Can jupyter lsp support Java Language Server e.g. eclipse jdtls

[l22141]

Description

Currently, I use jjava kernel to do some data analysis with java. I am not sure whether jupyter lab lsp support java language server or not . As per official language server list, i installed eclipse jdtls (a popular java language server) , and config the language server for eclipse jdtls. it seems that when i create a native file(.java) in jupyer lab, it can initialize the server successfully and autocomplete is OK. But when i create a notebook using jjava kernel, the server can't be initialized and autocomplete is also not work.
Below is the server config:

{
  "LanguageServerManager": {
    "language_servers":{
      "java_lsp": {
        "version": 2,
        "argv": [
            "java",
            "-Declipse.application=org.eclipse.jdt.ls.core.id1",
            "-Declipse.product=org.eclipse.jdt.ls.core.product",
            "-Dlog.level=ALL",
            "-Xmx1G", 
            "--add-modules=ALL-SYSTEM",
            "--add-opens", "java.base/java.util=ALL-UNNAMED", 
            "--add-opens", "java.base/java.lang=ALL-UNNAMED", 
            "-jar", "/Users/mac/jupyter_plugins/eclipse_jdtls/plugins/org.eclipse.equinox.launcher_1.6.900.v20240613-2009.jar", 
            "-configuration", "/Users/mac/jupyter_plugins/eclipse_jdtls/config_mac", 
            "-data", "/Users/mac/java_workspace"
        ],
        "languages": ["java"],
        "mime_types": ["text/x-java-source"],
        "display_name": "JDT LS"
      }
    }
  }
}

java lsp in the native file initialized successfully:

autocomplete is worked in the native file:

java lsp in the notebook can not be initialized:

jupyter lab version: 4.4.2
juperlab lsp version: 5.1.0
java language server (eclipse jdtls) : 1.43.0
java version: 17.0.7
python version 3.13.3
os version: MacOS 12.7.6

[krassowski]

If it works in .java file but not in notebooks, it means that the server could not be matched to kernel. Quote:

for notebooks the MIME type is derived from language_info/mimetype element of kernel_info response (with fallback on to cell metadata if missing from kernel response)

https://jupyterlab-lsp.readthedocs.io/en/latest/Configuring.html#language-servers

I would suggest to open the notebook with text editor, find out the mimetype in language_info and ensure this is in your LSP spec. For example for ipykernel it is:

[l22141]

Thanks for your reply.
As per your advice, i have checked the raw content of notebook, and the mimetype in language_info is same as the mime_types in the server config(text/x-java-source). below is the content of notebook:

btw, i also checked the console output after starting the jupyter lab(jupyter lab --debug ),

[D 2025-07-19 22:29:56.176 ServerApp] Received kernel info: {'protocol_version': '5.3', 'implementation': 'unknown', 'implementation_ver    sion': 'unknown', 'language_info': {'name': 'Java', 'version': '17.0.7+7', 'mimetype': 'text/x-java-source', 'file_extension': '.jshell'    , 'pygments_lexer': 'java', 'codemirror_mode': 'java'}, 'banner': 'Java 17.0.7+7 :: JJava kernel 1.0-a4 \nProtocol v5.3 implementation b    y unknown unknown', 'help_links': [{'text': 'Java tutorial', 'url': 'https://docs.oracle.com/javase/tutorial/java/nutsandbolts/index.htm    l'}, {'text': 'JJava homepage', 'url': 'https://github.com/dflib/jjava'}]}

the mimetype in language_info is also text/x-java-source. I am not sure the other fields have impacts on the matching.

[krassowski]

Hmm. Maybe languages should also include java-source? I am not sure why it is treated as a separate language.

[l22141]

Thanks for your quick response!
After adding java-source in the languages field of server config, java language server is initialized successfully! But it seems that the autocomplete prompt is from kernel not lsp.
see the autocomplete in the notebook:

compare with the autocomplete in the java file:

[krassowski]

I think this is because the file_extension returned by your kernel is .jshell not .java and the language server may be ignoring .jshell files.

[l22141]

I want to test if the file_extensionis changed from .jshell to .java, autocompletion from lsp is worked or not. So i change the notebook's raw contents manually, but it seems that the modified content is reverted back from .java to .jshell. Maybe it's controlled by kernel_info returned from jjava kernel. Does it need the jjava kernel to change from .jshell to .java, or else the lsp will not work in the notebook?

[krassowski]

Yes, the one from kernel_info is used. It looks like the author of that java kernel set java-shell and .jshell on purpose as per SpencerPark/IJava#46:

it should really be .jshell for the file extension and a mimetype for jshell as the syntax is slightly different that a java

It looks like there are three options:

• find a way to change file extension in IJava kernel to .java
• find a way for the jdtls to accept files with .jshell extension
• as last resource we could add an override in jupyterlab-lsp to allow overriding the file extension

[krassowski]

It looks like it was discussed in jdtls but not implemented so far:

• Provide support for JShell eclipse-jdtls/eclipse.jdt.ls#1653
• make .jsh useful  redhat-developer/vscode-java#1601

I guess you could get 90% of functionality by just coercing jdtls to accept .jshell files.

[l22141]

It seems that jdtls has already supported customized file extension.
eclipse-jdtls/eclipse.jdt.ls#3222

But i don't find methods how to do it from its document. I will try to find the methods and let you know if any progress.

Any way, very appreciate your help.

[krassowski]

Oh, based on eclipse-jdtls/eclipse.jdt.ls#3230 this looks doable - can you try:

{
  // ...
  "mime_types": ["text/x-java-source"],
  "display_name": "JDT LS",
  "workspace_configuration": {
    "java.associations": ["jshell", "java"]
  }
}

[l22141]

Do you mean add workspace_configuration to jupyer lsp server config file? I hava added to my java language server config:

{
  "LanguageServerManager": {
    "language_servers":{
      "java_lsp": {
        "version": 2,
        "argv": [
            "java",
            "-Declipse.application=org.eclipse.jdt.ls.core.id1",
            "-Declipse.product=org.eclipse.jdt.ls.core.product",
            "-Dlog.level=ALL",
            "-Xmx1G", 
            "--add-modules=ALL-SYSTEM",
            "--add-opens", "java.base/java.util=ALL-UNNAMED", 
            "--add-opens", "java.base/java.lang=ALL-UNNAMED", 
            "-jar", "/Users/mac/jupyter_plugins/eclipse_jdtls/plugins/org.eclipse.equinox.launcher_1.6.900.v20240613-2009.jar", 
            "-configuration", "/Users/mac/jupyter_plugins/eclipse_jdtls/config_mac", 
            "-data", "/Users/mac/java_workspace"
        ],
        "languages": ["java"],
        "mime_types": ["text/x-java-source"],
        "display_name": "JDT LS",
        "workspace_configuration": {
             "java.associations": ["jshell", "java"]
         }
      }
    }
  }
}

but, it still only has kernel autocompletion in the notebook.

[krassowski]

Yes, but I am not sure of the format required... Could you try:

"workspace_configuration": {
  "java.associations": {
    ".jshell": "java"
  }
}

[l22141]

same as before, only kernel prompts not lsp.
btw, i also changed .jshell to jshell, but same effect, no lsp prompt.

My question is : do we need to modify config file in jdtls? Is it enough only modify jupyter lsp server language json file.

[l22141]

@krassowski , i checked the eclipse-jdt-ls log(.metadata/.log), when i code in a notebook( ipynb ) file, eclipse jdtls outputs below log message:

!ENTRY org.eclipse.jdt.ls.core 1 0 2025-07-29 17:29:58.987
!MESSAGE Completion request completed

!ENTRY org.eclipse.jdt.ls.core 1 0 2025-07-29 17:29:59.937
!MESSAGE Completion request completed

!ENTRY org.eclipse.jdt.ls.core 1 0 2025-07-29 17:30:00.144
!MESSAGE Completion request completed

!ENTRY org.eclipse.jdt.ls.core 1 0 2025-07-29 17:30:00.387
!MESSAGE Completion request completed

!ENTRY org.eclipse.jdt.ls.core 1 0 2025-07-29 17:30:00.655
!MESSAGE Completion request completed

!ENTRY org.eclipse.jdt.ls.core 1 0 2025-07-29 17:30:00.789
!MESSAGE Completion request completed

!ENTRY org.eclipse.jdt.ls.core 1 0 2025-07-29 17:30:01.175
!MESSAGE Completion request completed

!ENTRY org.eclipse.jdt.ls.core 1 0 2025-07-29 17:30:01.692
!MESSAGE Completion request completed

!ENTRY org.eclipse.jdt.ls.core 4 0 2025-07-29 17:31:43.741
!MESSAGE  file:///Users/mac/java_workspace/.virtual_documents/Untitled2.ipynb  does not resolve to a ICompilationUnit

when i code in a .java file, it outputs below logs:

!ENTRY org.eclipse.jdt.ls.core 1 0 2025-07-29 17:36:21.351
!MESSAGE Reconciled 1. Took 0 ms

!ENTRY org.eclipse.jdt.ls.core 1 0 2025-07-29 17:36:21.403
!MESSAGE Completion request completed

!ENTRY org.eclipse.jdt.ls.core 1 0 2025-07-29 17:36:21.770
!MESSAGE begin problem for /Traffic.java

!ENTRY org.eclipse.jdt.ls.core 1 0 2025-07-29 17:36:21.771
!MESSAGE 2 problems reported for /Traffic.java

!ENTRY org.eclipse.jdt.ls.core 1 0 2025-07-29 17:36:21.786
!MESSAGE begin problem for /Traffic.java

!ENTRY org.eclipse.jdt.ls.core 1 0 2025-07-29 17:36:21.788
!MESSAGE 2 problems reported for /Traffic.java

!ENTRY org.eclipse.jdt.ls.core 1 0 2025-07-29 17:36:21.805
!MESSAGE begin problem for /Traffic.java

!ENTRY org.eclipse.jdt.ls.core 1 0 2025-07-29 17:36:21.806
!MESSAGE 2 problems reported for /Traffic.java

!ENTRY org.eclipse.jdt.ls.core 1 0 2025-07-29 17:36:21.807
!MESSAGE Validated 3. Took 53 ms

btw, /Users/mac/java_workspace is the directory where i run jupyter lab. And as for ..../.virtual_documents/Untitled2.ipynb , the contents of this file are same as code in the notebook file i created in jupyter lab.