Java实现的Sequence工具-阿里云开发者社区

Sequence是数据库应用中很重要的一个特性，可是有些数据库没有这个特性，比如很流行的开源数据库MySQL就没有Sequence，利用自增主键感觉很不爽，每个表的ID都要设置，通用性也不好。因此考虑为这些不支持Sequence的数据做一个Sequence工具。自动为整个系统提供主键生成策略。

下面是一个Sequence实现，以数据库MySQL为平台。源代码如下：

一、Sequence工具类

package sequence;

import java.util.HashMap;
import java.util.Map;
import java.sql.SQLException;

/**
* Created by IntelliJ IDEA. 
* User: leizhimin 
* Date: 2008-4-2 15:21:30 
* Note: Java实现的Sequence工具
*/
public class SequenceUtils {
 private static SequenceUtils _instance = new SequenceUtils();
 private Map<String, KeyInfo> keyMap = new HashMap<String, KeyInfo>(20); //Sequence载体容器
 private static final int POOL_SIZE = 10; //Sequence值缓存大小

 /**
 * 禁止外部实例化
 */
 private SequenceUtils() {
 }

 /**
 * 获取SequenceUtils的单例对象
 * @return SequenceUtils的单例对象
 */
 public static SequenceUtils getInstance() {
 return _instance;
 }

 /**
 * 获取下一个Sequence键值
 * @param keyName Sequence名称
 * @return 下一个Sequence键值
 */
 public synchronized long getNextKeyValue(String keyName) {
 KeyInfo keyInfo = null;
 Long keyObject = null;
 try {
 if (keyMap.containsKey(keyName)) {
 keyInfo = keyMap.get(keyName);
 } else {
 keyInfo = new KeyInfo(keyName, POOL_SIZE);
 keyMap.put(keyName, keyInfo);
 }
 keyObject = keyInfo.getNextKey();
 } catch (SQLException e) {
 e.printStackTrace();
 }
 return keyObject;
 }
}

二、Sequence载体

package sequence;

import java.sql.*;

/**
* Created by IntelliJ IDEA. 
* User: leizhimin 
* Date: 2008-4-2 15:24:52 
* Note: Sequence载体
*/
public class KeyInfo {
 private long maxKey; //当前Sequence载体的最大值
 private long minKey; //当前Sequence载体的最小值
 private long nextKey; //下一个Sequence值
 private int poolSize; //Sequence值缓存大小
 private String keyName; //Sequence的名称
 private static final String sql_update = "UPDATE KEYTABLE SET KEYVALUE = KEYVALUE + ? WHERE KEYNAME = ?";
 private static final String sql_query = "SELECT KEYVALUE FROM KEYTABLE WHERE KEYNAME = ?";

 public KeyInfo(String keyName, int poolSize) throws SQLException {
 this.poolSize = poolSize;
 this.keyName = keyName;
 retrieveFromDB();
 }

 public String getKeyName() {
 return keyName;
 }

 public long getMaxKey() {
 return maxKey;
 }

 public long getMinKey() {
 return minKey;
 }

 public int getPoolSize() {
 return poolSize;
 }

 /**
 * 获取下一个Sequence值
 *
 * @return 下一个Sequence值
 * @throws SQLException
 */
 public synchronized long getNextKey() throws SQLException {
 if (nextKey > maxKey) {
 retrieveFromDB();
 }
 return nextKey++;
 }

 /**
 * 执行Sequence表信息初始化和更新工作
 *
 * @throws SQLException
 */
 private void retrieveFromDB() throws SQLException {
 System.out.println("");
 Connection conn = DBUtils.makeConnection();
 //查询数据库
 PreparedStatement pstmt_query = conn.prepareStatement(sql_query);
 pstmt_query.setString(1, keyName);
 ResultSet rs = pstmt_query.executeQuery();
 if (rs.next()) {
 maxKey = rs.getLong(1) + poolSize;
 minKey = maxKey - poolSize + 1;
 nextKey = minKey;
 rs.close();
 pstmt_query.close();
 } else {
 System.out.println( "执行Sequence数据库初始化工作！");
 String init_sql = "INSERT INTO KEYTABLE(KEYNAME,KEYVALUE) VALUES('" + keyName + "',10000 + " + poolSize + ")";
 Statement stmt = conn.createStatement();
 stmt.executeUpdate(init_sql);
 maxKey = 10000 + poolSize;
 minKey = maxKey - poolSize + 1;
 nextKey = minKey;
 stmt.close();
 return;
 }

 //更新数据库
 conn.setAutoCommit( false);
 System.out.println( "更新Sequence最大值！");
 PreparedStatement pstmt_up = conn.prepareStatement(sql_update);
 pstmt_up.setLong(1, poolSize);
 pstmt_up.setString(2, keyName);
 pstmt_up.executeUpdate();
 pstmt_up.close();
 conn.commit();

 rs.close();
 pstmt_query.close();
 conn.close();
 }
}

三、简单的数据库连接工具

package sequence;

import java.sql.*;

/**
* 简单的数据连接工具
* File: DBUtils.java
* User: leizhimin
* Date: 2008-3-18 15:19:12
*/
public class DBUtils {
     public static final String url = "jdbc:mysql://127.0.0.1:3306/testdb";
    public static final String username = "root";
    public static final String password = "leizhimin";
    public static final String driverClassName = "com.mysql.jdbc.Driver";

    /**
     * 获取数据库连接Connection
     *
     * @return 数据库连接Connection
     */
    public static Connection makeConnection() {
        Connection conn = null;
        try {
            Class.forName(driverClassName);
        } catch (ClassNotFoundException e) {
            e.printStackTrace();
        }
        try {
            conn = DriverManager.getConnection(url, username, password);
        } catch (SQLException e) {
            e.printStackTrace();
        }
        return conn;
    }

    public static void main(String args[]) {
        testConnection();
    }

    /**
     * 测试连接方法
     */
    public static void testConnection() {
        Connection conn = makeConnection();
        try {
            Statement stmt = conn.createStatement();
            ResultSet rs = stmt.executeQuery("SELECT * FROM mysql.user");
            while (rs.next()) {
                String s1 = rs.getString(1);
                String s2 = rs.getString(2);
                System.out.println(s1 + "\t" + s2);
            }
            rs.close();
            stmt.close();
        } catch (SQLException e) {
            e.printStackTrace();
        } finally {
            try {
                conn.close();
            } catch (SQLException e) {
                e.printStackTrace();
            }
        }
    }
}

四、测试类（客户端）

package sequence;

/**
* Created by IntelliJ IDEA.
* User: leizhimin
* Date: 2008-4-2 15:31:30
* Company: LavaSoft([url]http://lavasoft.blog.51cto.com[/url])
* Sequence测试(客户端)
*/
public class TestSequence {
 /**
 * 测试入口
 * @param args
 */
 public static void main(String args[]) {
 test();
 }

 /**
 * 测试Sequence方法
 */
 public static void test() {
 System.out.println( "----------test()----------");
 for ( int i = 0; i < 20; i++) {
 long x = SequenceUtils.getInstance().getNextKeyValue( "sdaf");
 System.out.println(x);
 }
 }
}

五、Sequence表的代码（for MySQL5）

-- SQL for MySQL5
-- [url]http://lavasoft.blog.51cto.com[/url]
-- 2008年4月3日

-- 创建数据库testdb
create database if not exists testdb character set gbk collate gbk_chinese_ci;

-- 创建Sequence表
DROP TABLE IF EXISTS keytable;
CREATE TABLE keytable (
  KEYNAME varchar(24) NOT NULL COMMENT 'Sequence名称',
  KEYVALUE bigint(20) DEFAULT '10000' COMMENT 'Sequence最大值',
  PRIMARY KEY (KEYNAME)
) ENGINE=MyISAM DEFAULT CHARSET=gbk;

数据库操作的截屏：

六、运行测试类输出结果：

----------test()----------

更新Sequence最大值！
10061
10062
10063
10064
10065
10066
10067
10068
10069
10070

更新Sequence最大值！
10071
10072
10073
10074
10075
10076
10077
10078
10079
10080

Process finished with exit code 0

说明：这个Sequence工具很智能，当请求一个不存在的Sequence时，会自动生成一个Sequence，保存到数据库。当请求一个已经存在的Sequence时，会从Sequence载体的缓存中获取一个Sequence值返回，当Sequence载体的缓存值达到最大时，会自动重新初始化Sequence载体信息，并更新数据库记录信息。

本文转自 leizhimin 51CTO博客，原文链接：http://blog.51cto.com/lavasoft/69682，如需转载请自行联系原作者

Java实现的Sequence工具

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