Chapter 14: JSP Debugging

1
Chapter 14 JSP Debugging
Reference Beginning JSP
2
Objectives

• Debugging Art or Science?
• Debugging Techniques
• Compile Time Bugs
• Console Output
• Log4J
• Debugging with Tools
• NetBeans 4.1 JSP Debugger

3
Debugging Art or Science?

• Pages 436-437
• Science of Debugging
• Observation
• Program does not work as expected
• Hypothesis
• Guess what causes the unexpected behavior
• Prediction
• How a change in code or data might affect the
  observation
• Test
• Test the prediction by making the change and
  observing new results

4
Compile Time Bugs

• If JSP fails to compile
• Servlet container often returns a message to the
  browser explaining the problem
• In NetBeans 4.1, compile time bugs are caught
  when JSP page is compiled
• Example
• lt_at_taglib prefix"c" uri"http//java.sun.com/jsp/
  jstl/bore" gt
• In Netbeans 4.1
• Build Compile Example1.jsp shows
• org.apache.jasper.JasperException The absolute
  uri http//java.sun.com/jsp/jstl/bore cannot be
  resolved in either web.xml or the jar files
  deployed with this application

5
Debugging Scriptlets in JSP

• Pages 439-441
• In NetBeans 4.1
• Build Compile Example2.jsp
• Result Cannot find symbol
• symbol class Random
• location class org.apache.jsp.Example2_jsp
• Random r new Random()
• Solution Add import statement
• lt_at_page import"java.util.Random" gt

6
Instrument JSP Code

• Pages 442-444
• Using System.out.println
• Very basic, difficult to disable, etc.
• Run Example3.jsp and Example4.jsp
• Results logged under Output tab
• Bundled Tomcat 5.5.7

7
Servlet Container Logging System

• Pages 445-446
• Using the ServletContext.log()
• No control which messages to log
• Run Example5.jsp
• lt String message
• this.log( message )
• gt

8
Logging with JDK Logger

• Pages 447-454
• Using the Logger.info() depending on the logging
  level
• Set the level in ltJAVA_HOMEgt/jre/lib/logging.prope
  rties
• globally configured
• E.g., org.apache.jsp.level INFO

9
Logging with Log4j

• Pages 454-460
• Advantage
• Log4j JAR can be deployed as part of an
  application
• not globally configured
• Set the level in WEB-INF /classes/log4j.properties
  

10
Logging with Log4j and Tag Libraries (Scriptless
JSP)

• Pages 460-464
• Unlike previous approaches, does not require
  scriptlets in JSP
• E.g., http//localhost8084/ch14/LogTags.jsp

11
Debugging with Tools

• Using NetBeans 4.1
• Set breakpoints
• Examine variables
• Step through code
• Step Over, Step Into, Step Out, Continue,
  Terminate
• Remote debugging
• Attach to port 8084 (default)

12
Next Steps

• Try It Out sections of Textbook (Chapter 14)
• Examples 1 to 7
• LogTags.jsp
• You can skip the rest of the examples and
  exercises