Deadlocks in Java

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A deadlock is a situation where a thread is waiting for an object lock that another thread holds, and this second thread is waiting for an object lock that the first thread holds Since each thread is waiting for the other thread to relinquish a lock, they both remain waiting forever in the Blocked-for-lock-acquisition state The threads are said to be deadlocked
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CHAPTER 13: THREADS
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A deadlock is depicted in Figure 1310 Thread t1 has a lock on object o1, but cannot acquire the lock on object o2 Thread t2 has a lock on object o2, but cannot acquire the lock on object o1 They can only proceed if one of them relinquishes a lock the other one wants, which is never going to happen
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Deadlock
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thread t1 lock acquired
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o1 o2
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thread t2 lock acquired
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Thread t1 now waiting to acquire lock on o2 Thread t2 now waiting to acquire lock on o1
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The situation in Figure 1310 is implemented in Example 138 Thread t1 at (3) tries to synchronize at (4) and (5), first on string o1 at (1) then on string o2 at (2), respectively The thread t2 at (6) does the opposite It synchronizes at (7) and (8), first on string o2 then on string o1, respectively A deadlock can occur as explained previously However, the potential of deadlock in the situation in Example 138 is easy to fix If the two threads acquire the locks on the objects in the same order, then mutual lock dependency is avoided and a deadlock can never occur This means having the same locking order at (4) and (5) as at (7) and (8) In general, the cause of a deadlock is not always easy to discover, let alone easy to fix
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Example 138 Deadlock public class DeadLockDanger { String o1 = "Lock " ; String o2 = "Step "; Thread t1 = (new Thread("Printer1") { public void run() { while(true) { synchronized(o1) { synchronized(o2) { Systemoutprintln(o1 + o2); } } // (1) // (2) // (3)
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// (4) // (5)
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136: THREAD TRANSITIONS } } }); Thread t2 = (new Thread("Printer2") { public void run() { while(true) { synchronized(o2) { synchronized(o1) { Systemoutprintln(o2 + o1); } } } } }); public static void main(String[] args) { DeadLockDanger dld = new DeadLockDanger(); dldt1start(); dldt2start(); } } // (6)
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// (7) // (8)
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Possible output from the program:
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Lock Lock Lock Lock Lock _ Step Step Step Step Step
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1315 Which one of these events will cause a thread to die Select the one correct answer (a) (b) (c) (d) (e) 1316 The method sleep() is called The method wait() is called Execution of the start() method ends Execution of the run() method ends Execution of the thread s constructor ends
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Which statements are true about the following code
public class Joining { static Thread createThread(final int i, final Thread t1) { Thread t2 = new Thread() { public void run() { Systemoutprintln(i+1);
654 try { t1join(); } catch (InterruptedException ie) { } Systemoutprintln(i+2); } }; Systemoutprintln(i+3); t2start(); Systemoutprintln(i+4); return t2; }
CHAPTER 13: THREADS
public static void main(String[] args) { createThread(10, createThread(20, ThreadcurrentThread())); } }
Select the two correct answers (a) (b) (c) (d) (e) 1317 The first number printed is 13 The number 14 is printed before the number 22 The number 24 is printed before the number 21 The last number printed is 12 The number 11 is printed before the number 23
Which statements are true about the following program
public class ThreadAPI { private static Thread t1 = new Thread("T1") { public void run() { try { wait(1000); } catch (InterruptedException ie){} }}; private static Thread t2 = new Thread("T2") { public void run() { notify(); }}; private static Thread t3 = new Thread("T3") { public void run() { yield(); }}; private static Thread t4 = new Thread("T4") { public void run() { try { sleep(100); } catch (InterruptedException ie){} }}; public static void main(String[] args) { t1start(); t2start(); t3start(); t4start(); try { t4join(); } catch (InterruptedException ie){} } }