Java(TM) Boutique - Tutorial - Managing Data with the ThreadLocal Class

Tutorials : Managing Data with the ThreadLocal Class :

Coding the Servlet Filter

You may choose, for example, to store browser information and the CPU time used in your filter. If you have access to data from a logon process, it would be natural to store this in the filter as well (Listing 3).

Coding the Servlet

In the servlet class (a Struts Action class), you can also store the CPU time and call the logging system, shown in Listing 4 ( click here for the complete program).

Coding the Log Formatter

This example uses the JDK 1.4 logging system, and if you add your own "formatter" to it, it's utterly simple to fetch the data from the ThreadLocal object and write it out with the other logging data:

package hansen.playground;
. . .

public class MyFormatter extends Formatter {

  public String format(LogRecord record) {
    String myData = MyThreadLocal.getFormatted();
    return ... formatted data from "record" and "myData" ...  
  }
}

Coding the error.jsp Page

Finally, if you have an error page (error.jsp), you may print the contents of the ThreadLocal object. Like this:

<%@ page language="java" %>

<%@ page import="hansen.playground.*" %>

<h3>In error.jsp</h3>

<%
out.println("<pre>"+MyThreadLocal.getFormatted()+"</pre>");
%>

A Word of Warning

There are a couple of things you'll need to keep in mind when you're using ThreadLocals.

First, be careful to initialize the ThreadLocal object whenever the thread starts working, or you might end up connecting a logged-off user's data to another user's thread. When you're using ThreadLocals in a situation in which a filter initializes the object with user data, make sure there are no other entries to your servlet application—except through the filter.

Second, always remember that the ThreadLocal data is connected to the thread, not the user or anything else. A good idea is to delete the ThreadLocal data when it's no longer used. This addition to the filter class accomplishes that task:

. . .
// Go to the servlet
chain.doFilter(request, response);

MyThreadLocal.listData();

MyThreadLocal.set(null);
. . .

Whenever several non-related components running in the same thread need to share data, the ThreadLocal class is a viable solution. It's very simple to use, if you keep in mind that proper initialization and clean-up of local data is essential to avoid mixing data from different unit-of-works.

Other Resources

All the programs and files from the article are in this zip-file.
"Threading lightly, Part 3: Sometimes It's Best Not to Share," by Brian Goetz. This article contains more information on ThreadLocal.

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