What does this thread join code mean

Question

In this code  what does the two joins and break mean  t1 join   causes t2 to stop until t1 terminates   Thread t1   new Thread new EventThread  e1     t1 start    Thread t2   new Thread new EventThread  e2     t2 start    while  true       try         t1 join          t2 join          break       catch  InterruptedException e          e printStackTrace

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From oracle documentation page on Joins

The join method allows one thread to wait for the completion of another.

If t1 is a Thread object whose thread is currently executing,

t1.join() : causes the current thread to pause execution until t1's thread terminates.

If t2 is a Thread object whose thread is currently executing,

t2.join(); causes the current thread to pause execution until t2's thread terminates.

join API is low level API, which has been introduced in earlier versions of java. Lot of things have been changed over a period of time (especially with jdk 1.5 release) on concurrency front.

You can achieve the same with java.util.concurrent API. Some of the examples are

Using invokeAll on ExecutorService
Using CountDownLatch
Using ForkJoinPool or newWorkStealingPool of Executors(since java 8)

Refer to related SE questions:

wait until all threads finish their work in java

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Simply put   t1 join   returns after t1 is completed   It doesn t do anything to thread t1  except wait for it to finish  Naturally  code following  t1 join   will be executed only after  t1 join   returns

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This is a favorite Java interview question   Thread t1   new Thread new EventThread  e1     t1 start    Thread e2   new Thread new EventThread  e2     t2 start     while  true        try           t1 join       1         t2 join       2  These lines  1 2  are in in public static void main         break            t1 join   means  t1 says something like  I want to finish first   Same is the case with t2  No matter who started t1 or t2 thread  in this case the main method   main will wait until t1 and t2 finish their task   However  an important point to note down  t1 and t2 themselves can run in parallel irrespective of the join call sequence  on t1 and t2  It is the main daemon thread that has to wait

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What does this thread join code mean    To quote from the Thread join   method javadocs      join   Waits for this thread to die     There is a thread that is running your example code which is probably the main thread    The main thread creates and starts the t1 and t2 threads   The two threads start running in parallel  The main thread calls t1 join   to wait for the t1 thread to finish  The t1 thread completes and the t1 join   method returns in the main thread   Note that t1 could already have finished before the join   call is made in which case the join   call will return immediately  The main thread calls t2 join   to wait for the t2 thread to finish  The t2 thread completes  or it might have completed before the t1 thread did  and the t2 join   method returns in the main thread    It is important to understand that the t1 and t2 threads have been running in parallel but the main thread that started them needs to wait for them to finish before it can continue   That s a common pattern   Also  t1 and or t2 could have finished before the main thread calls join   on them   If so then join   will not wait but will return immediately      t1 join   means cause t2 to stop until t1 terminates    No   The main thread that is calling t1 join   will stop running and wait for the t1 thread to finish   The t2 thread is running in parallel and is not affected by t1 or the t1 join   call at all   In terms of the try catch  the join   throws InterruptedException meaning that the main thread that is calling join   may itself be interrupted by another thread    while  true       Having the joins in a while loop is a strange pattern   Typically you would do the first join and then the second join handling the InterruptedException appropriately in each case   No need to put them in a loop

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For me  Join   behavior was always  confusing because I was trying to remember who will wait for whom  Don t try to remember it that way      Underneath  it is pure wait   and notify   mechanism    We all know that  when we call wait   on any object t1   calling object main  is sent to waiting room Blocked state    Here  main thread is calling join   which is wait   under the covers  So main thread will wait until it is notified   Notification is given by t1 when it finishes it s run thread completion    After receiving the notification  main comes out of waiting room and proceeds it s execution

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A picture is worth a thousand words       Main thread-- gt ----- gt --- gt -- gt --block          continue--- gt ---- gt                                                       sub thread start                     join                                                                                     ---sub thread----- gt --- gt --- gt --finish       Hope to useful  for more detail click here

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When thread tA call tB join   its causes not only waits for tB to die or tA be interrupted itself but create happens-before relation between last statement in tB and next statement after tB join   in tA thread      All actions in a thread happen-before any other thread successfully returns from a join   on that thread    It means program  class App          shared  not synchronized variable   bad practice     static int sharedVar   0      public static void main String   args  throws Exception           Thread threadB   new Thread    - gt   sharedVar   1             threadB start            threadB join             while  true               System out print sharedVar             Always print   gt  gt  1111111111111111111111111       But program  class App          shared  not synchronized variable   bad practice     static int sharedVar   0      public static void main String   args  throws Exception           Thread threadB   new Thread    - gt   sharedVar   1             threadB start               threadB join     COMMENT JOIN          while  true              System out print sharedVar             Can print not only    gt  gt  0000000000     000000111111111111111111111111       But    gt  gt  00000000000000000000000000000000000000000000        Always only  0     Because Java Memory Model don t require  transfering  new value of  sharedVar  from threadB to main thread without heppens-before relation  thread start  thread join  usage of  synchonized  keyword  usage of AtomicXXX variables  etc

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let s say our main thread starts the threads t1 and t2  Now  when t1 join   is called  the main thread suspends itself till thread t1 dies and then resumes itself  Similarly  when t2 join   executes  the main thread suspends itself again till the thread t2 dies and then resumes   So  this is how it works   Also  the while loop was not really needed here

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join   means waiting for a thread to complete  This is a blocker method  Your main thread  the one that does the join    will wait on the t1 join   line until t1 finishes its work  and then will do the same for t2 join

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Hope it helps    package join   public class ThreadJoinApp        Thread th   new Thread  Thread 1             public void run                 System out println  Current thread execution -     Thread currentThread   getName                 for  int i   0  i  lt  10  i                      System out println  Current thread execution -     Thread currentThread   getName       at index -     i                                       Thread th2   new Thread  Thread 2             public void run                 System out println  Current thread execution -     Thread currentThread   getName                    Thread 2 waits until the thread 1 successfully completes              try               th join                  catch  InterruptedException ex                    System out println  Exception has been caught                               for  int i   0  i  lt  10  i                      System out println  Current thread execution -     Thread currentThread   getName       at index -     i                                       public static void main String   args            ThreadJoinApp threadJoinApp   new ThreadJoinApp            threadJoinApp th start            threadJoinApp th2 start                 Happy coding -- Parthasarathy S

[java] What does this thread join code mean?

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