JEP 544: Ahead-of-Time Code Compilation

SSkinney about 3 hours ago 19 commentsRead Article on openjdk.org

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cyberax•about 3 hours ago

Excelsior JET did that 25 years ago: https://en.wikipedia.org/wiki/Excelsior_JET

It's a shame that Sun/Oracle never partnered with them to bring native apps. This could have saved Java on the desktop.

pjmlp•about 1 hour ago

They were not the only ones, see PTC, Aicas, IBM, and a few others.

The big difference until GraalVM and OpenJ9 became available as open source, was that all those vendors made AOT compilation a commercial feature, and thus most devs never cared they existed.

In fact GraalVM was one of the main reasons Excelsior JET eventually closed doors.

wahern•about 2 hours ago

Don't forget GCJ. It wasn't removed from GCC until about 2016, a nearly 20-year long run, though interest had waned many years prior.

I don't remember if GCJ supported loading and running code dynamically with a built-in JIT or interpreter. I think it was just pure AOT, which caused some compatibility headaches.

java-man•about 2 hours ago

GCJ was great - I wrote a handset UI entirely in java for a startup in the early 2000's, basically android before android existed, using GCJ. I remember their native interface was somewhat more convenient to use than JNI.

I wish we had GCJ resurrected, now that the java libraries are GPL'd.

alex7o•30 minutes ago

Just ask claude to port it to latest gcc :P, pls don't

cyberax•about 1 hour ago

Yeah! I used it around 2003 to integrate a Java PDF parsing library into our C++ app.

Its biggest downside was the use of Boehm GC that had some issues with large heaps.

patwolf•about 3 hours ago

IBM's J9 had AOT for a long time too. I wonder how JEP 544 compares.

java-man•about 3 hours ago

I hope at some point we'll get AOT-only mode (or compile to native) and maybe even cross-compilation.

layer8•about 2 hours ago

That would place significant limitations on the applications that can be compiled. As the JEP explains: “Features such as dynamic class loading, dynamic linkage, dynamic dispatch, and dynamic reflection bring vast expressive power, and have been fundamental to the platform's success. HotSpot handles these features naturally, while static compilers struggle with them. Even heroic amounts of static analysis cannot make up for the fact that these features require many decisions to be made at run time. Implementors of static compilers for Java code have therefore resorted to incompatible constraints, such as closed-world assumptions, and to putting significant burdens on developers, such as having to identify in advance the classes eligible for reflection.”

Therefore I don’t see AOT-only becoming an integral part of standard Java in the foreseeable future.

jasomill•5 minutes ago

It works pretty well for greenfield projects on .NET, so long as you can live without third-party libraries that don't support AOT.

Existing code bases can be a pain, though, especially applications that heavily rely on things like C++/CLI and COM Interop that basically need to be rewritten from scratch.

java-man•37 minutes ago

we are not talking about fully supporting dynamic loading, because it's not needed in all the cases, and in some cases, the list of allowed classes in the application can (or must) be limited.

i guess what i am saying is that AOT-only case is not "standard java", but something that will make a (more) useful replacement for things like c/c++.

nirvdrum•about 2 hours ago

If you haven't checked it out yet, GraalVM's Native Image [1] already let's do AOT compilation suitable for distribution. You have to do some configuration to deal with reflection and dynamic class loading since there's no JVM in the produced build, but it comes with tooling to simplify that. And that restriction is being addressed by Project Crema [2].

[1] -- https://www.graalvm.org/latest/reference-manual/native-image... [2] -- https://github.com/oracle/graal/issues/11327

Twirrim•31 minutes ago

The slight drawback is that AOT compiled Java code's performance isn't as good as the JIT compiled form running on the JVM.

There's a reason why folks keep coming back to JIT compilation. It's hard to beat the value that can be gained from actually gathering data on how the code is actually used, which leads to a whole set of potential optimisations (which is the problem Profile Guided Optimisation attempts to solve for AOT compiled code.) The JVM is arguably one of the most advanced and capable JITing runtimes.

As with anything it's a trade off. Fast start times, pretty fast running (I think maybe less memory usage?); vs the full JIT speed you can get on the JVM. All depends on what you want to use the application for.

If you're talking something like a serverless function, go native. If you're talking production server where you're measuring runtime in more than dozens of minutes, probably better to stick to the JVM & JIT.

exabrial•about 2 hours ago

progress, not perfection! Java is on fire right now with new stuff landing. We'll get there!

invalidname•about 3 hours ago

We have that in Codename One, yes it's not really "Java" but it compiles bytecode AOT and cross compiles to some of the platforms.

SillyUsername•about 3 hours ago

So we've gone full circle again?

I suppose write once run anywhere is no longer a goal either.

"It is not a goal to support all CPU architectures currently supported by HotSpot."

This pretty much validates the point of view that VM design is now baggage, as a sandbox it has been flawed, for performance it's been prohibitive, and cross platform portability by virtue of being virtual, was just a convenient byproduct.

AI now handles the portability, sandbox security hasn't changed (cf. docker still has sandbox problems, LLMs have sandbox problems, it's always an ongoing concern) and that just leaves Java, as always, chasing performance.

randomUUID•about 2 hours ago

I don't get your point. This is what JEP says:

- "Improve startup and warmup time by making optimized native code for an application instantly available when the HotSpot Java Virtual Machine starts."

- "If the workload changes in production, regenerate native code dynamically for continued peak performance, providing the best of both ahead-of-time (AOT) and just-in-time (JIT) compilation."

- "Ensure that shifting from AOT-compiled code to JIT-compiled code is invisible to applications."

That is JEP544 doesn't substitute JVM and JIT optimizations. It just changes the nature of the JIT's outcome.

nirvdrum•about 2 hours ago

The very next sentence after the one you posted is "We expect normal porting activities to eventually add support for all major architectures." Goals and non-goals inform the scope of the proposal. Given this is effectively an "add-on", I don't think it's unreasonable to prioritize the most popular platforms. Others will continue to execute Java perfectly fine in JVM mode.

pjmlp•about 1 hour ago

Not at all, this is OpenJDK getting JIT cache feature like OpenJ9 has since 2008, or ART since Android 7.