set the placeholder in the log4j.properties file

[maven apache]

Hi:

I want the logs created by log4j put under the app dir,so I set the
placeholder in the log4j.properties:

# chartstdout is set to be a ConsoleAppender.
log4j.appender.chartstdout=org.apache.log4j.ConsoleAppender
log4j.appender.chartstdout.layout=org.apache.log4j.PatternLayout
log4j.appender.chartstdout.layout.ConversionPattern=%-4r [%t] %-5p %l %x -
%m%n


log4j.appender.chartfileout=org.apache.log4j.FileAppender
log4j.appender.chartfileout.File=${apppath}/logs/log.html
log4j.appender.chartfileout.Append=true
log4j.appender.chartfileout.layout=com.test.util.CustomHTMLLayout

log4j.rootLogger=debug, chartstdout,chartfileout

Then in my servlet whose "load-on-startup" attribute is "0" I set the
property "apppath",but it does not work.
It seems that the log4j read the log4j.properties and config the log before
I modify the property.
    public void init() {
        DbManager.init();
        // log
        String context =
getServletConfig().getServletContext().getRealPath("/");
        Properties p = new Properties();
        p.setProperty("apppath", context);
        try {

p.load(InitServlet.class.getResourceAsStream("/log4j.properties"));
        } catch (IOException e) {
            e.printStackTrace();
        }
        PropertyConfigurator.configure(p);
    }



Of course,I can rename the log4j.properties,then the log4j will not find the
file but I can. However some other libs which I used in my appliction
require log4j also,they need the log4j.properties to generate there logs.

Any ideas?