Vaadin Resource for FileDownload which consumes OutputStream

DownloadStreamWriter.java

import java.io.IOException;
import java.io.OutputStream;
import java.util.Iterator;
import java.util.function.Consumer;

import javax.servlet.http.HttpServletResponse;

import com.vaadin.server.Constants;
import com.vaadin.server.DownloadStream;
import com.vaadin.server.VaadinRequest;
import com.vaadin.server.VaadinResponse;

@SuppressWarnings("serial")
public class DownloadStreamWriter extends DownloadStream {

    private final Consumer<OutputStream> streamWriter;

    public DownloadStreamWriter(Consumer<OutputStream> streamWriter, String contentType, String fileName) {
        super(null, contentType, fileName);
        this.streamWriter = streamWriter;
    }

    @Override
    public void writeResponse(VaadinRequest request, VaadinResponse response) throws IOException {
        if (this.getParameter("Location") != null) {
            response.setStatus(HttpServletResponse.SC_MOVED_TEMPORARILY);
            response.setHeader("Location", this.getParameter("Location"));
            return;
        }

        OutputStream out = null;
        try {
            // Sets content type
            response.setContentType(this.getContentType());

            // Sets cache headers
            response.setCacheTime(this.getCacheTime());

            // Copy download stream parameters directly
            // to HTTP headers.
            final Iterator<String> i = this.getParameterNames();
            if (i != null) {
                while (i.hasNext()) {
                    final String param = i.next();
                    response.setHeader(param, this.getParameter(param));
                }
            }

            // Content-Disposition: attachment generally forces download
            String contentDisposition = this.getParameter(CONTENT_DISPOSITION);
            if (contentDisposition == null) {
                contentDisposition = getContentDispositionFilename(this.getFileName());
            }

            response.setHeader(CONTENT_DISPOSITION, contentDisposition);

            int bufferSize = this.getBufferSize();
            if (bufferSize <= 0 || bufferSize > Constants.MAX_BUFFER_SIZE) {
                bufferSize = Constants.DEFAULT_BUFFER_SIZE;
            }

            out = response.getOutputStream();

            this.streamWriter.accept(out);

        } finally {
            tryToCloseStream(out);
        }
    }

    /**
     * Helper method that tries to close an output stream and ignores any exceptions.
     *
     * @param out the output stream to close, <code>null</code> is also supported
     */
    static void tryToCloseStream(OutputStream out) {
        try {
            // try to close output stream (e.g. file handle)
            if (out != null) {
                out.close();
            }
        } catch (IOException e1) {
            // NOP
        }
    }

}

StreamWriterResource.java

import com.vaadin.server.ConnectorResource;
import com.vaadin.server.DownloadStream;
import com.vaadin.util.FileTypeResolver;

@SuppressWarnings("serial")
public class StreamWriterResource implements ConnectorResource {

    private final Consumer<OutputStream> streamWriter;
    private final String filename;
    private final String mimeType;

    public StreamWriterResource(Consumer<OutputStream> streamWriter, String filename) {
        this.streamWriter = streamWriter;
        this.filename = filename;
        this.mimeType = FileTypeResolver.getMIMEType(filename);
    }

    @Override
    public String getMIMEType() {
        return this.mimeType;
    }

    @Override
    public DownloadStream getStream() {
        return new DownloadStreamWriter(this.streamWriter, this.mimeType, this.filename);
    }

    @Override
    public String getFilename() {
        return this.filename;
    }

}