JDBC相关注意事项

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1. mysql中出现中文乱码解决方案：

在url中加上：

url=jdbc:mysql://localhost:3306/test?useSSL=false&useUnicode=true&characterEncoding=utf-8

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2.JDBC工具类写法：JDBCUtils

/**
 * @ClassName : JDBCUtils  
 * @Description : 操作数据库的工具类
 * @Author : Yemulin  
 * @Date : 2020/06/02 19:22  
 * @Version : 1.0
 */

public class JDBCUtils {

    /**
    * @Description : 获取数据库的连接
    * @Param : []
    * @author : Yemulin
    * @date : 2020/6/2 19:32
    * @return : java.sql.Connection
    * @throws : 
    */
    public static Connection getConnection() throws Exception {
        //1.读取配置文件中的4个基本信息
        InputStream is = ClassLoader.getSystemClassLoader().getResourceAsStream("jdbc.properties");
        Properties pros = new Properties();
        pros.load(is);

        String user = pros.getProperty("user");
        String password = pros.getProperty("password");
        String url = pros.getProperty("url");
        String driverClass = pros.getProperty("driverClass");

        //2.加载驱动
        Class.forName(driverClass);

        //3.获取连接
        Connection connection = DriverManager.getConnection(url, user, password);
        System.out.println(connection);
        return connection;
    }

    /**
    * @Description : 关闭连接和Statement的操作
    * @Param : [connection, ps]
    * @author : Yemulin
    * @date : 2020/6/3 17:26
    * @return : void
    * @throws : 
    */
    public static void closeResource(Connection connection, Statement ps){
        try {
            if (ps != null )
                ps.close();
        } catch (SQLException e) {
            e.printStackTrace();
        }
        try {
            if (connection != null)
                connection.close();
        } catch (SQLException e) {
            e.printStackTrace();
        }
    }

    /**
    * @Description : 关闭资源操作
    * @Param : [connection, ps, rs]
    * @author : Yemulin
    * @date : 2020/6/7 16:13
    * @return : void
    * @throws : 
    */
    public static void closeResource(Connection connection, Statement ps, ResultSet rs){
        try {
            if (ps != null )
                ps.close();
        } catch (SQLException e) {
            e.printStackTrace();
        }
        try {
            if (connection != null)
                connection.close();
        } catch (SQLException e) {
            e.printStackTrace();
        }
        try {
            if (rs != null)
                rs.close();
        } catch (SQLException e) {
            e.printStackTrace();
        }
    }

}

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3.使用PreparedStatement来替换Statement,实现对数据表的增删改操作

public class PreparedStatementUpdateTest {

    @Test
    public void testConmmonUpdate(){
//        String sql = "delete from customers where id = ?";
//        update(sql,3);

        String sql = "update `order` set order_name = ? where order_id = ?";
        update(sql,"DD","2");
    }
    
    //通用的增删改操作
    public void update(String sql, Object ...args)  {//sql中占位符的个数与可变形参的长度相同
        Connection conn = null;
        PreparedStatement ps = null;

        try {
            //1.获取数据库的连接
            conn = JDBCUtils.getConnection();
            //2.预编译sql语句，返回PrepareStatement的实例
            ps = conn.prepareStatement(sql);
            //3.填充占位符
            for (int i = 0 ; i < args.length ; i++){
                ps.setObject(i + 1,args[i]);//小心参数声明错误
            }
            //4.执行
            ps.execute();
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            //5.资源的关闭
            JDBCUtils.closeResource(conn,ps);
        }

    }

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4.针对于customers表的通用的查询操作

@Test
    public void testQueryForCustomers() throws Exception {
        String sql = "select id,name,birth,email from customers where id = ?";
        Customer customer = queryForCustomers(sql, 13);
        System.out.println(customer);

        sql = "select name,email from customers where name = ?";
        Customer customer1 = queryForCustomers(sql, "周杰伦");
        System.out.println(customer1);
    }
/**
    * @Description : 针对于customers表的通用的查询操作
    * @Param : []
    * @author : Yemulin
    * @date : 2020/6/7 16:26
    * @return : void
    * @throws : 
    */

    public Customer queryForCustomers(String sql,Object ...args) throws Exception {
        Connection conn = null;
        PreparedStatement ps = null;
        ResultSet rs = null;
        try {
            conn = JDBCUtils.getConnection();

            ps = conn.prepareStatement(sql);
            for (int i = 0; i < args.length; i++){
                ps.setObject(i + 1 ,args[i]);
            }

            rs = ps.executeQuery();
            //获取结果集的元数据：ResultSetMetaData
            ResultSetMetaData rsMetaData = rs.getMetaData();
            //通过ResultSetMetaData获取结果集中的列数
            int columnCount = rsMetaData.getColumnCount();
            if (rs.next()){
                Customer cust = new Customer();
                //处理结果集一行数据中的每一个列
                for (int i = 0 ; i < columnCount ; i++){
                    //获取列值
                    Object columnValue = rs.getObject(i + 1);

                    //获取每个列的列名
                    String columnName = rsMetaData.getColumnName(i + 1);

                    //给cust对象指定的columnName属性，赋值为columnValue:通过反射
                    Field field = Customer.class.getDeclaredField(columnName);
                    field.setAccessible(true);
                    field.set(cust, columnValue);

                }
                return cust;
            }
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            JDBCUtils.closeResource(conn, ps, rs);
        }
        return null;
    }

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5.针对于Order表的通用的查询操作

/**
 * @ClassName : OrderForQuery  
 * @Description :  针对于Order表的通用的查询操作
 * @Author : Yemulin  
 * @Date : 2020/06/13 15:31  
 * @Version : 1.0
 */

public class OrderForQuery {

    /*
    针对于表的字段名与类的属性名不相同的情况：
    1.必须声明sql时，使用类的属性名来命名字段的别名
    2.使用ResultSetMetaData时，需要使用getColumnLabel()来替换getColumnName()
        获取列的别名。

    说明：如果sql中没有给字段起别名，getColumnLabel()获取的就是列名
     */

    @Test
    public void testOrderForQuery(){
        String sql = "select order_id orderId,order_name orderName,order_date orderDate from `order` where order_id = ?";
        Order order = orderForQuery(sql, 1);
        System.out.println(order);
    }
    /**
    * @Description : 通用的针对于Order表的查询操作
    * @Param : []
    * @author : Yemulin
    * @date : 2020/6/25 14:46
    * @return : jdbc3bean.Order
    * @throws :
    */
    public Order orderForQuery(String sql, Object...args) {

        Connection conn = null;
        PreparedStatement ps = null;
        ResultSet rs = null;
        try {
            conn = JDBCUtils.getConnection();
            ps = conn.prepareStatement(sql);
            for (int i = 0; i < args.length; i++){
                ps.setObject(i + 1, args[i]);
            }

            //执行，获取结果集
            rs = ps.executeQuery();
            //获取结果集的元数据
            ResultSetMetaData rsmd = rs.getMetaData();
            //获取列数
            int columnCount = rsmd.getColumnCount();
            if (rs.next()){
                Order order = new Order();
                for (int i = 0; i < columnCount; i++){
                    //获取每个列的列值：通过ResultSet
                    Object columnValue = rs.getObject(i + 1);
                    //通过ResultSetMetaData
                    //获取每个列的列名：getColumnName() --不推荐
                    //获取每个列的别名：getColumnLabel()
//                    String columnName = rsmd.getColumnName(i + 1);
                    String columnLabel = rsmd.getColumnLabel(i + 1);

                    //通过反射，将对象指定名columnName的属性赋值为指定的值columnValue
                    Field field = Order.class.getDeclaredField(columnLabel);
                    field.setAccessible(true);
                    field.set(order, columnValue);
                }

                return order;
            }
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            JDBCUtils.closeResource(conn, ps, rs);
        }

        return null;
    }
}

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6.图解查询操作流程