log4j - Logging in Files

To write your logging information into a file, you would have to use org.apache.log4j.FileAppender.

FileAppender Configuration

FileAppender has the following configurable parameters:

Property	Description
immediateFlush	This flag is by default set to true, which means the output stream to the file being flushed with each append operation.
encoding	It is possible to use any character-encoding. By default, it is the platform-specific encoding scheme.
threshold	The threshold level for this appender.
Filename	The name of the log file.
fileAppend	This is by default set to true, which means the logging information being appended to the end of the same file.
bufferedIO	This flag indicates whether we need buffered writing enabled. By default, it is set to false.
bufferSize	If buffered I/O is enabled, it indicates the buffer size. By default, it is set to 8kb.

Following is a sample configuration file log4j.properties for FileAppender −

# Define the root logger with appender file
log4j.rootLogger = DEBUG, FILE

# Define the file appender
log4j.appender.FILE=org.apache.log4j.FileAppender

# Set the name of the file
log4j.appender.FILE.File=${log}/log.out

# Set the immediate flush to true (default)
log4j.appender.FILE.ImmediateFlush=true

# Set the threshold to debug mode
log4j.appender.FILE.Threshold=debug

# Set the append to false, overwrite
log4j.appender.FILE.Append=false

# Define the layout for file appender
log4j.appender.FILE.layout=org.apache.log4j.PatternLayout
log4j.appender.FILE.layout.conversionPattern=%m%n

If you wish to have an XML configuration file equivalent to the above log4j.properties file, then here is the content:

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE log4j:configuration SYSTEM "log4j.dtd">
<log4j:configuration>

<appender name="FILE" class="org.apache.log4j.FileAppender">

   <param name="file" value="${log}/log.out"/>
   <param name="immediateFlush" value="true"/>
   <param name="threshold" value="debug"/>
   <param name="append" value="false"/>
   
   <layout class="org.apache.log4j.PatternLayout">
      <param name="conversionPattern" value="%m%n"/>
   </layout>
</appender>

<logger name="log4j.rootLogger" additivity="false">
   <level value="DEBUG"/>
   <appender-ref ref="FILE"/>
</logger>

</log4j:configuration>

You can try log4j - Sample Program with the above configuration.

Logging in Multiple Files

You may want to write your log messages into multiple files for certain reasons, for example, if the file size reached to a certain threshold.
To write your logging information into multiple files, you would have to use org.apache.log4j.RollingFileAppender class which extends the FileAppender class and inherits all its properties.
We have the following configurable parameters in addition to the ones mentioned above for FileAppender −

Property	Description
maxFileSize	This is the critical size of the file above which the file will be rolled. Default value is 10 MB.
maxBackupIndex	This property denotes the number of backup files to be created. Default value is 1.

Following is a sample configuration file log4j.properties for RollingFileAppender.

# Define the root logger with appender file
log4j.rootLogger = DEBUG, FILE

# Define the file appender
log4j.appender.FILE=org.apache.log4j.RollingFileAppender

# Set the name of the file
log4j.appender.FILE.File=${log}/log.out

# Set the immediate flush to true (default)
log4j.appender.FILE.ImmediateFlush=true

# Set the threshold to debug mode
log4j.appender.FILE.Threshold=debug

# Set the append to false, should not overwrite
log4j.appender.FILE.Append=true

# Set the maximum file size before rollover
log4j.appender.FILE.MaxFileSize=5KB

# Set the the backup index
log4j.appender.FILE.MaxBackupIndex=2

# Define the layout for file appender
log4j.appender.FILE.layout=org.apache.log4j.PatternLayout
log4j.appender.FILE.layout.conversionPattern=%m%n

If you wish to have an XML configuration file, you can generate the same as mentioned in the initial section and add only additional parameters related to RollingFileAppender.
This example configuration demonstrates that the maximum permissible size of each log file is 5 MB. Upon exceeding the maximum size, a new log file will be created. Since maxBackupIndex is defined as 2, once the second log file reaches the maximum size, the first log file will be erased and thereafter, all the logging information will be rolled back to the first log file.
You can try log4j - Sample Program with the above configuration.

Daily Log File Generation

There may be a requirement to generate your log files on a daily basis to keep a clean record of your logging information.
To write your logging information into files on a daily basis, you would have to use org.apache.log4j.DailyRollingFileAppender class which extends the FileAppender class and inherits all its properties.
There is only one important configurable parameter in addition to the ones mentioned above for FileAppender:

Property	Description
DatePattern	This indicates when to roll over the file and the naming convention to be followed. By default, roll over is performed at midnight each day.

DatePattern controls the rollover schedule using one of the following patterns:

DatePattern	Description
'.' yyyy-MM	Roll over at the end of each month and at the beginning of the next month.
'.' yyyy-MM-dd	Roll over at midnight each day. This is the default value.
'.' yyyy-MM-dd-a	Roll over at midday and midnight of each day.
'.' yyyy-MM-dd-HH	Roll over at the top of every hour.
'.' yyyy-MM-dd-HH-mm	Roll over every minute.
'.' yyyy-ww	Roll over on the first day of each week depending upon the locale.

Following is a sample configuration file log4j.properties to generate log files rolling over at midday and midnight of each day.

# Define the root logger with appender file
log4j.rootLogger = DEBUG, FILE

# Define the file appender
log4j.appender.FILE=org.apache.log4j.DailyRollingFileAppender

# Set the name of the file
log4j.appender.FILE.File=${log}/log.out

# Set the immediate flush to true (default)
log4j.appender.FILE.ImmediateFlush=true

# Set the threshold to debug mode
log4j.appender.FILE.Threshold=debug

# Set the append to false, should not overwrite
log4j.appender.FILE.Append=true

# Set the DatePattern
log4j.appender.FILE.DatePattern='.' yyyy-MM-dd-a

# Define the layout for file appender
log4j.appender.FILE.layout=org.apache.log4j.PatternLayout
log4j.appender.FILE.layout.conversionPattern=%m%n

If you wish to have an XML configuration file, you can generate the same as mentioned in the initial section and add only additional parameters related to DailyRollingFileAppender.
You can try log4j - Sample Program with the above configuration.