JAVA Web出现这种问题
本人是初学者，没有JAVA Web的基础，为了满足教学任务
编写了一个图书信息系统，提交图书信息后报错
下面是Dao程序的代码
package Dao;
import com.beans.BookInfo;
import java.sql.*;
import java.util.ArrayList;
public class BookDao {
private Connection conn ;
private Statement stmt ;
private PreparedStatement prepStmt ;
private ResultSet rs;
// 构造函数
public BookDao() {
// 进行数据库连接
try {
Class.forName("com.mysql.jdbc.Driver");
String url = "jdbc:mysql://localhost:3306/book";
String username = "root";
String password = "cyk2003";
conn = DriverManager.getConnection(url, username, password);
} catch (ClassNotFoundException e) {
e.printStackTrace();
} catch (SQLException e) {
e.printStackTrace();
}
}
// 保存图书信息，插入到表 bookinfo
public void saveBook(BookInfo book) {
try {
String sql = "INSERT INTO bookinfo VALUES (?, ?, ?, ?, ?);";
prepStmt = conn.prepareStatement(sql);
prepStmt.setString(1, book.getBno());
prepStmt.setString(2, book.getBname());
prepStmt.setString(3, book.getBauthor());
prepStmt.setString(4, book.getBpush());
prepStmt.setFloat(5, book.getBprice());
prepStmt.executeUpdate();
} catch (SQLException e) {
e.printStackTrace();
} finally {
closeConnection();
}
}
// 根据编号删除图书信息
public void deleteBook(String bno) {
try {
String sql = "DELETE FROM bookinfo WHERE bno = ?;";
prepStmt = conn.prepareStatement(sql);
prepStmt.setString(1, bno);
prepStmt.executeUpdate();
} catch (SQLException e) {
e.printStackTrace();
} finally {
closeConnection();
}
}
// 修改图书信息
public void updateBook(BookInfo book) {
try {
String sql = "UPDATE bookinfo SET bname = ?, bauthor = ?, bpush = ?, bprice = ? WHERE bno = ?;";
prepStmt = conn.prepareStatement(sql);
prepStmt.setString(1, book.getBname());
prepStmt.setString(2, book.getBauthor());
prepStmt.setString(3, book.getBpush());
prepStmt.setFloat(4, book.getBprice());
prepStmt.setString(5, book.getBno());
prepStmt.executeUpdate();
} catch (SQLException e) {
e.printStackTrace();
} finally {
closeConnection();
}
}
// 根据编号查找图书信息
public BookInfo findBookInfo(String bno) {
try {
stmt = conn.createStatement();
String sql = "SELECT * FROM bookinfo WHERE bno = '" + bno + "';";
rs = stmt.executeQuery(sql);
if (rs.next()) {
BookInfo book = new BookInfo();
book.setBno(rs.getString("bno"));
book.setBname(rs.getString("bname"));
book.setBauthor(rs.getString("bauthor"));
book.setBpush(rs.getString("bpush"));
book.setBprice(rs.getFloat("bprice"));
return book;
}
} catch (SQLException e) {
e.printStackTrace();
} finally {
closeConnection();
}
return null;
}
// 查询所有书籍信息
public ArrayList<BookInfo> findAllBookInfo() {
ArrayList<BookInfo> books = new ArrayList<BookInfo>();
try {
stmt = conn.createStatement();
String sql = "SELECT * FROM bookinfo;";
rs = stmt.executeQuery(sql);
while (rs.next()) {
BookInfo book = new BookInfo();
book.setBno(rs.getString("bno"));
book.setBname(rs.getString("bname"));
book.setBauthor(rs.getString("bauthor"));
book.setBpush(rs.getString("bpush"));
book.setBprice(rs.getFloat("bprice"));
books.add(book);
}
} catch (SQLException e) {
e.printStackTrace();
} finally {
closeConnection();
}
return books;
}
// 关闭数据库连接
private void closeConnection() {
try {
if (rs != null) {
rs.close();
}
if (stmt != null) {
stmt.close();
}
if (prepStmt != null) {
prepStmt.close();
}
if (conn != null) {
conn.close();
}
} catch (SQLException e) {
e.printStackTrace();
}
}
}
没有连上数据库，检查下数据库连接的用户密码是否正确，其次如果你使用的mysql8,class_forname那个可以不设置，或者修改为：
class.forName("com.mysql.cj.jdbc.Driver");
this.conn 看下为什么是 null，是不是没有new初始化，还是没有打开
是不是数据库环境没安装？
这里为空了，你构造函数里边写的是连接数据库的代码，调用是怎么调用，是通过new么？或者就在上边 conn = DriverManager.getConnection(url, username, password)加个断点看是否连接成功
- 这篇文章：JAVA WEB 使用dao方式实现增加雇员、修改雇员信息，显示雇员图片等功能 也许有你想要的答案，你可以看看
- 除此之外, 这篇博客: java web 期末考试 简单的网页开发 进行增删改查操作中的 1.dao层 部分也许能够解决你的问题, 你可以仔细阅读以下内容或者直接跳转源博客中阅读:
PhoneDao：
package cn.dao; import java.sql.SQLException; import java.util.List; import org.apache.commons.dbutils.QueryRunner; import org.apache.commons.dbutils.handlers.BeanHandler; import org.apache.commons.dbutils.handlers.BeanListHandler; import cn.entity.Phone; import cn.util.C3p0Utils; public class PhoneDao { QueryRunner runner = new QueryRunner(C3p0Utils.getDataSource()); // 获取所有数据 public List<Phone> findPhone() throws SQLException { String sql = "select * from phone"; List<Phone> phones = runner.query(sql, new BeanListHandler<Phone>(Phone.class)); return phones; } // 增加数据 public int adddataPhone(Phone phone) throws SQLException { String sql = "insert into phone (id,sjname,sjprice) values(?,?,?)"; int row = runner.update(sql, phone.getId(), phone.getSjname(), phone.getSjprice()); return row; } // 删除数据,通过id public int deletePhoneById(int id) throws SQLException { String sql = "delete from phone where id = ?"; int row = runner.update(sql, id); return row; } // 修改数据,通过id public int updatePhone(Phone phone) throws SQLException { String sql = "update phone set sjname = ?,sjprice = ? where id=?"; int row = runner.update(sql, phone.getSjname(), phone.getSjprice(), phone.getId()); return row; } // 查询商品,通过id public Phone findPhoneById(int id) throws SQLException { String sql = "select * from phone where id = ?"; Phone phone = runner.query(sql, new BeanHandler<Phone>(Phone.class), id); return phone; } }UserDao
package cn.dao; import java.sql.SQLException; import java.util.ArrayList; import java.util.List; import org.apache.commons.dbutils.QueryRunner; import org.apache.commons.dbutils.handlers.BeanHandler; import org.apache.commons.dbutils.handlers.BeanListHandler; import cn.util.C3p0Utils; import cn.dao.*; import cn.entity.User; public class UserDao { public User findUandUpsw(String username, String password) throws SQLException { QueryRunner runner = new QueryRunner(C3p0Utils.getDataSource()); String sql = "select * from user where username = ? and password = ?"; User user = runner.query(sql, new BeanHandler<User>(User.class), username, password); return user; } }
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