The resource is declared inside the try (...) parentheses; when control leaves the block (normal exit, return, or exception) the runtime calls close() automatically. Compare this with the pre-Java-7 idiom of a try / finally that had to null-check and re-catch close exceptions: a 15-line dance shrinks to four lines. The compiler also wires up suppressed exceptions correctly, so a primary failure inside the block is not lost when close() itself throws.

Multiple resources are separated by semicolons inside the try parentheses. Closure order is the reverse of declaration order, which matters when one resource depends on another (a BufferedWriter wrapping a FileOutputStream, for instance). Java 9 added the ability to list already-declared final (or effectively final) variables instead of re-declaring them, useful when the resource is created elsewhere. Use this when an outer class owns the lifetime but a single block needs deterministic cleanup.

Implement AutoCloseable (or its checked-exception cousin Closeable) and any custom resource type plays nicely with the pattern. The output shows released B before released A: closures fire in reverse declaration order, even when an exception aborts the block. This is the same guarantee you rely on for I/O streams. Reach for a custom AutoCloseable when wrapping a connection pool checkout, a thread-pool task token, or any acquire/release pair where forgetting the release leaks resources.