Transactions are a high-level alternative for low-level concurrency-control mechanisms such as locks, semaphores, monitors. A recent proposal for integrating transactional features into programming languages is Transactional Featherweight Java (TFJ), extending Featherweight Java by adding transactions. With support for nested and multi-threaded transactions, its transactional model is rather expressive. In particular, the constructs governing transactions -to start and to commit a transaction- can be used freely with a non-lexical scope. On the downside, this flexibility also allows for an incorrect use of these constructs, e.g., trying to perform a commit outside any transaction. To catch those kinds of errors, we introduce a static type and effect system for the safe use of transactions for TFJ. We prove the soundness of our type system by subject reduction.
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