title: Jackson Unhandled Exception?
body: <p>I am new to android programming, and I was following this <a href="http://hmkcode.com/android-google-cloud-messaging-tutorial/" rel="nofollow">tutorial</a> 
  to create a GCM server program.  However, I came across a frustrating bug and would greatly appreciate any help.</p>
  
  <p><strong>This is my POST2GCM class:</strong></p>
  
  <pre><code>import java.io.BufferedReader;
  import java.io.DataOutputStream;
  import java.io.IOException;
  import java.io.InputStreamReader;
  import java.net.HttpURLConnection;
  import java.net.MalformedURLException;
  import java.net.URL;
  import com.fasterxml.jackson.databind.ObjectMapper;
  
  
  public class POST2GCM extends Content {
  
  private static final long serialVersionUID = 1L;
  
  public static void post(String apiKey, Content content){
  
          try{
  
          // 1. URL
          URL url = new URL("https://android.googleapis.com/gcm/send");
  
          // 2. Open connection
          HttpURLConnection conn = (HttpURLConnection) url.openConnection();
  
          // 3. Specify POST method
          conn.setRequestMethod("POST");
  
          // 4. Set the headers
          conn.setRequestProperty("Content-Type", "application/json");
          conn.setRequestProperty("Authorization", "key="+apiKey);
  
          conn.setDoOutput(true);
  
              // 5. Add JSON data into POST request body
  
              //`5.1 Use Jackson object mapper to convert Contnet object into JSON
              ObjectMapper mapper = new ObjectMapper();
  
              // 5.2 Get connection output stream
              DataOutputStream wr = new DataOutputStream(conn.getOutputStream());
  
              // 5.3 Copy Content "JSON" into
              mapper.writeValue(wr, content);
  
              // 5.4 Send the request
              wr.flush();
  
              // 5.5 close
              wr.close();
  
              // 6. Get the response
              int responseCode = conn.getResponseCode();
              System.out.println("\nSending 'POST' request to URL : " + url);
              System.out.println("Response Code : " + responseCode);
  
              BufferedReader in = new BufferedReader(
                      new InputStreamReader(conn.getInputStream()));
              String inputLine;
              StringBuffer response = new StringBuffer();
  
              while ((inputLine = in.readLine()) != null) {
                  response.append(inputLine);
              }
              in.close();
  
              // 7. Print result
              System.out.println(response.toString());
  
              } catch (MalformedURLException e) {
                  e.printStackTrace();
              } catch (IOException e) {
                  e.printStackTrace();
              }
          }
  }
  </code></pre>
  
  <p>I have included the "jackson-databind-2.5.1.jar" file but I get the error:</p>
  
  <blockquote>
    <p>Unhandled Exception: com.fasterxml.jackson.databind.JsonMappingException</p>
  </blockquote>
  
  <p>on the line <code>mapper.writeValue(wr, content);</code></p>
  
  <p><strong>What is causing this exception, and how can I fix it?</strong></p>