I had a class: MockServerHandler I use in my itests using Jetty 9 which I had to migrate to Jetty 12.
Before the migration (Jetty 9):

public class MockServerHandler extends DefaultHandler {
    @Override
    public void handle(String target, Request baseRequest, HttpServletRequest request, HttpServletResponse response)
            throws IOException, ServletException {
        try {
            log.debug("Got request for: {} request type {} ", target, request.getMethod());
            handleDownload(request, response);
        } catch (Exception e) {
            log.error("Mock handler caught and exception", e);
        }
    }

    private void handleDownload(HttpServletRequest request, HttpServletResponse response) {
        byte[] byteBuffer = getContentToReturn();
        long lastModified = getLastModified();
        String contentType = getContentType();
        int returnCode = getReturnCode();

        response.setStatus(returnCode);
        if (lastModified > 0L) {
            response.setDateHeader(HttpHeader.LAST_MODIFIED.asString(), lastModified);
        }
        response.setContentType(contentType);        
        response.setContentLength(byteBuffer.length);

        OutputStream outputStream = response.getOutputStream();
        outputStream.write(byteBuffer, 0, byteBuffer.length);
        response.flushBuffer();
        outputStream.close();
    }
}

After bumping to Jetty 12:

public class MockServerHandler extends DefaultHandler {
    @Override
    public boolean handle(Request request, Response response, Callback callback)
            throws IOException, ServletException {
        try {
            String target = request.getHttpURI().getPath();
            log.debug("Got request for: {} request type {} ", target, request.getMethod());
            handleDownload(request, response);
            callback.succeeded();
        } catch (Exception e) {
            callback.failed(e);
            response.setStatus(HttpStatus.SC_INTERNAL_SERVER_ERROR);
            log.error("Mock handler caught and exception", e);
        }
        return true;
    }

    private void handleDownload(Request request, Response response) {
        byte[] byteBuffer = getContentToReturn();
        long lastModified = getLastModified();
        String contentType = getContentType();
        int returnCode = getReturnCode();

        response.setStatus(returnCode);
        if (lastModified > 0L) {
            response.getHeaders().addDateField(HttpHeader.LAST_MODIFIED.asString(), lastModified);
        }
        response.getHeaders().add(HttpHeader.CONTENT_TYPE, contentType);

        OutputStream outputStream = Content.Sink.asOutputStream(response);
        outputStream.write(byteBuffer, 0, byteBuffer.length);
        outputStream.flush();
        outputStream.close();
    }
}

After these changes my integration tests started to fail. Some due to read timeout, others expect UTF-8 encoded response body and suddenly fail complaining the first byte isn’t compliant, and more.
I’m clearly missing something, as I tried to make the least amount of changes possible to retain behavior.

I noticed DefaultHandler passes super(InvocationType.NON_BLOCKING); which I did not expect (it wasn’t non-blocking on Jetty 9, right?). I tried extending Handler.Abstract directly but that didn’t change anything.

Am I violating the new Jetty contract somehow? I’ve read the docs carefully but couldn’t see anything I am doing blatantly wrong.
Any advice to understand how it works and what I’m missing is very much appreciated.