CODEWITHSUNDEEP
javamultithreading
bmanicus131
bmanicus131

Reputation: 53

Java MultiThreading not stopping

I have the following code for a kind of 'stopwatch' that extends the Thread class:

package StopWatch;

//Code taken from:
//https://stackoverflow.com/questions/9526041/how-to-program-for-a-stopwatch

public class Stopwatch extends Thread {
    private long startTime;
    private boolean started;

    public void startTimer() {
        this.startTime = System.currentTimeMillis();
        this.started = true;
        this.start();
    }

    public void run() {
        while(started){/*currentTimeMillis increases on its own */}
        System.out.println("timer stopped");
    }

    public int[] getTime() {
        long milliTime = System.currentTimeMillis() - this.startTime;
        int[] time = new int[]{0,0,0,0};
        time[0] = (int)(milliTime / 3600000);    //gives number of hours elapsed
        time[1] = (int)(milliTime / 60000) % 60; //gives number of remaining minutes elapsed
        time[2] = (int)(milliTime / 1000) % 60;  //gives number of remaining seconds elapsed
        time[3] = (int)(milliTime);              //gives number of remaining milliseconds elapsed
        return time;
    }

    public void stopTimer() {
        this.started = false;
    }
}

and I'm testing it in the following driver class: 

import StopWatch.Stopwatch;
public class StopWatchTest {
    public static void main(String[] args) {
        Stopwatch stopwatch = new Stopwatch();

        stopwatch.startTimer();
        int sum = 0;
        for (long i = 0; i < 100000; i++) {
            sum++;
        }
        int[] time = stopwatch.getTime();

        for (int i = 0; i < 4; i++) {
            if (i < 3) {
                System.out.print(time[i]+":");
            } else {
                System.out.print(time[i]);
            }
        }
        stopwatch.stopTimer();
    }
}

My intent is to use instances of class Stopwatch to measure the performance of various blocks of code (The for-loop in the driver class for instance) by having these Stopwatch objects in a main thread start a timer in separate thread before executing the blocks of code I want to evaluate, then have them (the Stopwatch objects) stop their timer once execution of said blocks in the main thread have finished. I understand that there are much simpler and easier ways to do this but I wanted to try doing it this way as sort of a "proof of concept" and to simply get better with multi-threading, but I'm encountering some problems:

1) When I run the driver class StopWatchTest I get seemingly random and arbitrary output each time (but mostly 0:0:0:0)

2) The main thread (or possibly the Stopwatch thread, I'm not even sure anymore) seems to never stop executing after I get outputs like 0:0:0:0

3) When I try debugging with breakpoints and the like I get completely unexpected behavior depending on where I put the breakpoints (The main thread does sometime finish execution but with random outputs like 0:0:13:2112 and other times I just get stuck in the Stopwatch thread)

Point 3 doesn't concern me as much as 1 and 2 as I have limited knowledge of how multi-threading behaves when one or several of the threads are paused at breakpoints for debugging (I suspect that when I break in the main thread the Stopwatch thread continues running). Points 1 and 2 bother me much more as I cannot see why they would be occurring.

Upvotes: 0

Views: 86

Answers (1)

GeertPt
GeertPt

Reputation: 17874

To get you started, you should flag the boolean started as volatile:

private volatile boolean started;

That should work, but it would make a busy loop, which is very bad for your CPU usage. You should look to wait()/notify() methods next.

Upvotes: 1

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