计时器任务无法启动

Question

I was writing code in the constructor of Class A to create an object of MetricData and start the timer in it. The timer will call method to fill the property array with random numbers every 2 seconds, and the data filling starts right on when start the timer. I got a NPE on propertyValue = propertyMetricData.getFirstDataPoint(); which I assume there is no data in the array when calling that method, but when I'm in debug mode executing command by command, it sometimes works(around 4/10 times works). So I guess maybe the problem has something to do with timing? Like when calling getFirstDataPoint() the fillData() in the timer hasn't been called yet? I could not find where the problem is, so please help me, .

================================ Constructor of class A ================================

public A(){

    propertyMetricData = new MetricData();
    propertyValue = propertyMetricData.getFirstDataPoint();
{

================================ Class MetricData ===================================

public class MetricData {

private Timer timer = new Timer();;
private DecimalFormat df = new DecimalFormat("###.##");
private Random rand = new Random();
private ArrayList<Double> dataPoints = new ArrayList<Double>();

public MetricData(){

    startTimer();   
}

public ArrayList<Double> getDataPoints(){
    return dataPoints;
}

public Double getFirstDataPoint(){
    return dataPoints.get(0);
}

private void startTimer(){

    timer.scheduleAtFixedRate(new TimerTask(){
        @Override
        public void run() {
            fillData();
        }
    }, 0, 2*1000);  
}

private void fillData(){

    dataPoints.clear();

    for(int i=0; i<100; i++){
        dataPoints.add(genRanNum(1,10));
    }
}

private Double genRanNum(int min, int max){

    double number = min + (rand.nextDouble()*(max - min));

    return Double.valueOf(df.format(number));
}

public void testTimer(){
    System.out.println(getFirstDataPoint());
}
}

Answer 1

First, for safety reasons, synchronize all access to dataPoints , since you'll be accessing it from different threads. Some weird synchronization issues could be solved by that.

public class MetricData {
...

public Double getFirstDataPoint(){
    synchronized(dataPoints) {
        return dataPoints.get(0);
    }
}

private void fillData(){
    synchronized (dataPoints) {    
        dataPoints.clear();

        for(int i=0; i<100; i++){
            dataPoints.add(genRanNum(1,10));
        }
    }
}
}

Secondly, you are executing your Timer asynchronously, so the second line on your A() constructor will not wait anything to run, so it'll most probably get nothing there.

It's a logic issue. I don't know what you meant by this code, but maybe if you move the propertyValue = propertyMetricData.getFirstDataPoint(); line from the constructor to a getter method would give you better results, since it'll be called only when needed, not on the object creation (and will give some time for the Timer to work):

public A() {
    propertyMetricData = new MetricData();
{

public Double getPropertyValue() {
    return propertyMetricData.getFirstDataPoint();
}

But my best advice is to take a deep breath and review your logic.

Answer 2

The quick do it dirty solution would be to call Thread.Sleep :

propertyMetricData = new MetricData();
Thread.sleep(2000);
propertyValue = propertyMetricData.getFirstDataPoint();

This will let the time to your timer which runs in its own Thread to populate the ArrayList of points.

However, I agree with all that has been said in the answer of Everton. You should probably review the logic so that it looks more elegant. You could probably lock the Main Thread waiting for the ArrayList to be populated by values. The Producer Thread (in this case the timer) would then notify the Consumer Thread so that it could call the getFirstDataPoint safely.

Or you could handle this with an Exception (I deliberately omit the synchronized) :

public Double getFirstDataPoint() throws Exception { // Or custom exception
    if(dataPoints.get(0).equals(null))
         throw new Exception("Array non populated yet!");
    return dataPoints.get(0);
}

And handle it so that the Main Thread tries to get the value later. (when the Array is populated)