Experimental feature in GraalVM

Jython Migration Guide

Most Jython code that uses Java integration will be based on a stable Jython release, and these only come in Python 2.x versions. GraalPy, in contrast, is only targeting Python 3.x. GraalPy does not provide full compatibility with these earlier 2.x versions of Jython. Thus, a significant migration step will have to be taken to migrate all your code to Python 3.

For Jython-specific features, follow this document to learn about migration to GraalPy.

Note that some features of Jython have a negative impact on runtime performance, and are disabled by default. To make migration easier, you can enable some features using a command line option: --python.EmulateJython.

Importing Java Packages #

There are certain features of Jython’s Java integration that are enabled by default on GraalPy. 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 GraalPy. However, on GraalPy only packages in the java namespace can be directly imported. Importing classes from packages outside the java namespace also requires the --python.EmulateJython option to be active.

Additionally, importing Java packages as Python modules is only supported under very specific circumstances. For example, this will work:

import java.lang as lang

This will not work:

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

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

import javax.swing.Window as Window

Basic Object Usage #

Constructing and working with Java objects and classes is achieved 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)
>>> rg.nextInt()
>>> boundNextInt = 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 also happens during data conversion. The goal here is to make using Java from Python as smooth as possible. The matching allowed here is similar to Jython, but GraalPy uses a more dynamic approach to matching—Python types emulating int or float are also converted to the appropriate Java types. This enables you, 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 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 #

The jarray module which is used to create primitive Java arrays is supported for compatibility.

>>> import jarray
>>> jarray.array([1,2,3], 'i')

Note that its usage is equivalent of constructing the array types using the java.type function and filling the array.

>>> 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 deceptive when using Java arrays as output parameters:

>>> 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]

Modules other than jarray are not supported.

Exceptions from Java #

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

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

Java Collections #

Java arrays and collections implementing java.util.Collection can be accessed using the [] syntax. Empty collections are considered false in boolean conversions. Their length is exposed by len built-in function. For example:

>>> from java.util import ArrayList
>>> l = ArrayList()
>>> l.add("foo")
>>> l.add("baz")
>>> l[0]
>>> l[1] = "bar"
>>> del l[1]
>>> len(l)
>>> bool(l)
>>> del l[0]
>>> bool(l)

Java iterables implementing java.lang.Iterable can be iterated over using a for loop or iter built-in function and are accepted by all built-ins that expect iterables. For example:

>>> [x for x in l]
['foo', 'bar']
>>> i = iter(l)
>>> next(i)
>>> next(i)
>>> next(i)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
>>> set(l)
{'foo', 'bar'}

Iterators can be iterated as well. For example:

>>> from java.util import ArrayList
>>> l = ArrayList()
>>> l.add("foo")
>>> i = l.iterator()  # Calls the Java iterator methods
>>> next(i)

Map collections implementing java.util.Map can be accessed using [] notation. Empty maps are considered false in boolean conversions. Iteration of maps yields the keys, consistent with dict.

>>> from java.util import HashMap
>>> m = HashMap()
>>> m['foo'] = 5
>>> m['foo']
>>> m['bar']
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
KeyError: bar
>>> [k for k in m]
>>> bool(m)
>>> del m['foo']
>>> bool(m)

Inheritance from Java #

Inheriting from a Java class or implementing an interface is supported with some syntactical differences from Jython. A class inheriting from a Java class can be created using an ordinary class statement where declared methods will override/implement the superclass methods when they match in name. Super calls are performed using a special attribute self.__super__. The created object won’t behave like a Python object but like a foreign Java object. Its Python-level members can be accessed using its this attribute. For example:

import atexit
from java.util.logging import Logger, Handler

class MyHandler(Handler):
    def __init__(self):
        self.logged = []

    def publish(self, record):

logger = Logger.getLogger("mylog")
handler = MyHandler()
# Make sure the handler is not used after the Python context has been closed
atexit.register(lambda: logger.removeHandler(handler))


# The python attributes/methods of the object have to be accessed through 'this' attribute
for record in handler.this.logged:
    print(f'Python captured message "{record.getMessage()}" at level {record.getLevel().getName()}')

Embedding Python into Java #

The other way to use Jython is to embed it into Java applications.

There are two options for embedding Jython in a Java application. One it to use the PythonInterpreter object that Jython provides. Existing code using Jython in this manner depends directly on the Jython package (for example, in the Maven configuration), because the Java code has references to Jython internal classes. These classes do not exist in GraalVM, and no equivalent classes are exposed. To migrate from this usage, switch to the GraalVM SDK. Using this SDK, no APIs particular to Python are exposed, everything is done through the GraalVM API, with maximum configurability of the Python runtime.

The other option to embed Jython in Java is via JSR 223 by using the classes of the the javax.script package, and, in particular, via the ScriptEngine class. We do not recommend this approach, since the ScriptEngine APIs are not a clean fit for the options and capabilities of GraalPy. However, to migrate existing code, the NetBeans project provides packages on Maven Central to help here. Remove Jython and add the following dependencies instead (using a Maven pom.xml file as an example):

  <version>RELEASE150</version> <!-- or any later release -->
  <version>RELEASE150</version> <!-- or any later release -->

Afterwards, basic usage of GraalPy can be achieved by replacing

ScriptEngine python = new ScriptEngineManager().getEngineByName("python");


import org.netbeans.api.scripting.Scripting;
// ...
ScriptEngineManager manager = Scripting.newBuilder().allowAllAccess(true).build();
ScriptEngine python = manager.getEngineByName("GraalVM:python");

It is important to note that either of those options will only work if your application is executed on GraalVM with the Python language installed. For more details, refer to the Embed Languages guide.

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