Writing a custom File Appender for log4j

log4j is a powerful and very versatile tool. here is a code snippet to write a custom file Appender.

Step 1

The way i desire, First we need to write a java class for making a custom appender.Let's call it "CustomFileAppender.java"

code will be as follows

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import java.text.SimpleDateFormat;
import java.util.Date;

import org.apache.log4j.FileAppender;

public class CustomFileAppender extends FileAppender {

@Override
public void setFile(String fileName) {
if (fileName.indexOf("%timestamp") >= 0) {
Date d = new Date();
SimpleDateFormat format = new SimpleDateFormat("yyyyMMddHHmmssSS");
fileName = fileName.replaceAll("%timestamp", format.format(d));
}
super.setFile(fileName);
}
}

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her we add the date and time in format "yyyyMMddHHmmssSS" after the end of file name. In otherwords as a suffix.

eg: then your log file name would be like  "log-2016063014401283"

I believe that everybody understand the numbers

for ease of everybody, i will explain that part also

the format we use "yyyyMMddHHmmssSS" and  example is "2016063014401283".
let's take part by part from left to right.

Date formatter	Description	example
yyyy	year	2016
MM	MM	06
dd	date	30
HH	hours	14
mm	minutes	40
Ss (simple)	Seconds in minute	12
SS  (capital)	Milliseconds	83

for more information  refer

https://docs.oracle.com/javase/7/docs/api/java/text/SimpleDateFormat.html

let's come back to logging.

Step 2

adding appender to log4j property file  "log4j.properties"

=============================================================

# Root logger option

log4j.rootLogger=DEBUG, stdout, file

# Redirect log messages to console

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=%m%n

####FullLog################

log4j.appender.file=com.research.log.CustomFileAppender

log4j.appender.file.File=D:\\Log\\mergeOperation-%timestamp.log

log4j.appender.file.layout=org.apache.log4j.PatternLayout

log4j.appender.file.layout.ConversionPattern=%m%n

==========================================================

explanation

you can see that "log4j.rootLogger"  has "file" in line number 1

# Root logger option

log4j.rootLogger=DEBUG, stdout, file

next line describe, how we handle the standard output when running program. eg: when you run the program in console. you can print the output as needed modifying this.This is not what we are looking for.

Creating appender for the log file, what we log actually.That is explained in the below set.

# Redirect log messages to console

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=%m%n

here is how we add the custom appender.

log4j.appender.file=com.research.log.CustomFileAppender

log4j.appender.file.File=D:\\Log\\newLog-%timestamp.log

log4j.appender.file.layout=org.apache.log4j.PatternLayout

log4j.appender.file.layout.ConversionPattern=%m%n

we have created file option using "CustomFileAppender.java " class. Give the name with the package except ".java"

log4j.appender.file=com.research.log.CustomFileAppender

Then we create the name of the log file in the specific destination we need.We are creating log file inside folder name "Log" in drive "D". name of the log file is "newLog.log". But as we need to ad an appender , the time as a suffix to the log name with hyphen, we give new name as  "newLog-%timestamp.log"

It will be like this

log4j.appender.file.File=D:\\Log\\newLog-%timestamp.log

next two lines describe the pattern. first you give the patter class (log4j.appender.file.layou) for the patter and after that you can decide the pattern of the log with "log4j.appender.file.layout.ConversionPattern" property.

log4j.appender.file.layout=org.apache.log4j.PatternLayout

log4j.appender.file.layout.ConversionPattern=%m%n