Question

If I have a class like this:

public class Example
{
    ArrayList<String> vector;

    public Example singleton = new Example();

    private Example()
    {
        //Read data from BD and fill the vector. Example vector:  ["foo","voo","faa","vuu","vee"]
    }   



    public synchronized removeElement()
    {
        vector.remove(0);
    }

    public synchronized changeElement()
    {
        vector.set(0,"fii");
    }
}

If there are multiple instances running and one of them executes the method removeElement, what happens with the values ​​of the other instance? And if one of them executes the method changeElement?

Answer 1

GAE can have multiple JVM instances running in parallel, so updating value in memory will only be visible on that instance.

You should use shared data storage: either memcache (free, fast, but volatile) or datastore (costly, slower, but consistent).