Java code to convert data: urls in html email to multipart/related mime messages with content id links

ImageReplacer.java

package com.orbitkit.blah.blah;

import com.google.common.base.Preconditions;
import com.google.common.io.BaseEncoding;
import lombok.SneakyThrows;
import lombok.Value;
import lombok.With;

import javax.mail.Session;
import javax.mail.internet.MimeBodyPart;
import javax.mail.internet.MimeMessage;
import javax.mail.internet.MimeMultipart;
import java.util.ArrayList;
import java.util.List;

public class ImageReplacer {

	@Value @With
	public static class Replaced {
		/** With data: urls replaced by cids */
		String message;
		/** The resulting image parts, with unique cids */
		List<ImagePart> images;

		@SneakyThrows
		public MimeMessage toMimeMessage() {
			final MimeMessage mimeMessage = new MimeMessage((Session)null);

			final MimeMultipart multipartRelated = new MimeMultipart("related");
			mimeMessage.setContent(multipartRelated);

			final MimeBodyPart htmlPart = new MimeBodyPart();
			htmlPart.setContent(message, "text/html; charset=utf-8");
			multipartRelated.addBodyPart(htmlPart);

			for (final ImagePart image : images) {
				final byte[] imageBytes = BaseEncoding.base64().decode(image.getBase64());

				final MimeBodyPart imagePart = new MimeBodyPart();
				imagePart.setContentID(image.getCid());
				imagePart.setContent(imageBytes, image.getContentType());

				multipartRelated.addBodyPart(imagePart);
			}

			return mimeMessage;
		}
	}

	@Value
	public static class ImagePart {
		String cid;
		String contentType;
		String base64;
	}

	public static Replaced replace(final String message) {
		final StringBuilder bld = new StringBuilder();
		final List<ImagePart> images = new ArrayList<>();

		int here = 0;

		while (true) {
			final int blockStart = message.indexOf("src=\"data:", here);
			if (blockStart < 0) {
				break;
			}

			final int dataStart = blockStart + "src=\"".length();

			// Everything up to the start of the data url
			bld.append(message, here, dataStart);

			final int dataEnd = message.indexOf("\"", dataStart);
			Preconditions.checkState(dataEnd > dataStart);

			final String dataUrl = message.substring(dataStart, dataEnd);
			final ImagePart part = partFromDataUrl(dataUrl, images.size());
			images.add(part);

			bld.append("cid:").append(part.getCid()).append('"');

			here = dataEnd + 1;
		}

		bld.append(message, here, message.length());

		return new Replaced(bld.toString(), images);
	}

	private static ImagePart partFromDataUrl(final String dataUrl, final int imageIndex) {
		final String[] splitProtocol = dataUrl.split(":");
		Preconditions.checkArgument(splitProtocol.length == 2);

		Preconditions.checkArgument(splitProtocol[0].equals("data"));
		final String startsWithContentType = splitProtocol[1];
		final String[] splitContentType = startsWithContentType.split(";");
		Preconditions.checkArgument(splitContentType.length == 2);

		final String contentType = splitContentType[0];
		final String startsWithBase64 = splitContentType[1];

		final String[] splitBase64 = startsWithBase64.split(",");
		Preconditions.checkArgument(splitBase64.length == 2);
		Preconditions.checkArgument(splitBase64[0].equals("base64"));
		final String base64Data = splitBase64[1];

		return new ImagePart("image" + imageIndex, contentType, base64Data);
	}
}