Java中的多线程和同步

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");
    }

Answer 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.

Answer 2

          //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());
            }
}

Answer 3

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.

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