Using system environment variables in log4j xml configuration

Question

Is it possible to reference system environment variables (as opposed to Java system properties) in a log4j xml configuration file?

I'd like to be able to do something like:

<level value="${env.LOG_LEVEL}" />

and have it get that from the system environment variables, so I can avoid having to pass in so many things with -D parameters.

Answer 1

I tried to do that recently and couldn't get it to work. What I ended up doing is sending a variable at startup. So say you have an environment variable called $LOG_LEVEL:

<level value="${log_level}" />

and at startup...

java -Dlog_level=$LOG_LEVEL your_app

Answer 2

This syntax is documented only in log4j 2.X so make sure you are using the correct version. It does not work on the 1.X versions.

    <Appenders>
    <File name="file" fileName="${env:LOG_PATH}">
        <PatternLayout>
            <Pattern>%d %p %c{1.} [%t] %m %ex%n</Pattern>
        </PatternLayout>
    </File>
</Appenders>

Answer 3

I think this is not supported, but basically you can do two things to bring in your environment variables:

1. Use System.setProperty before Log4J gets configured

2. Convert (your) environment variables to system properties in your launcher

The first option basically boils down to this:

for (Map<String,String>.Entry entry : System.getenv().entrySet()) {
  System.setProperty(entry.getKey(), entry.getValue());
}

... but the question is of course where to put this code. In particular if you're running within some sort of Tomcat container or similar, this might be troublesome.

The other largely depends on your environment. Basically if you have a shell script that starts your app, you can write some shell magic to set all environment variables as properties, or just the ones you need, eg:

java -DMY_ENV=$MY_ENV -DMY_OTHER_ENV=$MY_OTHER_ENV -cp ... com.example.Main

It's also possible to alter your server startup scripts to support this, eg catalina.sh or similar.

Answer 4

您需要在env和变量名称之间放置一个冒号，如下所示：

<level value="${env:LOG_LEVEL}" />

Answer 5

Create a system variable. I prefer to use setenv.bat for such variables.

@echo off
rem app specific log dir
set "APP_LOG_ROOTDIR=../app/app-log"
exit /b 0

Add reference in log4j.xml file

<appender name="fileAppender" class="org.apache.log4j.RollingFileAppender">
  <param name="Threshold" value="DEBUG" />
  <param name="MaxFileSize" value="512KB" />
  <param name="MaxBackupIndex" value="10" />
  <param name="File" value="${APP_LOG_ROOTDIR}/app.log"/>
  <layout class="org.apache.log4j.PatternLayout">
   <param name="ConversionPattern" value="%d %-5p %c{1} %m %n" />
  </layout>
</appender>