help/en/html/web/JsonServlet.shtml

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      <h1>JMRI JSON Protocol</h1>

      <p>The JMRI JSON Protocol provides access for a number of
      JMRI-defined entities, and is accessible through multiple
      methods:</p>

      <ul>
        <li><strong>The JMRI JSON servlet</strong>, providing both
        HTTP and WebSocket access</li>

        <li><strong>The JMRI JSON server</strong>, providing a
        standard network socket for streamed access</li>

        <li><strong>jquery.jmri</strong>, A jQuery-based JavaScript
        library that chooses the best method to communicate with a
        JMRI server using the JMRI JSON protocol</li>
      </ul>

      <h2>JMRI JSON Servlet</h2>

      <p>The <b>JMRI JSON Servlet</b> provides web access to lists
      and values for numerous JMRI-defined entities, in the modern
      <a href="https://en.wikipedia.org/wiki/JSON">JSON format</a>.
      It also provides a <a href=
      "https://en.wikipedia.org/wiki/WebSocket">WebSocket</a>
      interface for making changes to the state of those entities,
      and listening for external changes.</p>

      <h2>Setting up the JSON Servlet</h2>

      <p>The JMRI JSON Servlet is a feature that is part of the Web
      Server. In order to use this service, you'll need to do the
      following:</p>

      <ul>
        <li>Ensure the web server's port is unique:</li>

        <li style="list-style: none">
          <ul>
            <li>Click <b>Edit</b> and then <b>Preferences</b> to
            open the Preferences dialog</li>

            <li>Click <b>Web Server</b> on the left side</li>

            <li>Ensure the port number is unique (for example
            12080). Other ports are used by WiThrottle, JSON
            Server, etc.</li>

            <li>Click Save</li>
          </ul>
        </li>

        <li>Click <b>Actions</b> and <b>Start Web Server</b> to
        ensure the web server is running</li>

        <li>If you want the web server to run each time you start
        JMRI:</li>

        <li style="list-style: none">
          <ul>
            <li>Open the <b>Preferences</b> dialog</li>

            <li>Select <b>Start Up</b></li>

            <li>Click <b>Add &#9662;</b></li>

            <li>Select <b>Perform action...</b></li>

            <li>Select <b>Start Web Server</b></li>

            <li>Click <b>OK</b></li>
          </ul>
        </li>
      </ul>

      <h2>Using HTTP JSON</h2>

      <p>The <b>HTTP</b> feature of the servlet responds to the
      following URL patterns:</p>

      <ul>
        <li>/json/<i>&lt;listName&gt;</i> -- used with HTTP GET to
        retrieve a list from JMRI, or with PUT to add an item to a
        list</li>

        <li>/json/&lt;<i>typeName</i>&gt;/<i>&lt;itemName&gt;</i>
        -- used with HTTP GET to retrieve a single item from a
        list, or with POST to update an existing item</li>
      </ul>

      <p>There are a number of lists, and not all operations are
      supported on these lists and/or items within each list. Check
      the sections below for the operations that are currently
      supported.</p>

      <h3>List Names</h3>

      <p>Currently the servlet supports the following list names
      (not all of these are actually lists) used for GET operations
      to return lists.</p>

      <table class="data">
        <tr>
          <th>Name</th>

          <th>GET</th>

          <th>PUT</th>

          <th>POST</th>

          <th>Returns:</th>
        </tr>

        <tr>
          <td>blocks</td>

          <td>*</td>

          <td></td>

          <td></td>

          <td>block array</td>
        </tr>

        <tr>
          <td>cars</td>

          <td>*</td>

          <td></td>

          <td></td>

          <td>car array</td>
        </tr>

        <tr><td>configProfiles</td><td>*</td><td></td><td></td><td>configProfile array</td></tr>

        <tr>
          <td>consists</td>

          <td>*</td>

          <td></td>

          <td></td>

          <td>consist array</td>
        </tr>

        <tr>
          <td>engines</td>

          <td>*</td>

          <td></td>

          <td></td>

          <td>engine array</td>
        </tr>

        <tr>
          <td>hello</td>

          <td>*</td>

          <td></td>

          <td></td>

          <td>hello message and power message</td>
        </tr>

        <tr>
          <td>layoutBlocks</td>

          <td>*</td>

          <td></td>

          <td></td>

          <td>layoutBlock array</td>
        </tr>

        <tr>
          <td>lights</td>

          <td>*</td>

          <td></td>

          <td></td>

          <td>light array</td>
        </tr>

        <tr>
          <td>locations</td>

          <td>*</td>

          <td></td>

          <td></td>

          <td>location array</td>
        </tr>

        <tr>
          <td>memories</td>

          <td>*</td>

          <td></td>

          <td></td>

          <td>memory array</td>
        </tr>

        <tr>
          <td>metadata</td>

          <td>*</td>

          <td></td>

          <td></td>

          <td>metadata array</td>
        </tr>

        <tr>
          <td>networkServices</td>

          <td>*</td>

          <td></td>

          <td></td>

          <td>networkService array</td>
        </tr>

        <tr>
          <td>node</td>

          <td>*</td>

          <td></td>

          <td></td>

          <td>node message</td>
        </tr>

        <tr>
          <td>panels</td>

          <td>*</td>

          <td></td>

          <td></td>

          <td>panel array</td>
        </tr>

        <tr>
          <td>power</td>

          <td>*</td>

          <td>*</td>

          <td>*</td>

          <td>n/a</td>
        </tr>

        <tr>
          <td>railroad</td>

          <td>*</td>

          <td></td>

          <td></td>

          <td>railroad message</td>
        </tr>

        <tr>
          <td>reporters</td>

          <td>*</td>

          <td></td>

          <td></td>

          <td>reporter array</td>
        </tr>

        <tr>
          <td>roster</td>

          <td>*</td>

          <td></td>

          <td></td>

          <td>rosterEntry array</td>
        </tr>

        <tr>
          <td>rosterGroups</td>

          <td>*</td>

          <td></td>

          <td></td>

          <td>rosterGroup array</td>
        </tr>

        <tr>
          <td>routes</td>

          <td>*</td>

          <td></td>

          <td></td>

          <td>route array</td>
        </tr>

        <tr>
          <td>signalHeads</td>

          <td>*</td>

          <td></td>

          <td></td>

          <td>signalHead array</td>
        </tr>

        <tr>
          <td>signalMasts</td>

          <td>*</td>

          <td></td>

          <td></td>

          <td>signalMast array</td>
        </tr>

        <tr>
          <td>sensors</td>

          <td>*</td>

          <td></td>

          <td></td>

          <td>sensor array</td>
        </tr>

        <tr>
          <td>systemConnections</td>

          <td>*</td>

          <td></td>

          <td></td>

          <td>systemConnection array</td>
        </tr>

        <tr>
          <td>time</td>

          <td>*</td>

          <td></td>

          <td></td>

          <td>n/a</td>
        </tr>

        <tr>
          <td>trains</td>

          <td>*</td>

          <td></td>

          <td></td>

          <td>train array</td>
        </tr>

        <tr>
          <td>turnouts</td>

          <td>*</td>

          <td></td>

          <td></td>

          <td>turnout array</td>
        </tr>
      </table>

      <p>Note that these names are case sensitive, so you must
      supply them as shown above. As an example, let's say you
      issued the following GET command:</p>
      <pre>
    myjmri:12080/json/power
</pre>

      <p>The would return JSON that looks something like this:</p>
      <pre>
    {"type":"power","data":{"state":2}}
</pre>

      <p>The returned data consists of two parts:</p>

      <ul>
        <li><b>Type:</b> The type of item returned, which in this
        case is "power"</li>

        <li><b>Data:</b> Data that is specific to the type of
        object returned. For example, in this case the state of
        track power</li>
      </ul>

      <p>In order to understand all of the details on the data for
      different types, the best solution is to look at the source
      code. There are two files you can start with that will help a
      lot:</p>

      <ul>
        <li><a href=
        "https://github.com/JMRI/JMRI/blob/master/java/src/jmri/web/servlet/json/JsonServlet.java">
        JsonServlet.java</a> contains the code that handles the
        HTTP and Websocket requests</li>

        <li><a href=
        "https://github.com/JMRI/JMRI/blob/04a935d0c410174fbfa0cc6a2c34a818940b6b37/java/src/jmri/jmris/json/JsonUtil.java">
        JsonUtil.java</a> that is used by JsonServlet to return the
        data for different lists</li>
      </ul>

      <p>Continuing with the power example above, the JsonServlet
      code calls JsonUtil.getPower to get the reply. Looking this
      up in JsonUtil, the result is set to one of three values:
      OFF, ON, or UNKNOWN. These values, in turn, are defined as
      static imports. These names are too generic to be able to
      find easily with Github's search box for searching the
      repository. However, as UNKNOWN is defined like this:</p>
      <pre>
    import static jmri.jmris.json.JSON.UNKNOWN;
</pre>

      <p>it makes sense to browse the source tree for for a file
      called JSON.java under jmri/jmris/json. Sure enough, the file
      exists and contains definitions for these three values:</p>
      <pre>
    public static final int UNKNOWN = 0x00;
    public static final int ON = 0x02;
    public static final int OFF = 0x04;
</pre>

      <p>Therefore, the JSON returned above for power indicates
      that the power is currently on (state is set to 2).</p>

      <p>As you can see, given the number of lists and types, it's
      best to do some searching through the source for all of the
      details, as well as experimenting with a tool like Postman in
      Google's Chrome browser.</p>

      <h3>Type Names</h3>

      <p>Here are the different type names supported for the second
      pattern, along with which HTTP operations are supported:</p>

      <table class="data">
        <tr>
          <th>Name</th>

          <th>GET</th>

          <th>PUT</th>

          <th>POST</th>

          <th>DELETE</th>

          <th>Module</th>
        </tr>

        <tr>
          <td>block</td>

          <td>*</td>

          <td>*</td>

          <td>*</td>

          <td></td>

          <td>PanelPro</td>
        </tr>

        <tr>
          <td>car</td>

          <td>*</td>

          <td></td>

          <td></td>

          <td></td>

          <td>OperationsPro</td>
        </tr>

        <tr>
          <td>consist</td>

          <td>*</td>

          <td>*</td>

          <td>*</td>

          <td>*</td>

          <td>DecoderPro</td>
        </tr>

        <tr>
          <td>engine</td>

          <td>*</td>

          <td></td>

          <td></td>

          <td></td>

          <td>OperationsPro</td>
        </tr>

        <tr>
          <td>layoutBlock</td>

          <td>*</td>

          <td></td>

          <td>*</td>

          <td></td>

          <td>PanelPro</td>
        </tr>

        <tr>
          <td>light</td>

          <td>*</td>

          <td>*</td>

          <td>*</td>

          <td></td>

          <td>PanelPro</td>
        </tr>

        <tr>
          <td>location</td>

          <td>*</td>

          <td></td>

          <td></td>

          <td></td>

          <td>OperationsPro</td>
        </tr>

        <tr>
          <td>memory</td>

          <td>*</td>

          <td>*</td>

          <td>*</td>

          <td></td>

          <td>PanelPro</td>
        </tr>

        <tr>
          <td>metadata</td>

          <td>*</td>

          <td></td>

          <td></td>

          <td></td>

          <td>DecoderPro</td>
        </tr>

        <tr>
          <td>reporter</td>

          <td>*</td>

          <td>*</td>

          <td>*</td>

          <td></td>

          <td>PanelPro</td>
        </tr>

        <tr>
          <td>roster</td>

          <td>*</td>

          <td></td>

          <td></td>

          <td></td>

          <td>DecoderPro</td>
        </tr>

        <tr>
          <td>rosterEntry</td>

          <td>*</td>

          <td></td>

          <td></td>

          <td></td>

          <td>DecoderPro</td>
        </tr>

        <tr>
          <td>rosterGroup</td>

          <td>*</td>

          <td></td>

          <td></td>

          <td></td>

          <td>DecoderPro</td>
        </tr>

        <tr>
          <td>route</td>

          <td>*</td>

          <td></td>

          <td>*</td>

          <td></td>

          <td>PanelPro</td>
        </tr>

        <tr>
          <td>sensor</td>

          <td>*</td>

          <td>*</td>

          <td>*</td>

          <td></td>

          <td>PanelPro</td>
        </tr>

        <tr>
          <td>signalHead</td>

          <td>*</td>

          <td></td>

          <td>*</td>

          <td></td>

          <td>PanelPro</td>
        </tr>

        <tr>
          <td>signalMast</td>

          <td>*</td>

          <td></td>

          <td>*</td>

          <td></td>

          <td>PanelPro</td>
        </tr>

        <tr>
          <td>train</td>

          <td>*</td>

          <td></td>

          <td>*</td>

          <td></td>

          <td>OperationsPro</td>
        </tr>

        <tr>
          <td>turnout</td>

          <td>*</td>

          <td>*</td>

          <td>*</td>

          <td></td>

          <td>PanelPro</td>
        </tr>
      </table>

      <h2>Using HTTP JSON POST/PUT/DELETE</h2>

      <p>You can also insert, update, or delete items in the lists
      above using the HTTP POST, PUT, and DELETE methods. Not all
      combinations are supported.</p>

      <p>You can use these type names to set values. Continuing
      with the power example above, you can turn the power on by
      posting to a URL like this:</p>
      <pre>
    myjmri:12080/json/power
</pre>

      <p>With this as the body</p>
      <pre>
    {"state":2}
</pre>

      <h2>WebSocket</h2>

      <p>The <b>WebSocket</b> feature of this servlet currently
      handles JSON strings in four different forms:</p>

      <ul>
        <li>individual item state requests in the form:
        <code>{"type":"turnout","data":{"name":"LT14"}}</code> that
        are passed to type-specific handlers. These requests will
        initiate "listeners", which will send updated responses
        every time the item's state changes.

          <ul>
            <li>an item's state can be set by adding a
            <strong>state</strong> node to the <em>data</em> node
            in an HTTP POST request:
            <code>{"type":"turnout","data":{"name":"LT14","state":4}}</code></li>

            <li>individual types can be created if the request is
            an HTTP PUT request. Note that not all types support
            this.</li>
          </ul>
        </li>

        <li>list requests in the form:
        <code>{"list":"trains"}</code> or
        <code>{"type":"list","list":"trains"}</code> that return an
        array of items, or an empty array (<code>[]</code>). This request
        initiates a "listener" which will resend
        the entire list when items are added or removed, as well as
        set individual listeners for each item (see above).</li>

        <li>a heartbeat in the form <code>*</code> or
        <code>{"type":"ping"}</code>. The <code>*</code> heartbeat
        gets no response, while the JSON heartbeat causes a
        <code>{"type":"pong"}</code> response.</li>

        <li>a signoff in the form: <code>{"type":"goodbye"}</code>
        to which an identical response is sent before the
        connection gets closed.</li>
      </ul>

      <p>The <b>WebSocket</b> feature of the servlet can be seen by
      browsing to the <b>/json/</b> url, where a <b>JSON
      console</b> is providing for testing.<br>
      Example commands and responses:</p>

      <table id="jsonResponses">
        <tr>
          <th>command</th>

          <th>response</th>

          <th>notes</th>
        </tr>

        <tr>
          <td>
          <code>{"type":"sensor","data":{"name":"IS2"}}</code></td>

          <td>
          <code>{"type":"sensor","data":{"name":"IS2","state":4}}</code></td>

          <td>request current state of sensor IS2</td>
        </tr>

        <tr>
          <td>
          <code>{"type":"sensor","data":{"name":"IS2","state":4}}</code></td>

          <td>
          <code>{"type":"sensor","data":{"name":"IS2","state":4}}</code></td>

          <td>set sensor IS2 to InActive (4)</td>
        </tr>

        <tr>
          <td>
          <code>{"type":"turnout","data":{"name":"IT99","state":0}}</code></td>

          <td>
          <code>{"type":"turnout","data":{"name":"IT99","state":4}}</code></td>

          <td>sending state=0 is also treated as request for
          current state</td>
        </tr>

        <tr>
          <td><code>{"type":"power","data":{}}</code></td>

          <td><code>{"type":"power","data":{"state":2}}</code></td>

          <td>power does not need a name</td>
        </tr>

        <tr>
          <td>
          <code>{"type":"memory","data":{"name":"IMCURRENTTIME"}}</code></td>

          <td>
          <code>{"type":"memory","data":{"name":"IMCURRENTTIME","userName":null,"comment":null,"value":"2:53
          PM"}}</code></td>

          <td>get (fast)clock time, resent each minute</td>
        </tr>

        <tr>
          <td><code>{"type":"ping"}</code></td>

          <td><code>{"type":"pong"}</code></td>

          <td>ping request and response</td>
        </tr>

        <tr>
          <td><code>{"list":"panels"}</code></td>

          <td>
          <code>[{"type":"panel","data":{"name":"ControlPanel/R&amp;R","URL":"/panel/ControlPanel/R&amp;R?format=xml","userName":"R&amp;R","type":"Control
          Panel"}},{"type":"panel","data":{"name":"Layout/Waccamaw%20Coast%20Line","URL":"/panel/Layout/Waccamaw%20Coast%20Line?format=xml","userName":"Waccamaw
          Coast Line","type":"Layout"}}]</code></td>

          <td>request a list of panels, respond with array, example
          shows two panels</td>
        </tr>

        <tr>
          <td>
          <code>{"type":"throttle","data":{"throttle":"CSX754","address":754}}</code></td>

          <td>
          <code>{"type":"throttle","data":{"address":754,"speed":0.0,"forward":true,
          "F0":false,"F1":false,"F2":false,"F3":false,"F4":false,"F5":false,"F6":false,"F7":false,
          "F8":false,"F9":false,"F10":false,"F11":false,"F12":false,"F13":false,"F14":false,
          "F15":false,"F16":false,"F17":false,"F18":false,"F19":false,"F20":false,"F21":false,
          "F22":false,"F23":false,"F24":false,"F25":false,"F26":false,"F27":false,"F28":false,
          "throttle":"CSX754"}}</code></td>

          <td>request throttle for address 754, refer to it as
          "CSX754"</td>
        </tr>

        <tr>
          <td>
          <code>{"type":"throttle","data":{"throttle":"CSX754","speed":0.25}}</code></td>

          <td>
          <code>{"type":"throttle","data":{"speed":0.25,"throttle":"CSX754"}}</code></td>

          <td>request speed of throttle "CSX754" to 25%</td>
        </tr>

        <tr>
          <td>
          <code>{"type":"throttle","data":{"throttle":"CSX754","release":null}}</code></td>

          <td>
          <code>{"type":"throttle","data":{"release":null,"throttle":"CSX754"}}</code></td>

          <td>release the throttle "CSX754"</td>
        </tr>

        <tr>
          <td><code>{"list":"trains"}</code></td>

          <td><em>If list is empty:</em><code>[]</code></td>

          <td>Example showing list response if there are no objects
          of that type.</td>
        </tr>

        <tr>
          <td>
          <code>{"type":"block","data":{"name":"IB1"}}</code></td>

          <td>
          <code>{"type":"block","data":{"name":"IB1","userName":"AUTOBLK:1","comment":null,"value":"ns2608"}}</code></td>

          <td>request current value for block IB1</td>
        </tr>
      </table>

      <h2>JMRI JSON Server</h2>

      <p>The JMRI JSON server is functionally identical to the JSON
      WebSocket, but running over a standard network socket.</p>

      <h2>jquery.jmri</h2>

      <p><em>jquery.jmri</em> is a JavaScript library that ships
      with JMRI and makes the JSON protocol support just work on
      most current browsers.</p>

      <p>Examples of use of the jquery.jmri JavaScript library that
      ships with JMRI include (all links assume you are reading
      this on your JMRI web server):</p>

      <ul>
        <li><a href="/operations">Operations Conductor</a>
        (implemented in <a href=
        "/js/operations.js">/js/operations.js</a></li>

        <li><a href="/panel">Panel</a> (implemented in <a href=
        "/js/panel.js">/js/panel.js</a></li>
        <!-- the webThrottle does not use jquery-json, but instead uses its own implementation, so do not include it in this list
            <li><a href="/web/webThrottle.html">webThrottle</a> (implemented in <a href="/js/webThrottle.js">/js/webThrottle.js</a></li>
            -->

        <li><a href="/json">JSON Console</a> (implemented in
        <a href="/js/json-console.js">/js/json-console.js</a></li>

        <li><a href="/web/power.html">Power Demonstration</a> (view
        the page source for the implementation)</li>
      </ul><!--#include virtual="/Footer" -->
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