1. 对jdbc做一个简单的封装,select可以返回一个javabean对象,而不是resultset。主要用了反射。这是我之前写的代码,做了简单的修改。

  实现功能:a.对数据库的基本操作 增删改查

       b.对查询的单条记录返回一个指定类型的javabean对象,利用java反射,jdbc ResultSet类和ResultSetMetaData类

       c. 对查到的结果集返回一个List, 泛型

  数据源:用到的 数据库连接池是我自己简单实现的一个连接池:【java】简单实现数据库连接池,主要为了后续实现事务的简单实现

  用到的java知识 : 反射,泛型,jdbc

        


import
com.yeyeck.noob.ConnectionPollImpl; import com.yeyeck.noob.IConnectionPool; import java.lang.reflect.Field; import java.lang.reflect.InvocationTargetException; import java.lang.reflect.Method; import java.sql.*; import java.util.ArrayList; import java.util.List; public class DBUtil { public static int excuteUpdate(String sql, Object... objects) { IConnectionPool connectionPool = ConnectionPollImpl.getInstance(); Connection connection = connectionPool.getConnection(); PreparedStatement preparedStatement = null; try { preparedStatement = getStateMent(connection, sql, objects); return preparedStatement.executeUpdate(); //执行sql并返回结果 } catch (SQLException e) { e.printStackTrace(); } finally { if (preparedStatement != null) { try { preparedStatement.close(); } catch (SQLException e) { e.printStackTrace(); } } } return 0; } /** * 查询单条记录 * * @param sql 查询语句 * @param clazz 返回对象的class * @param objects 需要的参数,必须跟sql占位符的位置一一对应 * @param <T> 泛型返回 * @return 返回单个对象 */ public static <T> T queryForObject(String sql, Class<T> clazz, Object... objects) { IConnectionPool connectionPool = ConnectionPollImpl.getInstance(); Connection connection = connectionPool.getConnection(); PreparedStatement preparedStatement = null; ResultSet resultSet = null; T object = null; try { preparedStatement = getStateMent(connection, sql, objects); resultSet = getResultSet(preparedStatement); if (resultSet.next()) { object = invokeObject(resultSet, clazz); } } catch (SQLException | IllegalAccessException | InstantiationException | NoSuchFieldException | NoSuchMethodException | InvocationTargetException e) { e.printStackTrace(); } finally { close(preparedStatement, resultSet); //记得关闭 } return object; } /** *查询多条记录 * * @param sql 查询语句 * @param clazz 返回对象的class * @param objects 需要的参数,必须跟sql占位符的位置一一对应 * @param <T> 泛型返回 * * @return list */ public static <T> List<T> queryForList(String sql, Class<T> clazz, Object... objects) { IConnectionPool connectionPool = ConnectionPollImpl.getInstance(); Connection connection = connectionPool.getConnection(); PreparedStatement preparedStatement = null; ResultSet resultSet = null; List<T> list = new ArrayList<>(); try { preparedStatement = getStateMent(connection, sql, objects); resultSet = getResultSet(preparedStatement); while (resultSet.next()) { //调用 invokeObject方法,把一条记录封装成一个对象,添加到list中 list.add(invokeObject(resultSet, clazz)); } } catch (SQLException | IllegalAccessException | InstantiationException | NoSuchFieldException | NoSuchMethodException | InvocationTargetException e) { e.printStackTrace(); } finally { close(preparedStatement, resultSet); } return list.size() > 0 ? list : null; } private static void close(PreparedStatement preparedStatement, ResultSet resultSet) { try { if(resultSet != null) { resultSet.close(); } if (preparedStatement != null) { preparedStatement.close(); } } catch (SQLException e) { e.printStackTrace(); } } /** * 把数据库中的一条记录通过反射包装成相应的Bean * @param resultSet * @param clazz * @param <T> * @return * @throws IllegalAccessException * @throws InstantiationException * @throws SQLException * @throws NoSuchFieldException * @throws NoSuchMethodException * @throws InvocationTargetException */ private static <T> T invokeObject(ResultSet resultSet, Class<T> clazz) throws IllegalAccessException, InstantiationException, SQLException, NoSuchFieldException, NoSuchMethodException, InvocationTargetException { T object = clazz.getDeclaredConstructor().newInstance(); ResultSetMetaData metaData = resultSet.getMetaData(); for (int i = 0, count = metaData.getColumnCount(); i < count; i++) { String columnName = metaData.getColumnName(i + 1); //数据库返回结果的列名 String fieldName = StringUtil.camelName(columnName); //去掉列名中的下划线“_”并转为驼峰命名 Field field = clazz.getDeclaredField(fieldName); //根据字段名获取field String methName = setMethodName(fieldName); //拼set方法名 Class type = field.getType(); //获取字段类型 Method setMethod = clazz.getDeclaredMethod(methName, field.getType()); Object value = resultSet.getObject(i + 1); //获取字段值 setMethod.invoke(object, type.cast(value)); //强转并且赋值 } return object; } private static PreparedStatement getStateMent(Connection connection, String sql, Object... objects) throws SQLException { PreparedStatement preparedStatement = connection.prepareStatement(sql); for (int i = 0, len = objects.length; i < len; i++) { preparedStatement.setObject(i + 1, objects[i]); //给sql每个?占位符填上数据 } return preparedStatement; } private static ResultSet getResultSet(PreparedStatement statement) throws SQLException { if (statement == null) { return null; } else { return statement.executeQuery(); } } private static String setMethodName(String str) { return "set" + StringUtil.firstUpperCase(str); } }

其中用到的StringUtil的几个方法

public class StringUtil {

    /**
     * 转为驼峰命名
     * @param str
     * @return string
     */
    public static String camelName(String str) {
        if (!isEmpty(str)) {
            StringBuilder stringBuilder = new StringBuilder();
            for (int i = 0, len = str.length(); i < len; i++) {
                if (str.charAt(i) == '_') {
                    while (str.charAt(i + 1) == '_') {
                        i++;
                    }
                    stringBuilder.append(("" + str.charAt(++i)).toUpperCase());
                } else {
                    stringBuilder.append(str.charAt(i));
                }
            }
            return stringBuilder.toString();
        }
        return str;
    }

    /**
     * 判断是否为空串
     *
     * @param str
     * @return
     */
    public static boolean isBlank(String str) {
        if (str != null && str.length() > 0) {
            for (int i = 0, len = str.length(); i < len; i++) {
                if (!Character.isSpaceChar(str.charAt(i))) {
                    return false;
                }
            }
        }
        return true;
    }

    /**
     * 判断是否为空串 ?!!! 我怎么又写了个一样的方法?!!!
     * @param str
     * @return
     */
    public static boolean isEmpty(String str) {
        return str == null || str.length() == 0;
    }


    /**
     * 将第一个字母替换为大写
     * @param str
     * @return
     */
    public static String firstUpperCase(String str) {
        return str.substring(0, 1).toUpperCase() + str.substring(1, str.length());
    }
}

测试

1.测试用到的表

 CREATE TABLE `t_user` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `username` varchar(50) DEFAULT NULL,
  `age` int(11) NOT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=5 DEFAULT CHARSET=utf8 |

初始数据

测试代码

import java.util.List;

public class DBUtilTest {

    public static void main(String[] args) {


        List<User> users1 = DBUtil.queryForList("select * from t_user", User.class);
        System.out.println(users1);

        User user = new User();
        user.setUsername("刘能");
        user.setAge(12);
        String sql1 = "insert into `t_user`(username, age) values (?, ?)";
        System.out.println("insert 语句测试返回结果:" + DBUtil.excuteUpdate(sql1, user.getUsername(), user.getAge()));
        List<User> users2 = DBUtil.queryForList("select * from t_user", User.class);
        System.out.println(users2);

        String sql2 = "delete from t_user where id = ?";
        System.out.println("delete 语句测试返回结果:" + DBUtil.excuteUpdate(sql2, 3));

        String sql3 = "update t_user set age = ? where id = ?";
        System.out.println("update 语句测试返回结果:" + DBUtil.excuteUpdate(sql3, 100, 1));

        String sql4 = "select * from t_user where id = ?";
        User user2 = DBUtil.queryForObject(sql4, User.class, 2);
        System.out.println("select 语句测试返回结果: " + user2 );
        List<User> users3 = DBUtil.queryForList("select * from t_user", User.class);
        System.out.println(users3);


    }

}

测试结果

[User{id=1, username='小明', age=100}, User{id=2, username='小红', age=15}, User{id=4, username='小二', age=12}, User{id=5, username='刘能', age=12}]
insert 语句测试返回结果:1
[User{id=1, username='小明', age=100}, User{id=2, username='小红', age=15}, User{id=4, username='小二', age=12}, User{id=5, username='刘能', age=12}, User{id=6, username='二蛋', age=99}]
delete 语句测试返回结果:1
update 语句测试返回结果:1
select 语句测试返回结果: User{id=2, username='小红', age=15}
[User{id=1, username='小明', age=20}, User{id=2, username='小红', age=15}, User{id=4, username='小二', age=12}, User{id=6, username='二蛋', age=99}]

数据库数据

 

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