java 连接数据库出现问题

Question

连接数据库建表中出现如下报错，学生刚入门，求大神帮忙看一下原因
Exception in thread "main" java.lang.NullPointerException
at com.mysql.jdbc.Statement.executeUpdate(Statement.java:1214)
at com.mysql.jdbc.Statement.executeUpdate(Statement.java:1205)
at Week4.One.main(One.java:29)

代码如下

 package Week4;
import java.sql.*;
import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.SQLException;


public class One {
    public static void main(String[] args) throws Exception
    {
        Class.forName("com.mysql.jdbc.Driver");


        String url = "jdbc:mysql://127.0.0.1:3306/hello";   
        Connection conn = DriverManager.getConnection(url, "root", "");
        Statement stat = conn.createStatement();





        stat.close();
        conn.close();

        stat.executeUpdate("create table oe_class("
                + "clid int not null,"
                + "coid int not null,"
                + "seeid int not null,"
                + "uname int not null,"
                + "sesid int not null,"
                +"clweek int not null,"
                +"primary key (clid);"
                + ")");

        stat.executeUpdate("create table oe_course("
                + "coid int not null,"
                + "coname varchar(20) not null,"

                +"primary key (coid);"
                + ")");

Answer 1

简单点，就是将stat.close(); conn.close();放到最后。所有 stat.executeUpdate（）后面。primary key (clid);后面分号删掉。

Answer 2

执行 executeUpdate 方法要加 try/catch

stat.close()
conn.close() 关闭连接放到finally

Answer 3

添加
try{
Class.forName("com.mysql.jdbc.Driver");
.....
stat.executeUpdate("create table oe_course("
+ "coid int not null,"
+ "coname varchar(20) not null,"

            +"primary key (coid);"
            + ")");

}catch (){
......
}finally{
吧stat.close() 和conn.close() 放到finally 中
}

Answer 4

异常已经报出来了，是空指针异常，意思就是DriverManager并未成功获取Connection对象，所以conn 是null，
后面 的conn.createStatement()语句就会报nullpointer异常，
1. 添加数据库密码，DriverManager.getConnection(url, "root", "")
2. 检查驱动是否添加到类路径

Answer 5

一般的，finally块中用于释放资源，断开连接，尽量使用try ··· catch····· finally语句块：
try为你要进行的操作，一般的如过出现错误需要用catch进行捕获，最后不管成果功与否都需要将资源释放。
stat.close();
conn.close();
你过早的关闭了连接，造成了空指针异常，站讯不到对应的对象。
可以参考：
try{

    Class.forName("com.mysql.jdbc.Driver");


    String url = "jdbc:mysql://127.0.0.1:3306/hello";   
    Connection conn = DriverManager.getConnection(url, "root", "");
    Statement stat = conn.createStatement();
            ····························
            }catch(Exception e){
            e.printStackTrace();
            }fianlly{
                  stat.close();
                        conn.close();
            }

Answer 6

空指针异常啊，楼上已经说的很好了

Answer 7

把

 stat.close();
 conn.close();

这两行放到最后。