The J2EE™ Tutorial Second Edition

Isolation Levels

Transactions not only ensure the full completion (or rollback) of the statements that they enclose but also isolate the data modified by the statements. The isolation leveldescribes the degree to which the data being updated is visible to other transactions.

Suppose that a transaction in one program updates a customer's phone number, but before the transaction commits, another program reads the same phone number. Will the second program read the updated and uncommitted phone number, or will it read the old one? The answer depends on the isolation level of the transaction. If the transaction allows other programs to read uncommitted data, performance may improve because the other programs don't have to wait until the transaction ...