How to start a transaction in JDBC?

Connection.setTransactionIsolation(int) warns:

Note: If this method is called during a transaction, the result is implementation-defined.

This bring up the question: how do you begin a transaction in JDBC? It’s clear how to end a transaction, but not how to begin it.

If a Connection starts inside in a transaction, how are we supposed to invoke Connection.setTransactionIsolation(int) outside of a transaction to avoid implementation-specific behavior?

Answer

Answering my own question:

• JDBC connections start out with auto-commit mode enabled, where each SQL statement is implicitly demarcated with a transaction.
• Users who wish to execute multiple statements per transaction must turn auto-commit off.
• Changing the auto-commit mode triggers a commit of the current transaction (if one is active).
• Connection.setTransactionIsolation() may be invoked anytime if auto-commit is enabled.
• If auto-commit is disabled, Connection.setTransactionIsolation() may only be invoked before or after a transaction. Invoking it in the middle of a transaction leads to undefined behavior.

See JDBC Tutorial by Oracle.