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97 lines
3.6 KiB
Plaintext
97 lines
3.6 KiB
Plaintext
---
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title: Request Handlers
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order: 4
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layout: page
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---
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[[advanced.requesthandler]]
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= Request Handlers
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Request handlers are useful for catching request parameters or generating
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dynamic content, such as HTML, images, PDF, or other content. You can provide
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HTTP content also with stream resources, as described in
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<<../application/application-resources#application.resources.stream,"Stream
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Resources">>. The stream resources, however, are only usable from within a
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Vaadin application, such as in an [classname]#Image# component. Request handlers
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allow responding to HTTP requests made with the application URL, including GET
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or POST parameters. You could also use a separate servlet to generate dynamic
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content, but a request handler is associated with the user session and it can
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easily access data associated with the session and the user.
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To handle requests, you need to implement the [interfacename]#RequestHandler#
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interface. The [methodname]#handleRequest()# method gets the session, request,
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and response objects as parameters.
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If the handler writes a response, it must return [literal]#++true++#. This stops
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running other possible request handlers. Otherwise, it should return
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[literal]#++false++# so that another handler could return a response.
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Eventually, if no other handler writes a response, a UI will be created and
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initialized.
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In the following example, we catch requests for a sub-path in the URL for the
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servlet and write a plain text response. The servlet path consists of the
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context path and the servlet (sub-)path. Any additional path is passed to the
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request handler in the [parameter]#pathInfo# of the request. For example, if the
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full path is [filename]#/myapp/myui/rhexample#, the path info will be
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[filename]#/rhexample#. Also, request parameters are available.
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[source, java]
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----
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// A request handler for generating some content
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VaadinSession.getCurrent().addRequestHandler(
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new RequestHandler() {
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@Override
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public boolean handleRequest(VaadinSession session,
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VaadinRequest request,
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VaadinResponse response)
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throws IOException {
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if ("/rhexample".equals(request.getPathInfo())) {
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// Generate a plain text document
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response.setContentType("text/plain");
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response.getWriter().append(
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"Here's some dynamically generated content.\n");
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response.getWriter().format(Locale.ENGLISH,
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"Time: %Tc\n", new Date());
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// Use shared session data
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response.getWriter().format("Session data: %s\n",
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session.getAttribute("mydata"));
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return true; // We wrote a response
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} else
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return false; // No response was written
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}
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});
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----
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A request handler can be used by embedding it in a page or opening a new page
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with a link or a button. In the following example, we pass some data to the
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handler through a session attribute.
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[source, java]
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----
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// Input some shared data in the session
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TextField dataInput = new TextField("Some data");
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dataInput.addValueChangeListener(event ->
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VaadinSession.getCurrent().setAttribute("mydata",
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event.getProperty().getValue()));
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dataInput.setValue("Here's some");
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// Determine the base path for the servlet
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String servletPath = VaadinServlet.getCurrent()
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.getServletContext().getContextPath()
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+ "/book"; // Servlet
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// Display the page in a pop-up window
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Link open = new Link("Click to Show the Page",
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new ExternalResource(servletPath + "/rhexample"),
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"_blank", 500, 350, BorderStyle.DEFAULT);
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layout.addComponents(dataInput, open);
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----
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