How to log different modules in different log files?

Logging messages from different modules in different log files are commonly expected in a project.One can create as many loggers specific to different modules/packages inside the log4j properties file.

As for instance, we want to log workflow related log messages into a file named workflow.log while services related logs into a file named services.log.

The log4j.properties file will describe two seperate FileAppenders and each of them is associated with individual package.The log4j.properties file as shown below depicts two FileAppenders:

log4j.rootLogger=DEBUG

# WorkflowFileAppender - logs messages in the workflow.log file.
log4j.appender.WorkflowFileAppender=org.apache.log4j.FileAppender
log4j.appender.WorkflowFileAppender.File=workflow.log
log4j.appender.WorkflowFileAppender.layout=org.apache.log4j.PatternLayout
log4j.appender.WorkflowFileAppender.layout.ConversionPattern=%-4r [%t] %-5p %c %x - %m%n

# ServicesFileAppender - logs messages in the services.log file.
log4j.appender.ServicesFileAppender=org.apache.log4j.FileAppender
log4j.appender.ServicesFileAppender.File=services.log
log4j.appender.ServicesFileAppender.layout=org.apache.log4j.PatternLayout
log4j.appender.ServicesFileAppender.layout.ConversionPattern=%-4r [%t] %-5p %c %x - %m%n

log4j.logger.com.iqjava.workflow=,WorkflowFileAppender 
log4j.logger.com.iqjava.services=,ServicesFileAppender 

Here the root logger is set as 'DEBUG' which is just for information purpose, and not necessary.We have not set any logger level with individual file appenders. They both have been left blank so that rootLogger level would be inherited.We can surely change that if we want.

The code snippet contains two java files:

1. WorkflowExample.java

package com.iqjava.workflow;

import org.apache.log4j.Logger;
import org.apache.log4j.PropertyConfigurator;

public class WorkflowExample {

    static Logger logger = Logger.getLogger(WorkflowExample.class);

    public static void main(String[] args) {

        PropertyConfigurator.configure("log4j.properties");
        logger.debug("Here goes the debug message");
        logger.error("Here goes the error message");
        logger.fatal("Here goes the fatal message");
        logger.info("Here goes the info message");
        logger.warn("Here goes the warn message");

    }

}

2.ServicesExample.java

package com.iqjava.services;

import org.apache.log4j.Logger;
import org.apache.log4j.PropertyConfigurator;

public class ServicesExample {
    
    static Logger logger = Logger.getLogger(ServicesExample.class);

    public static void main(String[] args) {

        PropertyConfigurator.configure("log4j.properties");
        logger.debug("Here goes the debug message");
        logger.error("Here goes the error message");
        logger.fatal("Here goes the fatal message");
        logger.info("Here goes the info message");
        logger.warn("Here goes the warn message");

    }

}

If the individual java appenders are executed then output of workflow.log and services.log will look like as given below:

3.workflow.log

0    [main] DEBUG com.iqjava.workflow.WorkflowExample  - Here goes the debug message
33   [main] ERROR com.iqjava.workflow.WorkflowExample  - Here goes the error message
33   [main] FATAL com.iqjava.workflow.WorkflowExample  - Here goes the fatal message
33   [main] INFO  com.iqjava.workflow.WorkflowExample  - Here goes the info message
34   [main] WARN  com.iqjava.workflow.WorkflowExample  - Here goes the warn message

4.services.log

0    [main] DEBUG com.iqjava.services.ServicesExample  - Here goes the debug message
1    [main] ERROR com.iqjava.services.ServicesExample  - Here goes the error message
1    [main] FATAL com.iqjava.services.ServicesExample  - Here goes the fatal message
2    [main] INFO  com.iqjava.services.ServicesExample  - Here goes the info message
2    [main] WARN  com.iqjava.services.ServicesExample  - Here goes the warn message

If you want to change the logger level for these file appenders, you can always do that in log4j.properties file.