start-up slow when opening a single .java file in empty workspace

Use case

Open VS Code without opening any folder. Drag a .java file into the window. Java extension is expected to work immediately, but it takes several seconds.

Let's see if we have chance to improve it.

I find some clues:

• a) 2~3s after java extension starts to activate, JDTLS starts to write server logs. (client side code and launching JVM)
• b) it's another 5s before JavaLanguageServerPlugin is started. (mostly related to OSGi?)
• c) initialization and follow-up building jobs are finished within 1s.

Attaching the logs.

Extension host log

[2022-09-08 13:24:51.158] [exthost] [info] ExtensionService#_doActivateExtension redhat.java, startup: false, activationEvent: 'onLanguage:java'

JDTLS log

!SESSION 2022-09-08 13:24:53.994 -----------------------------------------------
eclipse.buildId=unknown
java.version=17.0.4.1
java.vendor=Eclipse Adoptium
BootLoader constants: OS=win32, ARCH=x86_64, WS=win32, NL=en_US
Command-line arguments:  -data C:\Users\yanzh\AppData\Local\Temp\vscodesws_68b2c\jdt_ws

!ENTRY org.eclipse.jdt.ls.core 1 0 2022-09-08 13:24:58.761
!MESSAGE class org.eclipse.jdt.ls.core.internal.JavaLanguageServerPlugin is started

!ENTRY org.eclipse.jdt.ls.core 1 0 2022-09-08 13:24:58.963
!MESSAGE Main thread is waiting

!ENTRY org.eclipse.jdt.ls.core 1 0 2022-09-08 13:24:59.023
!MESSAGE >> initialize

!ENTRY org.eclipse.jdt.ls.core 1 0 2022-09-08 13:24:59.025
!MESSAGE Initializing Java Language Server 1.16.0.202209070809

...

!ENTRY org.eclipse.jdt.ls.core 1 0 2022-09-08 13:24:59.741
!MESSAGE Creating the Java project jdt.ls-java-project

!ENTRY org.eclipse.jdt.ls.core 1 0 2022-09-08 13:24:59.771
!MESSAGE >> initialized

!ENTRY org.eclipse.jdt.ls.core 1 0 2022-09-08 13:24:59.921
!MESSAGE Finished creating the Java project jdt.ls-java-project

!ENTRY org.eclipse.jdt.ls.core 1 0 2022-09-08 13:25:00.409
!MESSAGE Workspace initialized in 691ms

!ENTRY org.eclipse.jdt.ls.core 1 0 2022-09-08 13:25:00.534
!MESSAGE >> initialization job finished

...

!ENTRY org.eclipse.jdt.ls.core 1 0 2022-09-08 13:25:00.833
!MESSAGE >> build jobs finished

...

まず onLanguage:java のアクティベーションと JDTLS の起動を追跡し、次に JavaLanguageServerPlugin の起動前の区間、特にレポートで言及されている OSGi の初期化を調査します。空のワークスペースで単一の .java ファイルを扱うフローを再現し、extension host と JDTLS のタイムスタンプを比較します。初期化やビルドの完了を遅延させずに起動遅延が短縮されれば完了です。