Alka JohnApache Log4j was the most popular logging framework for several years. It introduced the basic concept of hierarchical log levels and loggers, which are still used in modern logging frameworks. It is a trustworthy, quick, and adaptable logging framework written in Java and disseminated under the Apache Software Licence. It is an open-source logging framework.
Log4j is thread-safe and optimized for speed. It is easy to alter the format of the logger messages by extending the Layout class. Log4j is designed to manage Java exceptions from the start. The behavior of logging can be set at runtime using a configuration file.
Getting started with Log4j
Add the maven dependency for log4j in the pom.xml file as given below.
https://github.com/vmstate/article-100/blob/main/java/log4j/pom.xml
Logger
The logger object is declared in the following format.
private static final Logger logger = Logger.getLogger(Log4jExample.class); All levels of logger messages are added in the above example. We can decide on which logger messages to be passed to the appender. It is configured on the log4j.properties file. We can specify the log level in the field log4j.rootLogger.
log4j.rootLogger=INFO, console Here we have set the root logger level as INFO and the appender as ConsoleAppender. The logger messages of different levels will appear on the console based on the value of the root level.
| Root Level | Logger Levels |
|---|---|
| TRACE | TRACE, INFO, DEBUG, WARN, ERROR, FATAL |
| INFO | INFO, WARN, ERROR, FATAL |
| DEBUG | INFO, DEBUG, WARN, ERROR, FATAL |
| WARN | WARN, ERROR, FATAL |
| ERROR | ERROR, FATAL |
| FATAL | FATAL |
Filters
Filters allow processing the log messages based on selected criteria. Log4j supports filters at the logger level and the appender level. In the below-given example, we have added a filter in the appender. Include the following code in the log4j.properties to achieve the filtering.
log4j.appender.consoleAppender.filter.1=org.apache.log4j.varia.LevelMatchFilter log4j.appender.consoleAppender.filter.1.LevelToMatch=WARN log4j.appender.consoleAppender.filter.2=org.apache.log4j.varia.DenyAllFilter Here only the WARN messages will be printed in the console.
Appender
Appenders are used to define the destination of the logger messages. Log4j has different appenders, such as ConsoleAppender, FileAppender, and RollingFileAppender.
In this example, we have used ConsoleAppender to print the logger messages to the console.
log4j.appender.consoleAppender=org.apache.log4j.ConsoleAppender Layout
Layout is used to format the log events before they are given to the appenders. Log4j provides various appenders such as PatternLayout, HTMLLayout, and XMLLayout.
log4j.appender.consoleAppender.layout=org.apache.log4j.PatternLayout log4j.appender.consoleAppender.layout.ConversionPattern=%d %p %c - %m%n In the above-given example, PatternLayout is used to convert the logger messages in a pattern with date, package information, and message.
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