IllegalMonitorStateException

[Ana Martin]

Hi,

I have two classes running on different threads and both use the same resource.

"isUpdateNeeded" is the var that I want to update (the lock). The method "setUpdateVar()" change the value of this variable to true and notify the thread "UpdateVar" to use this variable. The issue occurs when notifyAll() is called:

java.lang.IllegalMonitorStateException: object not locked by thread before notifyAll()


Could anyone help me to understand how to lock/unlock properly in this example?



Many thanks in advance




public static void setUpdateVar() {
synchronized (isUpdateNeeded){
while(isUpdateNeeded == true){
try {
isUpdateNeeded.wait();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
isUpdateNeeded = true;
isUpdateNeeded.notifyAll();
}
}


private class UpdateVar extends Thread {
boolean isRunning = true;

public void run() {
while(isRunning) {
synchronized (isUpdateNeeded){
if (isUpdateNeeded == true){

isUpdateNeeded = false;

mHandler.post(new Runnable() {
public void run() {
ShowList();
}
});
}
}
}
}
}

[Steve Luke]

I assume isUpdateNeeded is a Boolean (or a boolean that autoboxes to a Boolean). If this is the case, then changing the value from true to false (or vice-versa) actually changes the Object referred to by isUpdateNeeded. IE When true, is is Boolean.TRUE and when false it is Boolean.FALSE, two separate Objects, which means you can't synchronize/notify between them.

To simplify things you should have a separate lock that can't change:

    final Object synchLock = new Object();
    Boolean isUpdateNeeded = true;

...

    public static void setUpdateVar() { 
        synchronized (synchLock){ 
            while(isUpdateNeeded == true){ 
                try { 
                    synchLock.wait(); 
                } catch (InterruptedException e) { 
                    e.printStackTrace(); 
                } 
            } 
            isUpdateNeeded = true; 
            synchLock.notifyAll(); 
        } 
    } 

...

[Ana Martin]

Thanks a lot. It works great with your solution!
Thanks for the explanation