Java 25 Support

[holgerfriedrich]

This issues is to summarise the current state of Java 25 support.
Java 25 is the next LTS release and is likely to be supported by OH in the future.

Java 25 brings a lot of improvements. I am curious especially about JEP 483: Ahead-of-Time Class Loading & Linking! How much faster will be the start on a RPi?

The migration will be done slowly. We will use Java 21 as the major and supported Java version.
In the background, we will work hard to remove blockers. I will add a Java 25 build to the CI, first only compiling with Java 25 compiler configured to generate Java 21 code. This will be extended over the next months once we are able to remove blockers.

Those are the steps to be taken:

Compile with Java 25, generate Java 21 bytecode

• Compile
• Unit-tests
• iTests - work with bndtools 7.2.1 and junit downgraded below v13 (due to transitive dependency) in 7.2.0
• Code Analysis
• bndtools 7.1 -> 7.2.1 (done for core, prepared for add-ons)
• Add JDK25 generating Java21 bytecode #5290
• openhab/openhab-addons#...
• upgrade from ecj 3.43 to 3.45, transition from Java 24 to Java 25 in ecj (currently core is on Java 24 due to issues in ecj 3.44)

Compile with Java 25, generate (mostly) Java 25 bytecode

• tools like maven-shade-plugin, ecj 3.43 are not supporting Java 25 bytecode -> limit this bundles to Java 24
• Code Analysis - tools to not yet support Java 25 bytecode -> do the necessary upgrades, release static-code-analysis
• Blocker: Karaf 4.4.x does not support Java 25

Run openHAB installation in Java 25

• Blocker: Karaf 4.4.x -> still relays on pax logging, calling an unsupported function due to removal of SecurityManager in JDK 25 -> wait for Karaf 4.5 targeted for YE 2025 2026
• Blocker: Some deprecated methods in JAAS cause Exceptions in JDK25, and need to be replaced. The new API is available in Java 21 (maybe before), but Karaf 4.5 branch needs some more upgrades to work with Java 21
• Distro has downgraded to bndtools 7.0 due to an issue with Eclipse -> check if 7.2.1 does work

[openhab-bot]

This issue has been mentioned on openHAB Community. There might be relevant details there:

https://community.openhab.org/t/are-there-any-experiences-and-recommendations-regarding-openhab-5-and-java-25-temurin/166906/4

[mstormi]

Since you mentioned startup on RPi:
With many rules startup is also slow because they're processed in sequence (DSL only?) although any RPi has 4 processor cores and could do so in parallel. It just does not.
I have no idea where to address this and how this relates to JEP483 (is that a prerequisite or unrelated?) so maybe put it up as an issue to be investigated in 5.x planning.

[holgerfriedrich]

Short update:

• SAT 0.18.0 is now released, supporting also J25 byte code.
• ecj 3.44 is released and supports J25 byte code, but has issues with annotation parsing. We will need to wait for 3.45 targeted Mar 2026.
• bndtools 7.2 is released, the limitation regarding Junit (and transitively to Mockito) remains. The is WIP, so I'd expect a fix for the next release, min 3 months ahead! 7.2.1 is available and in use in core
• Karaf 4.5 seems still far away. Mainline does not yet support J25. We are working on it.

[holgerfriedrich]

maven-shade-plugin 3.6.2 is now used in core. It should now fully support J25....