Retrieve email/message body in html using Gmail API

Both FULL and RAW will return you any text/html parts depending on how you'd like it. If you use FULL you'll get a parsed representation which will be nested json dictionaries that you'll have to walk over looking for the text/html part. If you opt for the RAW format you'll get the entire email in RFC822 format in the Message.raw field. You can pass that to the mime libraries in your chosen language and then use that to find the part you're interested in. Mime is complicated, you'll likely have a top-level "multipart" type with text/html as one of the direct children of it but no guarantees, it's an arbitrarily deep tree structure! :)

Email messages that have both HTML and plain text content will have multiple payload parts, and the part with the mimeType "text/html" will contains the HTML content. You can find it with logic like:

var part = message.parts.filter(function(part) {
  return part.mimeType == 'text/html';
});
var html = urlSafeBase64Decode(part.body.data);

Here is the full tutorial:

1- Assuming you already went through all credentials creation here

2- This is how you retrieve a Mime Message:

 public static String getMimeMessage(String messageId)
            throws Exception {

           //getService definition in -3
        Message message = getService().users().messages().get("me", messageId).setFormat("raw").execute();

        Base64 base64Url = new Base64(true);
        byte[] emailBytes = base64Url.decodeBase64(message.getRaw());

        Properties props = new Properties();
        Session session = Session.getDefaultInstance(props, null);

        MimeMessage email = new MimeMessage(session, new ByteArrayInputStream(emailBytes));

        return getText(email); //getText definition in at -4
    }

3- This is the piece that creates the Gmail instance:

private static Gmail getService() throws Exception {
    final NetHttpTransport HTTP_TRANSPORT = GoogleNetHttpTransport.newTrustedTransport();
    // Load client secrets.
    InputStream in = SCFManager.class.getResourceAsStream(CREDENTIALS_FILE_PATH);
    if (in == null) {
        throw new FileNotFoundException("Resource not found: " + CREDENTIALS_FILE_PATH);
    }
    GoogleClientSecrets clientSecrets = GoogleClientSecrets.load(JSON_FACTORY, new InputStreamReader(in));

    // Build flow and trigger user authorization request.
    GoogleAuthorizationCodeFlow flow = new GoogleAuthorizationCodeFlow.Builder(
            HTTP_TRANSPORT, JSON_FACTORY, clientSecrets, SCOPES)
            .setDataStoreFactory(new FileDataStoreFactory(new java.io.File(TOKENS_DIRECTORY_PATH)))
            .setAccessType("offline")
            .build();
    LocalServerReceiver receiver = new LocalServerReceiver.Builder().setPort(8888).build();
    Credential credential = new AuthorizationCodeInstalledApp(flow, receiver).authorize("user");

    return new Gmail.Builder(HTTP_TRANSPORT, JSON_FACTORY, credential)
            .setApplicationName(APPLICATION_NAME)
            .build();
}

4- And this is how you parse Mime Messages:

 public static String getText(Part p) throws
            MessagingException, IOException {
        if (p.isMimeType("text/*")) {
            String s = (String) p.getContent(); 
            return s;
        }

        if (p.isMimeType("multipart/alternative")) {
            // prefer html text over plain text
            Multipart mp = (Multipart) p.getContent();
            String text = null;
            for (int i = 0; i < mp.getCount(); i++) {
                Part bp = mp.getBodyPart(i);
                if (bp.isMimeType("text/plain")) {
                    if (text == null) {
                        text = getText(bp);
                    }
                    continue;
                } else if (bp.isMimeType("text/html")) {
                    String s = getText(bp);
                    if (s != null) {
                        return s;
                    }
                } else {
                    return getText(bp);
                }
            }
            return text;
        } else if (p.isMimeType("multipart/*")) {
            Multipart mp = (Multipart) p.getContent();
            for (int i = 0; i < mp.getCount(); i++) {
                String s = getText(mp.getBodyPart(i));
                if (s != null) {
                    return s;
                }
            }
        }

        return null;
    }

5- If you were wondering how to get the email id, this is how you list them:

 public static List<String> listTodayMessageIds() throws Exception {
        ListMessagesResponse response = getService()
                .users()
                .messages()
                .list("me") 
                .execute();  

        if (response != null && response.getMessages() != null && !response.getMessages().isEmpty()) {
            return response.getMessages().stream().map(Message::getId).collect(Collectors.toList());
        } else {
            return null;
        }
    }

Note:

If after this you want to query that html body in the "kind of Java Script way", I recommend you to explore jsoup library.. very intuitive and easy to work with:

Document jsoup = Jsoup.parse(body);

Elements tds = jsoup.getElementsByTag("td");
Elements ps = tds.get(0).getElementsByTag("p");

I hope this helps :-)