Unexpected long/integer value at the start of the logged message - log4j

Question

I'm facing an issue with log4j2, that prints a value as first part of a logged message.
I'm using the following configuration and it works as expected except for the un-wanted value:
<?xml version="1.0" encoding="UTF-8" ?>
<Configuration status="info">
<Appenders>
<RollingFile name="myappender-app" fileName="C:/Programs/apache-tomcat-8.5.29/logs/app.log" filePattern="C:/Programs/apache-tomcat-8.5.29/logs/%d_app.log">
<PatternLayout>
<pattern>[%t]%5level %d{ISO8601}[%X{mdc-uuid} %X{sessionId}] %c{2} - %m%n%r%throwable{5}</pattern>
</PatternLayout>
<Policies>
<TimeBasedTriggeringPolicy interval="1" modulate="true" />
</Policies>
</RollingFile>
</Appenders>
<Loggers>
<Logger name="com.mypack.app" level="debug" additivity="false">
<appender-ref ref="myappender-app" level="debug" />
</Logger>
</Loggers>
</Configuration>
The result is something like:
[https-jsse-nio-18443-exec-6] INFO 2018-04-02T10:37:24,344[ ] filters.UUIDFilter - First logged message
39736[https-jsse-nio-18443-exec-6] INFO 2018-04-02T10:37:24,596[localhost-1522658244353 ] controller.MyAppController - Second Message
39988[https-jsse-nio-18443-exec-6] INFO 2018-04-02T10:37:24,602[localhost-1522658244353 ].....
39994[https-jsse-nio-18443-exec-6]ERROR 2018-04-02T10:37:54,697[localhost-1522658244353 ]....
other messages
The first message is correct while the others have those values [39736,39988,..] at the start of each line.
I'm assuming is something related to the rolling policy, but I could not see it in other examples using the same configuration.
Does anyone knows which is the problem and how to solve it ?

Answer 1

Problem is %n%r%throwable{5} in below line -
<pattern>[%t]%5level %d{ISO8601}[%X{mdc-uuid} %X{sessionId}] %c{2} - %m%n%r%throwable{5}</pattern>
%n refers to new line character.
%r refers to number of milliseconds elapsed since the JVM was started until the creation of the logging event.
%throwable{5} refers to exception stack trace.
Because your pattern contains %n so it will create a new line in logs and then %r will log number of milliseconds (that unwanted value). Since there is no exception so %throwable{5} is printing nothing.
To resolve it, simply write %n at the end of the pattern so that a new log statement will get printed in next line. You can also remove %r if you do not want to print that unwanted value at all.