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Example project using clamp package
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Clamped: Demonstrating the Clamp package

The Clamp package supports functionality like the following in Java code:

import bar.clamped.BarClamp;  // yes, you can now just import Python classes!

public class UseClamped {

    public static void main(String[] args) {
        BarClamp barclamp = new BarClamp();
    try {
        System.out.println("BarClamp: " +;
    } catch (Exception ex) {
        System.err.println("Exception: " + ex);


In addition, Clamp also integrates with setuptools, in order to build jars in site-packages, and then to package up Jython, other dependent jars, the Python standard library, and installed packages into a single jar suitable for running on containers.

There are alternatives to Clamp, such as object factories (or more generally dependency injection). However, the object factories approach requires that the Java code must support factories, vs simple constructors. For projects like Apache Storm, you would have to write this object factory boilerplate code to use Python classes. In part to address such needs as Storm integration with Python, Clamp was created.

Release notes

Building jars with Clamp on Windows is not yet supported, but it will be once we have some development time to spare. (We are currently focused on getting Jython 2.7.0 out.) However, you can run such jars on Windows. (There's a small problem in using \ instead of / - easy to fix, however.)

Writing a Python class to use clamp

To write a clampable Python class, here's what it looks like. Note that such classes need to extend a Java class and/or Java interfaces. This Python class extends two Java interfaces, Serializable and Callable. By extending an interface or implementing class, you make it possible for Java code to know how to work with your Python classes. Note this part is no different than writing a Python callback for Java. However, the key extra step is the use of clamp_base, which also indicates that the root package for this class should be bar, or fully qualified, bar.clamped.BarClamp.

from import Serializable
from java.util.concurrent import Callable
from clamp import clamp_base

BarBase = clamp_base("bar")  # metaclass to map to Java packages

class BarClamp(BarBase, Callable, Serializable):

    def __init__(self):
        print "Being init-ed", self

    def call(self):
        print "Hello, world!"
        return 42

Next, a key piece of Clamp is that it supports setuptools and soon the PyPI ecosystem. This is all that is required to clamp a module, you simply need to specify it with the clamp keyword and require the clamp package:

import ez_setup

from setuptools import setup, find_packages
from clamp.commands import clamp_command

    name = "clamped",
    version = "0.1",
    packages = find_packages(),
    install_requires = ["clamp"],
    clamp = {
        "modules": ["clamped"]
    cmdclass = { "install": clamp_command }

Follow the Clamp installation steps.

Then install this example package. We will assume that you have jython27 mapped to the Jython launcher, $JYTHON_HOME/bin/jython:

$ git clone
$ cd clamped
$ jython27 clamp

The clamp command constructs a jar in $JYTHON_HOME/Lib/site-packages/jars/clamped-0.1.jar. It also ensures that this jar is automatically added to sys.path through the use of site-packages/jar.pth.

To make this more convenient, we can use a cmdclass with setuptools such that install uses Clamp's version, which does the clamp as part of its work:

    cmdclass = { "install": clamp_command }

You can use now this newly built jar for Java integration, but more likely you will need to build a single jar of your project, including all other clamped jars. To combine with the previous step:

$ jython27 install singlejar

which will construct a single jar, in our case clamped-0.1-single.jar. You can make this runnable if your toplevel directory specifies a file:

from clamped import BarClamp

x = BarClamp()

To run, simply do the following:

$ java -jar clamped-0.1-single.jar

which will result in output like the following:

Being init-ed bar.clamped.BarClamp@2798073c
Hello, world!

There you have it: Python code using a Java class to call Python, all packaged up in a single Java jar. Boggles the mind! Of course, it's up to you to write something more interesting than outputting Hello, world!. For another example, check out HelloWSGI.

With this single jar, you are now ready to directly integrate with Java. Let's say you have this class:

export CLASSPATH=`pwd`/clamped-0.1-single.jar:.
cd testjava
java UseClamped

and then you should expect to see output like the following, mostly debugging so we know it's still working :)

Being init-ed bar.clamped.BarClamp@129e4e49
Hello, world!
BarClamp: 42

You can decompile the proxy class to see exactly what's going on with these steps. First, download the Procyon decompiler. I used 0.5.21 when I did this step, but the most recent version when you try this out should be just fine.

Then unpack the jar and decompile with Procyon. You should do this in some unpacking directory, since jar unpacking will explode a lot of files at toplevel.

mkdir unpacked && cd unpacked
jar xf ../clamped-0.1.jar
java -jar /path/to/procyon-decompiler-0.5.21.jar bar/clamped/BarClamp.class

This will result in output like the following:

package bar.clamped;

import java.util.concurrent.*;
import org.python.core.*;
import org.python.compiler.*;

public class BarClamp implements PyProxy, Callable, Serializable, ClassDictInit
    protected PyObject __proxy;
    protected transient PySystemState __systemState;
    public static final long serialVersionUID;

    public void _setPyInstance(final PyObject _proxy) {
        this.__proxy = _proxy;

    public PyObject _getPyInstance() {
        return this.__proxy;

    public void _setPySystemState(final PySystemState _systemState) {
        this.__systemState = _systemState;

    public PySystemState _getPySystemState() {
        return this.__systemState;

    public void __initProxy__(final Object[] array) {
        Py.initProxy((PyProxy)this, "clamped", "BarClamp", array);

    public BarClamp() {

    public void finalize() {

    public Object clone() {
        return super.clone();

    public Object call() throws Exception {
        final PyObject findPython = ProxyMaker.findPython((PyProxy)this, "call");
        if (findPython != null) {
            final PyObject pyObject = findPython;
            try {
                return Py.tojava(pyObject._jcallexc((Object[])Py.EmptyObjects), (Class)Class.forName("java.lang.Object"));
            catch (Exception ex) {
                throw ex;
            catch (Throwable t) {
                return null;
        throw (Throwable)Py.NotImplementedError("");

    static {
        serialVersionUID = 1L;

    public static void classDictInit(final PyObject pyObject) {
        pyObject.__setitem__("__supernames__", Py.java2py((Object)new String[] { "clone", "finalize" }));

Note that the Python code still lives in another class - this is the just the proxy wrapper.

Known issues

Clamp is still subject to refactoring at this point; see the TODO list at the project page.

Also, it's not feasible to use __new__ in your Python classes that are clamped. Why not? Java expects that constructing an object for a given class returns an object of that class! The solution is simple: call a factory function, in Python or Java, to return arbitrary objects. This is just a simple, if fundamental, mismatch between Python and Java in its object model.


Clamp is a project that has been discussed for a long time in the Jython community as a way to replace the functionality of jythonc, which is no longer supported. Darjus Loktevic did much of the latest work to get this working, and I have added a few critical bits, including setuptools integration.

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