CODEWITHSUNDEEP
javamultithreadingmatrix
Yokupoku Maioku
Yokupoku Maioku

Reputation: 503

matrix is not printed well, when being accesses by threads

i want to increase +1 the numbers inside a row of matrix using threads.

what i wrote is that (only the shared resource method):

public void increaseRow(Integer row) {
        if (!mapForRow.containsKey(row))
            mapForRow.put(row, "not increased");
        if (mapForRow.get(row).equals("not increased")) {
            lock.lock();
            try {
                while (rowIncreased) {
                    condition.await();
                }
                mapForRow.get(row).equals("increased");
                rowIncreased = true;
                for (int j = 0; j < matrix.length; j++)
                    setMatrix(row, j, matrix[row][j] + 1);
                rowIncreased = false;
                // replace the value
                mapForRow.get(row).equals(" not increased");
            } catch (InterruptedException e) {
                e.printStackTrace();
            } finally {
                System.out.println();
                System.out.println("begin print matrix");
                for (int i = 0; i < row; i++) {
                    System.out.println();
                    for (int j = 0; j < column; j++)
                        System.out.print(matrix[i][j]);
                }
                System.out.println();
                System.out.println("end print matrix ");
                System.out.println();
                lock.unlock();
                condition.notifyAll();
            }
        }
    }

the matrix is initialized with 10 rows and 10 columns and the threads started are also 10

but via output i' m getting this:

begin print matrix

0000000000
0000000000
0000000000
0000000000
end print matrix 


begin print matrix

00Exception in thread "Thread-0" 00000000
0000000000
0000000000
end print matrix 

java.lang.IllegalMonitorStateException
[...]

i can understand the the thrown exception, but why the matrix is not fully displayed?

Upvotes: 1

Views: 47

Answers (1)

PeterK
PeterK

Reputation: 1723

Why don't you use an AtomicInteger for that. These can be safely accessed and manipulated from multiple threads without the need of locking.

If you imagine your code as in between the lines starting with //, you can see that, the NullPointerException will be caught and handled while the lock is in place, but if any other exception is thrown, one that is not caught in that block, the exception will progress up the stack trace until caught, and when it is caught, you lock will long have been released. 

try {

  // start of your code
  lock.lock();
  try {
    doStuff();
  } catch(InterruptedException e) {
    System.out.println("in lock");
  } finally {
    lock.unlock();
  }

  // end of your code

} catch(Exception e) { // IllegalMonitorStateException caught here
  System.out.println("not in lock");
}

Upvotes: 2

Related Questions