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A Servlet Example
When is it appropriate to use ThreadLocal? Whenever several components share the same thread—but not in a common data area. To illustrate this, we'll expand the above servlet case. When a request hits a Web server, it gets mapped to a servlet. Struts applications, for example, use the ActionServlet as the common entry point. And before the servlet is executed, one or more "filters" may be executed. A filter is a Java program implementing the servlet Filter interface. The filters are defined in web.xml like this:
. . .
<filter>
<filter-name>TestFilter</filter-name>
<filter-class>hansen.playground.TestFilter</filter-class>
</filter>
<filter-mapping>
<filter-name>TestFilter</filter-name>
<url-pattern>/*</url-pattern>
</filter-mapping>
. . .
The filter shown above will be run before every servlet. And because there is no direct relation between the filter and a servlet, a ThreadLocal object can be used to share data between the two. In this case, however, there is a more obvious solution: both the filter and the servlet share a request and session object. But what if the servlet calls a component—in the same thread—that does not have access to the request object or other servlet objects? In such a situation, it makes sense to pass the data through a ThreadLocal object.
A case that immediately springs to mind is when you're using a logging system. When something is logged, it's done through the parameters to the log methods. But when you're using a ThreadLocal object, the logging system may fetch its local thread data with a simple call like the one below:
list = MyThreadLocal.get();
The data may then be added to the log parameters. It's obvious that data such as user IDs, browser types, etc. might be useful to include in log messages—especially in system error messages.
Coding with ThreadLocals
Now it's time to see what code you'll need to implement a scenario with a filter, a servlet, a .jsp error-page, and a logging system.
First of all, in this example, the object stored in ThreadLocal is a list containing pairs of strings: a name part and value part. The data in the list can be written out in a format like this:
name1=value1
name2=value2
. . .
The string pairs are held in the Observed class, which is a simple bean (Listing 1).
To make using ThreadLocal object easier, wrap it in your own object (Listing 2).
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