how to use %classpath magic correctly?

[david-romano]

For context, I use this code:

%%sh
# Install java kernel
wget -q https://github.com/SpencerPark/IJava/releases/download/v1.3.0/ijava-1.3.0.zip
unzip -q ijava-1.3.0.zip
python install.py

# Install proxy for the java kernel
wget -qO- https://gist.github.com/SpencerPark/e2732061ad19c1afa4a33a58cb8f18a9/archive/b6cff2bf09b6832344e576ea1e4731f0fb3df10c.tar.gz | tar xvz --strip-components=1
python install_ipc_proxy_kernel.py --kernel=java --implementation=ipc_proxy_kernel.py

(from this gist) to load IJava as the kernel in a Google Colab notebook, but no matter what I do, the classpath is always

/usr/local/share/jupyter/kernels/java_tcp/ijava-1.3.0.jar

Here's a Google Colab gist that shows how I'm using %classpath and how the classpath isn't updated afterwards.

Am I using the magic incorrectly? I was hoping to be able import my own classes to use in a Google Colab notebook, but I can't get IJava to recognize them.

Thanks for any help!

[Spencer Park]

You're using the magic correctly (%classpath <path>).

The system property (System.getProperty("java.class.path")) isn't updated because that is the app classpath for the kernel itself but the managed JShell's classpath is still updated. When you run code in a cell it is compiled and run with a different classpath set through the various magic commands.

If you are distributing class files (rather than a jar) then they should be in folders matching their package structure and the root path added with %classpath.

e.g. with a class called HelloWorld in package com.example; located at /content/sample_data/com/example/HelloWorld.class, then you can use it with the following:

%classpath /content/sample_data
import com.example.HelloWorld;
System.out.println(HelloWorld.class);

[david-romano]

The system property (System.getProperty("java.class.path")) isn't updated because that is the app classpath for the kernel itself but the managed JShell's classpath is still updated. When you run code in a cell it is compiled and run with a different classpath set through the various magic commands.

Thanks for clearing that up! I'm guessing a distinction similar to the kernel/JShell one is why the output of HelloWorld doesn't appear as cell output for a cell whose code is:

Runtime.getRuntime().exec("java /content/sample_data/com/example/HelloWorld");

but that's outside the subject of the current thread, so I'll try to raise the question separately. Thanks again for the help!