数据库访问通常会对被查询的数据进行读锁定，在数据更新后会变成更新锁定。
The normal behaviour of a database access is to get a read lock on the queried data, which gets transformed into an update lock if data is updated.
如果您清楚可以对实体bean随时进行读操作且实体bean不会更新，那么该策略可以模拟只读锁定。
If you know that an entity bean will only ever be read and not updated, then this policy will simulate a read-only lock.
在发出独占锁定后，其他的读或写操作将不会再发生。
Once an exclusive lock has been issued, no other read or write operations can take place.
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