CODEWITHSUNDEEP
javamultithreadingsynchronization
Nikola
Nikola

Reputation: 890

Synchronized threads behaviour

I have a Cell class representing a single value and swapThread class whose run method just call the method swapValue() in Cell.

public static void main(String[] args) throws InterruptedException {

    Cell c1 = new Cell(15);
    Cell c2 = new Cell(23);

    Thread t1 = new swapThread(c1, c2);
    Thread t2 = new swapThread(c2, c1);

    t1.start();
    t2.start();
}

class Cell:

class Cell {
private static int counter = 0;
private int value, id;

public Cell(int v) {
    value = v;
    id = counter++;
}

synchronized int getValue() {
    return value;
}

synchronized void setValue(int v) {
    value = v;
}

void swapValue(Cell other) {

    int t = getValue();
    System.out.println("Swapping " + t);
    int v = other.getValue();
    System.out.println("with " + v);
    setValue(v);
    other.setValue(t);

    System.out.println("Cell is now " + getValue());
    System.out.println("Cell was " + other.getValue());
}

}

and class swapThread:

class swapThread extends Thread {
Cell cell, othercell;

public swapThread(Cell c, Cell oc) {
    cell = c;
    othercell = oc;
}

public void run() {

    cell.swapValue(othercell);

}
}

Usual output:

Swapping 15
Swapping 23
with 23
with 15
Cell is now 23
Cell is now 15
Cell was 15
Cell was 23

I can just wait for the thread1 to finish with Thread.join() in the main method but is there a way to avoid that by changing the synchronized methods.

Upvotes: 2

Views: 73

Answers (1)

Alex Salauyou
Alex Salauyou

Reputation: 14338

You can achieve serial execution of swapValues() by making this method static and synchronized:

static synchronized void swapValues(Cell c1, Cell c2) {

    int t = c1.getValue();
    System.out.println("Swapping " + t);
    int v = c2.getValue();
    System.out.println("with " + v);
    c1.setValue(v);
    c2.setValue(t);

    System.out.println("Cell is now " + c1.getValue());
    System.out.println("Cell was " + c2.getValue());
}

Thus, you synchronize it on Cell.class, making swapValues() to be performed sequentially.

Notice, now you need to pass 2 cells in it:

public void run() {
    Cell.swapValues(cell, othercell);
}

Upvotes: 1

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