Wicket 101: Making Use of the MDC Logging Context

Logging frameworks like log4j and logback usually offer a class called MDC (Mapped Diagnostic Context) to store contextual information of a user’s session to be used in log statements. As an example, here’s how to attach a session id to log statements using Wicket and log4j. Impatient? Skip right to the github example project

The Logging Configuration

Log4j is usually configured with a properties or xml file. If you want to output the session id with each log statement, you have to define the session with a variable like %X{sessionId} (see table “Conversion characters” in the javadoc of the PatternLayout class):

log4j.appender.Stdout.layout.conversionPattern=%-5p - %-26.26c{1} - %X{sessionId} - %m\n

This configuration reads the parameter “sessionId” out of the MDC context (if available) and prints it out with each log statement.

Filling the MDC context with Wicket

The session ID has to be stored in the MDC context, of course, before it can be printed out by the above logging configuration. Wicket offers the IRequestCycleListener interface for this purpose. You can extend AbstractRequestCycleListener if you don’t want to implement all methods of the interface:

public class AddSessionIdToMdcListener extends AbstractRequestCycleListener {
  public void onBeginRequest(RequestCycle cycle) {
    WebSession session = WebSession.get();
    // the check on isTemporary() is necessary, since temporary sessions
    // have a NULL sessionId
    if (!session.isTemporary()) {
      MDC.put("sessionId", session.getId());

All we do is simply calling MDC.put() at the beginning of each Wicket request. By the way: you can put anything in the context, it doesn’t have to be a session id. The listener can be added to your Wicket application with the following line within the init() method of the Application class:

public void init() {
  getRequestCycleListeners().add(new AddSessionIdToMdcListener());

On Wicket’s Session ID

Note that Wicket only creates a session id for stateful pages, i.e. if you have at least one stateful component on a page or if you explicitly call setStatelessHint(false). This should be the case for most Wicket pages, though.



One comment

  1. Ben Heiskell

    First of all, thanks! This saved my time hunting down the solution myself.

    However, the example is slightly off. If the session is temporary, the MDC context from the previous request wont be cleared. That will result in requests with temporary sessions logging the prior requests sessionId.

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