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

        synchronized (this) {
            if (timestamp != lastTimestamp) {
                lastTimestamp = timestamp;
                relative = Long.toString(timestamp - startTime);
            }
        }
        toAppendTo.append(relative);
    } 
                            #location 11 
                            #vulnerability type THREAD_SAFETY_VIOLATION
#fixed code
                                @Override
    public void format(final LogEvent event, final StringBuilder toAppendTo) {
        final long timestamp = event.getTimeMillis();
        toAppendTo.append(timestamp - startTime);
    }