While logging log create log file dynamically for that class and log into that file only in log4j

Question

I'm using log4j in my project for logging. I have one package and it contains 50+ java classes. While logging the logs from these if log statement comes from class A it should go in A.log file and etc for other classes. I want to do it at runtime. Don't want to write appenders in log4j.xml for all classes. At runtime it should be able to identify that log comes from class A so it log it into A.log file. I want achieve this by doing minimal or no changes in java file as its already implemented. Can I achieve by doing changes only in log4j.xml file of that package. System.setProperty("logfilename","className") is not useful for me. Any help appreciated.

Answer 1

Before anything else I feel that I should point out this seems like a very unusual logging strategy . Normally you would want to see a holistic view of what happened during a particular user's interaction with your program rather than having to refer to dozens of log files trying to trace through the flow of the logic. For example if a user runs your program by invoking class A and then class A uses Class B you already have to look at 2 logs in order to follow the user experience. Imagine having to do this for a complex user interaction that flows into many more classes and also imagine how difficult it would be to trace when the execution goes from A to B to A to B to C to B to A. Just in this small example you would have to look at the log for A and somehow recognize that execution is going to class B then look at the log for B and so on.

With all of the above said, if you still want to go this route then I don't believe you can achieve what you want with only a configuration change but it is possible with minimal code in addition to a configuration change.

If you take a look at the log4j2 manual in the section regarding Extending Log4j2 Lookups you'll see an example there of how to implement a lookup. You can create your own lookup for the logger name as follows:

package example;

import org.apache.logging.log4j.core.LogEvent;
import org.apache.logging.log4j.core.config.plugins.Plugin;
import org.apache.logging.log4j.core.lookup.StrLookup;

@Plugin(name = "logger", category = "Lookup")
public class LoggerLookup implements StrLookup{
    /**
     * Lookup the value for the key.
     * @param key  the key to be looked up, may be null
     * @return The value for the key.
     */
    public String lookup(String key) {
        return null;
    }


    /**
     * Lookup the value for the key using the data in the LogEvent.
     * @param event The current LogEvent.
     * @param key  the key to be looked up, may be null
     * @return The value associated with the key.
     */
    public String lookup(LogEvent event, String key) {
        if("name".equals(key)){
            return event.getLoggerName();
        }
        return null;
    }
}

Now, use the Routing Appender to dynamically create appenders as needed based on the logger name at runtime by configuring it similar to the following example:

    <Routing name="MyRoutingAppender">
        <Routes pattern="$${logger:name}">
            <Route>
                <File
                    fileName="logs/${logger:name}.txt"
                    name="appender-${logger:name}">
                    <PatternLayout>
                        <Pattern>%d{HH:mm:ss.SSS} [%t] %-5level ${logger:name} - %msg%n</Pattern>
                    </PatternLayout>
                </File>
            </Route>
        </Routes>
    </Routing>

Below are two sample classes that generate some logging.

First the main class:

package example;

import org.apache.logging.log4j.LogManager;
import org.apache.logging.log4j.Logger;

public class SomeClass {

    private static final Logger log = LogManager.getLogger();   

    public static void main(String[] args){
        log.info("Here's some info!");
        log.error("Some erorr happened!");

        AnotherClass ac = new AnotherClass();
        ac.logSomething();
    }
}

Second class:

package example;

import org.apache.logging.log4j.LogManager;
import org.apache.logging.log4j.Logger;

public class AnotherClass {
    private static final Logger log = LogManager.getLogger();   

    public void logSomething(){
        log.info("This is yet another info message");
    }
}

Below is the configuration file (log4j2.xml) I used to test with. Note that I used console appender at first to verify the new lookup was working and decided to leave it in as it may be helpful to those reading this answer.

<?xml version="1.0" encoding="UTF-8"?>
<Configuration status="WARN">
    <Appenders>
        <Console name="Console" target="SYSTEM_OUT">
            <PatternLayout pattern="%d{HH:mm:ss.SSS} [%t] %-5level ${logger:name} - %msg%n" />
        </Console>

        <Routing name="MyRoutingAppender">
            <Routes pattern="$${logger:name}">
                <Route>
                    <File
                        fileName="logs/${logger:name}.txt"
                        name="appender-${logger:name}">
                        <PatternLayout>
                            <Pattern>%d{HH:mm:ss.SSS} [%t] %-5level ${logger:name} - %msg%n</Pattern>
                        </PatternLayout>
                    </File>
                </Route>
            </Routes>
        </Routing>
    </Appenders>

    <Loggers>
        <Root level="debug">
            <AppenderRef ref="Console" level="info" />          
            <AppenderRef ref="MyRoutingAppender" level="info" />
        </Root>
    </Loggers>
</Configuration>

Running SomeClass generates two log files called "example.AnotherClass.txt" and "example.SomeClass.txt" in a directory named "logs". The logs in the first file were generated by AnotherClass and those in the second file were generated by SomeClass

All of this is just a proof of concept so you will need to modify it to fit your needs. Hopefully it illustrates the general way you could achieve what you want.