Using system environment variables in log4j xml co

问题:

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.

回答1:

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>

回答2:

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

回答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, e.g.:

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, e.g. catalina.sh or similar.

回答4:

You need to put a colon between env and the name of the variable, like this:

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

回答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>