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Thread Deadlock
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Perhaps the scariest thing that can happen to a Java program is deadlock. Deadlock occurs when two threads are blocked, with each waiting for the other s lock. Neither can run until it gives up the lock, so they ll sit there forever and ever and ever This can happen, for example, when thread A hits synchronized code, acquires a lock B, and then enters another method (still within the synchronized code it has the lock on) that s also synchronized. But thread A can t get the lock to enter this synchronized code lock C because another thread D has the lock already. So thread A goes off to the waiting for the C lock pool, hoping that thread D will hurry up and release the lock (by completing the synchronized method). But thread A will wait a very long time indeed, because while thread D picked up lock C, it then entered a method synchronized on lock B. Obviously, thread D can t get the lock B because thread A has it. And thread A won t release it until thread D releases lock C.
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Give Up Locks wait()
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Keep Locks notify() (Although the thread will probably exit the synchronized code shortly after this call, and thus give up its locks) join() sleep() yield()
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But thread D won t release lock C until after it can get lock B and continue. And there they sit. The following example demonstrates deadlock:
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1. public class DeadlockRisk { 2. private static class Resource { 3. public int value; 4. } 5. private Resource resourceA = new Resource(); 6. private Resource resourceB = new Resource(); 7. public int read() { 8. synchronized(resourceA) { // May deadlock here 9. synchronized(resourceB) { 10. return resourceB.value + resourceA.value; 11. } 12. } 13. } 14. 15. public void write(int a, int b) { 16. synchronized(resourceB) { // May deadlock here 17. synchronized(resourceA) { 18. resourceA.value = a; 19. resourceB.value = b; 20. } 21. } 22. } 23. }
Assume that read() is started by one thread and write() is started by another. If there are two different threads that may read and write independently, there is a risk of deadlock at line 8 or 16. The reader thread will have resourceA, the writer thread will have resourceB, and both will get stuck forever waiting for the other to back down. Code like this almost never results in deadlock because the CPU has to switch from the reader thread to the writer thread at a particular point in the code, and the chances of deadlock occurring are very small. The application may work fine 99.9 percent of the time. The preceding simple example is easy to fix; just swap the order of locking for either the reader or the writer at lines 16 and 17 (or lines 8 and 9). More complex deadlock situations can take a long time to figure out. Regardless of how little chance there is for your code to deadlock, the bottom line is: if you deadlock, you re dead. There are design approaches that can help avoid deadlock,
Thread Interaction (Exam Objective 7.4)
including strategies for always acquiring locks in a predetermined order. But that s for you to study and is beyond the scope of this book. We re just trying to get you through the exam. If you learn everything in this chapter, though, you ll still know more about threads then most Java programmers.
CERTIFICATION OBJECTIVE
Thread Interaction (Exam Objective 7.4)
Define the interaction among threads and object locks when executing synchronized wait, notify, and notifyAll. The last thing we need to look at is how threads can interact with one another to communicate about among other things their locking status. The java.lang.Object class has three methods wait(), notify(), and notifyAll() that help threads communicate about the status of an event that the threads care about. For example, if one thread is a mail-delivery thread and one thread is a mail-processor thread, the mail-processor thread has to keep checking to see if there s any mail to process. Using the wait and notify mechanism, the mail-processor thread could check for mail, and if it doesn t find any it can say, Hey, I m not going to waste my time checking for mail every two seconds. I m going to go hang out over here, and when the mail deliverer puts something in the mailbox, have him notify me so I can go back to runnable and do some work. In other words, wait and notify lets one thread put itself into a waiting room until some other thread notifies it that there s a reason to come back out. One key point to remember (and keep in mind for the exam) about wait/notify is this:
wait(), notify(), and notifyAll() must be called from within a synchronized context! A thread can t invoke a wait or notify method on an object unless it owns that object s lock.
Here we ll present an example of two threads that depend on each other to proceed with their execution, and we ll show how to use wait() and notify() to make them interact safely and at the proper moment.
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