common variable for log4j.xml configuration

Question

I have log4j.xml configuration like this:

<appender name="MyAppender"class="org.apache.log4j.DailyRollingFileAppender">   
     <param name="File"     value="/logs/custom/my.log"/>
...  
 </appender>

However the root directory of my file are the same for a lot of appender. Is there a way to defined "/logs/custom/" as a variable and reused it in all of my appender.

Thanks,

Sean

score 16 · Accepted Answer · edited May 23 '17 at 11:46

UPDATE: The original answer applies to Log4j 1.x

Log4j 2.x has much richer support for properties in configuration file, see the Log4j manual about Configuration with properties.

Log4j 1.x (the original answer):

The only way to achieve something similar when you are using log4j.xml is to set a system property at startup and then reference that from your log4j.xml.

At startup, you set your system property:

java -Dlog_dir=/var/logs/custom     com.yourorg.yourapp.Main

Or set it programmatically at runtime (before initializing Log4j):

System.setProperty("log_dir", "/var/logs/custom")

Then you can reference it like this:

<appender name="MyAppender"class="org.apache.log4j.DailyRollingFileAppender">   
     <param name="File" value="${log_dir}/my.log"/>
     ...  
</appender>

Or in properties file, like this:

log4j.appender.MyAppender.File = ${log_dir}/my.log

Source: I got inspiration for this answer from Using system environment variables in log4j xml configuration.

Also, if you are running under Tomcat, you can use ${catalina.home} variable, like this:

<appender name="MyAppender"class="org.apache.log4j.DailyRollingFileAppender">   
     <param name="File" value="${catalina.home}/logs/my.log"/>
     ...  
</appender>

what if i don't want it to pass it with java args. Is there any way where i can initialize in log4j.xml? — Nishant Lakhara, Feb 03 '16 at 07:26
You could always use System.setProperty() to set the property programmatically at runtime, before initializing Log4j. — Neeme Praks, Feb 05 '16 at 07:23

score 14 · Answer 2 · answered May 11 '12 at 20:18

14

It is possible in XML as well to define a variable and reuse it in the rest of the doc:

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE log4j:configuration SYSTEM "log4j.dtd" [
  <!ENTITY logHome "/logs/folder1/folder2">
]
>

Then, refer to this variable just defined, as &logHome;

<param name="File" value="&logHome;/folder3/my.log"/>

How about that?

(I believe I learnt about XML entity references some time ago at this link: http://www.ibm.com/developerworks/xml/library/x-tipentref/index.html)

answered May 11 '12 at 20:18

user185624

141
1
3

is there any way where i define a default param to this variable which gets overrided by system property set outside ..... may using java args? – Nishant Lakhara Feb 03 '16 at 07:27
No. But you can use a system property with different syntax, see the accepted answer. – Neeme Praks Feb 05 '16 at 07:36

score 6 · Answer 3 · answered Sep 30 '11 at 17:39

6

I don't believe this is possible using XML configuration, but it is in a .properties file configuration:

mysubdir = /logs/custom
...
log4j.appender.MyAppender.File = ${mysubdir}/my.log

answered Sep 30 '11 at 17:39

matt b

138,234
66
282
345

It is possible to use system properties in XML syntax, see the accepted answer. Not possible to define them in XML, though. – Neeme Praks Feb 05 '16 at 07:37
Actually, in Log4j 2.x, it is possible to define them also, in XML. https://logging.apache.org/log4j/2.x/manual/configuration.html#Configuration_with_Properties – Neeme Praks Feb 05 '16 at 07:43

common variable for log4j.xml configuration

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