#vulnerable code 
                                @Override
    public void format(final LogEvent event, final StringBuilder output) {
        final long timestamp = event.getMillis();

        synchronized (this) {
            if (timestamp != lastTimestamp) {
                lastTimestamp = timestamp;
                cachedDate = simpleFormat.format(timestamp);
            }
        }
        output.append(cachedDate);
    } 
                            #location 11 
                            #vulnerability type THREAD_SAFETY_VIOLATION
#fixed code
                                @Override
    public void format(final LogEvent event, final StringBuilder output) {
        final long timestamp = event.getMillis();

        synchronized (this) {
            if (timestamp != lastTimestamp) {
                lastTimestamp = timestamp;
                cachedDateString = simpleFormat.format(timestamp);
            }
        }
        output.append(cachedDateString);
    }