Simple Deadlock Examples

Question

I would like to explain threading deadlocks to newbies  I have seen many examples for deadlocks in the past  some using code and some using illustrations  like the famous 4 cars   There are also classic easily-deadlocked problems like The Dining Philosophers  but these may be too complex for a real newbie to fully grasp   I m looking for the simplest code example to illustrate what deadlocks are  The example should    Relate to a  real  programming scenario that makes some sense Be very short  simple and straight forward   What do you recommend

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I consider the Dining Philosophers problem to be one of the more simple examples in showing deadlocks though, since the 4 deadlock requirements can be easily illustrated by the drawing (especially the circular wait).

I consider real world examples to be much more confusing to the newbie, though I can't think of a good real world scenario off the top of my head right now (I'm relatively inexperienced with real-world concurrency).

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Here s a simple deadlock in C    void UpdateLabel string text       lock this          if MyLabel InvokeNeeded            IAsyncResult res    MyLable BeginInvoke delegate                  MyLable Text   text                           MyLabel EndInvoke res             else                MyLable Text   text                      If  one day  you call this from the GUI thread  and another thread calls it as well - you might deadlock  The other thread gets to EndInvoke  waits for the GUI thread to execute the delegate while holding the lock  The GUI thread blocks on the same lock waiting for the other thread to release it - which it will not because the GUI thread will never be available to execute the delegate the other thread is waiting for   ofcourse the lock here isn t strictly needed - nor is perhaps the EndInvoke  but in a slightly more complex scenario  a lock might be acquired by the caller for other reasons  resulting in the same deadlock

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Please see my answer to this question  Bottom line whenever two threads need to acquire two different resources  and do so in different orders then you can get deadlocks

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The producers-consumers problem together with the dining philosophers  problem is probably as simple as it s going to get  It has some pseudocode that illustrates it  as well  If those are too complex for a newbie they d better try harder to grasp them

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package ForkBlur   public class DeadLockTest     public static void main String args           final DeadLockTest t1   new DeadLockTest        final DeadLockTest t2   new DeadLockTest         Runnable r1   new Runnable               Override         public void run                 try                    synchronized  t1                        System out                              println  r1 has locked t1  now going to sleep                        Thread sleep 100                       System out                              println  r1 has awake   now going to aquire lock for t2                        synchronized  t2                            Thread sleep 100                                                         catch  Exception e                    e printStackTrace                                         Runnable r2   new Runnable               Override         public void run                 try                    synchronized  t2                        System out                              println  r2 has aquire the lock of t2 now going to sleep                        Thread sleep 100                       System out                              println  r2 is awake   now going to aquire the lock from t1                        synchronized  t1                            Thread sleep 100                                                         catch  Exception e                    e printStackTrace                                         new Thread r1  start        new Thread r2  start

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Let nature explain deadlock   Deadlock  Frog vs  Snake      I would love to have seen them go   their separate ways  but I was   exhausted   the photographer said     The frog was all the time trying to   pull the snake off  but the snake just   wouldn t let go

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CountDownLatch countDownLatch   new CountDownLatch 1   ExecutorService executorService   ExecutorService executorService   Executors newSingleThreadExecutor    executorService execute    - gt        Future lt   gt  future   executorService submit    - gt            System out println  generated task                countDownLatch countDown        try           future get          catch  InterruptedException e            e printStackTrace          catch  ExecutionException e             e printStackTrace                countDownLatch await    executorService shutdown

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public class DeadLock       public static void main String   args  throws InterruptedException           Thread mainThread   Thread currentThread            Thread thread1   new Thread new Runnable                  Override             public void run                     try                       mainThread join                      catch  InterruptedException e                        e printStackTrace                                                        thread1 start            thread1 join

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Deadlock can occur in a situation when a Girl1 is wanting to flirt with Guy2  who is caught by another Girl2  and Girl2 is wanting to flirt with a Guy1 that is caught by Girl1  Since  both girls are waiting for dumping each other  the condition is called deadlock    class OuchTheGirls       public static void main String   args                final String resource1    Guy1           final String resource2    Guy2               Girl1 tries to lock resource1 then resource2         Thread Girl1   new Thread    - gt                                                                            synchronized  resource1                                                                                    System out println  Thread 1  locked Guy1                                               try   Thread sleep 100    catch  Exception e                                                synchronized  resource2                                                                                            System out println  Thread 1  locked Guy2                                                                                                                                          Girl2 tries to lock Guy2 then Guy1         Thread Girl2   new Thread    - gt                                                                            synchronized  resource2                                                                                    System out println  Thread 2  locked Guy2                                               try   Thread sleep 100    catch  Exception e                                                synchronized  resource1                                                                                            System out println  Thread 2  locked Guy1                                                                                                                                        Girl1 start            Girl2 start

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Here s one simple deadlock in Java  We need two resources for demonstrating deadlock  In below example  one resource is class lock via sync method  and the other one is an integer  i   public class DeadLock        static int i      static int k       public static synchronized void m1            System out println Thread currentThread   getName     executing m1  Value of i   i            if k gt 0  i              while i  0               System out println Thread currentThread   getName     waiting in m1 for i to be  gt  0  Value of i   i               try   Thread sleep 10000    catch  InterruptedException e    e printStackTrace                           public static void main String   args             Thread t1   new Thread  t1                 public void run                     m1                                      Thread t2   new Thread  t2                 public void run                     try   Thread sleep 100    catch  InterruptedException e    e printStackTrace                      k                    m1                                      t1 start            t2 start

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Here s a code example from the computer science department of a university in Taiwan showing a simple java example with resource locking  That s very  real-life  relevant to me  Code below          Adapted from The Java Tutorial    Second Edition by Campione  M  and    Walrath  K Addison-Wesley 1998             This is a demonstration of how NOT to write multi-threaded programs     It is a program that purposely causes deadlock between two threads that    are both trying to acquire locks for the same two resources     To avoid this sort of deadlock when locking multiple resources  all threads    should always acquire their locks in the same order       public class Deadlock     public static void main String   args         These are the two resource objects        we ll try to get locks for     final Object resource1    resource1       final Object resource2    resource2         Here s the first thread        It tries to lock resource1 then resource2     Thread t1   new Thread           public void run               Lock resource 1         synchronized resource1             System out println  Thread 1  locked resource 1                Pause for a bit  simulating some file I O or              something  Basically  we just want to give the              other thread a chance to run  Threads and deadlock             are asynchronous things  but we re trying to force              deadlock to happen here              try               Thread sleep 50                catch  InterruptedException e                  Now wait  till we can get a lock on resource 2           synchronized resource2               System out println  Thread 1  locked resource 2                                                Here s the second thread          It tries to lock resource2 then resource1     Thread t2   new Thread          public void run              This thread locks resource 2 right away         synchronized resource2             System out println  Thread 2  locked resource 2                Then it pauses  for the same reason as the first              thread does           try              Thread sleep 50                catch  InterruptedException e                 Then it tries to lock resource1                But wait   Thread 1 locked resource1  and              won t release it till it gets a lock on resource2                This thread holds the lock on resource2  and won t             release it till it gets resource1                We re at an impasse  Neither thread can run               and the program freezes up            synchronized resource1               System out println  Thread 2  locked resource 1                                                Start the two threads         If all goes as planned  deadlock will occur         and the program will never exit      t1 start         t2 start

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Maybe a simple bank situation   class Account     double balance     void withdraw double amount        balance -  amount          void deposit double amount        balance    amount           void transfer Account from  Account to  double amount           sync from           sync to            from withdraw amount           to deposit amount            release to           release from              Obviously  should there be two threads which attempt to run transfer a  b  and transfer b  a  at the same time  then a deadlock is going to occur because they try to acquire the resources in reverse order    This code is also great for looking at solutions to the deadlock as well  Hope this helps

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Here s a simple example in C  11    include  lt mutex gt        mutex  include  lt iostream gt     cout   include  lt cstdio gt       getchar  include  lt thread gt       this thread  yield  include  lt future gt       async  include  lt chrono gt       seconds  using namespace std  mutex  m1  mutex  m2      Deadlock will occur because func12 and func21 acquires the two locks in reverse order  void func12         unique lock lt mutex gt  l1  m1       this thread  yield       hint to reschedule     this thread  sleep for  chrono  seconds 1         unique lock lt mutex gt  l2  m2       void func21         unique lock lt mutex gt  l2  m2       this thread  yield       hint to reschedule     this thread  sleep for  chrono  seconds 1         unique lock lt mutex gt  l1  m1      int main  int argc  char  argv           async func12       func21        cout  lt  lt   All done       this won t be executed because of deadlock     getchar

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One more simple deadlock example with two different resources and two thread waiting for each other to release resource  Directly from examples oreilly com jenut Deadlock java   public class Deadlock     public static void main String   args           These are the two resource objects we ll try to get locks for     final Object resource1    resource1       final Object resource2    resource2          Here s the first thread   It tries to lock resource1 then resource2     Thread t1   new Thread           public void run                Lock resource 1         synchronized resource1              System out println  Thread 1  locked resource 1                  Pause for a bit  simulating some file I O or something                 Basically  we just want to give the other thread a chance to              run   Threads and deadlock are asynchronous things  but we re              trying to force deadlock to happen here              try   Thread sleep 50     catch  InterruptedException e                   Now wait  till we can get a lock on resource 2           synchronized resource2                System out println  Thread 1  locked resource 2                                                 Here s the second thread   It tries to lock resource2 then resource1     Thread t2   new Thread           public void run                This thread locks resource 2 right away         synchronized resource2              System out println  Thread 2  locked resource 2                  Then it pauses  for the same reason as the first thread does           try   Thread sleep 50     catch  InterruptedException e                   Then it tries to lock resource1   But wait   Thread 1 locked              resource1  and won t release it  till it gets a lock on              resource2   This thread holds the lock on resource2  and won t              release it  till it gets resource1   We re at an impasse  Neither              thread can run  and the program freezes up            synchronized resource1                System out println  Thread 2  locked resource 1                                                 Start the two threads  If all goes as planned  deadlock will occur          and the program will never exit      t1 start         t2 start

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If method1   and method2   both will be called by two or many threads  there is a good chance of deadlock because if thread 1 acquires lock on String object while executing method1   and thread 2 acquires lock on Integer object while executing method2   both will be waiting for each other to release lock on Integer and String to proceed further  which will never happen  public void method1         synchronized  String class            System out println  quot Acquired lock on String class object quot             synchronized  Integer class                System out println  quot Acquired lock on Integer class object quot                       public void method2         synchronized  Integer class            System out println  quot Acquired lock on Integer class object quot             synchronized  String class                System out println  quot Acquired lock on String class object quot

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I found that a bit hard to understand when reading the dining philosophers  problem  deadlock IMHO is actually related to resource allocation   Would like to share a more simple example where 2 Nurse need to fight for 3 equipment in order to complete a task   Although it s written in java  A simple lock   method is created to simulate how the deadlock happen  so it can apply in other programming language as well  http   www justexample com wp example-of-deadlock

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One of the simple deadlock example I have come across   public class SimpleDeadLock      public static Object l1   new Object       public static Object l2   new Object       private int index     public static void main String   a          Thread t1   new Thread1          Thread t2   new Thread2          t1 start          t2 start            private static class Thread1 extends Thread         public void run              synchronized  l1                System out println  Thread 1  Holding lock 1                   try   Thread sleep 10                 catch  InterruptedException e                 System out println  Thread 1  Waiting for lock 2                   synchronized  l2                   System out println  Thread 2  Holding lock 1  amp  2                                                private static class Thread2 extends Thread         public void run              synchronized  l2                System out println  Thread 2  Holding lock 2                   try   Thread sleep 10                 catch  InterruptedException e                 System out println  Thread 2  Waiting for lock 1                   synchronized  l1                   System out println  Thread 2  Holding lock 2  amp  1

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Here is my detailed example for deadlock  after spending lots of time  Hope it helps     package deadlock   public class DeadlockApp        String s1    hello       String s2    world        Thread th1   new Thread             public void run                 System out println  Thread th1 has started                synchronized  s1      A lock is created internally  holds access of s1   lock will be released or unlocked for s1  only when it exits the block Line  23                 System out println  Executing first synchronized block of th1                     try                       Thread sleep 1000                     catch InterruptedException ex                        System out println  Exception is caught in th1                                      System out println  Waiting for the lock to be released from parrallel thread th1                    synchronized  s2      As another has runned parallely Line  32  lock has been created for s2                     System out println s1   s2                                                System out println  Thread th1 has executed                           Thread th2   new Thread             public void run                 System out println  Thread th2 has started                synchronized  s2      A lock is created internally  holds access of s2   lock will be released or unlocked for s2  only when it exits the block Line  44                 System out println  Executing first synchronized block of th2                     try                       Thread sleep 1000                     catch InterruptedException ex                        System out println  Exception is caught in th2                                      System out println  Waiting for the lock to be released from parrallel thread th2                    synchronized  s1      As another has runned parallely Line  11  lock has been created for s1                     System out println s1   s2                                                System out println  Thread th2 has executed                          public static void main String   args            DeadlockApp deadLock   new DeadlockApp            deadLock th1 start            deadLock th2 start              Line  51 and  52 runs parallely on executing the program  a lock is created inside synchronized method           A lock is nothing but  something like a blocker or wall  which holds access of the variable from being used by others            Locked object is accessible  only when it is unlocked  i e exiting  the synchronized block            Lock cannot be created for primitive types  ex  int  float  double            Dont forget to add thread sleep time  because if not added  then object access will not be at same time for both threads to create Deadlock  not actual runtime with lots of threads             This is a simple program  so we added sleep90 to create Deadlock  it will execute successfully  if it is removed                Happy coding -- Parthasarathy S

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package test concurrent  public class DeadLockTest      private static long sleepMillis     private final Object lock1   new Object       private final Object lock2   new Object        public static void main String   args           sleepMillis   Long parseLong args 0           DeadLockTest test   new DeadLockTest           test doTest             private void doTest            Thread t1   new Thread new Runnable                public void run                    lock12                                   Thread t2   new Thread new Runnable                public void run                    lock21                                   t1 start           t2 start             private void lock12            synchronized  lock1               sleep               synchronized  lock2                   sleep                                   private void lock21            synchronized  lock2               sleep               synchronized  lock1                   sleep                                   private void sleep            try              Thread sleep sleepMillis            catch  InterruptedException e               e printStackTrace                    To run the deadlock test with sleep time 1 millisecond  java -cp   test concurrent DeadLockTest 1

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A sneaky way to deadlock with just a single thread is to try to lock the same  non-recursive  mutex twice  This might not be the simple example you were looking for  but sure enough I encountered such cases already    include  lt mutex gt   include  lt iostream gt   int main       std  mutex m    m lock      m lock      std  cout  lt  lt   Expect never to get here because of a deadlock

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public class DeadLock        public static void main String   args            Object resource1   new Object            Object resource2   new Object            SharedObject s   new SharedObject resource1  resource2           TestThread11 t1   new TestThread11 s           TestThread22 t2   new TestThread22 s           t1 start            t2 start              class SharedObject       Object o1  o2      SharedObject Object o1  Object o2            this o1   o1          this o2   o2            void m1             synchronized o1                System out println  locked on o1 from m1                  synchronized o2                     System out println  locked on o2 from m1                                        void m2             synchronized o2                System out println  locked on o2 from m2                  synchronized o1                     System out println  locked on o1 from m2                                       class TestThread11 extends Thread       SharedObject s      TestThread11 SharedObject s            this s   s            public void run             s m1             class TestThread22 extends Thread       SharedObject s      TestThread22 SharedObject s            this s   s            public void run             s m2

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Go for the simplist possible scenario in which deadlock can occur when introducting the concept to your students  This would involve a minimum of two threads and a minimum of two resources  I think   The goal being to engineer a scenario in which the first thread has a lock on resource one  and is waiting for the lock on resource two to be released  whilst at the same time thread two holds a lock on resource two  and is waiting for the lock on resource one to be released    It doesn t really matter what the underlying resources are  for simplicities sake  you could just make them a pair of files that both threads are able to write to     EDIT  This assumes no inter-process communication other than the locks held

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I have created an ultra Simple Working DeadLock Example -  package com thread deadlock   public class ThreadDeadLockClient        public static void main String   args            ThreadDeadLockObject1 threadDeadLockA   new ThreadDeadLockObject1  threadDeadLockA            ThreadDeadLockObject2 threadDeadLockB   new ThreadDeadLockObject2  threadDeadLockB             new Thread new Runnable                   Override             public void run                     threadDeadLockA methodA threadDeadLockB                             start             new Thread new Runnable                   Override             public void run                     threadDeadLockB methodB threadDeadLockA                             start             package com thread deadlock   public class ThreadDeadLockObject1        private String name       ThreadDeadLockObject1 String name           this name   name             public  synchronized void methodA ThreadDeadLockObject2 threadDeadLockObject2            System out println  In MethodA     Current Object-- gt    this getName     Object passed as parameter-- gt    threadDeadLockObject2 getName             try               Thread sleep 1000             catch  InterruptedException e                   TODO Auto-generated catch block             e printStackTrace                      threadDeadLockObject2 methodB this              public String getName             return name             public void setName String name            this name   name              package com thread deadlock   public class ThreadDeadLockObject2        private String name       ThreadDeadLockObject2 String name           this name   name             public  synchronized void methodB ThreadDeadLockObject1 threadDeadLockObject1            System out println  In MethodB     Current Object-- gt    this getName     Object passed as parameter-- gt    threadDeadLockObject1 getName             try               Thread sleep 1000             catch  InterruptedException e                   TODO Auto-generated catch block             e printStackTrace                      threadDeadLockObject1 methodA this              public String getName             return name             public void setName String name            this name   name            In the above example 2 threads are executing the synchronized methods of two different objects  Synchronized methodA is called by object threadDeadLockA and synchronized methodB is called by object threadDeadLockB  In methodA a reference of threadDeadLockB is passed and in methodB a reference of threadDeadLockA is passed  Now each thread tries to get lock on the another object  In methodA the thread who is holding a lock on threadDeadLockA is trying to get lock on object threadDeadLockB and similarly in methodB the thread who is holding a lock on threadDeadLockB is trying to get lock on threadDeadLockA  Thus both the threads will wait forever creating a deadlock

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Simple example from https   docs oracle com javase tutorial essential concurrency deadlock html  public class Deadlock    public static void printMessage String message         System out println String format   s  s    Thread currentThread   getName    message        private static class Friend        private String name       public Friend String name            this name   name             public void bow Friend friend             printMessage  Acquiring lock on     this name            synchronized this                printMessage  Acquired lock on     this name               printMessage name     bows     friend name               friend bowBack this                         public void bowBack Friend friend             printMessage  Acquiring lock on     this name            synchronized  this                printMessage  Acquired lock on     this name               printMessage friend name     bows back                         public static void main String   args  throws InterruptedException        Friend one   new Friend  one        Friend two   new Friend  two         new Thread new Runnable              Override         public void run                 one bow two                    start         new Thread new Runnable              Override         public void run                 two bow one                    start           Output   Thread-0 Acquiring lock on one  Thread-1 Acquiring lock on two  Thread-0 Acquired lock on one  Thread-1 Acquired lock on two  Thread-1 two bows one  Thread-0 one bows two  Thread-1 Acquiring lock on one  Thread-0 Acquiring lock on two    Thread Dump   2016-03-14 12 20 09 Full thread dump Java HotSpot TM  64-Bit Server VM  25 74-b02 mixed mode     DestroyJavaVM   13 prio 5 os prio 0 tid 0x00007f472400a000 nid 0x3783 waiting on condition  0x0000000000000000     java lang Thread State  RUNNABLE   Thread-1   12 prio 5 os prio 0 tid 0x00007f472420d800 nid 0x37a3 waiting for monitor entry  0x00007f46e89a5000     java lang Thread State  BLOCKED  on object monitor      at com anantha algorithms ThreadJoin Friend bowBack ThreadJoin java 102      - waiting to lock  lt 0x000000076d0583a0 gt   a com anantha algorithms ThreadJoin Friend      at com anantha algorithms ThreadJoin Friend bow ThreadJoin java 92      - locked  lt 0x000000076d0583e0 gt   a com anantha algorithms ThreadJoin Friend      at com anantha algorithms ThreadJoin 2 run ThreadJoin java 141      at java lang Thread run Thread java 745    Thread-0   11 prio 5 os prio 0 tid 0x00007f472420b800 nid 0x37a2 waiting for monitor entry  0x00007f46e8aa6000     java lang Thread State  BLOCKED  on object monitor      at com anantha algorithms ThreadJoin Friend bowBack ThreadJoin java 102      - waiting to lock  lt 0x000000076d0583e0 gt   a com anantha algorithms ThreadJoin Friend      at com anantha algorithms ThreadJoin Friend bow ThreadJoin java 92      - locked  lt 0x000000076d0583a0 gt   a com anantha algorithms ThreadJoin Friend      at com anantha algorithms ThreadJoin 1 run ThreadJoin java 134      at java lang Thread run Thread java 745    Monitor Ctrl-Break   10 daemon prio 5 os prio 0 tid 0x00007f4724211000 nid 0x37a1 runnable  0x00007f46e8def000     java lang Thread State  RUNNABLE     at java net SocketInputStream socketRead0 Native Method      at java net SocketInputStream socketRead SocketInputStream java 116      at java net SocketInputStream read SocketInputStream java 170      at java net SocketInputStream read SocketInputStream java 141      at sun nio cs StreamDecoder readBytes StreamDecoder java 284      at sun nio cs StreamDecoder implRead StreamDecoder java 326      at sun nio cs StreamDecoder read StreamDecoder java 178      - locked  lt 0x000000076d20afb8 gt   a java io InputStreamReader      at java io InputStreamReader read InputStreamReader java 184      at java io BufferedReader fill BufferedReader java 161      at java io BufferedReader readLine BufferedReader java 324      - locked  lt 0x000000076d20afb8 gt   a java io InputStreamReader      at java io BufferedReader readLine BufferedReader java 389      at com intellij rt execution application AppMain 1 run AppMain java 93      at java lang Thread run Thread java 745    Service Thread   9 daemon prio 9 os prio 0 tid 0x00007f47240c9800 nid 0x3794 runnable  0x0000000000000000     java lang Thread State  RUNNABLE   C1 CompilerThread3   8 daemon prio 9 os prio 0 tid 0x00007f47240c6800 nid 0x3793 waiting on condition  0x0000000000000000     java lang Thread State  RUNNABLE   C2 CompilerThread2   7 daemon prio 9 os prio 0 tid 0x00007f47240c4000 nid 0x3792 waiting on condition  0x0000000000000000     java lang Thread State  RUNNABLE   C2 CompilerThread1   6 daemon prio 9 os prio 0 tid 0x00007f47240c2800 nid 0x3791 waiting on condition  0x0000000000000000     java lang Thread State  RUNNABLE   C2 CompilerThread0   5 daemon prio 9 os prio 0 tid 0x00007f47240bf800 nid 0x3790 waiting on condition  0x0000000000000000     java lang Thread State  RUNNABLE   Signal Dispatcher   4 daemon prio 9 os prio 0 tid 0x00007f47240be000 nid 0x378f waiting on condition  0x0000000000000000     java lang Thread State  RUNNABLE   Finalizer   3 daemon prio 8 os prio 0 tid 0x00007f472408c000 nid 0x378e in Object wait    0x00007f46e98c5000     java lang Thread State  WAITING  on object monitor      at java lang Object wait Native Method      - waiting on  lt 0x000000076cf88ee0 gt   a java lang ref ReferenceQueue Lock      at java lang ref ReferenceQueue remove ReferenceQueue java 143      - locked  lt 0x000000076cf88ee0 gt   a java lang ref ReferenceQueue Lock      at java lang ref ReferenceQueue remove ReferenceQueue java 164      at java lang ref Finalizer FinalizerThread run Finalizer java 209    Reference Handler   2 daemon prio 10 os prio 0 tid 0x00007f4724087800 nid 0x378d in Object wait    0x00007f46e99c6000     java lang Thread State  WAITING  on object monitor      at java lang Object wait Native Method      - waiting on  lt 0x000000076cf86b50 gt   a java lang ref Reference Lock      at java lang Object wait Object java 502      at java lang ref Reference tryHandlePending Reference java 191      - locked  lt 0x000000076cf86b50 gt   a java lang ref Reference Lock      at java lang ref Reference ReferenceHandler run Reference java 153    VM Thread  os prio 0 tid 0x00007f4724080000 nid 0x378c runnable    GC task thread 0  ParallelGC   os prio 0 tid 0x00007f472401f000 nid 0x3784 runnable    GC task thread 1  ParallelGC   os prio 0 tid 0x00007f4724021000 nid 0x3785 runnable    GC task thread 2  ParallelGC   os prio 0 tid 0x00007f4724022800 nid 0x3786 runnable    GC task thread 3  ParallelGC   os prio 0 tid 0x00007f4724024800 nid 0x3787 runnable    GC task thread 4  ParallelGC   os prio 0 tid 0x00007f4724026000 nid 0x3788 runnable    GC task thread 5  ParallelGC   os prio 0 tid 0x00007f4724028000 nid 0x3789 runnable    GC task thread 6  ParallelGC   os prio 0 tid 0x00007f4724029800 nid 0x378a runnable    GC task thread 7  ParallelGC   os prio 0 tid 0x00007f472402b800 nid 0x378b runnable    VM Periodic Task Thread  os prio 0 tid 0x00007f47240cc800 nid 0x3795 waiting on condition   JNI global references  16   Found one Java-level deadlock                                 Thread-1     waiting to lock monitor 0x00007f46dc003f08  object 0x000000076d0583a0  a com anantha algorithms ThreadJoin Friend     which is held by  Thread-0   Thread-0     waiting to lock monitor 0x00007f46dc006008  object 0x000000076d0583e0  a com anantha algorithms ThreadJoin Friend     which is held by  Thread-1   Java stack information for the threads listed above                                                       Thread-1       at com anantha algorithms ThreadJoin Friend bowBack ThreadJoin java 102      - waiting to lock  lt 0x000000076d0583a0 gt   a com anantha algorithms ThreadJoin Friend      at com anantha algorithms ThreadJoin Friend bow ThreadJoin java 92      - locked  lt 0x000000076d0583e0 gt   a com anantha algorithms ThreadJoin Friend      at com anantha algorithms ThreadJoin 2 run ThreadJoin java 141      at java lang Thread run Thread java 745   Thread-0       at com anantha algorithms ThreadJoin Friend bowBack ThreadJoin java 102      - waiting to lock  lt 0x000000076d0583e0 gt   a com anantha algorithms ThreadJoin Friend      at com anantha algorithms ThreadJoin Friend bow ThreadJoin java 92      - locked  lt 0x000000076d0583a0 gt   a com anantha algorithms ThreadJoin Friend      at com anantha algorithms ThreadJoin 1 run ThreadJoin java 134      at java lang Thread run Thread java 745   Found 1 deadlock   Heap  PSYoungGen      total 74752K  used 9032K  0x000000076cf80000  0x0000000772280000  0x00000007c0000000    eden space 64512K  14  used  0x000000076cf80000 0x000000076d8520e8 0x0000000770e80000    from space 10240K  0  used  0x0000000771880000 0x0000000771880000 0x0000000772280000    to   space 10240K  0  used  0x0000000770e80000 0x0000000770e80000 0x0000000771880000   ParOldGen       total 171008K  used 0K  0x00000006c6e00000  0x00000006d1500000  0x000000076cf80000    object space 171008K  0  used  0x00000006c6e00000 0x00000006c6e00000 0x00000006d1500000   Metaspace       used 3183K  capacity 4500K  committed 4864K  reserved 1056768K   class space    used 352K  capacity 388K  committed 512K  reserved 1048576K

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I recently realized that the fights between couples are nothing but a deadlock   where usually one of the process has to crash to resolve it  of course it s  the lesser priority one Boy       Here s the analogy     Process1  Girl G  Process2  Boy B  Resource1  Sorry Resource2  Accepting own mistake  Necessary Conditions  1  Mutual Exclusion  Only one of G or B can say sorry or accept own Mistake at a time  2  Hold and Wait  At a time  one is holding Sorry and other  Accepting own mistake  one is waiting for Accepting own mistake to  release sorry  and other is waiting for sorry to release accepting own  mistake  3  No preemption  Not even God can force B or G to release Sorry or Accepting own mistake  And voluntarily  Are you kidding me   4   Circular Wait  Again  the one holding sorry waits for other to accept  own mistakes  and one holding accept own mistakes want other to say  sorry first  So it s circular   So deadlocks occur when all these conditions are in effect at the same time  and that s always the case in a couple fight     Source  http   www quora com Saurabh-Pandey-3 Posts Never-ending-couple-fights-a-deadlock

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One example I can think of is the Table  Flashlight  and Batteries scenario  Imagine a flashlight and a pair of batteries placed on top of a table  If you were to walk to this table and grab the batteries while another person has the flashlight you both will be forced to awkwardly stare at each other while waiting for who will first place their item back on the table  This is an example of deadlock  You and the person are waiting for resources but none of you are giving up their resource   Similarly  in a program  deadlock occurs when two or more threads  you and the other person  are waiting for two or more locks  flashlight and batteries  to be freed and the circumstances in the program are such that the locks are never freed  you both have one piece of the puzzle    If you know java  this is how you can represent this problem   import java util concurrent locks     public class Deadlock1        public static class Table            private static Lock Flashlight   new ReentrantLock            private static Lock Batteries   new ReentrantLock                     public static void giveFlashLightAndBatteries                 try                   Flashlight lock                    Batteries lock                    System out println  Lights on                  finally                   Batteries unlock                    Flashlight unlock                                     public static void giveBatteriesAndFlashLight                 try                   Batteries lock                    Flashlight lock                    System out println  Lights on                  finally                   Flashlight unlock                    Batteries unlock                                       public static void main String   args               This thread represents person one         new Thread new Runnable                 public void run     Table giveFlashLightAndBatteries                 start                This thread represents person two         new Thread new Runnable                 public void run     Table giveBatteriesAndFlashLight                 start              If you run this example you will notice that sometimes things work nice and correctly  But sometimes your program will just not print anything  That is because one person has the batteries while another person has the flashlight which prevents them from turning on the flashlight causing a deadlock   This example is similar to the example given by the java tutorials  http   docs oracle com javase tutorial essential concurrency deadlock html  Another example is the loop example   public class Deadlock2        public static class Loop           private static boolean done   false           public static synchronized void startLoop   throws InterruptedException               while  done                    Thread sleep 1000                   System out println  Not done                                     public static synchronized void stopLoop                 done   true                        public static void main String   args               This thread starts the loop         new Thread new Runnable                 public void run                     try                       Loop startLoop                      catch  InterruptedException e                        e printStackTrace                                               start                This thread stops the loop         new Thread new Runnable                 public void run                     Loop stopLoop                             start              This example can either print  Not done  over and over or it can never print  Not done  at all  The first happens because the first thread acquires the class lock and never releases it preventing  stopLoop  from being accessed by the second thread  And the latest happens because the second thread started before the first thread causing the  done  variable to be true before the first thread executes

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public class DeadlockProg                    Gowtham Chitimi Reddy IIT BHU               public static void main String   args               TODO Auto-generated method stub         final Object ob1   new Object            final Object ob2   new Object            Thread t1   new Thread                public void run                    synchronized ob1                       try                          Thread sleep 100                                             catch InterruptedException e                           System out println  Error catched                                              synchronized ob2                                                                              Thread t2   new Thread                public void run                    synchronized ob2                       try                          Thread sleep 100                                             catch InterruptedException e                           System out println  Error catched                                              synchronized ob1                                                                                                                 t1 start            t2 start

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Let me explain more clearly using an example having more than 2 threads    Let us say you have n threads each holding locks L1  L2       Ln respectively  Now let s say  starting from thread 1  each thread tries to acquire its neighbour thread s lock  So  thread 1 gets blocked for trying to acquire L2  as L2 is owned by thread 2   thread 2 gets blocked for L3 and so on  The thread n gets blocked for L1  This is now a deadlock as no thread is able to execute   class ImportantWork     synchronized void callAnother                 synchronized void call ImportantWork work  throws InterruptedException       Thread sleep 100        work callAnother           class Task implements Runnable    ImportantWork myWork  otherWork    public void run        try         myWork call otherWork         catch  InterruptedException e                      class DeadlockTest    public static void main String args         ImportantWork work1 new ImportantWork        ImportantWork work2 new ImportantWork        ImportantWork work3 new ImportantWork        Task task1 new Task         task1 myWork work1      task1 otherWork work2       Task task2 new Task         task2 myWork work2      task2 otherWork work3       Task task3 new Task         task3 myWork work3      task3 otherWork work1       new Thread task1  start        new Thread task2  start        new Thread task3  start            In the above example  you can see that there are three threads holding Runnables task1  task2  and task3  Before the statement sleep 100  the threads acquire the three work objects  locks when they enter the call   method  due to the presence of synchronized   But as soon as they try to callAnother   on their neighbour thread s object  they are blocked  leading to a deadlock  because those objects  locks have already been taken

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