Chapter 17 Support for Transactions 17.6 Container Provider responsibilities 17.6.1 Bean-managed transaction demarcation
If a transaction is present the container will suspend it before the BMT bean method is run - regardless whether it will or will not start its own transaction (in fact, the container has no idea what the bean will or will not do).
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Building a Better World in your Backyard by Paul Wheaton and Shawn Klassen-Koop