Multiple threads and Synchronization in Java

Question

I'm working on a multithreaded program where each thread calculates the GCD for two numbers, stores the numbers and GCD into a TreeMap, and prints out the TreeMap after all the threads finish. What kind of method should I use to make sure that only one thread is storing the data at the same time, and how do I use the last thread to print the TreeMap when it is ready to print?

for (int i = 0; i < myList.size(); ++i) {
    for (int j = i + 1; j < myList.size(); ++j) {
        modulus1 = myList.get(i);
        modulus2 = myList.get(j);
        pool.execute(new ThreadProcessRunnable(modulus1, modulus2, myMap));
    }
}

public void run() {
    ThreadProcess process = null;
    try {
        // Only one thread should execute the following code
        for (Map.Entry<BigInteger, ArrayList<BigInteger>> entry : myMap.entrySet()) {
            System.out.println("key ->" + entry.getKey() + ", value->" + entry.getValue());
        }
    } catch (Exception e) {
        System.err.println("Exception ERROR");
    }

It appears that your for loop should only be an if check? List<BigInteger> list = myMap.get(FirstModulus); if (list == null) { list = new ArrayList<BigInteger>(); myMap.put(FirstModulus, list); } list.add(gcd); (this eliminates the need for the if (myMap.size() > 0) { ... } else { ... } and makes it a bit more readable) — pickypg
– pickypg, Commented Jan 17, 2013 at 6:08
Have a look at thread communication discussions like this one stackoverflow.com/questions/12274821/… — pickypg
– pickypg, Commented Jan 17, 2013 at 6:19

Andrey Atapin · Accepted Answer · 2013-01-17 05:29:21Z

1

You must use syncronize(myMap) {...} block in places where you need to guarantee single thread access to the map.

As for printing the result by the last thread, you can use a boolean flag as a signal of completeness and check it every time. Don't forget to make it volatile to let each thread see its value changes.

UPD: Brian Goetz "Java Concurrency In Practice" is a strongly recommended reading.

answered Jan 17, 2013 at 5:29

Andrey Atapin

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1 Comment

John Over a year ago

Thanks. the boolean flag works inside each thread? where should check the condition?

TheWhiteRabbit · Accepted Answer · 2013-01-17 05:25:19Z

0

          //Only one thread should executes the following code
synchronize{            
for (Map.Entry<BigInteger, ArrayList<BigInteger>> entry : myMap.entrySet()) {
                System.out.println("key ->" + entry.getKey() + ", value->" + entry.getValue());
            }
}

answered Jan 17, 2013 at 5:25

TheWhiteRabbit

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Comments

sujith · Accepted Answer · 2013-01-18 02:15:10Z

0

You can use Collections.synchronizedMap to make it thread safe. And use thread.join to make sure printing is done only when all threads are dead.

Edit: Do the printing in Main thread. Just before printing, call join on all threads.

edited Jan 18, 2013 at 2:15

answered Jan 17, 2013 at 5:54

sujith

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2 Comments

pickypg Over a year ago

The same issue relates to this as Quoi's solution.

John Over a year ago

Join waits for the thread to die. How do I know when all the thread finished.

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