Servlet Context, Collections and Serialization - servlets

Question

Often, I work on Java EE application. Today I'm facing an issue: serialize Collections in servlet context. In my case, my app contains a Servlet Context Listener and many servlets.
The context listener load a ConcurrentHashMap containing several lists of products at initialisation and a task scheduler to refresh this list.
The servlets are supposed to access the right list, based on user provided parameters.
Here the code of my contextInitialized Listener:
public void contextInitialized(ServletContextEvent event) {
app = event.getServletContext();
myMap = new ConcurrentHashMap<String, Catalog>();
myMap.put("FR", new Catalog());
myMap.put("UK", new Catalog());
app.setAttribute("catalogue", myMap);
scheduler = Executors.newSingleThreadScheduledExecutor();
scheduler.scheduleAtFixedRate(new AutomateRefresh(), 0, 60, TimeUnit.MINUTES);
}
In order to show my problem, i created a servlet that display everything which is a boolean or a ConcurrentHashMap in context
I'm not surprised to find this kind of results:
javax.servlet.context.tempdir is equal to...
Working is equal to... true
org.apache.catalina.resources is equal to...
org.apache.tomcat.InstanceManager is equal to...
org.apache.catalina.jsp_classpath is equal to...
javax.websocket.server.ServerContainer is equal to...
org.apache.jasper.compiler.TldCache is equal to...
catalogue is equal to...
org.apache.tomcat.JarScanner is equal to...
As you can see, my two custom keys (the boolean Working and the ConcurrentHashMap catalogue) exists. But catalogue is empty when not accessed inside the Listener.
I found that:
The serialization form of java.util.HashMap doesn't serialize the buckets themselves, and the hash code is not part of the persisted state.
Source: Serializing and deserializing a map with key as string
For many projects a serializable and thread-safe collection is useful. I am probably not the only one who is looking for that (see the amount of topic about servlet context).
ConcurrentHashMap is thread-safe but I am unable to retrieve my data in other servlet (in the same app). Is there an implementation of Collection which is thread-safe and serializable (due to WebLogic server policy) ? Or am I using it in a wrong way ?
EDIT: Code of "Display context servlet"
public void doGet( HttpServletRequest request, HttpServletResponse response ) throws ServletException, IOException{
System.out.println("List of all values in the context:");
Enumeration<?> e = getServletContext().getAttributeNames();
while (e.hasMoreElements())
{
String name = (String) e.nextElement();
System.out.print("\n" + name + " is equal to... ");
// Get the value of the attribute
Object value = this.getServletContext().getAttribute(name);
if (value instanceof ConcurrentHashMap) {
ConcurrentHashMap<String, Catalog> map = (ConcurrentHashMap<String, Catalog>) value;
Iterator<Entry<String, Catalog>> it = map.entrySet().iterator();
while (it.hasNext()) {
ConcurrentHashMap.Entry<String, Catalog> entry = (ConcurrentHashMap.Entry<String, Catalog>)it.next();
System.out.print("\t" + entry.getKey() + "=" + entry.getValue());
}
} else if (value instanceof Boolean) {
System.out.print((Boolean)value);
}
}
}
EDIT2: Like BalusC suggested the HashMap maybe null (a rookie mistake ?).
Here the task code. The task is in the Listener. The Listener initialize the HashMap with new empty object. The task refresh the objects when webapp starts and then every hour.
public class AutomateRefresh implements Runnable {
public void run() {
System.out.println("Scheduler trigger");
if(app.getAttribute("catalogue") instanceof ConcurrentHashMap){
myMap = (ConcurrentHashMap<String, Catalog>) app.getAttribute("catalogue");
//Autorefresh
Iterator<Entry<String, Catalog>> it = myMap.entrySet().iterator();
while (it.hasNext()) {
ConcurrentHashMap.Entry<String, Catalog> entry = (ConcurrentHashMap.Entry<String, Catalog>)it.next();
((Catalog)entry.getValue()).setValid(false);//Set as not valid anymore for further request
try {
((Catalog)entry.getValue()).refreshdb((String) entry.getKey());//TODO rework to use REST API
} catch (SQLException e) {
e.printStackTrace();
}
it.remove(); // avoids a ConcurrentModificationException
app.setAttribute("catalogue", myMap);
app.setAttribute("Working", true);
System.out.println((String)entry.getKey() + " = " + (Catalog)entry.getValue());
}
}
else{
System.out.println("Catalogue is not an instance of ConcurrentHashMap as expected.");
app.setAttribute("Working", false);
}
}
}
When the task triggered, for each Catalog stored in the Context, the task update the data stored by them. It also display data in console.
Results:
Refresh Catalog for UK with DB
UK = Catalog [list size is : 0 valid=true, lastToken=notoken]
Refresh Catalog for FR with DB
FR = Catalog [list size is : 30 valid=true, lastToken=notoken]
Catalog is a class with an ArrayList, a boolean and a String. Everything seems correct: UK is supposed to be empty but not null and FR is supposed to contains 30 products.
I still can not access this data in other servlets.

Answer 1

I found the origin of the problem, a rookie mistake as expected:
I tried to update this way, assuming it would have updated the object directly in the ConcurrentHashMap
((Catalog)entry.getValue()).refreshdb((String) entry.getKey());
I replace it by:
Catalog myCatalog = (Catalog)entry.getValue();
myCatalog.refreshdb((String) entry.getKey());
myMap.put((String)entry.getKey(), myCatalog);
And it works now.
I still don't know why my objects were accessible from the listener, they are not supposed to work that way. Maybe a strange behavior from my server ? Anyway, this issue is fixed.
Thanks to BalusC for his help.