Setting up a server with HttpServer

What do you write instead of the underscores to get the value of the parameter from the request URI?

public Headers handleGetRequest(HttpExchange exchange) throws IOException {
    String response = __________.toString().split("=")[1];

    exchange.sendResponseHeaders(200, response.getBytes().length);
    exchange.getResponseHeaders().set("Access-Control-Allow-Origin", "*");

    try (OutputStream stream = exchange.getResponseBody()) {
        stream.write(response.getBytes());
    }

    return exchange.getResponseHeaders();
}