@Override
    public void append(final LoggingEvent event) {

        // Reminder: the nesting of calls is:
        //
        // doAppend()
        // - check threshold
        // - filter
        // - append();
        // - checkEntryConditions();
        // - subAppend();

        if (!this.checkEntryConditions()) {
            return;
        }
        this.subAppend(event);
    }
This method is called by the {@link AppenderSkeleton#doAppend} method.

<p>
If the output stream exists and is writable then write a log statement to
the output stream. Otherwise, write a single warning message to
<code>System.err</code>.

<p>
The format of the output will depend on this appender's layout.
metamorphosis-client-extension/src/main/java/com/taobao/metamorphosis/client/extension/log4j/StreamAppender.java