java - 事务 与 更新丢失的问题？

Question

先查询某一行的值，然后在更新这个值。在高并发的情况下，A 用户 查出来的值比如是 8，这时候按着8进行处理过程中，有另外的用户B，将这个值改成了10，当A用户再去更新的时候，就会造成数据的更新丢失。

通过对查询更新方法设置事务，加入防重复读的隔离级别，也是解决不了更新丢失问题的。防重复读，只能保证第一次读到是8，后面在怎么读这条记录，结果都是8。

解决这个问题，在mysql 数据库层面，只有用for update （悲观锁）或是乐观锁来锁住这一行记录。

问题是，对于事务与mysql悲观锁的理解有点混沌了。请高人给指点迷津。

Answer 1

Enabling the default isolation level of mysql in the transaction state can already solve this problem.

Answer 2

For example
SET AUTOCOMMIT=0; BEGIN WORK;
SELECT quantity FROM products WHERE id=3 FOR UPDATE;
UPDATE products SET quantity = '1' WHERE id=3;
COMMIT WORK;

You can turn things on.
Then add FOR UPDATE when reading this row of records to lock this record
Using FOR UPDATE must use a transaction, because not using a transaction is similar to not using a FOR UPDATE, and using FOR in a transaction After UPDATE, before COMMIT/ROLLBACK,
If other sessions read this row with id=3, they will wait forever, wait for your transaction to end, and read new rows
Secondly, FOR UPDATE is best used with id = xx or id in (xx,xx) otherwise the database will be downgraded to a lock table

Answer 3

WHERE Riga conditions:

SELECT quantity FROM products WHERE id=3; 假设读到的quantity为8
UPDATE products SET quantity = '10' WHERE id=3 AND quantity=8;

Answer 4

There are 4 levels of transactions
Read committed, read uncommitted, rereadable, serialized
Please describe your needs carefully, let me take a look!