Java 17 compatibility for matchbox-engine — is Java 21 a hard requirement?

[bgrommes]

I'm looking to embed matchbox-engine as a library in a data pipeline running on AWS EMR (Spark 3.5), where the maximum supported Java runtime is Java 17. The current POM sets <maven.compiler.release>21</maven.compiler.release>, which means the compiled JAR won't load on Java 17.
Before forking and maintaining a Java 17 rebuild, I wanted to ask:

1. Does the matchbox-engine codebase actually use any Java 21-specific language features or APIs, or was the version bump to 21 conservative/precautionary?
2. Would a simple change of <maven.compiler.release> from 21 to 17 in the parent POM be expected to compile and run correctly, or are there known incompatibilities?
3. Is there any plan or appetite to support Java 17 as a minimum target going forward, given that Java 17 is the LTS version most widely supported by managed cloud platforms (AWS EMR, Snowflake, etc.)?

Thanks.

[bgrommes]

For more context it looks like the move to compile with Java 21 was made in release 4.0.0.

• See diff.
• See PR 354 - Upgrade to HAPI 8.0.0 and FHIR Core 6.5.15

I checked in the hapi-fhir and org.hl7.fhir.core projects and the current versions of those dependent libraries are compiled for Java 17.

[qligier]

Hi @bgrommes! From what I see, the only use of Java 21 in matchbox-engine is two List.getFirst() calls in tests. It would be easy to move back to Java 17 (but only for matchbox-engine, since matchbox-server makes much more use of Java 21 features).

From a quick test, it seems to work just fine like that, dependencies shouldn't be an issue.

Java 17 is the LTS version most widely supported by managed cloud platforms (AWS EMR

I didn't know that. Weird that Amazon doesn't support their own Corretto LTS versions.

[bgrommes1]

Thanks for the reply. Glad to hear Java 17 is feasible.

On the point about Amazon Corretto LTS versions, I think the main limitation is that Spark is written in Scala and has been slow to move to Scala 3.x, the first version that can run on Java 21. Amazon EMR is currently previewing a version that works with Scala 3.x and Java 21.

It would be easy to move back to Java 17 (but only for matchbox-engine, since matchbox-server makes much more use of Java 21 features).

What Java 21 features are being used in matchbox-server? I don't see any Java 21 language features besides the use of List.getFirst().

I forked the project and tested out compiling with Java 21 by replacing the List.getFirst() calls with List.get(0) in both matchbox-engine and matchbox-server. See this branch for all the changes. For the transform and validate usage I'm currently doing things seem to work fine in Java 17.

Let me know your thoughts on whether you would be willing to move the main project back to Java 17. Thanks.

[qligier]

matchbox-server makes also use of pattern matching in switches, e.g.

matchbox/matchbox-server/src/main/java/ch/ahdis/matchbox/mappinglanguage/StructureMapTransformProvider.java

Lines 302 to 321 in 4d4662b

	switch (theResource) {
	case final org.hl7.fhir.r4.model.StructureMap r4 -> {
	r4.getText().setStatus(NarrativeStatus.GENERATED);
	r4.getText().setDiv(new XhtmlNode(NodeType.Element, "div"));
	r4.getText().getDiv().addTag("pre").addText(render);
	}
	case final org.hl7.fhir.r4b.model.StructureMap r4b -> {
	r4b.getText().setStatus(org.hl7.fhir.r4b.model.Narrative.NarrativeStatus.GENERATED);
	r4b.getText().setDiv(new XhtmlNode(NodeType.Element, "div"));
	r4b.getText().getDiv().addTag("pre").addText(render);
	}
	case final StructureMap r5 -> {
	r5.getText().setStatus(org.hl7.fhir.r5.model.Narrative.NarrativeStatus.GENERATED);
	r5.getText().setDiv(new XhtmlNode(NodeType.Element, "div"));
	r5.getText().getDiv().addTag("pre").addText(render);
	}
	case null -> throw new IllegalStateException("StructureMapTransformProvider: theResource is null");
	default -> throw new MatchboxUnsupportedFhirVersionException("StructureMapTransformProvider",
	theResource.getStructureFhirVersionEnum());
	}

DummyValidationSupport is also implemented as a Record.

I'm unsure why you could compile at level 17 with these still in the code. But that shouldn't be an issue, we can just move matchbox-engine back to 17, that would be enough for your usage.

[bgrommes1]

@qligier my bad on the conclusion about Java 21 source compatibility scope. I only compiled matchbox-engine.

So yes, supporting Java 17 in just matchbox-engine will be perfect. Are you willing to make that switch in the next planned release? Thanks again.

[qligier]

Yes, I see no reason not to support Java 17 in the engine. I'll open a PR sometime this week.