How can I receive the file contents in my Vaadin service?

Absonegative :

I want to add an upload button to my web application. In the HTML template, I added:

<link rel="import" href="../../../bower_components/vaadin-upload/src/vaadin-upload.html">

<h2 id="header-Upload">Upload</h2>
<vaadin-upload nodrop/>

I now want to receive the stream in my backend Vaadin process. This stream can be then inserted in the database.

The documentation on https://vaadin.com/components/vaadin-upload/html-examples/upload-basic-demos doesn't provide this information.

I believe I should somehow link a StreamReceiver to the <vaadin-upload> in question, but I am not sure how to do that.

I am using Vaadin Flow (version 12).

Additional information

I tried the following:

In HTML:

<vaadin-upload id="upload" nodrop/>

In Java:

public class MyView extends PolymerTemplate<Model> {
    private final MemoryBuffer buffer = new MemoryBuffer();
    @Id("upload")
    private final Upload       upload = new Upload(buffer);

    public MyView() {
        upload.addSucceededListener(event -> {
            System.out.println(event.getFileName());
            System.out.println(buffer.getInputStream());
        });
    }
}

When I upload a file, I get the following exception:

[qtp162821120-20] ERROR com.vaadin.flow.server.DefaultErrorHandler - 
com.vaadin.flow.server.UploadException: Upload failed
    at com.vaadin.flow.server.communication.StreamReceiverHandler.streamToReceiver(StreamReceiverHandler.java:429)

Caused by: java.lang.IllegalStateException: Upload cannot be performed without a receiver set
    at com.vaadin.flow.component.upload.Upload$DefaultStreamVariable.getOutputStream(Upload.java:581)
    at com.vaadin.flow.server.communication.StreamReceiverHandler.streamToReceiver(StreamReceiverHandler.java:358)

kscherrer :

massive Edit: The actual solution to your problem was that you instantiated the Upload in your java class, when you should let Vaadin do this (because of the polymer template) and refer to this upload component using an identifier both in the html template (<vaadin-upload id="upload" nodrop/>) as well as in the java code (@Id("upload") private Upload upload;). You can now set the receiver of the upload in the java class using upload.setReceiver(buffer);.

public class MyView implements PolymerTemplate<Model> {

    @Id("upload") // com.vaadin.flow.component.polymertemplate
    private Upload upload; // NO INSTANTIATION!
    private private final MemoryBuffer buffer = new MemoryBuffer();

    public MyView (){
        upload.setReceiver(buffer);
        upload.addSucceededListener(event -> {
            System.out.println(event.getFileName());
            System.out.println(buffer.getInputStream());
        });
    }
}

My initial answer was using an unnecessary alternative approach that was not related to your problem (I thought so at first), but helped finding the real solution.