SWT for next release (4.39 aka 2026-03) will require Java 21 to run

[jonahgraham]

The SWT master branch has just had #2824 merged which changes BREE to Java 21 for all the SWT bundles.

SWT Devs

For SWT devs, this should mostly be an easy update, but there is one step that you need to take as the Snippets .classpath file needs a manual update:

If you have a warning like "The JRE container on the classpath is not a perfect match to the 'JavaSE-21' execution environment" (as pictured below) then you need to edit the .classpath manually.

You can do the edit manually, or better still press "Update the classpath setting" in the Overview page of the manifest editor:

The project was not built due to "release 21 is not found in the system"

Another warning/error you may hit is The project was not built due to "release 21 is not found in the system". Fix the problem, then try refreshing this project and building it since it may be inconsistent which seems to be this JDT issue: eclipse-jdt/eclipse.jdt.core#2509 - sometimes while I was developing this change and swapping branches I would see it.

The method threadId() is undefined for the type Thread

Once I finish cleaning up the warnings in SWT from the move to Java 21, if you have your snippets still configured as Java17 you will see the error The method threadId() is undefined for the type Thread for SnippetExplorer:

Fix this by making sure classpath is Java21 for the snippets.

[fedejeanne]

I ran into the first warning today. It was a breeze to fix thanks to you, @jonahgraham, proactively communicating this 🚀

🅰️➕
💯

[merks]

It took me a couple of tries to get rid of this error, even with the .classpath file containing the correct values. I deleted it and let Ooomph copy it again. But still and error, even though F3 navigated to the method and even with JRE System Library showing JavaSE-21. I finally deleted the project (from the workspace), imported it again (by resolving the target platform again via the targlets task), and then the error was finally gone. A bit strange...

[fedejeanne]

Hm, odd 😕

I can only report about the 1st error (the Warning in the MANIFEST.MF file) because when I fixed that one, the 2nd one (The compilation error saying that The method threadId() is undefined for the type Thread), which was also present in my WS, went away after pressing the Update the classpath settings in the Overview tab of the manifest.