profiler for performance measurement- java multithreading

hi guys

I am using executor service in my code. Is there a profiler i can use to check what the threads are doing and how much time it takes to finish the task.

Right now i am using System.currentTimeMillis() to calculate the time taken but its inaccurate

long exportStartTime= System.currentTimeMillis();                  
 ExecutorService executor = Executors.newFixedThreadPool(3);                        
 final HSSFWorkbook runnableWb = wb;
 Runnable expTask = new Runnable(){        
 public void run(){        
 getExportDocumentObject(true, "Excel",buffer, mReq,runnableWb);
 }
long exportEndTime= System.currentTimeMillis();                  
System.out.println("Time taken for task (VERY INACCURATE):" + exportEndTime-exportStartTime);


Are there any open source profilers which can tell me which thread is doing what and how much time it takes complete the task?

thanks