为什么在TimerTask中出现此错误？

Question

08-17 18:26:44.540: E/AndroidRuntime(2405): FATAL EXCEPTION: Scheduler
08-17 18:26:44.540: E/AndroidRuntime(2405): java.lang.IllegalThreadStateException: Thread already started.
08-17 18:26:44.540: E/AndroidRuntime(2405):     at java.lang.Thread.start(Thread.java:1045)
08-17 18:26:44.540: E/AndroidRuntime(2405):     at com.example.simpledownloader.scheduler.Scheduler.run(Scheduler.java:21)
08-17 18:26:44.540: E/AndroidRuntime(2405):     at java.util.Timer$TimerImpl.run(Timer.java:284)  

So, I have a TimerTask which checks all the pending downloads by brute forcing.
Downloads are all Thread stored in a Vector . As soon as the TimerTask sees the a thread is ready, it calls start() on it. here is the code:

Code:

package com.example.simpledownloader.scheduler;

import java.util.TimerTask;

import android.util.Log;

import com.example.simpledownloader.sharable.Sharable;
import com.example.simpledownloader.task.Task;

public class Scheduler extends TimerTask {
//------------------------------------------------------------------------------
    @Override
    public void run() {
        if(Sharable.getShouldLook()){ // Should we look for pending tasks ?
            for(Task each: Sharable.downloads){ // Yes, then loop
                if(each.isAlive()==false){ // Thread is not running ?
                    Log.v("SCHEDULER", "NOT RUNNING");
                    if(each.getReadiness() == true){ // Is it ready ?
                        each.start(); // Start !
                        break; // stop looping
                    }
                }
            }
        }
    }
//------------------------------------------------------------------------------
}  

Why do I keep getting that error ?
The Timer is set as such:

Sharable.schedulerTimer.scheduleAtFixedRate(Sharable.schedulerTask, 1000, 1000); // start scheduler

Answer 1

You're checking before you start the thread if isAlive() returns false. There are two ways that this could not be adequate to prevent you from getting the IllegalThreadStateException :

1. Some other thread is starting the download thread between the call to isAlive() and the call to start() , or

2. The download thread is terminating without being removed from your Sharable.downloads collection.

My suspicion is that the latter is more likely, so that's where I'd start looking. Alternatively, if you want to keep your threads in the downloads collection after they've finished, you may want to try each.getState() == Thread.State.NEW instead of each.isAlive() == false .