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Oracle的Sequence用爽了，发现MySQL没有Sequence，那么，自己写一个呗。
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; } }
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;
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原文地址：http://www.cnblogs.com/nick-huang/p/5641977.html
