log4j - power logging at your fingertips

log4j has been a popular logging framework in Java applications for several years. It is simple to implement, thread safe and light weight. While there are no current major releases planned, it is still a popular download among the Apache Logging Services toolset.

To add log4j to your project using Maven, add the following dependency snippet to your code

  log4j
  log4j
  1.2.16
 

Maven will download and add the log4j jar to your respository. Next, you need to specify a configuration file, which could be either a .properties file or a .xml file.The configuration file is usually named as log4j.properties or log4j.xml and need to be placed within the classpath for the application to find it.

If you are building an application that ships as an executable JAR, place this file within your src/main/resources or your src/test/resources directory depending upon whether you need your logging framework in the production code or not

The log4j manual describes a sample properties file.If you don't have access to a .properties file, you can use the entries shown in the example to create a .properties file for yourself.Now that you have a log4j properties file, you need to make sure that you have a Appender defined within it.

For logging messages to the console, use a ConsoleAppender. For logging to a file, use a FileAppender.

To control what should be logged with the log statement, set up a Layout and initialise it to a particular Pattern. A popular Layout is the PatternLayout that formats the output line with meta data using a pre-defined pattern.

For example, here is a snippet from the manual.

# Set root logger level to DEBUG and its only appender to A1.
log4j.rootLogger=DEBUG, A1

# A1 is set to be a ConsoleAppender.
log4j.appender.A1=org.apache.log4j.ConsoleAppender

# A1 uses PatternLayout.
log4j.appender.A1.layout=org.apache.log4j.PatternLayout
log4j.appender.A1.layout.ConversionPattern=%-4r [%t] %-5p %c %x - %m%n

The ConversionPattern can be used to specify meta-data about the logged statement, such as the Class from were the log originated, the date and time in different formats, the severity level etc.An exhaustive list is available in the API docs. It is worth noting the several warnings posted in the API docs regarding speed and efficiency before choosing a ConversionPattern character.

Another aspect of log4j logging performance that one needs to be aware of is the cost of creating a log statement that may not be used.To get around the parameter construction code, wrap the logging statements within a check:

if(logger.isDebugEnabled() {
    logger.debug("Log : " +  " String 1 " + "String 2"));
}

If the check is not place, the log statement will create 4 Strings (yes , we could have used a StringBuffer), but a simple check prevents this overhead. Ofcourse, there is now the cost of checking the LEVEL at which the logger is set, but this is a miniscule overhead.

Finally, now that the log4j is setup and has been configured, instantiate it 

static final Logger logger = Logger.getLogger(MyClass.class);

and start logging:

logger.info("Starting the APP");

Exceptions can be sent to the log file as an argument to the logger.error method.

catch (Exception exception) {
logger.error("Error in loading application! ", exception);

To conclude, if you need an easy and fast logging framework, log4j will fit your bill.However, if you are in the market for the next generation of logging, then have a look at Logback