No autocompletions in Java 1.8-based projects due to "Missing system library" error

This appears to be related to eclipse-jdt/eclipse.jdt.core#3298. I'm not sure when this changed but as of the update released yesterday I am now getting the following error in my logs any time I try to work on my Minecraft mod projects (e.g. glektarssza/minecraft-gtnh-customizer):

!ENTRY org.eclipse.jdt.core.manipulation 4 0 2025-10-29 09:43:37.638
!MESSAGE Error in JDT Core during AST creation
!STACK 0
java.lang.IllegalStateException: Missing system library
	at org.eclipse.jdt.core.dom.ASTParser.checkForSystemLibrary(ASTParser.java:311)
	at org.eclipse.jdt.core.dom.ASTParser.getClasspath(ASTParser.java:269)
	at org.eclipse.jdt.core.dom.ASTParser.internalCreateASTCached(ASTParser.java:1395)
	at org.eclipse.jdt.core.dom.ASTParser.lambda$1(ASTParser.java:1274)
	at org.eclipse.jdt.internal.core.JavaModelManager.cacheZipFiles(JavaModelManager.java:5709)
	at org.eclipse.jdt.core.dom.ASTParser.internalCreateAST(ASTParser.java:1274)
	at org.eclipse.jdt.core.dom.ASTParser.createAST(ASTParser.java:918)
	at org.eclipse.jdt.core.manipulation.CoreASTProvider$1.run(CoreASTProvider.java:294)
	at org.eclipse.core.runtime.SafeRunner.run(SafeRunner.java:47)
	at org.eclipse.jdt.core.manipulation.CoreASTProvider.createAST(CoreASTProvider.java:286)
	at org.eclipse.jdt.core.manipulation.CoreASTProvider.getAST(CoreASTProvider.java:199)
	at org.eclipse.jdt.ls.core.internal.JDTUtils.getAst(JDTUtils.java:1953)
	at org.eclipse.jdt.ls.core.internal.handlers.SemanticTokensHandler.full(SemanticTokensHandler.java:45)
	at org.eclipse.jdt.ls.core.internal.handlers.JDTLanguageServer.lambda$57(JDTLanguageServer.java:1205)
	at org.eclipse.jdt.ls.core.internal.BaseJDTLanguageServer.lambda$0(BaseJDTLanguageServer.java:87)
	at java.base/java.util.concurrent.CompletableFuture$UniApply.tryFire(CompletableFuture.java:667)
	at java.base/java.util.concurrent.CompletableFuture$Completion.exec(CompletableFuture.java:504)
	at java.base/java.util.concurrent.ForkJoinTask.doExec(ForkJoinTask.java:511)
	at java.base/java.util.concurrent.ForkJoinPool$WorkQueue.topLevelExec(ForkJoinPool.java:1450)
	at java.base/java.util.concurrent.ForkJoinPool.runWorker(ForkJoinPool.java:2019)
	at java.base/java.util.concurrent.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:187)

To replicate the issue:

1. Ensure you have Git LFS installed properly (see this comment on redhat-developer/vscode-java#4218 for why).
2. Clone the repository.
3. Install the extensions outlined in the Enabled Extensions Information section below.
4. Copy the appropriate template VSCode settings from ./templates/vscode/ from inside the repository above into your ./.vscode/settings.json file inside the repository above.
5. Replace the Java paths inside the copied templates to a version of Java 21.
6. Ensure your JAVA_HOME environment variable is set to a version of Java 17.
7. Run ./gradlew setupDecompWorkspace inside the repository above.
8. Run ./gradlew build inside the repository above.
9. Run ./gradlew eclipse inside the repository above.
10. Try to get autocompletion in any file.
11. Observe the issue.

再現用リポジトリ、その templates/vscode/ の設定、および issue に記載されている Gradle コマンドから始めます。Missing system library の失敗を ASTParser、CoreASTProvider、JDTUtils、SemanticTokensHandler を通して追跡し、その後 Java 1.8 プロジェクト設定での動作と比較します。記載された設定を繰り返した後、ログに記録されたエラーなしで自動補完が動作すれば完了です。