Populating Array from different threads

I'm trying to write a very simple program that creates several threads to send concurrent requests to a particular URL. I measure and store response times. The problem I’m having is that although I create my Response Times array as static and final, the values I store in this array exist only while I am inside of one of my generated threads开发者_如何学JAVA. As soon as I am out of the loop and into the main thread, the array is empty (contains 0 values). So the sum in my code snippet is always zero. I realize that my error is probably a very basic one but unfortunately I was unable to find similar topic online. Could you please point me to the right direction? Thanks.

public class MyClass {      
static final long[] respTimes = new long[l];

public static void sendRequest() {...}

public static void main(String[] args) throws Exception {  
    for(int i=0; i<l; i++) {
        new Thread("" + i) {
            public void run() { 
                long startTime = System.nanoTime();
                sendRequest();
                long estimatedTime = System.nanoTime() - startTime;
                respTimes[i] = estimatedTime;
            }
        }.start();
    }
        for(int i=0; i<l; i++) { sum += respTimes[i]; }
}  

}

That's not the problem. Your problem is that you print out the results before you had a chance to generate them. Do this:

Thread [] theThreads = new Thread[10];

for (...) {
  theThreads[i] = new Thread() { ... }.start();
}

// now make sure all the threads are done
for (...) {
  // this waits for the thread to finish
  theThreads[i].join();
}

// now print things out

Since I have a clinical aversion to static methods :P and a thing for organizing code (Also noticed you didn't handle any exceptions coming out of send method)

public class MultiClient
{
  private long[] responseTimes = null;
  private Throwable[] errors = null;
  public MultiClient(int count)
  {
    responseTimes = new long[count];
    errors = new Throwable[count];
  }
  protected void sendRequestWrapper(int index)
  {
    long startTime = System.nanoTime();
    try
    {
      //original send request
      sendRequest();
    }
    // optionally you may put a catch() here to process errors
    catch(Throwable t)
    {
      errors[index] = t;
    }
    finally
    {
      //this will guarantee response times filled up even in case of 
      //sendRequest raising an exception
      responseTimes[index] = System.nanoTime() - startTime;
    }
  }
  public void doStuff()
  {
    Thread[] threads = new Thread[count];
    //separete create thread calls
    for(int i = 0; i < threads.length; i++)
    {
        threads[i] = new Worker(i);
    }
    //... from start calls
    for(int i = 0; i < threads.length; i++)
    {
        threads[i].start();
    }
    // wait till all threads are done
    for(int i = 0; i < threads.length; i++)
    {
        threads[i].join();
    }
  }
  private class Worker extends Thread
  {
    private int index = -1;
    public Worker(int index)
    {
      super("worker "+index);
      this.index = index;
    }
    public void run()
    {
      MultiClient.this.sendRequestWrapper(index);
    }
  }
  public long[] getResponseTimes(){ return responeTimes; }
  public Throwable[] getErrors(){ return errors; }
  public boolean sendFailed(int indeX){ return errors[index] != null; }

  public static void main(String[] args)
  {
    int count = ...; // get count from somewhere (cmd line, hard coded etc.)
    MultiClient multiClient = new MultiClient(count); //create an array filler
    multiClient.doStuff(); //this will block till all the threads are done
    long[] responseTimes = multiClient.getResponseTimes();

    //do whatever you want with the response times
  }

}

You are probably experiencing a race condition. The for loop is completed before any of the threads have actually started and/or finished.

If you are using JDK 1.5 or higher, look into using the java.lang.concurrent classes. They make these types of thread joining operations quite easy.