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

...

Empieza siguiendo la activación de onLanguage:java y el lanzamiento de JDTLS; después inspecciona el intervalo anterior al inicio de JavaLanguageServerPlugin, especialmente la inicialización de OSGi mencionada en el informe. Reproduce el flujo de un espacio de trabajo vacío con un único archivo .java y compara las marcas de tiempo del extension host y de JDTLS; se considera terminado cuando se reduce el retraso de inicio sin retrasar la inicialización ni la finalización de la compilación.