mysql创建statement对象
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package com.yxq.toolsbean;
import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.sql.Statement;
public class DB
private final String url = "jdbc:mysql://localhost:3306/mysql";
private final String userName = "root";
private final String password = "";
private Connection conn = null;
private Statement stm=null;
/* 通过构造方法加载数据库驱动 */
public DB()
try
Class.forName("com.mysql.jdbc.Driver");
System.out.println("加载数据库成功！");
catch (Exception e)
e.printStackTrace();
System.out.println("加载数据库驱动失败！");
/* 创建数据库连接 */
public void createCon()
try
Connection conn = DriverManager.getConnection(url, userName, password);
System.out.println("获取数据库连接成功！");
catch (Exception e)
e.printStackTrace();
System.out.println("获取数据库连接失败！");
/* 获取Statement对象 */
public void getStm()
createCon();
try
Statement stm=conn.createStatement();
System.out.println("创建Statement对象成功！");
catch (SQLException e)
e.printStackTrace();
System.out.println("创建Statement对象失败！");
我用测试类test测试以上代码
package com.yxq.toolsbean;
public class Test
public static void main(
String[] args)
DB db=new DB();
db.createCon();
db.getStm();
报错为
加载数据库成功！
获取数据库连接成功！
获取数据库连接成功！
Exception in thread "main" java.lang.NullPointerException
at com.yxq.toolsbean.DB.getStm(DB.java:40)
at com.yxq.toolsbean.Test.main(Test.java:10)
有空你们自己把代码粘贴到myeclipse看一下
不要思维定式在 Statement stmt = new Statement();才是创建对象 这种思维上.
创建对象方式有很多 我给你写了个类似的 名字也用了Statement不过只是个名字而已 真实的Statement实现方法你可以看源码,我的代码只是告诉你"创建对象方式有很多"
public interface Statement
public class Preparedment implements Statement
public static Statement getSatementInstance()
return new Preparedment();
现在你可以用以下来创建 一个接口 的对象
Statement stm1 = Preparedment.getSatementInstance();
Statement stm2 = new Preparedment();
参考技术A import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.sql.Statement;
public class DBConn
private Connection conn;
String strMysqlURL = "jdbc:mysql://127.0.0.1:3306/bookroom?user=root&password=arone";
public Connection getConnection()
try
Class.forName("com.mysql.jdbc.Driver").newInstance();
conn = DriverManager.getConnection(strMysqlURL);
catch (Exception e)
e.printStackTrace();
return conn;
public static void main(String args[])
DBConn con = new DBConn();
con.getConnection();
public DBConn() // 初始化时建立数据源的连接
this.getConnection();
public synchronized Statement getStmt() throws Exception
Statement stmt;
stmt = conn.createStatement();
return stmt;
// 创建预处理会话对象
public synchronized PreparedStatement getPstmt(String sql) throws Exception
PreparedStatement pstmt=null;
pstmt = conn.prepareStatement(sql);
return pstmt;
// 执行查询操作
public synchronized ResultSet query(String sql) throws Exception
try
Statement stmt;
stmt = conn.createStatement();
ResultSet rs = stmt.executeQuery(sql);
return rs;
catch (Exception e)
System.out.println(e.toString());
return null;
public boolean hasUserName(String sql) // 判断重复
try
Statement stmt;
stmt = getConnection().createStatement();
ResultSet rs = stmt.executeQuery(sql);
rs.next();
if (rs.getString("num").equals("0"))
return false;
else
return true;
catch (SQLException e)
// TODO Auto-generated catch block
e.printStackTrace();
return false;
// 执行更新操作
public synchronized int update(String sql) throws Exception
int nn=0;
Statement stmt=null;
try
stmt = conn.createStatement();
nn = stmt.executeUpdate(sql);
catch (Exception e)
System.out.println(e.toString());
nn = 0;
return nn;
// 关闭数据库连接
public void DBclose() throws SQLException
if (conn != null)
try
conn.close();
catch (SQLException e)
e.printStackTrace();
参考技术B 。。靠了，肯定错啊，你那个createCon中的Connection conn是局部变量而且他的值改变又不影响类中的那个Connection对象。你把createCon中别定义Connection conn，直接conn，肯定对
直接用createCon() 返回Connection对象多好，一般都这么写
还有连接两次数据库，没必要，也很危险。追问
把修改后的代码发一下
参考技术C Statement stm=conn.createStatement();错误的意思是你这里的conn为空
JDBC之Java连接mysql实现增删改查
使用软件：mysql、eclipse
链接步骤：
1.注册驱动
2.创建一个连接对象
3.写sql语句
4.执行sql语句并返回一个结果或者结果集
5.关闭链接（一般就是connection、statement、setresult）这三个连接对象，关闭顺序一般是（setresult ---> statement --> setresult ）
一、直接连接方法：（这种方法就是讲sql语句和结果所有的步骤写在一起） 不建议使用该方法
1 public static void main(String[] args) { 2 String url = "jdbc:mysql://localhost:3306/students"; 3 String user = "root"; 4 String password = "admin"; 5 Connection conn = null; 6 Statement st = null; 7 8 try { 9 // 1. 注册驱动 10 Class.forName("com.mysql.jdbc.Driver"); 11 // 2. 创建一个链接对象 12 conn = DriverManager.getConnection(url,user,password); 13 // 3. 创建一个sql语句的发送命令对象 14 String sql = "insert into student values(‘2001‘,‘Tom‘,‘20‘,‘7000‘)"; 15 st= conn.createStatement(); 16 // 4. 执行sql语句，拿到查询的结果集对象 17 st.executeQuery(sql);20 } catch (Exception e) { 21 e.printStackTrace(); 22 }finally { 23 // 5. 关闭链接 ，命令对象 ，结果集 24 if(st != null) { 25 try { 26 st.close(); 27 } catch (Exception e) { 28 e.printStackTrace(); 29 } 30 } 31 if(conn != null) { 32 try { 33 conn.close(); 34 } catch (Exception e) { 35 e.printStackTrace(); 36 } 37 } 38 }
二、建立工具类方法，将必要的几步写一个类，使用的时候直接调用（建议使用）
1.注册驱动、创建连接对象、关闭资源 这三部一般可以写一个类，由于写sql语句和执行sql语句的结果不一致，所以可以将其在用到的时候在写
2.一般写工具类都是写成静态方法，以方便调用
//这是工具类：
import java.sql.Connection; import java.sql.DriverManager; import java.sql.ResultSet; import java.sql.Statement; public class JdbcUtils { private static String driverName = "com.mysql.jdbc.Driver"; private static String url = "jdbc:mysql://localhost:3306/student_achievement_system"; private static String user = "root"; private static String password = "admin"; /** * 链接数据库 */ static { try { Class.forName(JdbcUtils.driverName); } catch (Exception e) { e.printStackTrace(); } } /** * 获取链接对象connection * @return */ public static Connection getConnection() { try { return DriverManager.getConnection(JdbcUtils.url, JdbcUtils.user, JdbcUtils.password); } catch (Exception e) { e.printStackTrace(); } return null; } /** * 关闭资源 * @param conn * @param st * @param rs */ public static void close(Connection conn,Statement st,ResultSet rs) { if(rs != null) { try { rs.close(); } catch (Exception e) { e.printStackTrace(); } } if(st != null) { try { st.close(); } catch (Exception e) { e.printStackTrace(); } } if(conn != null) { try { conn.close(); } catch (Exception e) { e.printStackTrace(); } } } }
//这是对数据库的基本操作类
// 增加、删除、更新、查找一条、查找所有的方法
public class StudentsDaoImpl implements IStudentsDao { //增加 @Override public int save(Students student) { Connection conn = null; PreparedStatement ps = null; try { conn = JdbcUtils.getConnection(); String sql = "insert into students values(?,?,?,?,?,?)"; ps = conn.prepareStatement(sql); ps.setInt(1, student.getStudentId()); ps.setString(2, student.getStudentName()); ps.setString(3, student.getSex()); ps.setString(4, student.getPhoneNo()); ps.setString(5, student.getAddress()); ps.setDate(6, (Date) student.getBirthday()); int row = ps.executeUpdate(); return row; } catch (Exception e) { e.printStackTrace(); }finally { JdbcUtils.close(conn, ps, null); } return 0; } //删除 @Override public int delete(int studentId) { Connection conn = null; PreparedStatement ps = null; try { conn = JdbcUtils.getConnection(); String sql = "delete from students where studentId=?"; ps = conn.prepareStatement(sql); ps.setInt(1, studentId); int row = ps.executeUpdate(); return row; } catch (Exception e) { e.printStackTrace(); }finally { JdbcUtils.close(conn, ps, null); } return 0; } //更新 @Override public int update(int studentId, Students student) { Connection conn = null; PreparedStatement ps = null; try { conn = JdbcUtils.getConnection(); String sql = "update students set studentName=?,sex=?,phoneNo=?,address=?,birthday=? where studentId=?"; ps = conn.prepareStatement(sql); ps.setString(1, student.getStudentName()); ps.setString(2, student.getSex()); ps.setString(3, student.getPhoneNo()); ps.setString(4, student.getAddress()); ps.setDate(5, ((Date) student.getBirthday())); ps.setInt(6, studentId); int row = ps.executeUpdate(); return row; } catch (Exception e) { e.printStackTrace(); }finally { JdbcUtils.close(conn, ps, null); } return 0; } //查找一条数据 @Override public Students getByStudentId(int studentId) { Connection conn = null; PreparedStatement ps = null; ResultSet rs = null; try { conn = JdbcUtils.getConnection(); String sql = "select * from students where studentId=?"; ps = conn.prepareStatement(sql); ps.setInt(1, studentId); rs = ps.executeQuery(); if(rs.next()) { Students student = new Students(); student.setStudentId(rs.getInt("studentId")); student.setStudentName(rs.getString("studentName")); student.setSex(rs.getString("sex")); student.setPhoneNo(rs.getString("phoneNo")); student.setAddress(rs.getString("address")); student.setBirthday(rs.getDate("birthday")); return student; } } catch (Exception e) { e.printStackTrace(); }finally { JdbcUtils.close(conn, ps, rs); } return null; } //查找所有数据 @Override public List<Students> getAll() { Connection conn = null; PreparedStatement ps = null; ResultSet rs = null; try { conn = JdbcUtils.getConnection(); String sql = "select * from students"; ps = conn.prepareStatement(sql); rs = ps.executeQuery(); List<Students> studentsList = new ArrayList<>(); while(rs.next()) { Students student = new Students(); student.setStudentId(rs.getInt("studentId")); student.setStudentName(rs.getString("studentName")); student.setSex(rs.getString("sex")); student.setPhoneNo(rs.getString("phoneNo")); student.setAddress(rs.getString("address")); student.setBirthday(rs.getDate("birthday")); studentsList.add(student); } return studentsList; } catch (Exception e) { e.printStackTrace(); }finally { JdbcUtils.close(conn, ps, rs); } return null; }
}
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