Chapter 9: Using the Apache Log Tag Library - BeKnowledge

Chapter 9: Using the Apache Log Tag Library
Figure 9-2: The SimpleLogDump.jsp page
Creating Custom Tags with the Log Tag Library to Use a
Custom Level
In Chapter 8, we saw how to write a custom Level class, TRACE, and use it with log4j. If we want to
use the TRACE custom level along with the Log tag library, we have to extend the Log tag library
framework. Typically, we can do so by performing these tasks:
� Write a new Tag class to represent the custom level TRACE.
� Make the new Tag and Level classes available to the application.
� Modify the "taglib-descriptor" file to describe the new tag.
In the next sections, we will examine how we can achieve these tasks to use the TRACE level with the
Log tag library.
Creating a New Tag
In the Log tag library world, all tag classes subclass from the abstract class LoggerTag. The
LoggerTag class itself is a subclass of the BodyTagSupport class and does the entire job of handling
tag-related operations. It also defines an abstract method, getPriority(), that is implemented in all
other subclasses of the LoggerTag class. All other tag classes such as ErrorTag, InfoTag, etc., are
subclasses of LoggerTag and provide implementations for the getPriority() method. Thus, to define
a new tag, we simply need to extend the LoggerTag class and provide an implementation of the
getPriority() method that returns the custom level TRACE. Listing 9-5, TraceTag.java,
demonstrates how to write a custom tag using the custom level TRACE.
Listing 9-5: TraceTag.java
package com.apress.logging.log4j.customtag;
import org.apache.taglibs.log.LoggerTag;
import org.apache.log4j.Priority;
import com.apress.logging.log4j.CustomLevel;
public class TraceTag extends LoggerTag
{
protected Priority getPriority()
{
return CustomLevel.TRACE;
}
}
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