analysis libraries exist that can help us detect the possible deadlocks, it is still necessary to be able to detect them during runtime and get some information which can help us fix the issue or alert us so we can restart our application or whatever.
Detect deadlocks programmatically using ThreadMXBean class
Java 5 introduced ThreadMXBean - an interface that provides various monitoring methods for threads. I recommend you to check all of the methods as there are many useful operations for monitoring the performance of your application in case you are not using an external tool. The method of our interest is findMonitorDeadlockedThreads, or, if you are using Java 6,findDeadlockedThreads. The difference is that findDeadlockedThreads can also detect deadlocks caused by owner locks (java.util.concurrent), while findMonitorDeadlockedThreads can only detect monitor locks (i.e. synchronized blocks). Since the old version is kept for compatibility purposes only, I am going to use the second version. The idea is to encapsulate periodical checking for deadlocks into a reusable component so we can just fire and forget about it.
One way to impement scheduling is through executors framework - a set of well abstracted and very easy to use multithreading classes.
ScheduledExecutorService scheduler = Executors.newScheduledThreadPool(1);
this.scheduler.scheduleAtFixedRate(deadlockCheck, period, period, unit);
Simple as that, we have a runnable called periodically after a certain amount of time determined by period and time unit. Next, we want to make our utility is extensive and allow clients to supply the behaviour that gets triggered after a deadlock is detected. We need a method that receives a list of objects describing threads that are in a deadlock: