How to calculate the total time it takes for multiple threads to finish executing?

Question

I am starting multiple threads(around 1000) from my code. They are Runnable threads called from a while loop. How do I calculate the total time taken for all threads to finish executing?

Also I am opening a DB Connection and have a set of queries for which I am starting these threads(1 thread for 1 query). When do I close the connection?

Answer 1

I would use an ExecutorService

long start = System.nanoTime();
ExecutorService service = Executors.newWhatEverPool();
for(loop)
   service.submit(new MyRunnable());

service.shutdown();
service.awaitTermination(1, TimeUnit.HOUR); // or longer.    
long time = System.nanoTime() - start;
System.out.printf("Tasks took %.3f ms to run%n", time/1e6);

Close the connection when you have finished with it. It is a common pattern for the creator of a resource to also close it. e.g. If the main thread creates the connection, it could close after all the threads have finished.