HTTP 500 The Website Cannot display the page

Question

This is the Java Code To receive the Data from C Server using Sockets and store the data in Google Data Store and Deploy the app on Google app Engine.

package pack.exp;

@SuppressWarnings("serial")
public class CToJavaToCloudServlet extends HttpServlet 
{
static List<String> obj1; 

public static void main(String[] args) throws UnknownHostException, IOException 
{
        Socket s = new Socket("10.9.11.55", 6870);
    InputStream stream = s.getInputStream();
    byte[] data = new byte[13];
    int read;
    String can_Id= null;


    while((read = stream.read(data)) != -1)
    {
        can_Id= String.format("%02X%02X%02X%02X", data[0], data[1], data[2],    
            data[3]);

    String can_Data= String.format("%02X:%02X:%02X:%02X:%02X:%02X:%02X:%02X", 
            data[4], data[5], data[6], data[7], data[8], data[9], data[10], data[11]);

    obj1= new ArrayList<String>();
    obj1.add(can_Data.substring(0, 2));
    obj1.add(can_Data.substring(3, 5));
    obj1.add(can_Data.substring(6, 8));
    obj1.add(can_Data.substring(9, 11));
    obj1.add(can_Data.substring(12, 14));
    obj1.add(can_Data.substring(15, 17));
    obj1.add(can_Data.substring(18, 20));
    obj1.add(can_Data.substring(21, 23));
    }
}

public void doGet(HttpServletRequest req, HttpServletResponse resp) throws 
           IOException 
{
    Key k1 = KeyFactory.createKey("C","0C F0 0400"); 

    String statusEEC1= obj1.get(0);
    String driversDemandEnginePercentTorque= obj1.get(1);
    String actualEnginePercentTorque= obj1.get(2);
    String engineSpeed= obj1.get(3) + obj1.get(4);
    String sourceAddressOfControllingDeviceForEngineControl= obj1.get(5);
    String engineStarterMode= obj1.get(6);
    String engineDemandPercentTorque= obj1.get(7); 

    Entity can1 = new Entity(k1);

    can1.setProperty("Status EEC", statusEEC1);     

            can1.setProperty("Drivers Demand Engine Percent Torque", 
            driversDemandEnginePercentTorque); 

            can1.setProperty("Actual Engine Percent Torque", 
            actualEnginePercentTorque);

            can1.setProperty("Engine Speed", engineSpeed);

            can1.setProperty("Source Address Of Controlling Device For Engine Control", 
            sourceAddressOfControllingDeviceForEngineControl);

            can1.setProperty("Engine Starter Mode", engineStarterMode);

            can1.setProperty("Engine Demand Percent Torque", 
            engineDemandPercntTorque);


    DatastoreService datastore = DatastoreServiceFactory.getDatastoreService();
    datastore.put(can1);

    Entity can11 = null;

    try 
    {
        can11= datastore.get(k1);
    }

    catch (EntityNotFoundException e) 
    {
        e.printStackTrace();
    }


    String statusEEC11= (String) can11.getProperty("Status EEC");

            String driversDemandEnginePercentTorque1= (String) 
            can11.getProperty("Drivers Demand Engine Percent Torque");

            String actualEnginePercentTorque1= (String) can11.getProperty("Actual 
            Engine Percent Torque");

            String engineSpeed1= (String) can11.getProperty("Engine Speed");

            String sourceAddressOfControllingDeviceForEngineControl1= (String)  
            can11.getProperty("Source Address Of Controlling Device For Engine  
            Control");
    String engineStarterMode1= (String) can11.getProperty("Engine Starter 
            Mode");
    String engineDemandPercentTorque1= (String) can11.getProperty("Engine 
            Demand Percent Torque");


    resp.setContentType("text/plain");
    resp.getWriter().println("Can id 0C F0 0400------>");
    resp.getWriter().println("");
    resp.getWriter().println("Status EEC--- " + statusEEC11);  
    resp.getWriter().println("Drivers Demand Engine Percent Torque---" +  
            driversDemandEnginePercentTorque1);  
    resp.getWriter().println("Actual Engine Percent Torque---" + 
            actualEnginePercentTorque1);  
    resp.getWriter().println("Engine Speed---" + engineSpeed);  
    resp.getWriter().println("Source Address Of Controlling Device For Engine 
            Control---" + sourceAddressOfControllingDeviceForEngineControl1);  
    resp.getWriter().println("Engine Starter Mode---" + engineStarterMode1); 
    resp.getWriter().println("Engine Demand Percent Torque---" +                 
            engineDemandPercentTorque1); 
    resp.getWriter().println("");
    resp.getWriter().println("");
    resp.getWriter().println("");
    resp.getWriter().println("");
   }
}

On deploying the app to google cloud engine I am getting the error. The website cannot display the page HTTP 500 Most likely causes: •The website is under maintenance. •The website has a programming error.

Can anybody help me out with this problem.

Answer 1

The javax.servlet and javax.servlet.http packages provide interfaces and classes for writing servlets. All servlets must implement the Servlet interface, which defines lifecycle methods. When implementing a generic service, you can use or extend the GenericServlet class provided with the Java Servlet API. The HttpServlet class provides methods, such as doGet and doPost, for handling HTTP-specific services.

Since you are implementing HttpServlet interface, you have to implement doGet and/or doPost.

Servlet init methods allows a servlet to perform one-time initialization prior to servicing requests. One common mistake when implementing init method is in this form:

So your main method is not called at all. Move the code from main into init. Another problem with your code is that you are opening a socket expecting data. This should be done in doGet and/or doPost methods since they are responsible for receiving data.