{"id":13576,"date":"2014-08-26T11:00:13","date_gmt":"2014-08-26T08:00:13","guid":{"rendered":"http:\/\/examples.javacodegeeks.com\/?p=13576"},"modified":"2014-09-03T06:19:33","modified_gmt":"2014-09-03T03:19:33","slug":"log4j-appender-example","status":"publish","type":"post","link":"https:\/\/examples.javacodegeeks.com\/java-development\/enterprise-java\/log4j\/log4j-appender-example\/","title":{"rendered":"log4j – Appender Example"},"content":{"rendered":"

In this post, we are going to discuss about log4j<\/a><\/code> appenders<\/em> and how you configure it using an log4j.xml file.<\/p>\n

1. What is log4j?<\/h2>\n

log4j <\/strong>is a tool to help the programmer output log statements to a variety of output targets.<\/p>\n

In case of problems with an application, it is helpful to enable logging so that the problem can be located. With log4j<\/em> it is possible to enable logging at runtime without modifying the application binary. The log4j<\/em> package is designed so that log statements can remain in shipped code without incurring a high performance cost. It follows that the speed of logging (or rather not logging) is capital.<\/p>\n

At the same time, log output can be so voluminous that it quickly becomes overwhelming. One of the distinctive features of log4j<\/em> is the notion of hierarchical loggers. Using loggers it is possible to selectively control which log statements are output at arbitrary granularity.<\/p>\n

log4j<\/em> is designed with three goals in mind: reliability, speed and flexibility. There is a tight balance between these requirements. We believe that log4j strikes the right balance.<\/p>\n

log4j<\/em> has three main components: loggers<\/em>, appenders<\/strong> and layouts<\/em>. These three types of components work together to enable developers to log messages according to message type and level, and to control at runtime how these messages are formatted and where they are reported<\/strong>.<\/p>\n

1.1 What are log4j appenders?<\/h3>\n

log4j<\/em> allows logging requests to print to multiple destinations. In log4j<\/em> speak an output destination is called an appender<\/em>. Currently, appenders exist for the console, files, Swing components, remote socket servers, JMS, NT Event Loggers, and remote UNIX Syslog daemons. Log4j allows attaching multiple appenders to any logger. Appenders can be added to and removed from a logger at any time. A logger can make use of one and only one level.<\/p>\n

1.2 log4j layouts<\/h3>\n

More often than not, users wish to customize not only the output destination but also the output format. This is accomplished by associating a layout with an appender. The layout is responsible for formatting the logging request according to the user’s wishes, whereas an appender takes care of sending formatted output to its destination. Most layouts are not designed to be shared by multiple appenders. It follows that each appender must have its own \u201cprivate\u201d layout.<\/p>\n

A common layout called the PatternLayout<\/a><\/code>, part of the standard log4j<\/em> distribution, lets the user specify the output format according to conversion patterns similar to the C language\u2019s printf function<\/em>. For example, a PatternLayout<\/em> with the conversion pattern “%r [%t] %-5p %c – %m%n”<\/em> will output something akin to:

<\/div> <\/div><\/p>\n
\r\n199 [main] INFO com.foo.Bar - Hello there.\r\n<\/pre>\n
The first field is the number of milliseconds elapsed since the start of the program. The second field is the thread that executed the log request. The third field is the level of the log statement. The fourth field is the name of the logger associated with the log request. The text after the ‘-‘ is the message of the statement.<\/p>\n
1.3 log4j \u2013 Configuration scripts<\/h3>\n
The log4j<\/em> environment is fully configurable programmatically. However, it is far more flexible to configure log4j<\/em> using configuration files<\/strong>. Currently, configuration files can be written Java properties (key=value) format or in XML<\/em>.<\/p>\n
Let’s see an example of how easy is to configure appenders<\/em> using a configuration script.<\/p>\n
log4j.xml<\/span><\/em><\/p>\n
\r\n<?xml version=\"1.0\" encoding=\"UTF-8\" ?>\r\n<!DOCTYPE log4j:configuration SYSTEM \"log4j.dtd\">\r\n<log4j:configuration debug=\"false\" xmlns:log4j=\"http:\/\/jakarta.apache.org\/log4j\/\">\r\n\r\n\t<!-- Console Appender -->\r\n\t<appender name=\"console\" class=\"org.apache.log4j.ConsoleAppender\">\r\n\t\t<layout class=\"org.apache.log4j.PatternLayout\">\r\n\t\t\t<param name=\"ConversionPattern\" value=\"%d{yyyy-MM-dd HH:mm:ss} %-5p %c{1}:%L - %m%n\" \/>\r\n\t\t<\/layout>\r\n\t<\/appender>\r\n\r\n\t<!-- File Appender -->\r\n\t<appender name=\"file\" class=\"org.apache.log4j.RollingFileAppender\">\r\n\t\t<param name=\"append\" value=\"false\" \/>\r\n\t\t<param name=\"maxFileSize\" value=\"10KB\" \/>\r\n\t\t<param name=\"maxBackupIndex\" value=\"5\" \/>\r\n\t\t<param name=\"file\" value=\"\/Users\/afibarra\/Tmp\/FooBar.log\" \/>\r\n\t\t<layout class=\"org.apache.log4j.PatternLayout\">\r\n\t\t\t<param name=\"ConversionPattern\" value=\"%d{yyyy-MM-dd HH:mm:ss} %-5p %c{1}:%L - %m%n\" \/>\r\n\t\t<\/layout>\r\n\t<\/appender>\r\n\r\n\t<!-- Foo package -->\r\n\t<logger name=\"com.javacodegeeks.examples.log4jappenders.foo\">\r\n\t\t<level value=\"INFO\" \/>\r\n\t\t<appender-ref ref=\"console\" \/>\r\n\t\t<appender-ref ref=\"file\" \/>\r\n\t<\/logger>\r\n\r\n\t<!-- Bar package -->\r\n\t<logger name=\"com.javacodegeeks.examples.log4jappenders.bar\">\r\n\t\t<level value=\"WARN\" \/>\r\n\t\t<appender-ref ref=\"console\" \/>\r\n\t\t<appender-ref ref=\"file\" \/>\r\n\t<\/logger>\r\n\r\n\t<!-- Root logger option -->\r\n\t<root>\r\n\t\t<level value=\"DEBUG\" \/>\t\t\r\n\t\t<appender-ref ref=\"console\" \/>\r\n\t\t<appender-ref ref=\"file\" \/>\r\n\t<\/root>\r\n\r\n<\/log4j:configuration>\r\n<\/pre>\n
Here, he have defined two of the most used appenders:<\/p>\n