【MySQL】MySQL的Sequence

Oracle的Sequence用爽了，发现MySQL没有Sequence，那么，自己写一个呗。

> 最简单的实现

先建一个表存储当前值：

CREATE TABLE `t_sequence` (
`sequence_name`  varchar(64) CHARACTER SET utf8 COLLATE utf8_general_ci NOT NULL COMMENT '序列名称' ,
`value`  int(11) NULL DEFAULT NULL COMMENT '当前值' ,
PRIMARY KEY (`sequence_name`)
)
ENGINE=InnoDB
DEFAULT CHARACTER SET=utf8 COLLATE=utf8_general_ci
ROW_FORMAT=COMPACT
;

获取当前值的函数：

CREATE DEFINER = `root`@`localhost` FUNCTION `currval`(sequence_name varchar(64))
 RETURNS int(11)
BEGIN
    declare current integer;
    set current = 0;
    select t.value into current from t_sequence t where t.sequence_name = sequence_name;
    return current;
end;

获取下一个值：

CREATE DEFINER = `root`@`localhost` FUNCTION `nextval`(sequence_name varchar(64))
 RETURNS int(11)
BEGIN
    declare current integer;
    set current = 0;
    
    update t_sequence t set t.value = t.value + 1 where t.sequence_name = sequence_name;
    select t.value into current from t_sequence t where t.sequence_name = sequence_name;

    return current;
end;

写一个多线程程序测试下并发：

import java.sql.Connection;
import java.sql.DriverManager;
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;

public class JDBCTools {
    
    public static void main(String[] args) throws Exception {
        JDBCTools.execute("update t_balance t set t.balance = 300 where t.user_id = 2");
        JDBCTools.query("select * from t_balance t");
    }
    
    public static String HOST = "localhost";
    public static String PORT = "3306";
    public static String DATABASE_NAME = "demo";
    public static String USER_NAME = "root";
    public static String PASSWORD = "123456";
    
    /**
     * 获取数据库连接
     * @return 数据库连接
     */
    public static Connection getConn() throws Exception {
        Class.forName("com.mysql.jdbc.Driver");
        
        String url = "jdbc:mysql://" + HOST + ":" + PORT + "/" + DATABASE_NAME + "?user=" + USER_NAME + "&password=" + PASSWORD + "&useUnicode=true&characterEncoding=UTF8";
        Connection connection = DriverManager.getConnection(url);
        return connection;
    }
    
    /**
     * 关闭资源
     */
    public static void closeResource(Connection conn, Statement st, ResultSet rs) {
        if (rs != null) {
            try {
                rs.close();
            } catch (SQLException e) {
                // TODO 处理异常
                e.printStackTrace();
            }
        }
        if (st != null) {
            try {
                st.close();
            } catch (SQLException e) {
                // TODO 处理异常
                e.printStackTrace();
            }
        }
        if (conn != null) {
            try {
                conn.close();
            } catch (SQLException e) {
                // TODO 处理异常
                e.printStackTrace();
            }
        }
    }

    /**
     * 查询SQL
     * @param sql 查询语句
     * @return 数据集合
     * @throws SQLException
     */
    public static List<Map<String, String>> query(String sql) throws Exception {
        Connection connection = null;
        Statement statement = null;
        ResultSet resultSet = null;
        List<Map<String, String>> resultList = null;
        
        try {
            connection = JDBCTools.getConn();
            
            statement = connection.createStatement();
            resultSet = statement.executeQuery(sql);
            
            ResultSetMetaData resultSetMetaData = resultSet.getMetaData();
            int columnCount = resultSetMetaData.getColumnCount();
            String[] columnNames = new String[columnCount + 1];
            for (int i = 1; i <= columnCount; i++) {
                columnNames[i] = resultSetMetaData.getColumnName(i);
            }

            resultList = new ArrayList<Map<String, String>>();
            Map<String, String> resultMap = new HashMap<String, String>();
            resultSet.beforeFirst();
            while (resultSet.next()) {
                for (int i = 1; i <= columnCount; i++) {
                    resultMap.put(columnNames[i], resultSet.getString(i));
                }
                resultList.add(resultMap);
            }
            // System.out.println("成功查询数据库，查得数据：" + resultList);
        } catch(Throwable t) {
            // TODO 处理异常
            t.printStackTrace();
        } finally {
            JDBCTools.closeResource(connection, statement, resultSet);
        }
        
        return resultList;
    }
    
    /**
     * 执行SQL
     * @param sql 执行的SQL
     * @return 操作条数
     */
    public static int execute(String sql) throws Exception {
        Connection connection = null;
        Statement statement = null;
        ResultSet resultSet = null;
        int num = 0;
        
        try {
            connection = JDBCTools.getConn();
            
            statement = connection.createStatement();
            num = statement.executeUpdate(sql);
            
        } catch(Throwable t) {
            // TODO 处理异常
            t.printStackTrace();
        } finally {
            JDBCTools.closeResource(connection, statement, resultSet);
        }
        
        return num;
    }
    
}

import java.util.List;
import java.util.Map;
import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;


public class Call {
    
    public static ConcurrentHashMap<String, String> map = new ConcurrentHashMap<String, String>();

    public static void main(String[] args) {
        ExecutorService es = Executors.newCachedThreadPool();
        
        for (int i = 0; i < 100; i++) {
            es.execute(new Runnable() {
                
                @Override
                public void run() {
                    String key = getSequence();
                    if (!map.contains(key)) {
                        System.out.println("put " + key);
                        map.put(key, key);
                    } else {
                        System.out.println(key + " is exists.");
                    }
                }
            });
        }

    }
    
    public static String getSequence() {
        List<Map<String, String>> list = null;
        try {
            list = JDBCTools.query("select nextval('user_code') as current");
        } catch (Exception e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
        String temp = list.get(0).get("current");
        return temp;
    }

}

> 用排他锁的方式

上面的程序虽然测试并无发现问题（有可能测试程序写得不好），理论上高并发会出现重复值，用排他锁重写了一下，不过效率会比较低：

CREATE DEFINER = `root`@`localhost` FUNCTION `nextval_safe`(sequence_name varchar(64))
 RETURNS int(11)
BEGIN
    declare current integer;
    set current = 0;
    
    select t.value into current from t_sequence t where t.sequence_name = sequence_name for update;
    update t_sequence t set t.value = t.value + 1 where t.sequence_name = sequence_name;
    set current = current + 1;

    return current;
end;