Trouble Accessing Java Classes from Python Using GraalPy and GraalVM

[ShuzoShinagawa1102]

Hello everyone,

What I want to do

I'm trying to access a Java class from a Python script using GraalPy (GraalPython) with GraalVM, but I'm encountering an error that I can't seem to resolve.

Python Script (main.py)

import java

# Get the Java classpath and print it
System = java.type("java.lang.System")
classpath = System.getProperty("java.class.path")
print(f"Java Classpath: {classpath}")

# Attempt to access the Java class
User = java.type('com.example.model.User')

# Create an instance of the Java class
user = User('Alice')
print(user.getName())

Java Class(User.java)

package com.example.model;

public class User {

    private String name;

    public User(String name) {
        this.name = name;
    }

    public String getName() {
        return this.name;
    }
}

I've compiled the Java class and packaged it into a JAR file named gpdemo-0.0.1-SNAPSHOT-plain.jar, and I set it as classpath for zshrc.

Execution command

graalpy --jvm \
  --vm-cp=$CLASSPATH \
  ./main.py

Error Message:

User = java.type('com.example.model.User')
KeyError: 'host symbol com.example.model.User is not defined or access has been denied'

What I've Tried:

• Verified that the JAR file is in the correct location and that the class is properly packaged.
• Printed out the Java classpath within the Python script to ensure that the JAR file is included.
• Ensured that GraalVM is installed correctly and that I'm using the --jvm option to enable JVM interop.

Environment Details:

GraalVM Version: Oracle GraalVM 23.0.1+11.1
GraalPy Version: GraalPy 3.10.8 (GraalVM CE JVM 23.1.2)
Operating System: macOS

Questions:

• Is there something I'm missing in the way I'm specifying the classpath when running the script?
• Do I need to adjust any settings related to host access or permissions to allow Python to access the Java class?
• Has anyone else encountered a similar issue and found a solution?

[ShuzoShinagawa1102]

additionally, here is the contained classes which are showed by
jar tf $CLASSPATH
command.

MacBook-Pro-4 gpdemo % jar tf $CLASSPATH
META-INF/
META-INF/MANIFEST.MF
com/
com/example/
com/example/demo/
com/example/demo/GpdemoApplication.class
com/example/model/
com/example/model/User.class
application.properties

[msimacek]

Hi @ShuzoShinagawa1102, the option for supplying classpath when running through the graalpy launcher is --vm.cp=$CLASSPATH (not -vm-cp).

[ShuzoShinagawa1102]

@msimacek Thank you for pointing that out. It was just a typo on my part, and I am indeed using "--vm.cp" when runnng the command.

[msimacek]

I tried your example and it printed Alice with no errors. What I did:

• Save your java code as com/example/model/User.java and your python code as main.py
• javac com/example/model/User.java
• graalpy --vm.cp=. main.py

The differences I have from you is that I used the latest release, I'm on Linux and I didn't build a JAR. Could you please try the latest release (24.1.1, the graalpy-jvm variant)?

[ShuzoShinagawa1102]

@msimacek
Thank you for your kind response.

I installed GraalPy latest release version (24.1.1) that you suggested (https://github.com/oracle/graalpython/releases) and it worked.
The command which I executed is here.

graalpy --jvm \
  --vm.cp=gpdemo-0.0.1-SNAPSHOT-plain.jar \
  ./main.py

it returned ↓