public Class ServletTester

ServletTester is not best practice and may be deprecated and eventually removed in future Jetty versions.

ServletTester is a just a wrapper around a ServletContextHandler, with a LocalConnector to accept HTTP/1.1 requests, so there is no value that this class adds to already existing classes.

Replace its usages with:

Server server = new Server();
LocalConnector connector = new LocalConnector(server);
server.addConnector(connector);
ServletContextHandler context = new ServletContextHandler(server, "/contextPath");
// Configure the context here.
server.start();

You can configure the context by adding Servlets and Filters, attributes, etc. even after it has been started. Use HttpTester and LocalConnector to make HTTP/1.1 requests, in this way:

// Generate the request.
HttpTester.Request request = HttpTester.newRequest();
request.setMethod("GET");
request.setURI("/contextPath/servletPath");
request.put(HttpHeader.HOST, "localhost");
ByteBuffer requestBuffer = request.generate();

// Send the request buffer and get the response buffer.
ByteBuffer responseBuffer = connector.getResponse(requestBuffer);

// Parse the response buffer.
HttpTester.Response response = HttpTester.parseResponse(responseBuffer);
assert response.getStatus() == HttpStatus.OK_200;

Alternatively, you can use raw strings for requests and responses, but you must be sure the request strings are in the correct HTTP/1.1 format:

String rawRequest = "" +
        "GET /contextPath/servletPath HTTP/1.1\r\n" +
        "Host: localhost\r\n" +
        "\r\n";
String rawResponse = connector.getResponse(rawRequest);
HttpTester.Response response = HttpTester.parseResponse(rawResponse);