Hi Rob, Can you share an actual piece of code that would benefit from such a lambda migration? I have checked the library code of java.base module; the try-with-resources and finally are mostly used for I/O and security cleanups, none of which would significantly benefit from this improvement. In particular, I noted this form would only be more useful than finally with these preconditions:
1. The cleanup action must be throwing some exception 2. The cleanup action doesn't make use of local variables Which IMO makes this usage much more niche. Finally, for your proposed solution, I think we can accomplish that with a subinterface of AutoCloseable that has this type parameter - this does not require existing implementors to update, yet offers the same benefits. Regards, Chen Liang Confidential – Oracle Internal ________________________________ From: core-libs-dev <[email protected]> on behalf of Rob Spoor <[email protected]> Sent: Tuesday, October 7, 2025 2:43 PM To: [email protected] <[email protected]> Subject: RFE: make AutoCloseable generic in its exception type Hi all, AutoCloseable, as added in Java 7 and unchanged ever since, declares to throw Exception from its close() method. That means that anytime someone needs to have an AutoCloseable that throws a different exception, they must create an interface that extends AutoCloseable and override close() only to throw a different (or no) exception. That can become quite annoying if you have to do it often. But without such interfaces, we cannot use lambdas or method references in try-with-resources without having to deal with Exception: try (AutoCloseable _ = this::cleanup) { } // need to catch or re-throw Exception I think that making AutoCloseable generic in its exception type is a change that's quite simple, yet makes life easier for developers. The interface would become this: public interface AutoCloseable<X extends Exception> { /** * <existing javadoc minus throws * * @throws X if this resource cannot be closed */ void close() throws X; } In its raw form, the interface is exactly what we already have: close() throws Exception. Therefore, any use of AutoCloseable that doesn't fill in the generic type (including extending interfaces) will work as they do now. They can still override close() and change the thrown exception type. Interfaces that extend AutoCloseable can fill in the generic type, and if the close() method doesn't add additional documentation it can be removed completely. Interfaces like Closeable will only have to change the extends clause: public interface Closeable extends AutoCloseable<IOException> { // keep close for its documentation } That doesn't add much value yet, but we can now use it in try-with-resources without having to deal with Exception: try (AutoCloseable<RuntimeException> c = this::cleanup) { } // no need to catch or re-throw RuntimeException There is one other possible change that can make it even easier, and that is adding a factory method like Gatherer.Integrator.of: public interface AutoCloseable<X extends Exception> { // documentation omitted void close() throws X; // documentation omitted static <X extends Exception> AutoCloseable<X> of(AutoCloseable<X> closeable) { return closeable; } } The try-with-resources can now become even simpler: try (var _ = AutoCloseable.of(this::cleanup)) { } There is one possible issue with this "of" method, and that is possible incompatibilities with existing implementing classes or extending interfaces that already have an "of(AutoCloseable)" method with a different return type, or that are not static. I think the chances of that are not great, but it still is something to consider. Any thoughts?
