So I should use the following:
public void sessionCreated(HttpSessionEvent event) {
synchronized (this) {
activeSessions++;
}
} public void sessionDestroyed(HttpSessionEvent event) {
synchronized (this) {
if (activeSessions > 0) {
activeSessions--;
}
}
}Ralph Einfeldt wrote:
You have to synchronize the -- and ++ operations. Otherwise you will have unpredictable results.
You have to keep in mind that activeSessions++ is not atomic, so another thread can get between the computation of the value and the assignment.
One scenario: Thread A: read activeSessions = 0 Thread B: read activeSessions = 0 Thread A: compute activeSessions + 1= 1 Thread B: compute activeSessions + 1= 1 Thread A: store the result in activeSessions = 1 Thread B: store the result in activeSessions = 1
Now activeSessions is 1 although 2 Sessions are active. If the sessions are not destroyed in a very close gap the result will get negative.
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