reactive-mail-server/backend/src/main/java/com/mailserver/imap/MimeSource.java

package com.mailserver.imap;

import com.mailserver.domain.Attachment;
import com.mailserver.domain.Message;
import jakarta.activation.DataHandler;
import jakarta.activation.FileDataSource;
import jakarta.mail.MessagingException;
import jakarta.mail.Session;
import jakarta.mail.internet.InternetAddress;
import jakarta.mail.internet.MimeBodyPart;
import jakarta.mail.internet.MimeMessage;
import jakarta.mail.internet.MimeMultipart;
import jakarta.mail.internet.MimeUtility;

import java.io.ByteArrayOutputStream;
import java.io.IOException;
import java.nio.file.Files;
import java.nio.file.Path;
import java.util.ArrayList;
import java.util.Date;
import java.util.List;
import java.util.Properties;

/**
 * MIME text of a message that has no original (written in the web client): built from the stored fields and
 * attachment files. The boundary and Message-ID come from the message id, so the result is the same every time.
 */
public final class MimeSource {

    private MimeSource() {
    }

    public static byte[] build(Message message, List<Attachment> attachments, String hostname)
            throws MessagingException, IOException {
        Properties props = new Properties();
        props.put("mail.mime.splitlongparameters", "true");
        Session session = Session.getInstance(props);
        String messageId = message.getMessageId() == null || message.getMessageId().isBlank()
                ? message.getId() + "@" + hostname
                : message.getMessageId().replaceAll("^<|>$", "");
        if (!messageId.contains("@")) {
            messageId = messageId + "@" + hostname;
        }
        String finalMessageId = "<" + messageId + ">";
        MimeMessage mime = new MimeMessage(session) {
            @Override
            protected void updateMessageID() throws MessagingException {
                setHeader("Message-ID", finalMessageId);
            }
        };

        String from = message.getSenderEmail() == null ? "unknown@" + hostname : message.getSenderEmail();
        mime.setFrom(new InternetAddress(from, message.getSenderName(), "UTF-8"));
        List<InternetAddress> to = addresses(message.getRecipientEmail());
        if (!to.isEmpty()) {
            mime.setRecipients(jakarta.mail.Message.RecipientType.TO, to.toArray(new InternetAddress[0]));
        }
        List<InternetAddress> cc = addresses(message.getCcEmail());
        if (!cc.isEmpty()) {
            mime.setRecipients(jakarta.mail.Message.RecipientType.CC, cc.toArray(new InternetAddress[0]));
        }
        mime.setSubject(message.getSubject() == null ? "" : message.getSubject(), "UTF-8");
        if (message.getReceivedAt() != null) {
            mime.setSentDate(Date.from(message.getReceivedAt()));
        }

        String body = message.getBody() == null ? "" : message.getBody();
        String subtype = message.isHtml() ? "html" : "plain";
        if (attachments.isEmpty()) {
            mime.setText(body, "UTF-8", subtype);
        } else {
            MimeMultipart mixed = new FixedBoundaryMultipart("mixed", "=_rms_" + message.getId());
            MimeBodyPart text = new MimeBodyPart();
            text.setText(body, "UTF-8", subtype);
            mixed.addBodyPart(text);
            for (Attachment attachment : attachments) {
                Path file = Path.of(attachment.getFilePath());
                if (!Files.isRegularFile(file)) {
                    continue;
                }
                MimeBodyPart part = new MimeBodyPart();
                FileDataSource source = new FileDataSource(file.toFile()) {
                    @Override
                    public String getContentType() {
                        return attachment.getContentType() == null ? "application/octet-stream" : attachment.getContentType();
                    }
                };
                part.setDataHandler(new DataHandler(source));
                part.setFileName(MimeUtility.encodeText(attachment.getFilename(), "UTF-8", null));
                part.setDisposition(attachment.getContentId() != null ? MimeBodyPart.INLINE : MimeBodyPart.ATTACHMENT);
                if (attachment.getContentId() != null) {
                    part.setContentID("<" + attachment.getContentId().replaceAll("^<|>$", "") + ">");
                }
                mixed.addBodyPart(part);
            }
            mime.setContent(mixed);
        }
        mime.saveChanges();
        ByteArrayOutputStream out = new ByteArrayOutputStream();
        mime.writeTo(out);
        return out.toByteArray();
    }

    private static List<InternetAddress> addresses(String list) {
        List<InternetAddress> result = new ArrayList<>();
        if (list == null || list.isBlank()) {
            return result;
        }
        for (String one : list.split("[,;]")) {
            String address = one.trim();
            if (!address.isEmpty()) {
                try {
                    result.add(new InternetAddress(address));
                } catch (MessagingException ignored) {
                    // a broken address is left out, the message is still served
                }
            }
        }
        return result;
    }

    /** MimeMultipart with a given boundary instead of a random one. */
    private static final class FixedBoundaryMultipart extends MimeMultipart {
        FixedBoundaryMultipart(String subtype, String boundary) {
            super(subtype);
            this.contentType = "multipart/" + subtype + "; boundary=\"" + boundary + "\"";
        }
    }
}