[Isomorphic]

Threading is a very large topic, trying some Google searches, or, if you want to just not have to worry about it in your servlet, implement the SingleThreadModel interface, which causes only one thread to go through each instance of your servlet and hence will work around most threading issues.

You absolutely *must* get your Developer Console working (it's basically bad to post at all without checking the Developer Console first) so make sure you have followed the complete installation instructions and get it working before doing anything else.

[jwpartners]

So,

I implemented the SingleThreadModel to quickly see if that solves the problem:

YES.

So I'll try your solution in a 2nd time.

Thanks you very much

[jwpartners]

Hello,

I revised the rule of servlet programming and I found my mistake!
I set the property "out" (PrintWriter out = response.getWriter ();) as an instance variable of the servlet.

I'm sorry and thank you again.