#vulnerable code 
                                @Override
    public long appendMessage(MimeMessage message,
                              Flags flags,
                              Date receivedDate) {
        long uid = nextUid;
        nextUid++;

        try {
            message.setFlags(flags, true);
            message.setFlag(Flags.Flag.RECENT, true);
        } catch (MessagingException e) {
            throw new IllegalStateException("Can not set flags", e);
        }
        StoredMessage storedMessage = new StoredMessage(message,
                receivedDate, uid);

        int newMsn;
        synchronized (mailMessages) {
            mailMessages.add(storedMessage);
            newMsn = mailMessages.size();
        }

        // Notify all the listeners of the new message
        synchronized (_mailboxListeners) {
            for (FolderListener _mailboxListener : _mailboxListeners) {
                _mailboxListener.added(newMsn);
            }
        }

        return uid;
    } 
                            #location 5 
                            #vulnerability type THREAD_SAFETY_VIOLATION
#fixed code
                                @Override
    public long appendMessage(MimeMessage message,
                              Flags flags,
                              Date receivedDate) {
        final long uid = nextUid.getAndIncrement();

        try {
            message.setFlags(flags, true);
            message.setFlag(Flags.Flag.RECENT, true);
        } catch (MessagingException e) {
            throw new IllegalStateException("Can not set flags", e);
        }
        StoredMessage storedMessage = new StoredMessage(message,
                receivedDate, uid);

        int newMsn;
        synchronized (mailMessages) {
            mailMessages.add(storedMessage);
            newMsn = mailMessages.size();
        }

        // Notify all the listeners of the new message
        synchronized (_mailboxListeners) {
            for (FolderListener _mailboxListener : _mailboxListeners) {
                _mailboxListener.added(newMsn);
            }
        }

        return uid;
    }