public synchronized void setFile(String file) {
        // Trim spaces from both ends. The users probably does not want
        // trailing spaces in file names.
        String val = file.trim();
        fileName = val;
    }
The <b>File</b> property takes a string value which should be the name of
the file to append to.

<p>
<font color="#DD0044"><b>Note that the special values "System.out" or
"System.err" are no longer honored.</b></font>

<p>
Note: Actual opening of the file is made when {@link #activateOptions} is
called, not when the options are set.
metamorphosis-client-extension/src/main/java/com/taobao/metamorphosis/client/extension/log4j/FileStreamAppender.java