public void setAppend(boolean flag) {
        fileAppend = flag;
    }
The <b>Append</b> option takes a boolean value. It is set to
<code>true</code> by default. If true, then <code>File</code> will be
opened in append mode by {@link #setFile setFile} (see above). Otherwise,
{@link #setFile setFile} will open <code>File</code> in truncate mode.

<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