JDBC--DAO设计模式

public class DAO {
    public int update(String sql, Object ... args){
        Connection conn = null;
        PreparedStatement ps = null;
        int rowNum = 0;
        try{
            conn = JDBCUtils.getConnection();
            ps = conn.prepareStatement(sql);
            for(int i = 0; i < args.length; i++){
                ps.setObject(i + 1, args[i]);
            }
            
            rowNum = ps.executeUpdate();
            return rowNum;
        }catch(Exception e){
            e.printStackTrace();
        }finally{
            JDBCUtils.release(conn, ps, null);
        }
        return rowNum;
    }
    
    public <T> T get(Class<T> clazz, String sql, Object ...args){
        T entity = null;
        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();
            Map<String, Object> map = new HashMap<String, Object>();
            List<String> columnLabels = getColumnLabels(rs);
            
            if(rs.next()){
                for(String columnLabel : columnLabels){
                    Object columnValue = rs.getObject(columnLabel);
                    map.put(columnLabel, columnValue);
                }
            }
            
            if(map.size() > 0){
                entity = clazz.newInstance();
                for(Map.Entry<String, Object> entry : map.entrySet()){
                    String fieldName = entry.getKey();
                    Object value = entry.getValue();
                    //使用BeanUtils工具类来为属性赋值
                    BeanUtils.setProperty(entity, fieldName, value);
                    /**使用反射的方式为属性赋值
                    Field field = clazz.getDeclaredField(key);
                    field.setAccessible(true);
                    field.set(entity, value);*/
                }
                return entity;
            }
            
        }catch(Exception e){
            e.printStackTrace();
        }finally{
            JDBCUtils.release(conn, ps, rs);
        }
        return entity;
    }
    
    public <T> List<T> getForList(Class<T> clazz, String sql, Object ... args){
        List<T> list = new ArrayList<>();
        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();
            /**
             * 将得到的ResultSet结果集转换为Map<String, Object>列表，
             * 其中key值为rs表的别名，value值为对应别名的值
             */
            List<Map<String, Object>> data = handleResultSetToMapList(rs);
            
            /**
             * 将MapList转换为对应的BeanList
             */
            list = transferMapListToBeanList(clazz, data);
        }catch(Exception e){
            e.printStackTrace();
        }finally{
            JDBCUtils.release(conn, ps, rs);
        }
        return list;
    }

    private <T> List<T> transferMapListToBeanList(Class<T> clazz,List<Map<String, Object>> data) throws Exception {
        List<T> list = new ArrayList<>();
        if(data.size() > 0){
            for(Map<String, Object> map : data){
                T entity = clazz.newInstance();
                for(Map.Entry<String, Object> entry : map.entrySet()){
                    String fieldName = entry.getKey();
                    Object value = entry.getValue();
                    
                    BeanUtils.setProperty(entity, fieldName, value);;
                }
                list.add(entity);
            }
        }
        return list;
    }

    public static <E> E getForValue(String sql, Object ... args){
        E entity = null;
        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();
            
            if(rs.next()){
                return (E)rs.getObject(1);
            }
            
        }catch(Exception e){
            e.printStackTrace();
        }finally{
            JDBCUtils.release(conn, ps, rs);
        }
        return entity;
    }
    
    /**
     * 将获取到的结果集处理成MapList
     * @param rs
     * @return
     * @throws Exception
     */
    private List<Map<String, Object>> handleResultSetToMapList(ResultSet rs)
            throws Exception {
        List<String> columnLabels = getColumnLabels(rs);
        List<Map<String, Object>> data = new ArrayList<Map<String, Object>>();
        while(rs.next()){
            Map<String, Object> map = new HashMap<String, Object>();
            for(String columnLabel : columnLabels){
                Object columnValue = rs.getObject(columnLabel);
                map.put(columnLabel, columnValue);
            }
            data.add(map);
        }
        return data;
    }
    
    
    /**
     * 根据结果集获取所有列的别名
     * @param rs
     * @return
     * @throws Exception
     */
    private List<String> getColumnLabels(ResultSet rs) throws Exception{
        List<String> list = new ArrayList<>();
        ResultSetMetaData rsmd = rs.getMetaData();
        for(int i = 0; i < rsmd.getColumnCount(); i++){
            list.add(rsmd.getColumnLabel(i + 1));
        }
        return list;
    }
}