Handling errors in extensions and custom code on startup

[andrus]

Not yet ready to commit to this solution, but wanted to document the idea.

Kernel may potentially load a bunch of user code on startup as extensions and custom code snippets (via JJAVA_STARTUP_SCRIPT). Any one of them can result in an error. Currently, the errors are printed in console, the kernel process dies, Jupyter tries to restart it, resulting in a restart loop (5 restart tries), and then still spinning not able to assign the kernel to the notebook.

I wonder if a more sensible behavior would be to catch and report the exceptions in the console, still start the kernel in a partially-broken state, and then on first eval, print the startup errors in the notebook to alert the user?

I guess no matter what we do here, identifying the boot step where the kernel failed, and pointing to a specific extension location or to the JJAVA_STARTUP_SCRIPT as the source of trouble would help users a lot. Maybe this part can be addressed simply by printing each startup and extension loading step as a debug message.

# intentionally bad code
$ export JJAVA_STARTUP_SCRIPT='var x = y' 

$ jupyter lab
...
[I 2025-11-08 09:26:38.905 ServerApp] Kernel started: a9c2319b-4365-4f8f-9122-69064bb204b3
[I 2025-11-08 09:26:38.962 KernelLauncher] Starting kernel: [java, -Xmx20480m, --add-opens=java.base/java.nio=ALL-UNNAMED, 
--add-opens, jdk.jshell/jdk.jshell=ALL-UNNAMED, -jar, /Users/andrus/Library/Jupyter/kernels/java/jjava.jar, 
/Users/andrus/Library/Jupyter/runtime/kernel-a9c2319b-4365-4f8f-9122-69064bb204b3.json]
Exception in thread "main" org.dflib.jjava.kernel.execution.CompilationException: [snippet: [var x = y;], status: REJECTED]
        at org.dflib.jjava.kernel.execution.CodeEvaluator.evalSingle(CodeEvaluator.java:118)
        at org.dflib.jjava.kernel.execution.CodeEvaluator.eval(CodeEvaluator.java:131)
        at org.dflib.jjava.kernel.JavaKernel.evalRaw(JavaKernel.java:234)
        at java.base/java.util.ArrayList.forEach(ArrayList.java:1596)
        at org.dflib.jjava.distro.JJava.main(JJava.java:82)
[I 2025-11-08 09:26:39.714 KernelLauncher] Kernel terminated.

[I 2025-11-08 09:26:41.892 ServerApp] AsyncIOLoopKernelRestarter: restarting kernel (1/5), new random ports
[I 2025-11-08 09:26:41.984 KernelLauncher] Starting kernel: [java, -Xmx20480m, --add-opens=java.base/java.nio=ALL-UNNAMED, 
--add-opens, jdk.jshell/jdk.jshell=ALL-UNNAMED, -jar, /Users/andrus/Library/Jupyter/kernels/java/jjava.jar, 
/Users/andrus/Library/Jupyter/runtime/kernel-a9c2319b-4365-4f8f-9122-69064bb204b3.json]
Exception in thread "main" org.dflib.jjava.kernel.execution.CompilationException: [snippet: [var x = y;], status: REJECTED]
        at org.dflib.jjava.kernel.execution.CodeEvaluator.evalSingle(CodeEvaluator.java:118)
        at org.dflib.jjava.kernel.execution.CodeEvaluator.eval(CodeEvaluator.java:131)
        at org.dflib.jjava.kernel.JavaKernel.evalRaw(JavaKernel.java:234)
        at java.base/java.util.ArrayList.forEach(ArrayList.java:1596)
        at org.dflib.jjava.distro.JJava.main(JJava.java:82)
[I 2025-11-08 09:26:42.734 KernelLauncher] Kernel terminated.
[I 2025-11-08 09:26:44.916 ServerApp] AsyncIOLoopKernelRestarter: restarting kernel (2/5), new random ports

... after 5 restart attempts

[W 2025-11-08 09:41:35.608 ServerApp] wrote error: 'Kernel does not exist: a9c2319b-4365-4f8f-9122-69064bb204b3'
    Traceback (most recent call last):
      File "/opt/homebrew/Cellar/jupyterlab/4.4.0/libexec/lib/python3.13/site-packages/tornado/web.py", line 1790, in _execute
        result = await result
                 ^^^^^^^^^^^^
      File "/opt/homebrew/Cellar/jupyterlab/4.4.0/libexec/lib/python3.13/site-packages/jupyter_server/auth/decorator.py", line 73, in inner
        return await out
               ^^^^^^^^^
      File "/opt/homebrew/Cellar/jupyterlab/4.4.0/libexec/lib/python3.13/site-packages/jupyter_server/services/kernels/handlers.py", line 75, in get
        model = await ensure_async(km.kernel_model(kernel_id))
                                   ~~~~~~~~~~~~~~~^^^^^^^^^^^
      File "/opt/homebrew/Cellar/jupyterlab/4.4.0/libexec/lib/python3.13/site-packages/jupyter_server/services/kernels/kernelmanager.py", line 508, in kernel_model
        self._check_kernel_id(kernel_id)
        ~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^
      File "/opt/homebrew/Cellar/jupyterlab/4.4.0/libexec/lib/python3.13/site-packages/jupyter_server/services/kernels/kernelmanager.py", line 539, in _check_kernel_id
        raise web.HTTPError(404, "Kernel does not exist: %s" % kernel_id)
    tornado.web.HTTPError: HTTP 404: Not Found (Kernel does not exist: a9c2319b-4365-4f8f-9122-69064bb204b3)