join() waiting forever when exception occurs, jvm shutdown hook not working - multithreading

Question

I am trying to shutdown the application, whenever any Fatal
Error/Exception comes but before shut down the application my current
thread/task should complete, so I have written mainThread.join()
inside run(), its working fine when there is no exception. But whenever my
doTask() throwing exception that time join() waiting forever.
public class POC
{
public void doTask() throws Exception
{
throw new Exception("Fatal Error");
//throw new Exception("Fatal Error"); By commenting working fine.
}
public static void main(String[] args)
{
POC ob = new POC();
final Thread mainThread = Thread.currentThread();
Runtime.getRuntime().addShutdownHook(new Thread()
{
public void run()
{
try
{
System.out.println("Join() Start");
mainThread.join();
System.out.println("Join() End"); //Why this is not printing?
}
catch (InterruptedException e)
{
e.printStackTrace();
}
}
});
try
{
System.out.println("Before doTask()");
ob.doTask(); //User Defined Run()
System.out.println("After doTask()");
}
catch (Exception ex) // FATAL
{
System.err.println("Exception : " + ex.getLocalizedMessage());
System.exit(-1);
}
}
}
OutPut : 0
Before Run()
Exception : Fatal Error
Join() Start
Why System.out.println("Join() End"); is not printing?

Answer 1

You have a simple deadlock.
When you throw an exception, exception handler call System.exit(-1), which is blocking, see javadoc:
Terminates the currently running Java virtual machine by initiating its shutdown sequence
...
The virtual machine's shutdown sequence consists of two phases. In the first phase all registered #addShutdownHook shutdown hooks, if any, are started in some unspecified order and allowed to run concurrently until they finish.
So main thread is waiting in System#exit call until all shutdown hook will be finished and your only shutdown hook blocks and waits until main thread will finish (which is waiting in System#exit ... GOTO 1).