log4j.xml配置的公共变量

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

Answer 1

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 )

Answer 2

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>

Answer 3

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