12. join() drawbacks

1. How join() will break the application functionality.

1. join() is a synchronized method, so current thread needs object lock of specified thread object.
   t2.join() → this line is executed by a thread t1
2. In above example t1 thread needs t2 object lock for entering into join() of t2 object.

Case 1 : If t2 object lock is not available

• t1 thread calling t2.join(), join() is synchronized method so t1 thread needs t2 object lock to execute t2.join()
• If may be another thread like t3 may holds t2 object lock and executing user defined synchronized methods of t2 object.
• Suppose if t3 thread completed execution of synchronized method of t2 object in 1min, then t1 thread need to wait 1 min for executing t2.join()

public class ThreadDemo extends Thread { Thread mainThread; @Override public  void run() { try { Thread.sleep(2000l); } catch (InterruptedException e) { e.printStackTrace(); } System.out.println("main thread state is = " + mainThread.getState()); System.out.println("completed t1 thread"); } public static void main(String[] args) throws InterruptedException { ThreadDemo t1 = new ThreadDemo(); t1.mainThread = Thread.currentThread(); t1.start(); ThreadDemo2 t2 = new ThreadDemo2(); t2.t1 = t1; t2.start(); Thread.sleep(100l); System.out.println("main thrad calling join()"); t1.join(); System.out.println("i am out of join"); } } class ThreadDemo2 extends Thread{ ThreadDemo t1 = null; @Override public void run() { m(); } public void m() { synchronized (t1) { System.out.println("i am t2 and executing inside m() by locking t1 object"); try { Thread.sleep(20000); } catch (InterruptedException e) { e.printStackTrace(); } System.out.println("i am t2 and releasing the lock and end of m() of t2"); } } }

Output

i am t2 and executing inside m() by locking t1 object main thrad calling join() main thread state is = BLOCKED completed t1 thread i am t2 and releasing the lock and end of m() of t2 i am out of join → executes after 20 secs

Case 2 : If t1 completes then join() notify the all threads wait() on t1 will come to runnable state

• t1 thread calling t2.join(), join() is synchronized method so t1 thread needs t2 object lock to execute t2.join()
• Now t1 thread calls t2.wait() internally(join() logic), so t1 thread will go to waiting state and releases the lock of t2 object lock.
• After releasing t2 object lock t1 thread will go to waiting state.
• t1 thread will come back to runnable state in the below cases only
• Any thread executes t1.interrupt()
• Any thread executes t2.notifyAll() or t2.notify()
• So JVM will execute t2.notifyAll() after completing t2 thread, then t1 thread will come back to Runnable state.
• Suppose one more thread t3 is also executes t2.wait() (we wrote this logic explicitly) and waiting for t2 object lock in waiting state.
• Now JVM will execute t2.notifyAll() after completing t2 thread, then t1, t3 both threads are coming out from waiting state.
• So if you use t2.join() then don’t use t2.wait() in your coding part.

Case 3 : If we call explicitly notify() on t2 object then t1 will come to runnable state

• t1 thread calling t2.join(), join() is synchronized method so t1 thread needs t2 object lock to execute t2.join()
• Now t1 thread calls t2.wait() internally(join() logic), so t1 thread will go to waiting state and releases the lock of t2 object lock.
• After releasing t2 object lock t1 thread will go to waiting state.
• t1 thread will come back to runnable state in the below cases only
• Any thread executes t1.interrupt()
• Any thread executes t2.notifyAll() or t2.notify()
• Before completing t1 thread, if we explicitly called t2.notifyAll() then t1 thread come back to Runnable state and executes while loop written inside of join()
  while(t2.isAlive()){
  t2.wait();
  }
• T1 thread will come to runnable state and take the t2 object lock and executes while condition, it will returns true because t1 thread is still alive.
• T2 thread go to inside loop once again and executes t2.wait()
• It again goes to waiting state until t1 thread dead only it will come out of while loop only.
• In above case we can bring the thread t1 from waiting state to runnable state, but not from join() to our application before t2 deads.

public class ThreadDemo extends Thread { Thread mainThread; @Override public  void run() { while (true) { try { Thread.sleep(100); m(); } catch (InterruptedException e) { e.printStackTrace(); } System.out.println("main thread state is = " + mainThread.getState()); } } public synchronized void m() { this.notifyAll(); } public static void main(String[] args) throws InterruptedException { ThreadDemo t1 = new ThreadDemo(); t1.mainThread = Thread.currentThread(); t1.start(); t1.join(); System.out.println("i am out of join"); } } main thread state is = WAITING main thread state is = BLOCKED main thread state is = WAITING main thread state is = WAITING main thread state is = BLOCKED main thread state is = BLOCKED main thread state is = RUNNABLE main thread state is = BLOCKED main thread state is = BLOCKED main thread state is = BLOCKED main thread state is = BLOCKED main thread state is = WAITING ……..