Furthermore, if the service isn't available (there isn't a database, or the database has been temporarily stopped), then this method returns null to indicate that no such service is available.

此外，如果服务不可用(数据库不存在，或数据库临时停掉了)，那么这个方法会返回null以说明该服务不可用。

youdao

If this function returns NULL, no exception has been raised and the caller should assume no current thread state is available.

如果这个函数返回NULL，没有抛出异常，调用者会假定当前线程状态无效。

youdao

College endowments have recovered somewhat, but both their investment policies and the availability of alternative investments have changed substantially, so the stellar returns of years past are no longer available to reduce the out-of-pocket cost to students.

FORBES: College Aid Announcements Sign of Predictable Change