Passing data between a JSP page and servlet using JSON and JQuery (part 2)

In part 1, I discussed the client side code and gave examples of how to use JSON in a web page using JavaScript.  Part 2 discusses the server side code and gives an example of how to use JSON inside a Java servlet.  I won’t spend any time in review here so if you need a refresher you can re-read part 1.

The following block of code is a stub servlet that contains the doGet() and doPost() methods.  Notice that the doPost() method simply calls the doGet() method.  This is a shortcut for convenience – you can choose to implement each method with unique behaviors if you choose.  This example application submits the form to the server using $.getJSON(), which invokes the GET HTTP request, so the doGet() method is the entry point into the servlet.

package com.windchill101.example;

import java.io.IOException;
import java.io.PrintWriter;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import org.json.simple.JSONObject;
import org.json.simple.JSONValue;

public class ExportServlet extends HttpServlet {
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    }

    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        doGet(request, response);
    }
}

The first thing we want to do inside the doGet() method is to set the content type to text/json in the HttpServletResponse object so the web browser knows how to process the response once it’s returned from the server:

response.setContentType("text/json");

Since the json object is in essence just a String, we can use the PrintWriter from the response object to send the JSON object back to the browser:

 PrintWriter out = response.getWriter();

Now we can start harvesting information from the request object, including the json object and any other parameters we need.  In part 1 we sent two parameters to the server:

1. action:”export”
2. json:jsonData

The first parameter is a simple string and the second parameter is our json object.  Lets pull both of these values from the request object:

String action = request.getParameter("action");
String json = request.getParameter("json");

The action parameter can be used as-is since it’s a simple string.  The json parameter needs to be converted to a JSONObject before we can use it – we do this with the static JSONValue.parse() method:

JSONObject jsonDataObject = (JSONObject) JSONValue.parse(json);

At this point, we can get the variable values by invoking the get() method, passing the variable name as a parameter.  Since the JSONObject can hold objects, we need to cast the value to a String:

String ecoStatus = ((String)jsonData.get("ecoStatus"));
String emailName = ((String)jsonData.get("emailName"));
String exportFormat2D = ((String)jsonData.get("exportFormat2D"));
String exportFormat3D = ((String)jsonData.get("exportFormat3D"));
String exportSupplier = ((String)jsonData.get("exportSupplier"));
String fromDrawingNumber = ((String)jsonData.get("fromDrawingNumber"));
String toDrawingNumber = ((String)jsonData.get("toDrawingNumber"));
String partNumbers = ((String)jsonData.get("partNumbers"));

For the sake of consistency, I’m using the same variable names in this example on both ends of the transaction.  In the example above, the variable ecoStatus is used both within the jsonData object as well as in the servlet and in the JavaScript function within the web page.  On the form however, the form field name is ‘status’.  You can choose to use different names at each step, but I prefer to use consistent names.

Now that the servlet knows the form field values from the web page, it can do some processing of the data and prepare to send a message back to the page.  In this example, I’ll use another Java class named ExportModel to do the actual work.  I’m a strong believer in keeping programs modular when at all possible – it improves the maintainability of code, makes it easier to break the program up for testing, and improves my chances of reusing code for other projectes.   

ExportModel model = new ExportModel();
model.setFromDrawingNumber(fromDrawingNumber);
model.setToDrawingNumber(toDrawingNumber);
model.setEcoStatus(ecoStatus);
model.setExportFormat2D(exportFormat2D);
model.setExportFormat3D(exportFormat3D);
model.setExportSupplier(exportSupplier);
model.setEmailName(emailName);
model.processExport();
String status = model.getStatusMessage();

The ExportModel has a getStatusMessage() method that provides a human readable message that we will display on the web page so the user knows if the operation was successful.

Now we need to create a new JSONObject and add the status message so we can return it to the web page:

JSONObject result = new JSONObject();
result.put("message",status);

Now we need to convert the JSONObject to a string so we can send it back using the PrintWriter handle that we got from the response object.  The JSONObject has a static method that takes a JSONObject instance as a parameter and returns a properly formatted String object.

String jsonResult = JSONObject.toJSONString(result);

Now that we have the properly formatted JSON string, we can simply print it using the PrintWriter object “out”:

out.println(jsonResult);
out.flush();
out.close();

Here’s the complete doGet() method when everything discussed above is assembled together.  Note that I’ve added a try/catch/finally statement to handle any exceptions that may be thrown:

protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    response.setContentType("text/json");
    PrintWriter out = response.getWriter();

    try {
        String action = request.getParameter("action");
        String json = request.getParameter("json");
        JSONObject jsonData = (JSONObject) JSONValue.parse(json);
        String fromDrawingNumber = ((String)jsonData.get("fromDrawingNumber"));
        String toDrawingNumber = ((String)jsonData.get("toDrawingNumber"));
        String ecoStatus = ((String)jsonData.get("ecoStatus"));
        String exportFormat2D = ((String)jsonData.get("exportFormat2D"));
        String exportFormat3D = ((String)jsonData.get("exportFormat3D"));
        String exportSupplier = ((String)jsonData.get("exportSupplier"));
        String partNumbers = ((String)jsonData.get("partNumbers"));
        String emailName = ((String)jsonData.get("emailName"));

        ExportModel model = new ExportModel();
        model.setFromDrawingNumber(fromDrawingNumber);
        model.setToDrawingNumber(toDrawingNumber);
        model.setEcoStatus(ecoStatus);
        model.setExportFormat2D(exportFormat2D);
        model.setExportFormat3D(exportFormat3D);
        model.setExportSupplier(exportSupplier);
        model.setPartNumbers(partNumbers);
        model.setEmailName(emailName);

        String status = "{"message":"Error - action was neither 'copy' or 'export'"}";
        if (action.equals("copy")) {
            model.processSaveAs();
            status = model.getStatusMessage();
        } else if (action.equals("export")) {
            model.processExport();
            status = model.getStatusMessage();
        }

        JSONObject result = new JSONObject();
        result.put("message",status);
        String jsonResult = JSONObject.toJSONString(result);
        out.println(jsonResult);
    } catch (Exception ex) {
        out.println("{"message":"Error - caught exception in ExportServlet"}");
    } finally {
        out.flush();
        out.close();
    }
}

That wraps up this example – I apologize for the length but there are a lot of pieces involved.  Hopefully it’s crystal clear and you will be able to use this concept in your own web application.  Feel free to leave comments or ask questions below.