log4j-core/src/main/java/org/apache/logging/log4j/core/net/SmtpManager.java [110:305]:
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    @Override
    public void sendEvents(final Layout<?> layout, final LogEvent appendEvent) {
        if (message == null) {
            connect(appendEvent);
        }
        try {
            final LogEvent[] priorEvents = removeAllBufferedEvents();
            // LOG4J-310: log appendEvent even if priorEvents is empty

            final byte[] rawBytes = formatContentToBytes(priorEvents, appendEvent, layout);

            final String contentType = layout.getContentType();
            final String encoding = getEncoding(rawBytes, contentType);
            final byte[] encodedBytes = encodeContentToBytes(rawBytes, encoding);

            final InternetHeaders headers = getHeaders(contentType, encoding);
            final MimeMultipart mp = getMimeMultipart(encodedBytes, headers);

            final String subject = data.getSubjectSerializer().toSerializable(appendEvent);

            sendMultipartMessage(message, mp, subject);
        } catch (final MessagingException | IOException | RuntimeException e) {
            logError("Caught exception while sending e-mail notification.", e);
            throw new LoggingException("Error occurred while sending email", e);
        }
    }

    LogEvent[] removeAllBufferedEvents() {
        return buffer.removeAll();
    }

    protected byte[] formatContentToBytes(final LogEvent[] priorEvents, final LogEvent appendEvent,
                                          final Layout<?> layout) throws IOException {
        final ByteArrayOutputStream raw = new ByteArrayOutputStream();
        writeContent(priorEvents, appendEvent, layout, raw);
        return raw.toByteArray();
    }

    private void writeContent(final LogEvent[] priorEvents, final LogEvent appendEvent, final Layout<?> layout,
                              final ByteArrayOutputStream out)
        throws IOException {
        writeHeader(layout, out);
        writeBuffer(priorEvents, appendEvent, layout, out);
        writeFooter(layout, out);
    }

    protected void writeHeader(final Layout<?> layout, final OutputStream out) throws IOException {
        final byte[] header = layout.getHeader();
        if (header != null) {
            out.write(header);
        }
    }

    protected void writeBuffer(final LogEvent[] priorEvents, final LogEvent appendEvent, final Layout<?> layout,
                               final OutputStream out) throws IOException {
        for (final LogEvent priorEvent : priorEvents) {
            final byte[] bytes = layout.toByteArray(priorEvent);
            out.write(bytes);
        }

        final byte[] bytes = layout.toByteArray(appendEvent);
        out.write(bytes);
    }

    protected void writeFooter(final Layout<?> layout, final OutputStream out) throws IOException {
        final byte[] footer = layout.getFooter();
        if (footer != null) {
            out.write(footer);
        }
    }

    protected String getEncoding(final byte[] rawBytes, final String contentType) {
        final DataSource dataSource = new ByteArrayDataSource(rawBytes, contentType);
        return MimeUtility.getEncoding(dataSource);
    }

    protected byte[] encodeContentToBytes(final byte[] rawBytes, final String encoding)
        throws MessagingException, IOException {
        final ByteArrayOutputStream encoded = new ByteArrayOutputStream();
        encodeContent(rawBytes, encoding, encoded);
        return encoded.toByteArray();
    }

    protected void encodeContent(final byte[] bytes, final String encoding, final ByteArrayOutputStream out)
            throws MessagingException, IOException {
        try (final OutputStream encoder = MimeUtility.encode(out, encoding)) {
            encoder.write(bytes);
        }
    }

    protected InternetHeaders getHeaders(final String contentType, final String encoding) {
        final InternetHeaders headers = new InternetHeaders();
        headers.setHeader("Content-Type", contentType + "; charset=UTF-8");
        headers.setHeader("Content-Transfer-Encoding", encoding);
        return headers;
    }

    protected MimeMultipart getMimeMultipart(final byte[] encodedBytes, final InternetHeaders headers)
        throws MessagingException {
        final MimeMultipart mp = new MimeMultipart();
        final MimeBodyPart part = new MimeBodyPart(headers, encodedBytes);
        mp.addBodyPart(part);
        return mp;
    }

    /**
     * @deprecated Please use the {@link #sendMultipartMessage(MimeMessage, MimeMultipart, String)} method instead.
     */
    @Deprecated
    protected void sendMultipartMessage(final MimeMessage msg, final MimeMultipart mp) throws MessagingException {
        synchronized (msg) {
            msg.setContent(mp);
            msg.setSentDate(new Date());
            Transport.send(msg);
        }
    }

    protected void sendMultipartMessage(final MimeMessage msg, final MimeMultipart mp, final String subject) throws MessagingException {
        synchronized (msg) {
            msg.setContent(mp);
            msg.setSentDate(new Date());
            msg.setSubject(subject);
            Transport.send(msg);
        }
    }

    private synchronized void connect(final LogEvent appendEvent) {
        if (message != null) {
            return;
        }
        try {
            message = createMimeMessage(data, session, appendEvent);
        } catch (final MessagingException e) {
            logError("Could not set SmtpAppender message options", e);
            message = null;
        }
    }

    /**
     * Factory to create the SMTP Manager.
     */
    public static class SMTPManagerFactory implements MailManagerFactory {

        @Override
        public SmtpManager createManager(final String name, final FactoryData data) {
            final String smtpProtocol = data.getSmtpProtocol();
            final String prefix = "mail." + smtpProtocol;

            final Properties properties = PropertiesUtil.getSystemProperties();
            properties.setProperty("mail.transport.protocol", smtpProtocol);
            if (properties.getProperty("mail.host") == null) {
                // Prevent an UnknownHostException in Java 7
                properties.setProperty("mail.host", NetUtils.getLocalHostname());
            }

            final String smtpHost = data.getSmtpHost();
            if (null != smtpHost) {
                properties.setProperty(prefix + ".host", smtpHost);
            }
            if (data.getSmtpPort() > 0) {
                properties.setProperty(prefix + ".port", String.valueOf(data.getSmtpPort()));
            }

            final Authenticator authenticator = buildAuthenticator(data.getSmtpUsername(), data.getSmtpPassword());
            if (null != authenticator) {
                properties.setProperty(prefix + ".auth", "true");
            }

            if (smtpProtocol.equals("smtps")) {
                final SslConfiguration sslConfiguration = data.getSslConfiguration();
                if (sslConfiguration != null) {
                    final SSLSocketFactory sslSocketFactory = sslConfiguration.getSslSocketFactory();
                    properties.put(prefix + ".ssl.socketFactory", sslSocketFactory);
                    properties.setProperty(prefix + ".ssl.checkserveridentity", Boolean.toString(sslConfiguration.isVerifyHostName()));
                }
            }

            final Session session = Session.getInstance(properties, authenticator);
            session.setProtocolForAddress("rfc822", smtpProtocol);
            session.setDebug(data.isSmtpDebug());
            return new SmtpManager(name, session, null, data);
        }

        private Authenticator buildAuthenticator(final String username, final String password) {
            if (null != password && null != username) {
                return new Authenticator() {
                    private final PasswordAuthentication passwordAuthentication =
                        new PasswordAuthentication(username, password);

                    @Override
                    protected PasswordAuthentication getPasswordAuthentication() {
                        return passwordAuthentication;
                    }
                };
            }
            return null;
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log4j-jakarta-smtp/src/main/java/org/apache/logging/log4j/smtp/SmtpManager.java [89:284]:
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    @Override
    public void sendEvents(final Layout<?> layout, final LogEvent appendEvent) {
        if (message == null) {
            connect(appendEvent);
        }
        try {
            final LogEvent[] priorEvents = removeAllBufferedEvents();
            // LOG4J-310: log appendEvent even if priorEvents is empty

            final byte[] rawBytes = formatContentToBytes(priorEvents, appendEvent, layout);

            final String contentType = layout.getContentType();
            final String encoding = getEncoding(rawBytes, contentType);
            final byte[] encodedBytes = encodeContentToBytes(rawBytes, encoding);

            final InternetHeaders headers = getHeaders(contentType, encoding);
            final MimeMultipart mp = getMimeMultipart(encodedBytes, headers);

            final String subject = data.getSubjectSerializer().toSerializable(appendEvent);

            sendMultipartMessage(message, mp, subject);
        } catch (final MessagingException | IOException | RuntimeException e) {
            logError("Caught exception while sending e-mail notification.", e);
            throw new LoggingException("Error occurred while sending email", e);
        }
    }

    LogEvent[] removeAllBufferedEvents() {
        return buffer.removeAll();
    }

    protected byte[] formatContentToBytes(final LogEvent[] priorEvents, final LogEvent appendEvent,
                                          final Layout<?> layout) throws IOException {
        final ByteArrayOutputStream raw = new ByteArrayOutputStream();
        writeContent(priorEvents, appendEvent, layout, raw);
        return raw.toByteArray();
    }

    private void writeContent(final LogEvent[] priorEvents, final LogEvent appendEvent, final Layout<?> layout,
                              final ByteArrayOutputStream out)
        throws IOException {
        writeHeader(layout, out);
        writeBuffer(priorEvents, appendEvent, layout, out);
        writeFooter(layout, out);
    }

    protected void writeHeader(final Layout<?> layout, final OutputStream out) throws IOException {
        final byte[] header = layout.getHeader();
        if (header != null) {
            out.write(header);
        }
    }

    protected void writeBuffer(final LogEvent[] priorEvents, final LogEvent appendEvent, final Layout<?> layout,
                               final OutputStream out) throws IOException {
        for (final LogEvent priorEvent : priorEvents) {
            final byte[] bytes = layout.toByteArray(priorEvent);
            out.write(bytes);
        }

        final byte[] bytes = layout.toByteArray(appendEvent);
        out.write(bytes);
    }

    protected void writeFooter(final Layout<?> layout, final OutputStream out) throws IOException {
        final byte[] footer = layout.getFooter();
        if (footer != null) {
            out.write(footer);
        }
    }

    protected String getEncoding(final byte[] rawBytes, final String contentType) {
        final DataSource dataSource = new ByteArrayDataSource(rawBytes, contentType);
        return MimeUtility.getEncoding(dataSource);
    }

    protected byte[] encodeContentToBytes(final byte[] rawBytes, final String encoding)
        throws MessagingException, IOException {
        final ByteArrayOutputStream encoded = new ByteArrayOutputStream();
        encodeContent(rawBytes, encoding, encoded);
        return encoded.toByteArray();
    }

    protected void encodeContent(final byte[] bytes, final String encoding, final ByteArrayOutputStream out)
            throws MessagingException, IOException {
        try (final OutputStream encoder = MimeUtility.encode(out, encoding)) {
            encoder.write(bytes);
        }
    }

    protected InternetHeaders getHeaders(final String contentType, final String encoding) {
        final InternetHeaders headers = new InternetHeaders();
        headers.setHeader("Content-Type", contentType + "; charset=UTF-8");
        headers.setHeader("Content-Transfer-Encoding", encoding);
        return headers;
    }

    protected MimeMultipart getMimeMultipart(final byte[] encodedBytes, final InternetHeaders headers)
        throws MessagingException {
        final MimeMultipart mp = new MimeMultipart();
        final MimeBodyPart part = new MimeBodyPart(headers, encodedBytes);
        mp.addBodyPart(part);
        return mp;
    }

    /**
     * @deprecated Please use the {@link #sendMultipartMessage(MimeMessage, MimeMultipart, String)} method instead.
     */
    @Deprecated
    protected void sendMultipartMessage(final MimeMessage msg, final MimeMultipart mp) throws MessagingException {
        synchronized (msg) {
            msg.setContent(mp);
            msg.setSentDate(new Date());
            Transport.send(msg);
        }
    }

    protected void sendMultipartMessage(final MimeMessage msg, final MimeMultipart mp, final String subject) throws MessagingException {
        synchronized (msg) {
            msg.setContent(mp);
            msg.setSentDate(new Date());
            msg.setSubject(subject);
            Transport.send(msg);
        }
    }

    private synchronized void connect(final LogEvent appendEvent) {
        if (message != null) {
            return;
        }
        try {
            message = createMimeMessage(data, session, appendEvent);
        } catch (final MessagingException e) {
            logError("Could not set SmtpAppender message options", e);
            message = null;
        }
    }

    /**
     * Factory to create the SMTP Manager.
     */
    public static class SMTPManagerFactory implements MailManagerFactory {

        @Override
        public SmtpManager createManager(final String name, final FactoryData data) {
            final String smtpProtocol = data.getSmtpProtocol();
            final String prefix = "mail." + smtpProtocol;

            final Properties properties = PropertiesUtil.getSystemProperties();
            properties.setProperty("mail.transport.protocol", smtpProtocol);
            if (properties.getProperty("mail.host") == null) {
                // Prevent an UnknownHostException in Java 7
                properties.setProperty("mail.host", NetUtils.getLocalHostname());
            }

            final String smtpHost = data.getSmtpHost();
            if (null != smtpHost) {
                properties.setProperty(prefix + ".host", smtpHost);
            }
            if (data.getSmtpPort() > 0) {
                properties.setProperty(prefix + ".port", String.valueOf(data.getSmtpPort()));
            }

            final Authenticator authenticator = buildAuthenticator(data.getSmtpUsername(), data.getSmtpPassword());
            if (null != authenticator) {
                properties.setProperty(prefix + ".auth", "true");
            }

            if (smtpProtocol.equals("smtps")) {
                final SslConfiguration sslConfiguration = data.getSslConfiguration();
                if (sslConfiguration != null) {
                    final SSLSocketFactory sslSocketFactory = sslConfiguration.getSslSocketFactory();
                    properties.put(prefix + ".ssl.socketFactory", sslSocketFactory);
                    properties.setProperty(prefix + ".ssl.checkserveridentity", Boolean.toString(sslConfiguration.isVerifyHostName()));
                }
            }

            final Session session = Session.getInstance(properties, authenticator);
            session.setProtocolForAddress("rfc822", smtpProtocol);
            session.setDebug(data.isSmtpDebug());
            return new SmtpManager(name, session, null, data);
        }

        private Authenticator buildAuthenticator(final String username, final String password) {
            if (null != password && null != username) {
                return new Authenticator() {
                    private final PasswordAuthentication passwordAuthentication =
                        new PasswordAuthentication(username, password);

                    @Override
                    protected PasswordAuthentication getPasswordAuthentication() {
                        return passwordAuthentication;
                    }
                };
            }
            return null;
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