java连mysql抛空指针异常
我新手，主要想实现一个简单的注册、登陆功能。
一个写了4个类：
USer类：
package cn.pzhu.domain;import java.util.Date;public class User { private String id; private String username; private String email; private String password; private Date birthday; private String nickname; public String getId() { return id; } public void setId(String id) { this.id = id; } public String getUsername() { return username; } public void setUsername(String username) { this.username = username; } public String getEmail() { return email; } public void setEmail(String email) { this.email = email; } public String getPassword() { return password; } public void setPassword(String password) { this.password = password; } public Date getBirthday() { return birthday; } public void setBirthday(Date birthday) { this.birthday = birthday; } public String getNickname() { return nickname; } public void setNickname(String nickname) { this.nickname = nickname; }}package cn.pzhu.dao.impl;import java.sql.*;import cn.pzhu.dao.UserDao;import cn.pzhu.domain.User;public class UserDaoImpl implements UserDao { static Connection connection; static Statement statement; static PreparedStatement preparedStatement; static String sql; static{ try { Class.forName("com.mysql.jdbc.Driver"); connection = DriverManager.getConnection("jdbc:mysql://localhost:3306/test1?user=root&password=pzhu"); } catch (ClassNotFoundException e1) { throw new RuntimeException(e1); }catch (SQLException e2) { throw new RuntimeException(e2); } } //数据库user表字段顺序：username password id email birthday nickname public void add(User user) { sql = "insert into user values(?,?,?,?,?,?)"; try { preparedStatement = connection.prepareStatement(sql); preparedStatement.setString(1, user.getUsername()); preparedStatement.setString(2, user.getPassword()); preparedStatement.setString(3, user.getId()); preparedStatement.setString(4, user.getEmail()); preparedStatement.setDate(5,new java.sql.Date(user.getBirthday().getTime())); preparedStatement.setString(6, user.getNickname()); preparedStatement.executeUpdate(); } catch (SQLException e3) { throw new RuntimeException(e3); } try { if(preparedStatement!=null){ preparedStatement.close(); preparedStatement = null; } if (connection!=null) { connection.close(); connection = null; } } catch (Exception e4) { throw new RuntimeException(e4); } } //数据库user表字段顺序：username password id email birthday nicknameusername password id email birthday nickname public User find(String username,String password ) { sql = "select * from user where username='"+username+"' and password='"+password+"'"; try { statement = connection.createStatement(); ResultSet resultSet = statement.executeQuery(sql); if(resultSet.next()){ User user = new User(); user.setBirthday(resultSet.getDate(5)); user.setEmail(resultSet.getString(4)); user.setId(resultSet.getString(3)); user.setNickname(resultSet.getString(6)); user.setPassword(resultSet.getString(2)); user.setUsername(resultSet.getString(1)); return user; }else { return null; } } catch (SQLException e5) { throw new RuntimeException(e5); } finally{ try { if (statement!=null) { statement.close(); statement=null; } if (connection!=null) { connection.close(); connection=null; } } catch (SQLException e6) { throw new RuntimeException(e6); } } } //用户注册时检测数据库中是否已经存在该用户名 public boolean find(String username) { sql = "select * from user where username='"+username+"'"; try { statement = connection.createStatement(); ResultSet resultSet = statement.executeQuery(sql); if(resultSet.next()){ return true; }else { return false; } } catch (SQLException e5) { throw new RuntimeException(e5); } finally{ try { if (statement!=null) { statement.close(); statement=null; } if (connection!=null) { connection.close(); connection=null; } } catch (SQLException e6) { throw new RuntimeException(e6); } } }}
package cn.pzhu.service.impl;import cn.pzhu.dao.impl.UserDaoImpl;import cn.pzhu.domain.User;import cn.pzhu.exception.UserExistException;//对web层提供所有的服务public class BusinessServiceImpl { UserDaoImpl userDaoImpl = new UserDaoImpl(); //提供注册服务 public void register(User user) throws UserExistException { //判断用户是否已经存在 boolean b = userDaoImpl.find(user.getUsername()); if (b) { throw new UserExistException();//用户已经存在，向web层抛一个用户已存在异常(测试的时候，如果用户存在，整个程序到这里可以正常执行) }else { userDaoImpl.add(user);//但是如果不存在用户，向数据库添加的时候，就有异常了 } } //提供登陆服务 public User login(String username,String password ) { return userDaoImpl.find(username, password); }}package cn.pzhu.exception;public class UserExistException extends Exception { public UserExistException() { // TODO Auto-generated constructor stub } public UserExistException(String message) { super(message); // TODO Auto-generated constructor stub } public UserExistException(Throwable cause) { super(cause); // TODO Auto-generated constructor stub } public UserExistException(String message, Throwable cause) { super(message, cause); // TODO Auto-generated constructor stub }} public boolean find(String username) { String sql = "select * from user where username='"+username+"'"; Connection cn = getConnection(); // 这里负责返回一个连接; try { Statement statement = cn.createStatement(); ResultSet resultSet = statement.executeQuery(sql); if(resultSet.next()){ return true; }else { return false; } } catch (SQLException e5) { throw new RuntimeException(e5); } finally{ closeConnection(cn); } }