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To the lotus feet of Bhagavan Sri Ramakrishna Contents at a Glance About the Author . xiii About the Technical Reviewer. xv Acknowledgments . xvii Introduction. xix CHAPTER 1 Introduction to Application Logging. 1 CHAPTER 2 Understanding Apache log4j . 9 CHAPTER 3 Destination of Logging—The Appender Objects. 35 CHAPTER 4 Formatting Logging Information in log4j . 77 CHAPTER 5 Filtering, Error Handling, and Special Rendering of Log Messages . 95 CHAPTER 6 Extending log4j to Create Custom Logging Components. 115 CHAPTER 7 A Complete log4j Example . 149 CHAPTER 8 Log4j and J2EE . 157 CHAPTER 9 Using the Apache Log Tag Library . 163 CHAPTER 10 Best Practices and Looking Forward to 1.3 . 175 APPENDIX A The log4j Configuration Parameters . 187 APPENDIX B The log4j DTD . 191 INDEX. 195 v Contents About the Author . xiii About the Technical Reviewer. xv Acknowledgments . xvii Introduction. xix ■CHAPTER 1 Introduction to Application Logging . 1 What Is Logging?. 1 Logging Is Systematic. 2 Logging Is Controlled . 2 Logging Information Represents the Application State . 2 Advantages of Logging. 3 Disadvantages of Logging . 3 How Logging Works . 4 Evaluating a Logging Package . 5 Popular Java-Based Logging APIs . 6 JDK Logging API. 6 Apache log4j. 6 Commons Logging API . 6 The Road Ahead. 7 ■CHAPTER 2 Understanding Apache log4j . 9 Installing log4j . 9 Overview of the log4j Architecture . 10 Configuring log4j . 12 Naming and Placing the Configuration File . 12 What We Can Configure . 13 XML-Style Configuration. 15 Notable Points About Configuration . 16 Log4j Initialization . 16 Initialization Through VM Parameters. 16 Creating a Log4j Configuration File . 17 Running the Program . 18 Why the Message Printed Twice . 18 vii viii ■CONTENTS Configuring log4j Programmatically . 19 Dynamic Loading of Configuration . 19 Configuring log4j in Web Applications . 20 Setting Up Tomcat . 20 An Example Servlet . 22 Level Object . 23 Logger Object. 24 How to Obtain a Logger . 25 Logging Information . 26 Configuration Methods . 28 Conditions of Successful Logging. 28 A Logger Example . 29 LogManager Object. 31 Nested Diagnostic Context (NDC) . 32 Message Diagnostic Context (MDC). 32 Conclusion . 34 ■CHAPTER 3 Destination of Logging—The Appender Objects . 35 Properties of Appender. 36 Adding Appenders to Loggers . 37 Logger-to-Appender Collaboration . 38 Thread-Safety in log4j . 38 WriterAppender . 38 Properties of WriterAppender. 39 Variations of WriterAppender . 40 ConsoleAppender . ..