Prevent a Thread : Join()

• If a Thread wants to wait until completing some other Thread then we should go for join() method.

• Example: If a Thread t1 wants to wait until completing t2 then t1 has to call t2.join().

• If t1 executes t2.join() then immediately t1 will be entered into waiting state until t2 completes.

• Once t2 completes then t1 can continue its execution.

 Scenario:

 

• Wedding cards printing thread(t2) has to wait until venue fixing Thread (t1) completion, hence t2 has to call t1.join().

• Wedding cards distribution Thread(t3) has to wait until wedding cards printing thread(t2) completion, hence t3 has to call t2.join(). 

Prototypes of join():

1. public final void join()throws InterruptedException
2. public final void join(long ms) throws InterruptedException // wait till the mentioned time in millisecs.
3. public final void join(long ms, int ns) throws InterruptedException 

NOTE:- Every join() throws InterruptedException which is checked exception. Hence compulsory we should handle this exception either by using try catch or by throws keyword otherwise we will get compile time error.

Join Life cycle:-

Case :-1 : Example:

class MyThread extends Thread {             public void run()             {                         for(int i=0;i<5;i++)                         {                                     Thread.yield(); System.out.println("child thread");                         }             } } class ThreadYieldDemo {             public static void main(String[] args)             {                         MyThread t=new MyThread(); t.start();                         for(int i=0;i<5;i++)                         {                                     System.out.println("main thread");                         }             } }

•  If we are commenting line 1 then both Threads will be executed simultaneously and we can't expect exact execution order.

• If we are not commenting line 1 then main Thread will wait until completing child Thread in this the output is Child Thread 10 times followed by Parent Thread 10 times.

Case:-2 : Waiting of child Thread until completing main Thread : Example:

class MyThread extends Thread {             static Thread mt;             public void run()             {                         try                         {                                     mt.join();                         }                         catch (InterruptedException e){}                         for(int i=0;i<10;i++)                         {                                     System.out.println("Child Thread");                         }             } } class ThreadJoinDemo {             public static void main(String[] args)throws InterruptedException             {                         MyThread mt=Thread.currentThread();                         MyThread t=new MyThread();                         t.start();                         for(int i=0;i<10;i++)                         {                                     Thread.sleep(2000);                                     System.out.println("Main Thread");                         }             } }

Output:-
Main Thread
…
…10 times
Main Thread
Chile Thread
…
… 10 times
Child Thread

Case:-3:

• If main thread calls join() on child thread object and child thread called join() on main thread object then both threads will wait for each other forever and the program will be hanged(like deadlock if a Thread class join() method on the same thread itself then the program will be hanged).

Case:-4: Example:

• If a Thread call join() on the same Thread itself than the program will be stucked (this is something like deadlock). In this case Thread has to wait infinite amount of time.

class ThreadDemo {             public static void main() throws InterruptedException {                         Thread.currentThread().join(); // main thread waiting for             } }

Multithreading Agenda

Thanks..!!