How does try-with-resources work?

A resource is anything that implements AutoCloseable. The compiler generates a finally block that calls close, so you cannot forget cleanup. If the body throws and close also throws, the body's exception remains primary and the close failure is added as a suppressed exception.

try (BufferedReader reader =
         Files.newBufferedReader(Path.of("data.txt"))) {
    return reader.readLine();
}

Details worth knowing

• Multiple resources close in reverse declaration order
• Java 9 allows an effectively final existing variable in the header
• Use getSuppressed on the primary exception to inspect close failures
• A resource that does not need closing should not be put in the header

Try-with-resources is the standard way to manage files, sockets, JDBC statements and locks that implement AutoCloseable. If you write a resource class, implement close idempotently and release native handles promptly rather than waiting for a finalizer.