Logging is an essential part of any Java application as it helps in understanding the application’s behavior and troubleshooting issues. Log4j is a popular logging framework in Java that provides a flexible and configurable way to generate log statements.

By default, Log4j comes with a set of predefined log levels such as DEBUG, INFO, WARN, ERROR, and FATAL. However, there may be scenarios where you need to define custom log levels to handle application-specific requirements. In this blog post, we will discuss how to implement custom log levels in Log4j for specific Java application requirements.

Step 1: Configure Log4j

The first step is to configure Log4j in your Java application. You can achieve this by creating a log4j.properties file or a log4j2.xml file, depending on the version of Log4j you are using. This configuration file specifies the log levels, appenders, and other logging settings.

# Example log4j.properties

# Set root logger level
log4j.rootLogger=INFO, stdout

# Custom log levels
log4j.logger.com.example.custom=DEBUG

# Console appender
log4j.appender.stdout=org.apache.log4j.ConsoleAppender
log4j.appender.stdout.Target=System.out
log4j.appender.stdout.layout=org.apache.log4j.PatternLayout
log4j.appender.stdout.layout.ConversionPattern=%d{yyyy-MM-dd HH:mm:ss} %-5p %c{1}:%L - %m%n

In the above example, we have defined a custom log level DEBUG for the package com.example.custom. You can change the package name and log level according to your application’s requirements.

Step 2: Implement Custom Log Levels

To implement custom log levels in Log4j, you need to extend the org.apache.log4j.Level class and override its methods as per your requirements. Below is an example of implementing a custom log level called VERBOSE:

import org.apache.log4j.Level;

public class VerboseLevel extends Level {
  
    private static final long serialVersionUID = 1L;
  
    public static final int VERBOSE_INT = INFO_INT + 100;
    public static final Level VERBOSE = new VerboseLevel(VERBOSE_INT, "VERBOSE", 7);

    protected VerboseLevel(int level, String levelStr, int syslogEquivalent) {
        super(level, levelStr, syslogEquivalent);
    }
}

In the above code, we have extended the Level class and defined a new log level called VERBOSE with an integer value greater than INFO. You can add any additional logic or behavior to your custom log level as required by your application.

Step 3: Update Log4j Configuration

After implementing the custom log level, you need to update your Log4j configuration file to use the newly created log level.

# Example log4j.properties

# Set root logger level
log4j.rootLogger=INFO, stdout

# Custom log levels
log4j.logger.com.example.custom=VERBOSE

# Console appender
log4j.appender.stdout=org.apache.log4j.ConsoleAppender
log4j.appender.stdout.Target=System.out
log4j.appender.stdout.layout=org.apache.log4j.PatternLayout
log4j.appender.stdout.layout.ConversionPattern=%d{yyyy-MM-dd HH:mm:ss} %-5p %c{1}:%L - %m%n

In the above example, we have updated the log level for the com.example.custom package to use the custom log level VERBOSE instead of the default log levels.

Conclusion

Implementing custom log levels in Log4j allows you to tailor your logging exactly to your application’s requirements. By defining and using custom log levels, you can have more fine-grained control over the logging behavior, making troubleshooting and debugging easier.