Calling jython script from java

Question

I am currently working on a simple hello world program using jython and java.

The program is designed in way that a jython method accepts a name parameter and returns welcome message.

My problem is whenever I am accessing the jython method from java, it shows nullponter exception

My jython Script (JythonHello.py):

class JythonHello:
    def __init__(self, name):
        self.name = name   
    def sayHello(self):
        return "Hello "+ self.name

and my java code:

public static void main(String[] args) {

    PythonInterpreter interpreter = new PythonInterpreter();
    interpreter.execfile("src/jython/JythonHello.py");

    PyObject callFunction = interpreter.get("sayHello");
    PyObject result = callFunction.__call__(new PyString("Boban"));
    String msg = (String) result.__tojava__(String.class);
    System.out.println("output: " + msg);
}

Any suggestions?

Yes. Add the exception stack trace. And just for the record: you are sure that "src/...." is a valid path from where you call your java main? — GhostCat
– GhostCat, Commented Apr 3, 2017 at 7:45
it shows only java.lang.NullPointerException. I am not sure about the path.Is there any other way of loading script file? — Boban Thomas
– Boban Thomas, Commented Apr 3, 2017 at 7:52
java.lang.NullPointerException at javaSide.JavaClass.main(JavaClass.java:16) — Boban Thomas
– Boban Thomas, Commented Apr 3, 2017 at 7:53
error in PyObject result = callFunction.__call__(new PyString("Boban")); — Boban Thomas
– Boban Thomas, Commented Apr 3, 2017 at 7:55

GhostCat · Accepted Answer · 2017-04-03 08:01:47Z

2

Looking at your code; your python code defines a class and a member method:

class JythonHello:
  def __init__(self, name): ...
  def sayHello(self): ...

And it seems that you intend to call that method:

 PyObject callFunction = interpreter.get("sayHello");
 PyObject result = callFunction.__call__(new PyString("Boban"));

But please note: sayHello() doesn't take any arguments. That self parameter is an indication that you have to call it on an object; but without any other parameters!

So, in pure python you would say:

helloVar = JythonHello("Boban")
helloVar.sayHello()

But your java code tries to call it like

sayHello("Boban")

So, the real answer is: step back; and re-think what you really intend to do; and then write code that works that way.

I would start by not adding the "class" part on the python side; instead try to simply invoke a function that takes a string argument for example!

And finally: could it be that you are on the wrong path altogether? The main point of jython is to write simply python code to do "debug" work within a running JVM. You are writing complicated Java code to use a bit of python code on the other hand ...

answered Apr 3, 2017 at 8:01

GhostCat

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2 Comments

Luatic Over a year ago

I thought if you've got something like Java - harder, but more efficient + Python - easier, less efficient, the idea is implementing core features in Java, and writing extensions in Python, so, are you really sure the main point of jython is debbugging ?

GhostCat Over a year ago

@user7185318 Well, it is in my context ;-)

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