Jython Compatibility

Full Jython compatibility is not a goal of this project. One major reason for this is that most Jython code that uses Java integration will be based on a stable Jython release, and these only come in Python 2.x versions. The GraalVM implementation of Python, in contrast, is only targeting Python 3.x.

Nonetheless, there are certain features of Jython’s Java integration that we can offer similarly. Here is an example:

>>> import java.awt as awt
>>> win = awt.Frame()
>>> win.setSize(200, 200)
>>> win.setTitle("Hello from Python!")
>>> win.getSize().toString()
>>> win.show()

This example works exactly the same on both Jython and Python on GraalVM. Some features of Jython are more expensive at runtime, and thus are hidden behind a command line flag on Graal: --python.EmulateJython.

Importing #

Import statements allow you to import Java classes, but (unlike Jython), only packages in the java namespace can be directly imported. Importing classes from packages outside java namespace also requires the --python.EmulateJython option to be active. This will work:

import java.lang as lang

But this will not:

import javax.swing as swing
from javax.swing import *

Instead, you will have to import one of the classes you are interested in directly:

import javax.swing.JWindow as JWindow

Basic Object Usage #

Constructing and working with Java objects and classes is done with natural Python syntax. The methods of Java objects can also be retrieved and passed around as first class objects (bound to their instance) the same as Python methods:

>>> from java.util import Random
>>> rg = Random(99)
>>> boundNextInt = rg.nextInt
>>> rg.nextInt()
>>> boundNextInt()

Java-to-Python Types: Automatic Conversion #

Method overloads are resolved by matching the Python arguments in a best-effort manner to the available parameter types. This is also when data conversion happens. The goal here is to make using Java from Python as smooth as possible. The matching we do here is similar to Jython, but Graal Python uses a more dynamic approach to matching — Python types emulating int or float are also converted to the appropriate Java types. This allows, for example, to use Pandas frames as double[][] or NumPy array elements as int[] when the elements fit into those Java primitive types.

Java type Python type
null None
boolean bool
byte, short, int , long int, any object that has an __int__ method
float, double float, any object that has a __float__ method
char str of length 1
java.lang.String str
byte[] bytes, bytearray, wrapped Java array, Python list with only the appropriate types
Java arrays Wrapped Java array or Python list with only the appropriate types
Java objects Wrapped Java object of the appropriate type
java.lang.Object Any object

Special Jython Modules #

Any of the special Jython modules are not available. For example, the jarray module on Jython allows construction of primitive Java arrays. This can be achieved as follows on GraalPython:

>>> import java
>>> java.type("int[]")(10)

The code that only needs to pass a Java array can also use Python types. However, implicitly, this may entail a copy of the array data, which can be deceiving when using Java arrays as output parameters:

>>> # This example needs the --python.EmulateJython flag for the java.io import
>>> import java
>>> i = java.io.ByteArrayInputStream(b"foobar")
>>> buf = [0, 0, 0]
>>> i.read(buf) # buf is automatically converted to a byte[] array
>>> buf
[0, 0, 0] # the converted byte[] array got lost
>>> jbuf = java.type("byte[]")(3)
>>> i.read(jbuf)
>>> jbuf
[98, 97, 122]

Exceptions from Java #

Catching all kinds of Java exceptions comes with a performance penalty and is only enabled with the --python.EmulateJython flag.

>>> import java
>>> v = java.util.Vector()
>>> try:
...    x = v.elementAt(7)
... except java.lang.ArrayIndexOutOfBoundsException as e:
...    print(e.getMessage())
7 >= 0

Java Collections #

There is no automatic mapping of the Python syntax for accessing dictionary elements to the java.util mapping and list classes’ ` get, set, or put` methods. To use these mapping and list clases, you must call the Java methods:

>>> # This example needs the --python.EmulateJython flag for the java.util import
>>> import java
>>> ht = java.util.Hashtable()
>>> ht.put("foo", "bar")
>>> ht.get("foo")

The Python-style iteration of Java java.util.Enumerable, java.util.Iterator, or java.lang.Iterable is not supported. For these, you will have to use a while loop and use the hasNext() and next() (or equivalent) methods.

No Inheriting from Java #

Python classes cannot inherit from Java classes. A workaround can be to create a flexible subclass in Java, compile it and use delegation instead. Take this example:

import java.util.logging.Handler;

public class PythonHandler extends Handler {
    private final Value pythonDelegate;

    public PythonHandler(Value pythonDelegate) {
        this.pythonDelegate = pythonDelegate;

    public void publish(LogRecord record) {
        pythonDelegate.invokeMember("publish", record);

    public void flush() {

    public void close() {

Then you can use it like this in Python:

# This example needs the --python.EmulateJython flag for the java.util import
from java.util.logging import LogManager, Logger

class MyHandler():
    def publish(self, logRecord): print("[python]", logRecord.toString())​
    def flush(): pass​
    def close(): pass
LogManager.getLogManager().addLogger(Logger('my.python.logger', None, MyHandler()))

Embedding Python into Java #

The other way to use Jython is to embed it into Java applications. Where above, Graal Python offered some measure of compatibility with existing Jython code, we do not offer any in this case. Existing code using Jython depends directly on the Jython package (for example, in the Maven configuration), because the Java code has references to Jython internal classes such as PythonInterpreter.

For Graal Python, no dependency other than on the GraalVM SDK is required. There are no APIs particular to Python that are exposed, and everything is done through the GraalVM API. Important to know is that as long as your application is executed on a GraalVM with the Python language installed, you can embed Python in your programs. For more detail, refer to the Embed Languages reference.