Why we should use Join in threads?

Question

I have 2 threads T1 and T2 ,both have different jobs so usually we prefer to accomplish this task by thread Joins.

But we can do this with out using join(). We can add T2 thread's code inside T1 thread. What difference does this make ?

Answer 1

Joining a thread means that one waits for the other to end, so that you can safely access its result or continue after both have finished their jobs.

Example: if you start a new thread in the main thread and both do some work, you'd join the main thread on the newly created one, causing the main thread to wait for the second thread to finish. Thus you can do some work in parallel until you reach the join.

If you split a job into two parts which are executed by different threads you may get a performance improvement, if

• the threads can run independently, i.e. if they don't rely on each other's data, otherwise you'd have to synchronize which costs performance
• the JVM is able to execute multiple threads in parallel, i.e. you have a hyperthreading/multicore machine and the JVM utilizes that

Answer 2

usually we prefer to accomplish this task by thread Joins.

No we don't. We accomplish this task by starting two threads. There is no obligation to use join() so there is no 'should' about it. If you want to pause the current thread while another thread completes, do so. If you don't, don't.

Answer 3

If you call T1.join(); from T2 it will wait for T1 to die (finish). It is a form of thread synchronization, but from what you describe you can simply fire of two thread and simply do not use join. If you use two threads then the work will be done in parallel, if you put the code only in one thread then the work will be done sequentially.

Answer 4

Here is the reason to use join: You use it when final result depends on result of two tasks which could run at the same time.

Example1: After user clicks submit button the program has to call two external webservices to update their respective systems. It can be done at the same time that is why we would create a separate thread for one of webservices.

The user will sit before the screen and wait for a notification: Your submission is OK! The screen should say OK only after both threads finished.