How to insert current webapp's folder name in log4j filename

Question

I'm writing a Java web application to deploy in Tomcat and i'm using log4j for logging. I like to insert automatically the web application's folder name in the generated log file's name.

Currently the filename setting looks like this in log4j.properties:

log4j.appender.R.File=${catalina.home}/logs/mywebapp.log

and i need something like this:

log4j.appender.R.File=${catalina.home}/logs/${current.webapp.folder}.log

Is there some kind of environment variable for this to specify in the properties file, or i have to instantiate the logger from code?

Answer 1

You can achieve that using a context listener to set up a system property, and then use the property in your log4j configuration.

Step 1: set up a system property

You can first set a system property (e.g., contextPath) to the value of the application's Tomcat context path. You could do this in a context listener.

package my.package.listener;

import javax.servlet.ServletContext;
import javax.servlet.ServletContextEvent;
import javax.servlet.ServletContextListener;

public class ContextListener implements ServletContextListener {

    @Override
    public void contextDestroyed(ServletContextEvent arg0) {
    }

    @Override
    public void contextInitialized(ServletContextEvent event) {
        defineContextPath(event);
    }
    
    private void defineContextPath(ServletContextEvent event) {
        ServletContext context = event.getServletContext();
        String contextPath = context.getContextPath();
        
        if (contextPath != null) {
            String pattern = ".*/(.*)";
            String contex = contextPath.replaceAll(pattern, "$1");
            System.setProperty("contextPath", contex);
            System.out.println("contextPath: " + contex);
        } else {
            System.out.println("contextPath not found");
        }
    }
}

Declare the listener in your web.xml:

<!-- Be sure to keep my.package.ContextListener as the first listener 
     if you have more listeners as below --> 
<listener>
    <listener-class>my.package.ContextListener</listener-class>
</listener>
<listener>
    <listener-class>org.springframework.web.context.ContextLoaderListener</listener-class>
</listener>

Step 2: use the system property to set your log4j filename

Then in your log4j configuration file, you can use the system property contextPath to set your log4j filename.

<?xml version="1.0" encoding="UTF-8"?>
<Configuration>
    <Appenders>
        <Console name="STDOUT" target="SYSTEM_OUT">
            <PatternLayout pattern="%d %5p %c{1}:%L - %m%n" />
        </Console>
        <RollingFile name="RollingFile" fileName="${catalina.base}/logs/${tomcat.hostname}/${contextPath}.log" 
            filePattern="${catalina.base}/logs/${tomcat.hostname}/$${date:yyyy-MM}/${contextPath}-%d{MM-dd-yyyy}-%i.log.gz">
            <PatternLayout pattern="%d{yyyy.MM.dd 'at' HH:mm:ss z} %-5level %class{36} %L %M - %msg%xEx%n" />
            <TimeBasedTriggeringPolicy />
            <SizeBasedTriggeringPolicy size="500 MB" />
        </RollingFile>
    </Appenders>
    <Loggers>
        <Logger name="org.apache.log4j.xml" level="info" />
        <Root level="info">
            <AppenderRef ref="STDOUT" />
            <AppenderRef ref="RollingFile"/> 
        </Root>
    </Loggers>
</Configuration>

Reference

• https://blog.oio.de/2013/06/10/using-the-context-path-of-a-web-app-as-log-filename-with-log4j/

Answer 2

It's not a configuration file only solution, so i keep the question open for a while.

I've renamed the log4j.properties to myapp-log4j.properties and modified the log file name property like this:

log4j.appender.R.File=${catalina.base}/logs/#{context.name}.log

Because i have a servlet which is loaded at startup, i'm initializing log4j in the init() function.

String contextPath = getServletContext().getContextPath();

// First reconfigure log4j
String contextName = contextPath.substring(1);
if (!configureLog4J(contextName)) {
  return;
}

The function for this:

  public static final String LOG4JAPPENDERRFILE = "log4j.appender.R.File";

  private boolean configureLog4J(String contextName) {
    Properties props = new Properties();
    try {
       InputStream configStream = getClass().getResourceAsStream("/myapp-log4j.properties");
       props.load(configStream);
       configStream.close();
    } catch(IOException e) {
       System.out.println("FATAL! Cannot load log4j configuration file from classpath.");
       e.printStackTrace(System.out);
       return false;
    }

    String logFile = props.getProperty(LOG4JAPPENDERRFILE);
    logFile=logFile.replace("#{context.name}", contextName);
    props.setProperty(LOG4JAPPENDERRFILE, logFile);
    LogManager.resetConfiguration();
    PropertyConfigurator.configure(props);
    return true;
  }

It seems like to work fine, but i don't really like that i have to modify the properties file in code.