Question

I want to convert pdfs to image files within appengine. Ideally I would upload the pdf as a blob and store both the pdf and an image of the pdf. The conversion could also be done at a different time (taskqueue).

I have not found any working samples or good documentation of doing this.
The official documentation is here. Here is my implementation on my upload servlet.

@SuppressWarnings("serial")
public class UploadBlobServlet extends HttpServlet {

  private static final Logger log = Logger.getLogger(UploadBlobServlet.class.getName());

public void doPost(HttpServletRequest req, HttpServletResponse resp)
        throws IOException {

    BlobstoreService blobstoreService = BlobstoreServiceFactory.getBlobstoreService();
    Map<String, BlobKey> blobs = blobstoreService.getUploadedBlobs(req);
    BlobKey blobKey = blobs.get("data");
    log.log(Level.WARNING,"blobKey: "+blobKey.getKeyString());



        if (blobKey != null) {
        resp.getWriter().println(blobKey.getKeyString());

        BlobstoreInputStream in=new BlobstoreInputStream(blobKey);
        byte[] b = IOUtils.toByteArray(is);
      //  try{
            in.read(b);
            Asset asset = new Asset(
                    "application/pdf", b, "testfile.pdf");
                Document document = new Document(asset);
                Conversion conversion = new Conversion(document, "image/png");

                ConversionService service =
                    ConversionServiceFactory.getConversionService();
                ConversionResult result = service.convert(conversion);

                if (result.success()) {
                  // Note: in most cases, we will return data all in one asset,
                  // except that we return multiple assets for multi-page images.
                FileService fileService=FileServiceFactory.getFileService(); 
                for (Asset ass : result.getOutputDoc().getAssets()) {
                    AppEngineFile file=fileService.createNewBlobFile("image/png", "testfile.png");
                    FileWriteChannel writeChannel=fileService.openWriteChannel(file, false);
                    writeChannel.write(ByteBuffer.wrap(b));
                    writeChannel.closeFinally();
                  }
                } else {
                    log.log(Level.WARNING,"error");

                }

Update: Have added byte[]=IOUtils.toByteArray(is); and still getting a NPE…

I am also curious as to the quality of the conversion if anyone has experience.

Answer 1

Here is the working code to receive an upload pdf and convert it to a png using the Conversion api. The upload is completed with a multi-part post to an upload url must be obtained through:

     String url=blobstoreService.createUploadUrl("/upload");

Just place this code in a servlet and map it to “upload” in your web.xml.

The Conversion is good quality, however I did notice just a little blurriness around text. In my case the png was about 25% larger.

public void doPost(HttpServletRequest req, HttpServletResponse resp)
        throws IOException {

    BlobstoreService blobstoreService = BlobstoreServiceFactory.getBlobstoreService();
    Map<String, BlobKey> blobs = blobstoreService.getUploadedBlobs(req);
    BlobKey blobKey = blobs.get("data");



        if (blobKey != null) {
        resp.getWriter().println(blobKey.getKeyString());
        BlobstoreInputStream in=new BlobstoreInputStream(blobKey);

        byte[] b = IOUtils.toByteArray(in);
        if(b!=null){
            log.log(Level.WARNING,"blobsize: "+b.length);
        }else{
            log.log(Level.WARNING,"b is null");

        }
            in.read(b);
            Asset asset = new Asset(
                    "application/pdf", b, "testfile.pdf");
                Document document = new Document(asset);
                Conversion conversion = new Conversion(document, "image/png");

                ConversionService service =
                    ConversionServiceFactory.getConversionService();
                ConversionResult result = service.convert(conversion);

                if (result.success()) {
                  // Note: in most cases, we will return data all in one asset,
                  // except that we return multiple assets for multi-page images.
                FileService fileService=FileServiceFactory.getFileService(); 
                for (Asset ass : result.getOutputDoc().getAssets()) {
                    AppEngineFile file=fileService.createNewBlobFile("image/png", "test3file.png");
                    FileWriteChannel writeChannel=fileService.openWriteChannel(file, true);
                    writeChannel.write(ByteBuffer.wrap(ass.getData()));

                    writeChannel.closeFinally();
                  }
                } else {
                    log.log(Level.WARNING,"error");

                }