Use instance fields and properties from Python package in Java app

This article provides an introduction to cross-technology handling of static fields and properties.
Javonet allows you to reference and use modules or packages written in (Java/Kotlin/Groovy/Clojure, C#/VB.NET, Ruby, Perl, Python, JavaScript/TypeScript) like they were created in your technology. If have not yet created your first project check Javonet overview and quick start guides for your technology.

With Javonet you can interact with instance field and properties from Python package like they were available in Java but invocation must be performed through Javonet SDK API.

Javonet allows you to pass any Java value type as argument to method from Python package. For reference type arguments (instances of other classes) you can create such instance with Javonet and pass the Invocation Context variable referencing that object as argument of method invocation.

Custom Python package with instance fields and properties

With Javonet it is possible to reference any custom Python package and interact with public instance fields and properties declared on types defined within that module almost the same as with any other Java library.

Snippet below represents the sample code from Python package which contains class with fields:

class TestClass:
    def __init__(self, public_value, private_value):
        self.public_value = public_value
        self.private_value = private_value

    static_value = 3
static_value = 3

It is possible to get one of the declared instance fields from Python package using following Java code.

import com.javonet.sdk.*;

public class SampleProgram {

    private static final String resourcesDirectory = ".";
    public static void main(String []args) {

        // use activate only once in your app
        Javonet.activate("your-license-key");
        
        // create called runtime context
        RuntimeContext calledRuntime = Javonet.inMemory().python();
        
        // set up variables
        // path to directory with .py files
        String libraryPath = resourcesDirectory;
        String className = "TestClass.TestClass";
        
        // load custom library
        calledRuntime.loadLibrary(libraryPath);
        
        // create type's instance
        InvocationContext instance = calledRuntime.getType(className).createInstance(18, 19).execute();
        
        // get instance's field
        InvocationContext response = instance.getInstanceField("public_value").execute();
        
        // get value from response
        int result = (int) response.getValue();
        
        // write result to console
        System.out.println(result);
     }
}class TestClass:
    def __init__(self, public_value, private_value):
        self.public_value = public_value
        self.private_value = private_value

    static_value = 3
// use activate only once in your app
Javonet.activate("your-license-key");

// create called runtime context
RuntimeContext calledRuntime = Javonet.inMemory().python();

// set up variables
// path to directory with .py files
String libraryPath = resourcesDirectory;
String className = "TestClass.TestClass";

// load custom library
calledRuntime.loadLibrary(libraryPath);

// create type's instance
InvocationContext instance = calledRuntime.getType(className).createInstance(18, 19).execute();

// get instance's field
InvocationContext response = instance.getInstanceField("public_value").execute();

// get value from response
int result = (int) response.getValue();

// write result to console
System.out.println(result);

This uses in memory runtime bridging to load the Python package, and next retrieves reference to specific type and gets the instance field. Result of the invocation is returned as regular Java value and can be used for further processing. It is possible to set one of the declared instance fields from Python package using following Java code.

import com.javonet.sdk.*;

public class SampleProgram {

    private static final String resourcesDirectory = ".";
    public static void main(String []args) {

        Javonet.activate("your-license-key");
        
        // create called runtime context
        RuntimeContext calledRuntime = Javonet.inMemory().python();
        
        // set up variables
        // path to directory with .py files
        String libraryPath = resourcesDirectory;
        String className = "TestClass.TestClass";
        
        // load custom library
        calledRuntime.loadLibrary(libraryPath);
        
        // get type from runtime
        InvocationContext calledRuntimeType = calledRuntime.getType(className).execute();
        
        // create type's instance
        InvocationContext instance = calledRuntimeType.createInstance(18, 19).execute();
        
        // set instance's field
        instance.setInstanceField("public_value", 44).execute();
        
        // get instance's field
        InvocationContext response = instance.getInstanceField("public_value").execute();
        
        // get value from response
        int result = (int) response.getValue();
        
        // write result to console
        System.out.println(result);
     }
}class TestClass:
    def __init__(self, public_value, private_value):
        self.public_value = public_value
        self.private_value = private_value

    static_value = 3
Javonet.activate("your-license-key");

// create called runtime context
RuntimeContext calledRuntime = Javonet.inMemory().python();

// set up variables
// path to directory with .py files
String libraryPath = resourcesDirectory;
String className = "TestClass.TestClass";

// load custom library
calledRuntime.loadLibrary(libraryPath);

// get type from runtime
InvocationContext calledRuntimeType = calledRuntime.getType(className).execute();

// create type's instance
InvocationContext instance = calledRuntimeType.createInstance(18, 19).execute();

// set instance's field
instance.setInstanceField("public_value", 44).execute();

// get instance's field
InvocationContext response = instance.getInstanceField("public_value").execute();

// get value from response
int result = (int) response.getValue();

// write result to console
System.out.println(result);

This uses in memory runtime bridging to load the Python package, and next retrieves reference to specific type, then sets and gets the instance field. Result of the invocation is returned as regular Java value and can be used for further processing.

Framework instance fields and properties

The same steps are required to get types and fields from framework Python package:

This snippet doesn't support selected combination of technologies.

The same operation can be performed remotely by just changing the new Runtime Context invocation from in memory to tcp that will create and interact with your Python package objects on any remote node, container or service that hosts Javonet Code Gateway. This way you can preserve the same logic in your application and instantly switch between monolithic and microservices architecture without the need to implement the integration layer based on web services or other remote invocation methods.

Read more about use cases and software architecture scenarios where Javonet runtime bridging technology can support your development process.

Use instance fields and properties from Python package in Java app

Custom Python package with instance fields and properties

Framework instance fields and properties

Additional references