Passing java object to python

Question

I am prototyping an interface to our application to allow other people to use python, our application is written in java. I would like to pass some of our data from the java app to the python code but I am unsure how to pass an object to python. I have done a simple java->python function call using simple parameters using Jython and found it very useful for what I am trying to do. Given the class below, how can I then use it in Python/Jython as an input to a function/class:

public class TestObject 
{
   private double[] values;
   private int length;
   private int anotherVariable;

   //getters, setters
 }

Answer 1

One solution. You could use some sort of message system, queue, or broker of some sort to serialize/deserialize, or pass messages between python and java. Then create some sort workers/producer/consumers to put work on the queues to be processed in python, or java.

Also consider checking out for inspiration: https://www.py4j.org/

py4j is used heavily by/for pyspark and hadoop type stuff.

To answer your question more immediately.

Example using json-simple.:

import org.apache.commons.io.FileUtils;
import org.json.simple.JSONObject;
 //import org.json.simple.JSONObject;
  

public class TestObject 
{
   private double[] values;
   private int length;
   private int anotherVariable;
   private boolean someBool;
   private String someString;

   //getters, setters

   public String toJSON() {
       JSONObject obj=new JSONObject();
       obj.put("values",new Double(this.values));
       obj.put("length",new Integer(this.length));
       obj.put("bool_val",new Boolean(this.SomeBool));
       obj.put("string_key",this.someString);
       StringWriter out = new StringWriter();
       obj.writeJSONString(out);
       return out.toString();
   }

   public void writeObject(){
          Writer writer = new BufferedWriter(
                              new OutputStreamWriter(
                                  new FileOutputStream("anObject.json"), "utf-8")
                              )
                           )

          writer.write(this.toJSON());
   }

   public static void setObject(){
       values = 100.134;
       length = 12;
       anotherVariable = 15;
       someString = "spam";
   }
 }

And in python:

class DoStuffWithObject(object):
    def __init__(self,obj):
        self.obj = obj
        self.changeObj()
        self.writeObj()

    def changeObj(self):
        self.obj['values'] = 100.134;
        self.obj['length'] = 12;
        self.obj['anotherVariable'] = 15;
        self.obj['someString'] = "spam";

    def writeObj(self):
        ''' write back to file '''
        with open('anObject.json', 'w') as f:
            json.dump(self.obj, f)

    def someOtherMethod(self, s):
       ''' do something else '''
       print('hello {}'.format(s))

import json
with open('anObject.json','r') as f:
    obj = json.loads(f.read())
# print out obj['values'] obj['someBool'] ...
for key in obj:
    print(key, obj[key])

aThing = DoStuffWithObject(obj)
aThing.someOtherMethod('there')

And then in java read back the object. There are solutions that exist implementing this idea (JSON-RPC, XML-RPC, and variants). Depending, you may may also want to consider using something like ( http://docs.mongodb.org/ecosystem/drivers/java/ ) the benefit being that mongo does json.

See:

• https://spring.io/guides/gs/messaging-reactor/
• http://spring.io/guides/gs/messaging-rabbitmq/
• http://spring.io/guides/gs/scheduling-tasks/
• Celery like Java projects
• Jedis
• RabbitMQ
• ZeroMQ

A more comprehensive list of queues:

• http://queues.io/

Resources referenced:

• http://www.oracle.com/technetwork/articles/java/json-1973242.html
• How do I create a file and write to it in Java?
• https://code.google.com/p/json-simple/wiki/EncodingExamples