Shuffling array in multiple threads

I have an array of size N. I want to shuffle its elements in 2 threads (or more). Each thread should work with it's own part of the array.

Lets say, the first thread shuffles elements from 0 to K, and the second thread shuffles elements from K to N (where 0 < K < N). So, it can look like this:

//try-catch stuff is ommited
static void shuffle(int[] array) {
   Thread t1 = new ShufflingThread(array, 0, array.length / 2);
   Thread t2 = new ShufflingThread(array, array.length / 2, array.length);
   t1.start();
   t2.start();
   t1.join();
   t2.join();
}

public static void main(String[] args) {
   int array = generateBigSortedArray();
   shuffle(array);
}

Are there any guaranties from JVM that I will see changes in the array from the main method after such shuffling?

How should I implement ShufflingThread (or, how should I run it, maybe within a synch开发者_如何转开发ronized block or whatever else) in order to get such guaranties?

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The join() calls are sufficient to ensure memory coherency: when t1.join() returns, the main thread "sees" whatever operations thread t1 did on the array.

Also, Java guarantees that there is no word-tearing on arrays: distinct threads may use distinct elements of the same array without needing synchronization.

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I think this is a good exercise in thread control, where (1) a job can be broken up into several parts (2) the parts can run independently and asynchronously and (3) A master thread monitors the completion of all such jobs in their respective threads. All you need is for this master thread to wait() and be notify()-ed jobCount times, every time a thread completes execution. Here is a sample code that you can compile/run. Uncomment the println()'s to see more.

Notes: [1] JVM doesnt guarantee the order of execution of the threads [2] You need to synchronize when your master thread access the big array, to not have corrupted data....

public class ShufflingArray {

private int nPart = 4,      // Count of jobs distributed, resource dependent
        activeThreadCount,  // Currently active, monitored with notify
        iRay[];         // Array the threads will work on

    public ShufflingArray (int[] a) {
    iRay = a;
    printArray (a);
}

private void printArray (int[] ia) {
    for (int i = 0 ; i < ia.length ; i++)
        System.out.print (" " + ((ia[i] < 10) ? " " : "") + ia[i]);
    System.out.println();
    }

public void shuffle () {
    int startNext = 0, pLen = iRay.length / nPart;  // make a bunch of parts
    for (int i = 0 ; i < nPart ; i++, activeThreadCount++) {
        int start = (i == 0) ? 0 : startNext,
            stop = start + pLen;
        startNext = stop;
        if (i == (nPart-1))
            stop = iRay.length;
        new Thread (new ShuffleOnePart (start, stop, (i+1))).start();
    }
    waitOnShufflers (0);        // returns when activeThreadCount == 0
    printArray (iRay);
}

synchronized private void waitOnShufflers (int bump) {
    if (bump == 0) {
        while (activeThreadCount > 0) {
            // System.out.println ("Waiting on " + activeThreadCount + " threads");
            try {
                wait();
            } catch (InterruptedException intex) {
    }}} else {
        activeThreadCount += bump;
        notify();
}}

public class ShuffleOnePart implements Runnable {
    private int startIndex, stopIndex;      // Operate on global array iRay

    public ShuffleOnePart (int i, int j, int k) {
        startIndex = i;
        stopIndex = j;
        // System.out.println ("Shuffler part #" + k);
    }

    // Suppose shuffling means interchanging the first and last pairs
    public void run () {
        int tmp = iRay[startIndex+1];
        iRay[startIndex+1] = iRay[startIndex];  iRay[startIndex] = tmp;
        tmp = iRay[stopIndex-1];
        iRay[stopIndex-1] = iRay[stopIndex-2];  iRay[stopIndex-2] = tmp;
        try {   // Lets imagine it needs to do something else too
            Thread.sleep (157);
        } catch (InterruptedException iex) { }
        waitOnShufflers (-1);
}}

public static void main (String[] args) {
    int n = 25, ia[] = new int[n];
    for (int i = 0 ; i < n ; i++)
        ia[i] = i+1;
    new ShufflingArray(ia).shuffle();

}}

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Thread.start() and Thread.join() are enough to give you the happens-before relationships between the array initialisation, its hand-off to the threads and then the read back in the main method.

Actions that cause happens-before are documented here.

As mentioned elsewhere, ForkJoin is very well suited to this kind of divide-and-conquer algorithm and frees you from a lot of the book-keeping that you'd otherwise need to implement.

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using ExecutorService from java.util.Concurrent package along with Callable Task to return the part of the array's from each thread run, once both thread are completed is another way to do for consistent behaviour.

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Well, they can't BOTH be accessing the same array and if you use a lock, or a mutex or any other synchronizing mechanism, you kinda lose the power of the threads (since one will have to wait for another, either to finish the shuffling or finish a bit of the shuffling). Why don't you just divide the array in half, give each thread its bit of the array and then merge the two arrays?