How to find that a thread is alive or not

Question

Hi all,
I've create a thread to make a http request in every 10 second and get a string as the response from the server. Requesting process start on the window open. And also I can dispose the window, so my processing is not stop. That is exactly what I want to do.
Say after disposing the window I open it again. So the same thread start again while the first one is running. It messup my work. So I think of it. Every window open try to check that thread is alive or not. If alive stop the thread and start is again.
So I try this simply.
    public boolean ThreadIsAlive(){
        if(processThread.isAlive())
            return true;
        else
            return false;
    }If I found the thread true on the above code, I want to stop it. So I try this.
    public void stop(){
        processThread = null;
    }I think up to now what I have done is ok. Then on the ActionListner I do this.
                ActionListener actionListener = new ActionListener(){
                    public void actionPerformed(ActionEvent e){
                        if(appProcess.IsThreadAlive()){
                        appProcess.stop();
                        System.out.println("Main Dialog!");
                        java.awt.EventQueue.invokeLater(new Runnable() {
                            public void run() {
                                MainDialog dialog = new MainDialog(new javax.swing.JFrame(), true);
                                dialog.setVisible(true);
                };No compile error. But I got a runtime exception.
Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
        at mytrayicon.Processing.IsThreadAlive(Processing.java:82)
        at mytrayicon.PanelRadarMain$3.actionPerformed(PanelRadarMain.java:73)
        at java.awt.TrayIcon.processActionEvent(TrayIcon.java:750)
        at java.awt.TrayIcon.processEvent(TrayIcon.java:708)
        at java.awt.TrayIcon.dispatchEvent(TrayIcon.java:690)
        at java.awt.EventQueue.dispatchEvent(EventQueue.java:604)
        at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:273)
        at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:183)
        at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:173)
        at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:168)
        at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:160)
        at java.awt.EventDispatchThread.run(EventDispatchThread.java:121)Can anybody help me to solve this.
Thanks

Answer 1

ItsJava wrote:
Hi all,
I've create a thread to make a http request in every 10 second and get a string as the response from the server. Requesting process start on the window open. And also I can dispose the window, so my processing is not stop. That is exactly what I want to do.
Say after disposing the window I open it again. So the same thread start again while the first one is running. It messup my work. So I think of it. Every window open try to check that thread is alive or not. If alive stop the thread and start is again.
So I try this simply.
public boolean ThreadIsAlive(){
if(processThread.isAlive())
return true;
else
return false;
}If I found the thread true on the above code, I want to stop it. So I try this.
public void stop(){
processThread = null;
That does not "stop" the thread, it simply removes that reference to it.
>
I think up to now what I have done is ok. Then on the ActionListner I do this.
ActionListener actionListener = new ActionListener(){
public void actionPerformed(ActionEvent e){
if(appProcess.IsThreadAlive()){
appProcess.stop();
System.out.println("Main Dialog!");
java.awt.EventQueue.invokeLater(new Runnable() {
public void run() {
MainDialog dialog = new MainDialog(new javax.swing.JFrame(), true);
dialog.setVisible(true);
};No compile error. But I got a runtime exception.
Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at mytrayicon.Processing.IsThreadAlive(Processing.java:82)
at mytrayicon.PanelRadarMain$3.actionPerformed(PanelRadarMain.java:73)
at java.awt.TrayIcon.processActionEvent(TrayIcon.java:750)
at java.awt.TrayIcon.processEvent(TrayIcon.java:708)
at java.awt.TrayIcon.dispatchEvent(TrayIcon.java:690)
at java.awt.EventQueue.dispatchEvent(EventQueue.java:604)
at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:273)
at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:183)
at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:173)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:168)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:160)
at java.awt.EventDispatchThread.run(EventDispatchThread.java:121)
I'm sorry, but you set a reference to null, then attempt to derference it, and you wonder why you get an NPE?
>
Can anybody help me to solve this.
ThanksTo stop the thread, have a volatile variable that you can change the value of using a synchronized method, and have the thread
periodically check the value of this variable and stop itself when the value equals a specific value. That is one way to properly
stop the thread.