Java Multithreading concept and join method

Question

I m confused in join   method used in Threads in Java  In the following code      Using join   to wait for threads to finish  class NewThread implements Runnable        String name     name of thread     Thread t       NewThread String threadname            name   threadname          t   new Thread this  name           System out println  New thread      t           t start       Start the thread          This is the entry point for thread       public void run             try               for  int i   5  i  gt  0  i--                    System out println name          i                   Thread sleep 1000                           catch  InterruptedException e                System out println name     interrupted                       System out println name     exiting              class DemoJoin        public static void main String args              NewThread ob1   new NewThread  One            NewThread ob2   new NewThread  Two            NewThread ob3   new NewThread  Three            System out println  Thread One is alive                      ob1 t isAlive             System out println  Thread Two is alive                      ob2 t isAlive             System out println  Thread Three is alive                      ob3 t isAlive        wait for threads to finish         try               System out println  Waiting for threads to finish                 ob1 t join                ob2 t join                ob3 t join              catch  InterruptedException e                System out println  Main thread Interrupted                      System out println  Thread One is alive                      ob1 t isAlive             System out println  Thread Two is alive                      ob2 t isAlive             System out println  Thread Three is alive                      ob3 t isAlive             System out println  Main thread exiting               Sample output from this program is shown here   New thread  Thread One 5 main  New thread  Thread Two 5 main  New thread  Thread Three 5 main  Thread One is alive  true Thread Two is alive  true Thread Three is alive  true Waiting for threads to finish  One  5 Two  5 Three  5 One  4 Two  4 Three  4 One  3 Two  3 Three  3 One  2 Two  2 Three  2 One  1 Two  1 Three  1 Two exiting  Three exiting  One exiting  Thread One is alive  false Thread Two is alive  false Thread Three is alive  false Main thread Exiting   In the above code     I m not able to understand the flow of execution of the program  And when ob1 is created then the constructor is called where  t start   is written but still run   method is not executed rather main   method continues execution  So why is this happening  join   method is used to wait until the thread on which it is called does not terminates  but here in output we see alternate outputs of the thread why     And if the use of join is this then what is the use of synchronized    I know I m missing a basic concept here  but I m not able to figure it out so please help

User · Answer

The JVM and the underlying OS have considerable freedom when scheduling things. The fact that you get all the way to "Waiting for threads to finish" before you see the output from individual threads may simply mean that thread start-up takes a bit longer (i.e. it takes some time between the moment when a thread becomes "alive" and when the run() method actually starts executing). You could conceivably see thread output sooner but it's not guaranteed either way.

As for join(), it only guarantees that whatever is after it will only happen once the thread you are joining is done. So when you have three join() calls in a row it doesn't mean the threads should end in a particular order. It simply means that you will wait for ob1 first. Once ob1 finishes, ob2 and ob3 may be still running or they may already be finished. If they are finished, your other join() calls will return immediately.

synchronized is used specifically when multiple threads access the same object and make changes to it. A synchronized block is guaranteed never to be executed by two threads simultaneously - i.e. the thread that executes it has the synchronized object all to itself.

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First rule of threading -  Threading is fun         I m not able to understand the flow of execution of the program  And   when ob1 is created then the constructor is called where t start   is   written but still run   method is not executed rather main   method   continues execution  So why is this happening    This is exactly what should happen   When you call Thread start  the thread is created and schedule for execution  it might happen immediately  or close enough to it   it might not   It comes down to the thread scheduler   This comes down to how the thread execution is scheduled and what else is going on in the system   Typically  each thread will be given a small amount of time to execute before it is put back to  sleep  and another thread is allowed to execute  obviously in multiple processor environments  more than one thread can be running at time  but let s try and keep it simple      Threads may also yield execution  allow other threads in the system to have chance to execute   You could try  NewThread String threadname        name   threadname      t   new Thread this  name       System out println  New thread      t       t start       Start the thread        Yield here     Thread yield        And it might make a difference to the way the threads run   equally  you could sleep for a small period of time  but this could cause your thread to be overlooked for execution for a period of cycles  sometimes you want this  sometimes you don t         join   method is used to wait until the thread on which it is called   does not terminates  but here in output we see alternate outputs of   the thread why     The way you ve stated the question is wrong   join will wait for the Thread it is called on to die before returning   For example  if you depending on the result of a Thread  you could use join to know when the Thread has ended before trying to retrieve it s result   Equally  you could poll the thread  but this will eat CPU cycles that could be better used by the Thread instead

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join   is a instance method of java lang Thread class which we can use join   method to ensure all threads that started from main must end in order in which they started and also main should end in last  In other words waits for this thread to die   Exception  join   method throws InterruptedException   Thread state  When join   method is called on thread it goes from running to waiting state  And wait for thread to die   synchronized block  Thread need not to acquire object lock before calling join   method i e  join   method can be called from outside synchronized block   Waiting time  join    Waits for this thread to die   public final void join   throws InterruptedException    This method internally calls join 0   And timeout of 0 means to wait forever   join long millis      synchronized method Waits at most millis milliseconds for this thread to die  A timeout of 0 means to wait forever   public final synchronized void join long millis      throws InterruptedException   public final synchronized void join long millis  int nanos      throws InterruptedException    Example of join method  class MyThread implements Runnable        public void run                String threadName   Thread currentThread   getName               Printer print  run   method of   threadName              for int i 0 i lt 4 i                     Printer print  i   i    Thread   threadName                                   public class TestJoin        public static void main String   args  throws InterruptedException              Printer print  start main                     MyThread runnable   new MyThread               Thread thread1 new Thread runnable              Thread thread2 new Thread runnable               thread1 start               thread1 join                thread2 start               thread2 join                Printer print  end main                class Printer        public static void print String str               System out println str             Output       start main           run   method of Thread-0      i 0  Thread Thread-0      i 1  Thread Thread-0      i 2  Thread Thread-0      i 3  Thread Thread-0      run   method of Thread-1      i 0  Thread Thread-1      i 1  Thread Thread-1      i 2  Thread Thread-1      i 3  Thread Thread-1      end main     Note  calling thread1 join    made main thread to wait until Thread-1 dies   Let   s check a program to use join long millis   First  join 1000  will be called on Thread-1  but once 1000 millisec are up  main thread can resume and start thread2  main thread won   t wait for Thread-1 to die    class MyThread implements Runnable        public void run                String threadName   Thread currentThread   getName               Printer print  run   method of   threadName              for int i 0 i lt 4 i                     try                        Thread sleep 500                     catch  InterruptedException e                         e printStackTrace                                      Printer print  i   i    Thread   threadName                                   public class TestJoin        public static void main String   args  throws InterruptedException              Printer print  start main                     MyThread runnable   new MyThread               Thread thread1 new Thread runnable              Thread thread2 new Thread runnable               thread1 start                   once 1000 millisec are up                main thread can resume and start thread2             thread1 join 1000               thread2 start               thread2 join                Printer print  end main                class Printer        public static void print String str               System out println str             Output       start main           run   method of Thread-0      i 0  Thread Thread-0      run   method of Thread-1      i 1  Thread Thread-0      i 2  Thread Thread-0      i 0  Thread Thread-1      i 1  Thread Thread-1      i 3  Thread Thread-0      i 2  Thread Thread-1      i 3  Thread Thread-1      end main     For more information see my blog   http   javaexplorer03 blogspot in 2016 05 join-method-in-java html

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when ob1 is created then the constructor is called where  t start    is written but still run   method is not executed rather main   method is executed further  So why is this happening    here your threads and main thread has equal priority Execution of equal priority thread totally depends on the Thread schedular You can t expect which to execute first      join   method is used to wait until the thread on which it is called does not terminates  but here in output we see alternate outputs of the thread why     Here your calling below statements from main thread         ob1 t join         ob2 t join         ob3 t join      So main thread waits for ob1 t ob2 t ob3 t threads to die look into Thread join doc  So all three threads executes successfully and main thread completes after that

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See the concept is very simple   1  All threads are started in the constructor and thus are in ready to run state  Main is already the running thread   2  Now you called the t1 join    Here what happens is that the main thread gets knotted behind the t1 thread  So you can imagine a longer thread with main attached to the lower end of t1   3  Now there are three threads which could run  t2  t3 and combined thread t1   main    4 Now since till t1 is finished main can t run  so the execution of the other two join statements has been stopped   5  So the scheduler now decides which of the above mentioned in point 3  threads run which explains the output

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You must understand   threads scheduling is controlled by thread scheduler So  you cannot guarantee the order of execution of threads under normal circumstances   However  you can use join   to wait for a thread to complete its work   For example  in your case  ob1 t join      This statement will not return until thread t has finished running   Try this   class Demo      Thread t   new Thread                   new Runnable                          public void run                                 do something                                                       Thread t1   new Thread                   new Runnable                          public void run                                 do something                                                       t start       Line 15     t join        Line 16     t1 start        In the above example  your main thread is executing  When it encounters line 15  thread t is available at thread scheduler  As soon as main thread comes to line 16  it will wait for thread t to finish   NOTE that t join did not do anything to thread t or to thread t1   It only affected the thread that called it  i e   the main   thread    Edited   t join    needs to be inside the try block because it throws the InterruptedException exception  otherwise you will get an error at compile time  So  it should be   try      t join     catch InterruptedException e

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I m not able to understand the flow of execution of the program  And when ob1 is created then the constructor is called where  t start   is written but still run   method is not executed rather main   method continues execution  So why is this happening    This depends on Thread Scheduler as main shares the same priority order  Calling start   doesn t mean run   will be called immediately  it depends on thread scheduler when it chooses to run your thread       join   method is used to wait until the thread on which it is called does not terminates  but here in output we see alternate outputs of the thread why     This is because of the Thread sleep 1000  in your code  Remove that line and you will see ob1 finishes before ob2 which in turn finishes before ob3  as expected with join     Having said that it all depends on when ob1 ob2 and ob3 started  Calling sleep will pause thread execution for    1 second  in your code   giving scheduler a chance to call other threads waiting  same priority

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Thread scheduler is responsible for scheduling of threads  So every time you run the program  there is no guarantee to the order of execution of threads   Suppose you have a thread object named threadOne and if join   is called on threadOne like this   threadOne join    then all currently executing threads will be paused until thread1 has finished its execution or terminates    Consider the following piece of code   class RunnableSample implements Runnable       private Thread t      private String threadName       public RunnableSample String name            this threadName   name            public void run             try               for int i   4  i  gt   1  i--                    System out println Thread currentThread   getName            i               Thread sleep 500                           catch  InterruptedException e                System out println threadName     interrupted                        public void start             if t    null              t   new Thread this  threadName           t start            try               t join              catch Exception e                System out println e                     public class RunnableDemo       public static void main String   args            RunnableSample r1   new RunnableSample  threadOne            r1 start             RunnableSample r2   new RunnableSample  threadTwo            r2 start             RunnableSample r3   new RunnableSample  threadThree            r3 start               The output of the above program will be   threadOne  4 threadOne  3 threadOne  2 threadOne  1 threadTwo  4 threadTwo  3 threadTwo  2 threadTwo  1 threadThree  4 threadThree  3 threadThree  2 threadThree  1   Since join   is called on threadOne first  threadTwo and threadThree will be paused until threadOne terminates   NOTE that threadOne  threadTwo and ThreadThree all have started   Now the threads are executing in a specific order   If join   is not called on a thread in our example  then there will be no order of execution of threads   public void start         if t    null          t   new Thread this  threadName       t start        Its output will be   threadOne  4 threadThree  4 threadTwo  4 threadTwo  3 threadThree  3 threadOne  3 threadOne  2 threadThree  2 threadTwo  2 threadOne  1 threadThree  1 threadTwo  1   Coming to synchronization  which is useful if you want to control the access of multiple threads on any shared resource  If you want to restrict only one thread to access shared resources then synchronization is the best way to do it

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I came across the join   while learning about race condition and I will clear the doubts I was having  So let us take this small example   Thread t2   new Thread               new Runnable                      public void run                             do something                                       Thread t1   new Thread               new Runnable                      public void run                             do something                                       t2 start      Line 11 t1 start      Line 12 t2 join       Line 13 t1 join       Line 14 System out print   lt Want to print something that was being modified by t2 and t1 gt      My AIM  Three threads are running namely t1  t2 and the main thread  I want to print something after the t1 and t2 has finished  The printing operation is on my main thread therefore for the expected answer I need to let t1 and t2 finish and then print my output   So t1 join   just makes the main thread wait  till the t1 thread completes before going to the next line in program    Here is the definition as per GeeksforGeeks      java lang Thread class provides the join   method which allows one   thread to wait until another thread completes its execution    Here is one question that might solve your doubt     Q-  Will t1 thread get the time slice to run by the thread scheduler  when the program is processing the t2 join   at Line   13       ANS-  Yes it will be eligible to get the time slice to run as we have already made it eligible by running the line t1 start   at Line   11   t2 join   only applies the condition when the JVM will go   to next line  that is Line 14  It might be also possible that t1   might get finished processing at Line 13

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My Comments   When I see the output  the output is mixed with One  Two  Three which are the thread names and they run simultaneously  I am not sure when you say thread is not running by main method   Not sure if I understood your question or not  But I m putting my answer what I could understand  hope it can help you   1  Then you created the object  it called the constructor  in construct it has start method which started the thread and executed the contents written inside run   method    So as you created 3 objects  3 threads - one  two  three   all 3 threads started executing simultaneously   2  Join and Synchronization They are 2 different things  Synchronization is when there are multiple threads sharing a common resource and one thread should use that resource at a time  E g  Threads such as DepositThread  WithdrawThread etc  do share a common object as BankObject  So while DepositThread is running  the WithdrawThread will wait if they are synchronized  wait    notify    notifyAll   are used for inter-thread communication  Plz google to know more   about Join    it is when multiple threads are running  but you join  e g  if there are two thread t1 and t2 and in multi-thread env they run  the output would be  t1-0 t2-0 t1-1 t2-1 t1-2 t2-2  and we use t1 join    it would be  t1-0 t1-1 t1-2 t2-0 t2-1 t2-2  This is used in realtime when sometimes you don t mix up the thread in certain conditions and one depends another to be completed  not in shared resource   so you can call the join   method

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First of all  when you create ob1 then constructor is called and it starts execution  At that time t start   also runs in separate thread  Remember when a new thread is created  it runs parallely to main thread  And thats why main start execution again with next statement    And Join   statement is used to prevent the child thread from becoming orphan  Means if you did nt call join   in your main class  then main thread will exit after its execution and child thread will be still there executing the statements  Join   will wait until all child thread complete its execution and then only main method will exit   Go through this article  helps a lot

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No words just running code      Thread class public class MyThread extends Thread        String result   null       public MyThread String name            super name              public void run             for  int i   0  i  lt  1000  i                   System out println  Hello from     this getName                       result    Bye from     this getName              Main Class  public class JoinRND       public static void main String   args             System out println  Show time               Creating threads         MyThread m1   new MyThread  Thread M1            MyThread m2   new MyThread  Thread M2            MyThread m3   new MyThread  Thread M3                Starting out Threads         m1 start            m2 start            m3 start               Just checking current value of thread class variable         System out println  M1 before      m1 result           System out println  M2 before      m2 result           System out println  M3 before      m3 result              After starting all threads main is performing its own logic in            parallel to other threads         for  int i   0  i  lt  1000  i                   System out println  Hello from Main                       try                System out                      println  Main is waiting for other threads to get there task completed                m1 join                m2 join                m3 join              catch  InterruptedException e                   TODO Auto-generated catch block             e printStackTrace                       System out println  M1 after    m1 result           System out println  M2 after    m2 result           System out println  M3 after    m3 result            System out println  Show over

[java] Java Multithreading concept and join() method

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