标签：
丢失更新：一个事务的更新被另一个事务的更新覆盖了；
时间点 | 事务1 | 事务2 |
t1? | 开始事务 | ? |
t2? | ? | 开始事务 |
t3? | 查询pid=p1的记录结果为[pid=p1,pname=zhangSan,age=23,sex=male] | ? |
t4 | ? | 查询pid=p1的记录结果为[pid=p1,pname=zhangSan,age=23,sex=male] |
t5? | 修改age=24，其它保留原值，即： update person set pname=‘zhangSan‘, age=24,sex=‘male‘ where pid=‘p1‘; | ? |
t6? | 提交事务 | ? |
t7? | ? | 修改sex=female，其它保留原值 update person set pname=‘zhangSan‘, age=23,sex=‘female‘ where pid=‘p1‘; |
t8 | ? | 提交事务 |
?
事务2覆盖了事务1的更新操作。结果为：[pid=p1,pname=zhangSan,age=23,sex=female]。因为事务2没有在事务1的基础上进行更新，而是在自己的查询基础上进行更新。
?
public class Demo1 { ????private static Connection getConnection() throws Exception { ????????String driverClassName = "com.mysql.jdbc.Driver"; ????????String url = "jdbc:mysql://localhost:3306/day12?useUnicode=true&characterEncoding=utf8"; ????????String username = "root"; ????????String password = "123"; ? ????????Class.forName(driverClassName); ????????return DriverManager.getConnection(url, username, password); ????} ? ????public Person load(Connection con, String pid) throws Exception { ????????String sql = "select * from t_person where pid=?"; ????????PreparedStatement pstmt = con.prepareStatement(sql); ????????pstmt.setString(1, pid); ????????ResultSet rs = pstmt.executeQuery(); ????????if (rs.next()) { ????????????return new Person(rs.getString(1), rs.getString(2), rs.getInt(3), ????????????????????rs.getString(4)); ????????} ????????return null; ????} ???? ????public void update(Connection con, Person p) throws Exception { ????????String sql = "update t_person set pname=?, age=?, gender=? where pid=?"; ????????PreparedStatement pstmt = con.prepareStatement(sql); ????????pstmt.setString(1, p.getPname()); ????????pstmt.setInt(2, p.getAge()); ????????pstmt.setString(3, p.getGender()); ????????pstmt.setString(4, p.getPid()); ???????? ????????pstmt.executeUpdate(); ????} ???? ????@Test ????public void fun1() throws Exception { ????????Connection con = getConnection(); ????????con.setAutoCommit(false); ???????? ? //[pid=p1,pname=zs,age=24,gender=male] ????????Person p = load(con, "p1"); ????????p.setAge(42);//断点 ????????update(con, p); ???????? ????????con.commit(); ????} ????@Test ????public void fun2() throws Exception { ????????Connection con = getConnection(); ????????con.setAutoCommit(false); //[pid=p1,pname=zs,age=24,gender=male] ????????Person p = load(con, "p1"); ????????p.setGender("female");//断点 ????????update(con, p); ???????? ????????con.commit(); ????} } |
?
处理丢失更新：
?
7.1 悲观锁解决丢失更新
只需要修改上面代码的load()方法中select语句即可：
select * from t_person where pid=? for update
?
????public Person load(Connection con, String pid) throws Exception { ????????String sql = "select * from t_person where pid=? for update"; ????????PreparedStatement pstmt = con.prepareStatement(sql); ????????pstmt.setString(1, pid); ????????ResultSet rs = pstmt.executeQuery(); ????????if (rs.next()) { ????????????return new Person(rs.getString(1), rs.getString(2), rs.getInt(3), ????????????????????rs.getString(4)); ????????} ????????return null; ????} |
?
悲观锁：悲观的思想，认为丢失更新问题总会出现，在select语句中添加for update为事务添加排他锁，这会让其他事务等待当前事务结束后才能访问。当然，其他事物的select语句中也要加上for update语句才会等待；
悲观锁的性能低！
?
7.2 乐观锁
乐观锁与数据库锁机制无关；
我们需要修改t_person表，为其添加一个字段表示当前记录的版本。例如给t_person表添加version字段，默认值为1。
当事务查询记录时得到version=1，再执行update时需要比较当前version的值是否与查询到的version相同，决定update是否执行成功。如果update成功，还要把version的值加1。
????public void update(Connection con, Person p) throws Exception { ????????String sql = "update t_person set pname=?, age=?, gender=?, version=version+1 where pid=? and version=?"; ????????PreparedStatement pstmt = con.prepareStatement(sql); ????????pstmt.setString(1, p.getPname()); ????????pstmt.setInt(2, p.getAge()); ????????pstmt.setString(3, p.getGender()); ????????pstmt.setString(4, p.getPid()); ????????pstmt.setInt(5, p.getVersion()); ???????? ????????pstmt.executeUpdate(); ????} |
?
?
乐观锁：与数据库锁机制无关，乐观的思想，认为丢失更新不是总出现；通过给表添加版本字段来决定update操作是否成功。即查询时和更新时的版本必须一致！
标签：
原文地址：http://www.cnblogs.com/Prozhu/p/5541980.html
