--- title: Request Handlers order: 4 layout: page --- [[advanced.requesthandler]] = Request Handlers Request handlers are useful for catching request parameters or generating dynamic content, such as HTML, images, PDF, or other content. You can provide HTTP content also with stream resources, as described in <>. The stream resources, however, are only usable from within a Vaadin application, such as in an [classname]#Image# component. Request handlers allow responding to HTTP requests made with the application URL, including GET or POST parameters. You could also use a separate servlet to generate dynamic content, but a request handler is associated with the user session and it can easily access data associated with the session and the user. To handle requests, you need to implement the [interfacename]#RequestHandler# interface. The [methodname]#handleRequest()# method gets the session, request, and response objects as parameters. If the handler writes a response, it must return [literal]#++true++#. This stops running other possible request handlers. Otherwise, it should return [literal]#++false++# so that another handler could return a response. Eventually, if no other handler writes a response, a UI will be created and initialized. In the following example, we catch requests for a sub-path in the URL for the servlet and write a plain text response. The servlet path consists of the context path and the servlet (sub-)path. Any additional path is passed to the request handler in the [parameter]#pathInfo# of the request. For example, if the full path is [filename]#/myapp/myui/rhexample#, the path info will be [filename]#/rhexample#. Also, request parameters are available. [source, java] ---- // A request handler for generating some content VaadinSession.getCurrent().addRequestHandler( new RequestHandler() { @Override public boolean handleRequest(VaadinSession session, VaadinRequest request, VaadinResponse response) throws IOException { if ("/rhexample".equals(request.getPathInfo())) { // Generate a plain text document response.setContentType("text/plain"); response.getWriter().append( "Here's some dynamically generated content.\n"); response.getWriter().format(Locale.ENGLISH, "Time: %Tc\n", new Date()); // Use shared session data response.getWriter().format("Session data: %s\n", session.getAttribute("mydata")); return true; // We wrote a response } else return false; // No response was written } }); ---- A request handler can be used by embedding it in a page or opening a new page with a link or a button. In the following example, we pass some data to the handler through a session attribute. [source, java] ---- // Input some shared data in the session TextField dataInput = new TextField("Some data"); dataInput.addValueChangeListener(event -> VaadinSession.getCurrent().setAttribute("mydata", event.getProperty().getValue())); dataInput.setValue("Here's some"); // Determine the base path for the servlet String servletPath = VaadinServlet.getCurrent() .getServletContext().getContextPath() + "/book"; // Servlet // Display the page in a pop-up window Link open = new Link("Click to Show the Page", new ExternalResource(servletPath + "/rhexample"), "_blank", 500, 350, BorderStyle.DEFAULT); layout.addComponents(dataInput, open); ---- 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223 224 225 226 227 228 229 230 231 232 233 234 235 236 237 238 239 240 241 242 243 244 245 246 247 248 249 250 251 252 253 254 255 256 257 258 259 260 261 262 263 264 265 266 267 268 269 270 271 272 273 274 275 276 277 278 279 280 281 282 283 284 285 286 287