Java实现的Sequence工具
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.<br>
* <b>User</b>: leizhimin<br>
* <b>Date</b>: 2008-4-2 15:21:30<br>
* <b>Note</b>: 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.<br>
* <b>User</b>: leizhimin<br>
* <b>Date</b>: 2008-4-2 15:24:52<br>
* <b>Note</b>: 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。
参考文档：《Java与模式》多例模式一章。以上代码仅仅是阎宏博士一个例子的具体实现。
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