Java Web学习总结(九) JDBC
Java Web学习总结(九) JDBC
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什么是JDBC
JDBC的全称是Java数据库连接（Java Databse Connectivity)，它是一套用于执行SQL语句的Java API.
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JDBC的一般步骤
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加载驱动
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连接数据库
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执行SQL代码
- 初始化SQL代码
- 构建SQL代码
- 执行SQL代码
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关闭连接
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示例：
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创建示例数据库
CREATE DATABASE jc; USE jc; CREATE TABLE `user` ( `Id` int(11) NOT NULL AUTO_INCREMENT, `username` varchar(20) DEFAULT NULL, `password` varchar(20) DEFAULT NULL, PRIMARY KEY (`Id`) )engine = InnoDB DEFAULT CHARSET=utf8; USE jc; insert into user (username, password) values('zs', '123'); insert into user (username, password) values('ls', '123'); insert into user (username, password) values('ww', '123'); -
创建实体类User
package ink.heat.entity; public class User { private int id; private String username; private String password; public User() { } public User(String username, String password) { super(); this.username = username; this.password = password; } public User(int id, String username, String password) { super(); this.id = id; this.username = username; this.password = password; } public int getId() { return id; } public void setId(int id) { this.id = id; } public String getUsername() { return username; } public void setUsername(String username) { this.username = username; } public String getPassword() { return password; } public void setPassword(String password) { this.password = password; } @Override public String toString() { return "User [id=" + id + ", username=" + username + ", password=" + password + "]"; } } -
创建UserDao类（对User的增、删、改、查操作）
package ink.heat.dao; import java.sql.Connection; import java.sql.DriverManager; import java.sql.PreparedStatement; import java.sql.ResultSet; import java.sql.SQLException; import java.util.ArrayList; import java.util.List; import ink.heat.entity.User; public class UserDao { /** * 对User的增加， 删除， 修改， 查询 */ private Connection conn = null; private PreparedStatement pstat = null; private ResultSet rs = null; private String driver = "com.mysql.jdbc.Driver"; private String url = "jdbc:mysql://localhost:3306/jc?characterEncoding=UTF-8"; private String dbUsername = "root"; private String dbPassword = "123456"; /** * 获得数据库链接 * @return Connection 对象 * @throws ClassNotFoundException * @throws SQLException */ public Connection getConnection() throws ClassNotFoundException, SQLException { Class.forName(driver);//1.加载驱动 return (Connection) DriverManager.getConnection(url, dbUsername, dbPassword);//2.获得数据库连接 } /** * 关闭所有连接 */ public void closeAll() { if (conn != null) { try { conn.close(); } catch (SQLException e) { e.printStackTrace(); } } if (pstat != null) { try { pstat.close(); } catch (SQLException e) { e.printStackTrace(); } } if (rs != null) { try { rs.close(); } catch (SQLException e) { e.printStackTrace(); } } } /** * 打印数据库返回的users * @param users */ public void printUsers(List<User> users) { for (User user : users) { System.out.println(user); } } /** * 查询所有对象 * @return 所有用户对象 */ public List<User> queryAllUser() { List<User> users = new ArrayList<User>(); try{ conn = getConnection(); String sql = "select * from user"; pstat = conn.prepareStatement(sql);//初始化SQL代码 rs = pstat.executeQuery();//执行SQL代码 while (rs.next()) { User user = new User(); user.setId(rs.getInt("id")); user.setUsername(rs.getString("username")); user.setPassword(rs.getString("password")); users.add(user); } } catch (ClassNotFoundException e) { e.printStackTrace(); } catch (SQLException e) { e.printStackTrace(); } finally { closeAll(); } return users; } /** * 增加一个User * @param user * @return */ public int addUser(User user) { int result = -1; try { conn = getConnection(); String sql = "insert into user (username, password) values(?,?)"; pstat = conn.prepareStatement(sql); pstat.setString(1, user.getUsername()); pstat.setString(2, user.getPassword()); result = pstat.executeUpdate(); } catch (ClassNotFoundException e) { e.printStackTrace(); } catch (SQLException e) { e.printStackTrace(); } finally { closeAll(); } return result; } /** * 更新一个用户 * @param user * @return */ public int updateUser(User user) { int result = -1; try { conn = getConnection(); String sql = "update user set username=?,password=? where id=?"; pstat = conn.prepareStatement(sql); pstat.setString(1, user.getUsername()); pstat.setString(2, user.getPassword()); pstat.setInt(3, user.getId()); result = pstat.executeUpdate(); } catch (ClassNotFoundException e) { e.printStackTrace(); } catch (SQLException e) { e.printStackTrace(); } finally { closeAll(); } return result; } /** * 删除一个用户 * @param id * @return */ public int deleteUser(int id) { int result = -1; try { conn = getConnection(); String sql = "delete from user where id=?"; pstat = conn.prepareStatement(sql); pstat.setInt(1, id); result = pstat.executeUpdate(); } catch (ClassNotFoundException e) { e.printStackTrace(); } catch (SQLException e) { e.printStackTrace(); } finally { closeAll(); } return result; } } -
创建测试类
package ink.heat.test; import java.util.List; import ink.heat.dao.UserDao; import ink.heat.entity.User; public class JDBCTest { public static void main(String[] args) { List<User> users = null; UserDao dao = new UserDao(); //查询 System.out.println("--------【查询】- - - - - - - "); users = dao.queryAllUser(); dao.printUsers(users); System.out.println("--------【增加】- - - - - - - "); User u = new User("zl", "123"); dao.addUser(u); users = dao.queryAllUser(); dao.printUsers(users); System.out.println("--------【修改】- - - - - - - "); User u1 = new User(4, "zl", "1234"); dao.updateUser(u1); users = dao.queryAllUser(); dao.printUsers(users); System.out.println("--------【删除】- - - - - - - "); dao.deleteUser(1); users = dao.queryAllUser(); dao.printUsers(users); } }
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关于Statement和PreparedStatement
Statement 由于存在使用不方便以及存在SQL注入问题， 所以不推荐使用。
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创建DBUtils
上面的例子中，已经将重复的部分代码提取来作为方法了， 但是这些方法都只对UserDao有效，将来项目Dao层多时， 代码又会重复， 所以将它们提取到一个工具类是十分有必要的。
package ink.heat.util; import java.io.IOException; import java.io.InputStream; import java.sql.Connection; import java.sql.DriverManager; import java.sql.ResultSet; import java.sql.SQLException; import java.sql.Statement; import java.util.Properties; public class DBUtils { private static String driver; private static String url; private static String dbUsername; private static String dbPassword; static { Properties properties = new Properties(); InputStream in = DBUtils.class.getClassLoader().getResourceAsStream("db.properties"); try { properties.load(in); driver = properties.getProperty("driver"); url = properties.getProperty("url"); dbUsername = properties.getProperty("dbUsername"); dbPassword = properties.getProperty("dbPassword"); } catch (IOException e) { // TODO Auto-generated catch block e.printStackTrace(); } } public static Connection getConnection() throws ClassNotFoundException, SQLException { Class.forName(driver); return (Connection) DriverManager.getConnection(url, dbUsername, dbPassword); } public static void closeAll(Connection conn, Statement stat, ResultSet rs) { if (conn != null) { try { conn.close(); } catch (SQLException e) { e.printStackTrace(); } } if (stat != null) { try { stat.close(); } catch (SQLException e) { e.printStackTrace(); } } if (rs != null) { try { rs.close(); } catch (SQLException e) { e.printStackTrace(); } } } }在src目录下新建db.properties文件， 文件内容如下：
driver=com.mysql.jdbc.Driver url=jdbc:mysql://localhost:3306/jc?characterEncoding=UTF-8 dbUsername=root dbPassword=123456