import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
import java.sql.ResultSetMetaData;
import java.sql.SQLException;
import java.sql.Statement;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

/**
 * 
 * <p>类描述：  mysql jdbc示例</p>
 * <p>创建人：whg  </p>
 * <p>创建时间：2021年7月8日 下午7:25:16  </p>
 */
public class MysqlUtil  {
    public static final String DRIVER = "com.mysql.jdbc.Driver";

    private static Connection conn = null;

    /**
     * 通过静态代码块 注册数据库驱动
     */
    static {
        try {
            Class.forName(DRIVER);
        } catch (ClassNotFoundException e) {
            e.printStackTrace();
        }
    }

    /**
     * 获得Connection
     *
     * @return
     */
    public  Connection getConnection(String url,String username,String password) {
        try {
            if(null == conn || conn.isClosed()){
                conn = DriverManager.getConnection(url, username, password);
            }
        } catch (SQLException e) {
            e.printStackTrace();
        }
        return conn;
    }

    /**
     * 获得Statement
     *
     * @return
     */
    public  Statement getStatement() {
        Statement st = null;
        try {
            if(null != conn  || conn.isClosed()){
                st = conn.createStatement();
            }
        } catch (SQLException e) {
            e.printStackTrace();
        }
        return st;
    }

    /**
     * 关闭ResultSet
     *
     * @param rs
     */
    public  void closeResultSet(ResultSet rs) {
        if (null != rs) {
            try {
                rs.close();
            } catch (SQLException e) {
                e.printStackTrace();
            }
        }
    }

    /**
     * 关闭Statement
     *
     * @param st
     */
    public  void closeStatement(Statement st) {
        if (null != st) {
            try {
                st.close();
            } catch (SQLException e) {
                e.printStackTrace();
            }
        }
    }

    /**
     * 关闭Connection
     *
     * @param conn
     */
    public  void closeConnection(Connection conn) {
        if (null != conn) {
            try {
                conn.close();
            } catch (SQLException e) {
                e.printStackTrace();
            }
        }
    }

    /**
     * 关闭全部
     *
     * @param rs
     * @param sta
     * @param conn
     */
    public  void closeAll(ResultSet rs, Statement sta, Connection conn) {
        closeResultSet(rs);
        closeStatement(sta);
        closeConnection(conn);
    }

    /**
     * 查询
     *
     * @return
     */
    public  List<Map<String, Object>> query(String sql,String url,String username,String password) {

        List<Map<String, Object>> resultList  = new ArrayList<Map<String, Object>>();

        PreparedStatement pst = null;
        ResultSet rs = null;
        Connection conn = getConnection(url,username,password);
        try {
            pst = conn.prepareStatement(sql);

            rs = pst.executeQuery();
            //获取结果集的元数据
            ResultSetMetaData md = rs.getMetaData();
            //获取结果集元数据中的列数
            int columnCount = md.getColumnCount();

            while (rs.next()) {
                Map<String, Object> map = new HashMap<>();
                for (int i = 0; i < columnCount; i++) {
                    Object colDate = rs.getObject(i + 1);
                    String colName = md.getColumnLabel(i + 1);

                    //放入数组
                    map.put(colName,colDate);
                }
                //放入集合
                resultList.add(map);
                /*map.clear();*/
            }
        } catch (SQLException e) {
            e.printStackTrace();
        } finally {
            closeAll(rs,pst,conn);
        }
        return resultList;
    }

}