Question

I am not sure whether such code is safe or not. I need to read a name from the DB, and I need to use, say 10 threads, each one take a name, then inside the run it calls a function that needs this name as an argument.

Is 10 threads a suitable number for pc running Intel core i7 with 8GB RAM ?? How can I know the acceptable number of threads I can create? Is this code correct and safe?

I find in Dietel book that they create new thread before the Executor. They add line like: PrintTask task1 = new PrintTask( "thread1" );

check: http://www.deitel.com/articles/java_tutorials/20051126/JavaMultithreading_Tutorial_Part4.html) , but find the below method (without the new statement) in: http://www.vogella.com/articles/JavaConcurrency/article.html

In other word, should I create them before the loop? Two references use different methods and I’m confused. Which code is correct, this:

        ExecutorService Executor = Executors.newFixedThreadPool(10);

        while(resultSet.next()) 
           {
              name=resultSet.getString("hname");
              MyRunnable worker = new  MyRunnable(name);

               Executor.execute( worker );
               Counter++;
        }


     Executor.shutdown();
     System.out.println("thread shutdown");

// Wait until all threads are finish
while (! Executor.isTerminated()) {

}
System.out.println("Finished all threads");

OR

MyRunnable task1 = new MyRunnable(name );
MyRunnable task2 = new MyRunnable(name );
MyRunnable task3 = new MyRunnable(name );
MyRunnable task4 = new MyRunnable(name );
MyRunnable task5 = new MyRunnable(name );
MyRunnable task6 = new MyRunnable(name );
MyRunnable task7 = new MyRunnable(name );
MyRunnable task8 = new MyRunnable(name );
MyRunnable task9 = new MyRunnable(name );
MyRunnable task10 = new MyRunnable(name );


        ExecutorService Executor = Executors.newFixedThreadPool(10);

        while(resultSet.next()) 
           {
              name=resultSet.getString("hname");
              MyRunnable worker = new  MyRunnable(name);

               Executor.execute( worker );
               Counter++;
        }


     Executor.shutdown();
     System.out.println("thread shutdown");

// Wait until all threads are finish
while (! Executor.isTerminated()) {

}
System.out.println("Finished all threads");

Also, in the constructor of MyRunnable class that implements run, do i have to start the thread explicitly or does the Executor.execute( worker ) enough in this case.

Answer 1

I would have rewritten it like that:

ExecutorService executor = Executors.newFixedThreadPool(10);

while(resultSet.next())
{
    name=resultSet.getString("hname");
    MyRunnable worker = new  MyRunnable(name);
    executor.submit(worker);
    counter++;
}


executor.shutdown();
System.out.println("thread shutdown");

executor.awaitTermination(60, TimeUnit.SECONDS);
System.out.println("Finished all threads");

variable in lower case, awaitTermination instead of a loop.

The number of threads to use depends on several factors: the machine, the number of tasks to execute, the “size” of the task, etc.