Question

我不知道从哪里开始我已经弄乱了我的代码，我仍然得到一个空指针异常。让我相信也许我的联系搞砸了......无论如何，不确定任何帮助都会很酷。

package callassstatement;
    import java.sql.CallableStatement;
    import java.sql.Connection;
    import java.sql.DriverManager;
    import java.sql.PreparedStatement;
    import java.sql.ResultSet;
    import java.sql.SQLException;
    import java.util.Scanner;


    public class CallAssStatement {
    private static Connection conn;


    public static void printOptions() {

    System.out.println("enter 1 to get employee");
    System.out.println("enter 2 to get department");
    System.out.println("enter 3 to exit program");

}

public static String getEmployeeMethod(String id) {
    String abc = null;
    try {

        CallableStatement cs = conn.prepareCall(" { call sp_GetEmployee(1)}");


        cs.setString(1, id);
        //register the OUT parameter before calling the stored procedure
         cs.registerOutParameter(2, java.sql.Types.VARCHAR);
        cs.registerOutParameter(3, java.sql.Types.VARCHAR);
        cs.registerOutParameter(4, java.sql.Types.VARCHAR);
        cs.registerOutParameter(5, java.sql.Types.VARCHAR);

        cs.execute();

        //read the OUT parameter now
        String employeeId = cs.getString(1);
        String lastName = cs.getString(2);
        String firstName = cs.getString(3);
        String departmentId = cs.getString(4);
        String startDate = cs.getString(5);



            abc = ("EmplyeeID: " + employeeId + " " + lastName + "," + firstName + "" + " in " 
                    + departmentId + " Department "+ ", StartDate:"+ startDate);
            return abc;

    } catch (SQLException ex) {
        System.out.println(ex.getMessage());
    }
    return abc;
}

public static void main(String[] args) {

    CandDLoader.createConn();

    printOptions();
    Scanner s = new Scanner(System.in);
    Scanner id = new Scanner(System.in);

    String input = s.nextLine();


    switch (input) {
        case "1":
            System.out.println("calling get employee");
            System.out.println(" Enter employeeID:");
            String ab = id.nextLine();
            getEmployeeMethod(ab);
            break;
        case "2":
            System.out.print("calling get department");
            break;
        case "3":
            System.out.print("exiting");
            System.exit(0);
        default:
            System.out.print("what are you trying to do");
            printOptions();




    }
    }

这是我的连接类。

package callassstatement;

   import java.sql.Connection;
   import java.sql.DriverManager;
   import java.sql.SQLException;


   public class CandDLoader {


   public static Connection createConn(){
Connection conn = null;
try{

Class.forName("com.mysql.jdbc.Driver");
conn = DriverManager.getConnection("jdbc:mysql://localhost/timeclock", "timeclockuser", "password_1234");
    }
    catch(SQLException | ClassNotFoundException ex){}


    return conn;
    }
    }

Answer 1

在createConn() return conn main()，CandDLoader.createConn();，您说

conn

因此，对返回的conn = CandDLoader.createConn();执行任何操作并基本上将其丢弃。

将该行更改为此

private static Connection conn;

或更改

private static Connection conn = CandDLoader.createConn();

到这个

null

声明没有赋值的对象字段时，它们将被指定为private static Connection conn;。

例如

private static Connection conn = null;

与说

完全相同

@echo off
setlocal

set _project=project.vcxproj

call :do_build "%_project%" Release Win32
call :do_build "%_project%" Debug   Win32
call :do_build "%_project%" Release x64
call :do_build "%_project%" Debug   x64

endlocal
exit /b 0

:do_build
setlocal
set _proj=%~1
set _conf=%~2
set _arch=%~3
set _code=0
if "%_arch%"=="Win32" (set _vc_arg=x86) else (set _vc_arg=amd64)
call "%VS100COMNTOOLS%..\..\VC\vcvarsall.bat" %_vc_arg%
msbuild /t:build /p:Configuration="%_conf%" /p:Platform=%_arch% %_proj% || set "_code=1"
endlocal & exit /b %_code%

Answer 2

检查代码库的以下行

conn = DriverManager.getConnection("jdbc:mysql://localhost/timeclock", "timeclockuser", "password_1234");

此处提供的网址不包含端口号，应如下所示：

conn = DriverManager.getConnection("jdbc:mysql://localhost:3306/timeclock", "timeclockuser", "password_1234");

Answer 3

private static Connection conn;类中的

CallAssStatement始终为空，您在CandDLoader.createConn();中创建了一个连接，但实际上并未将其分配给conn中的CallAssStatement变量}。

无法在JDBC中执行MySQL存储过程获取nullpointer异常

3 个答案: