使用App Engine Java Servlet动态创建和存储CSV文件

Question

I have a java web application which graphs data using amCharts. The data I want to graph is stored in a SQL database. I have a servlet that queries the database for the data I would like to graph and formats it into a comma delimited csv format. The part I am having an issues with is storing this formatted data into a file after it is created, and then being able to access this file later (through the real path) so I can update the graph.

This is what I have so far for the servlet:

public class CreateCsvServlet extends HttpServlet{

public void doGet(HttpServletRequest req, HttpServletResponse response)  throws IOException, ServletException{
    String macAdd = (String) req.getSession().getAttribute("macAddress");

    String path =  getServletContext().getRealPath("/powerData.txt");   //the file "powerData.txt" is an empty file that was deployed with the web app

    final String DB_URL="jdbc:google:mysql://mydatabase:iot?user=root";
    Statement stmt = null;
    Connection conn = null;            

      try{
         // Register JDBC driver
         Class.forName("com.mysql.jdbc.GoogleDriver");

         // Open a connection
         conn = DriverManager.getConnection(DB_URL);

         // Execute SQL query
         stmt = conn.createStatement();

         String sql = "SELECT `wattSeconds`,`time` FROM `iot_data`.`Usage Data` WHERE `MACAddress`='" + macAdd +"'";

         rs = stmt.executeQuery(sql);

         PrintWriter csvWriter = new PrintWriter(new File(path));

         // Extract data from result set
         while(rs.next()){
            //Retrieve by column name
            String watt  = rs.getString("wattSeconds");
            String time = rs.getString("time");
            String row = watt + "," + time;

            csvWriter.println(row) ;
         } 

         csvWriter.close(); 

         // Clean-up environment
         rs.close();
         stmt.close();
         conn.close();
      }catch(SQLException se){
         //Handle errors for JDBC
         se.printStackTrace();
      }

I am trying to use a PrintWriter to write to the file, but I am not sure if this is the right approach. Also, I am giving the path for a file that already exists to the new File(path) and I feel like this is incorrect. Want I want to actually happen is to check if the file exists, and if it does it should overwrite the file with this new content.

Answer 1

You cannot write to the files on App Engine. You have two options:

1. Store this file as a long String object in the datastore (it must be under 1MB).

2. Write it to a file in Google Cloud Storage.