java servlet如何从init（）方法获取值并在方法之外使用它们？

Question

Help please I'm new on servlet I'm trying to initialize value on the init method and use them after but I get nullpointerexception this is my classe Hello it contains 2 methods init() and jdbcinfo() i need to get data base connection once

 package com.Ws;
    //imports..    
    public class Hello extends HttpServlet {

        public static Connection con;


        @Override
        public void init() throws ServletException
        {





                  try {
                    Class.forName("net.sourceforge.jtds.jdbc.Driver");  
                     con =DriverManager.getConnection("jdbc:jtds:sqlserver://localhost:6543/Dbname","user","");


                } catch (ClassNotFoundException e) {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
                    System.out.println("--printStackTrace--"+e);

                } catch (SQLException e) {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
                    System.out.println("--printStackTrace2--"+e);
                }
              }


     }
    //I get nullpointerexception here con = null

        public String jdbcInfo(String req) {

            PreparedStatement statementT;

            try {




    connection =con;

                PreparedStatement statement = connection.prepareStatement(req);
                ResultSet result = statement.executeQuery();

                while (result.next()) {

                    ///

                }
            }

            catch (Exception e) {
                e.printStackTrace();
                System.out.println("exception: Serveur base de donnée indosponnible");

            }



            if (res == "1")
                return res;
            else
                return "false";
        }



        }

my web.xml

 <servlet>
    <servlet-name>Hello</servlet-name>
    <servlet-class>com.Ws.Hello</servlet-class>
    <load-on-startup>1</load-on-startup>

</servlet>

Answer 1

@geert3 is correct. You do not want to create a connection and save it in a field in a servlet's init() method. Servlets are singletonscan handle requests from multiple threads, so all fields should reference objects that are deeply immutable and thread-safe. Connection objects are niether.

Instead, you should use a database connection pool . Don't build your own connection pool code; there are many choices out there. If you are running the code inside an application server, your application server might have built-in database pool support.

As for the particular problem with the code, the best way to trouble-shoot it is to look at the printed stack trace.

Answer 2

//I get nullpointerexception here con = null

Your Instance variable of Class Connection con can be null if it didn't get initialized . Now how can this be possible :

At this line

  Class.forName("net.sourceforge.jtds.jdbc.Driver");  

if jar file is not present in your class Path this line will throw ClassNotFoundException and in that case it comes out of try block without executing this line

  con =DriverManager.getConnection("jdbc:jtds:sqlserver://localhost:6543/Dbname","user","");

and in this case your con will be null So Just check if the jar file is actually present in your class path or not