一个生成全局唯一Sequence ID的高并发工厂类 (Java）

一个生成全局唯一Sequence ID的高并发工厂类 (Java）

发表于10个月前(2013-03-21 17:26)   阅读（593） | 评论（1） 14人收藏此文章, 我要收藏

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java sequence

Sequence是数据库系统按照一定规则自动增加的数字序列。这个序列一般作为代理主键（因为不会重复），没有其他任何意义。 

Sequence是数据库系统的特性，有的数据库实现了Sequence，有的则没有。比如Oracle、DB2、PostgreSQL数据库实现Sequence, 而MySQL、SQL Server、Sybase等数据库没有Sequence。

那么如何给一个不支持Sequence的数据库增加支持呢？

下面将给出Java版本的代码。这个代码还实现了对 Sequence ID的高效缓存，并不是每一次调用都会对数据库进行访问，在高并发环境下性能优异。

1. SequenceId.java

01	package uuid;

02

03	public class SequenceId {

04	public static final long NOT_FOUND = 0;

05	private static final long STEP = 100;

06	private final SequenceIdProvider provider;

07	private final String name;

08	private final long beginValue;

09	private long value;

10

11	protected SequenceId(SequenceIdProvider provider, String name, long beginValue) {

12	this.provider = provider;

13	this.name = name;

14	this.beginValue = beginValue;

15	this.value = -1;

16

17	if (beginValue <= 0) {

18	throw new IllegalArgumentException("begin value must be great than zero.");

19

}

20

}

21

22	public String getName() {

23	return name;

24

}

25

26	public synchronized long nextVal() {

27	if (value < 0) {

28	value = provider.load(name);

29	if (value <= NOT_FOUND) {

30	value = beginValue - 1;

31

}

32	provider.store(name, value + STEP);

33

}

34

35

value++;

36

37	if (value % STEP == 0) {

38	provider.store(name, value + STEP);

39

}

40

41	return value;

42

}

43

}

2. SequenceIdProvider.java

01	package uuid;

02

03	public interface SequenceIdProvider {

04

05	public SequenceId create(String name);

06

07	public SequenceId create(String name, long begin);

08

09	public long load(String name);

10

11	public void store(String name, long value);

12

13

}

3. JdbcSequenceIdProvider.java

001	package uuid;

002

003	import java.sql.*;

004	import javax.sql.DataSource;

005

006	public class JdbcSequenceIdProvider implements SequenceIdProvider {

007

008	private static final String TABLE_NAME = "_SEQUENCE_";

009	private final DataSource dataSource;

010

011	public JdbcSequenceIdProvider(DataSource dataSource) {

012	this.dataSource = dataSource;

013

014	confirmTableExists();

015

}

016

017

@Override

018	public SequenceId create(String name) {

019	return new SequenceId(this, name, 1);

020

}

021

022

@Override

023	public SequenceId create(String name, long begin) {

024	return new SequenceId(this, name, begin);

025

}

026

027	private void confirmTableExists() {

028	Connection conn = null;

029

try {

030	conn = dataSource.getConnection();

031

032	ResultSet rs = conn.getMetaData().getTables(null, null, TABLE_NAME, null);

033	boolean found = rs.next();

034	rs.close();

035

036	if (!found) {

037	Statement stmt = conn.createStatement();

038	String sql = "create table " + TABLE_NAME + " (name varchar(50) not null, next_val long not null, primary key(name))";

039	stmt.execute(sql);

040	stmt.close();

041

}

042	} catch (SQLException e) {

043	throw new RuntimeException(e);

044	} finally {

045	close(conn);

046

}

047

}

048

049	private void close(Connection conn) {

050	if (conn != null) {

051

try {

052	conn.close();

053	} catch (SQLException e) {

054	throw new RuntimeException(e);

055

}

056

}

057

}

058

059

@Override

060	public long load(String name) {

061	long value = SequenceId.NOT_FOUND;

062	Connection conn = null;

063

try {

064	conn = dataSource.getConnection();

065	String sql = "select next_val from " + TABLE_NAME + " where name=?";

066	PreparedStatement ps = conn.prepareStatement(sql);

067	ps.setString(1, name);

068	ResultSet rs = ps.executeQuery();

069	if (rs.next()) {

070	value = rs.getLong(1);

071

}

072	rs.close();

073	ps.close();

074	} catch (SQLException e) {

075	throw new RuntimeException(e);

076	} finally {

077	close(conn);

078

}

079	return value;

080

}

081

082

@Override

083	public void store(String name, long value) {

084	Connection conn = null;

085

try {

086	conn = dataSource.getConnection();

087	String sql = "update " + TABLE_NAME + " set next_val=? where name=?";

088	PreparedStatement ps = conn.prepareStatement(sql);

089	ps.setLong(1, value);

090	ps.setString(2, name);

091	int updated = ps.executeUpdate();

092	ps.close();

093

094	if (updated == 0) {

095	sql = "insert into " + TABLE_NAME + " (name, next_val) values (?,?)";

096	ps = conn.prepareStatement(sql);

097	ps.setString(1, name);

098	ps.setLong(2, value);

099	ps.executeUpdate();

100	ps.close();

101

}

102	} catch (SQLException e) {

103	throw new RuntimeException(e);

104	} finally {

105	close(conn);

106

}

107

}

108

}

4. GlobalSequenceIdProvider.java

01	package uuid;

02

03	import javax.sql.DataSource;

04

05	public class GlobalSequenceIdProvider {

06

07	private static final SequenceId global;

08	private static final SequenceId table1;

09	private static final SequenceId table2;

10

11

static {

12	SequenceIdProvider provider = new JdbcSequenceIdProvider(getDataSource());

13	global = provider.create("global");

14	table1 = provider.create("table1");

15	table2 = provider.create("table2", 1000);

16

}

17

18	public static long nextVal() {

19	return global.nextVal();

20

}

21

22	public static long nextVal_table1() {

23	return table1.nextVal();

24

}

25

26	public static long nextVal_table2() {

27	return table2.nextVal();

28

}

29

30	private static DataSource getDataSource() {

31

// TODO:

32	return null;

33

}

34

35

}