jythonc Guide

What is jythonc

jythonc transforms Python source code into Java source code then invokes a Java compiler to turn it into .class files. This allows Python to be integrated into Java in several places that regular Jython currently doesn't support. It also processes special annotations in docstrings on methods in Python code to determine the static type information to expose when turning a dynmically typed Python method into a statically typed Java method.

jythonc is unmaintained and will not be present in its current form in Jython 2.5. While jythonc handles all the language features of Python 2.2, it doesn't support Python 2.3 and later features such as generators. As such, we cannot maintain the current jythonc going forward; however, Jython 2.5 will include a replacement (likely also called jythonc) which produces class files directly instead of generating Java source files. The current jythonc is included in Jython 2.2.x to support existing users and for these features:

  1. Running in a JVM with a classloader that will not load dynamically created classes
  2. Declaring Java method signatures in Python code
  3. Loading Python classes dynamically from Java with Class.forName

Most uses of the second feature, adding method declarations to docstrings, can be handled by declaring a Java interface to implement with a Python class. Each implementing Python method inherits its type signature from the corresponding method in the Java interface. Exposing the Python class as an instance of that type to Java code is explained in Accessing Jython from Java Without Using jythonc and its followup, Simple and Efficient Jython Object Factories.

Using jythonc

A script called jythonc should have been installed in the Jython installation directory when you installed the package. If it was not, it can be invoked as follows: jython Tools\jythonc\jythonc.py.

The jythonc tool generates actual Java source code, and then invokes a Java compiler over this source code to create the .class files. Thus, you will need access to a Java compiler in order to use jythonc. It is recommended that you use Sun's standard javac compiler, or IBM's excellent jikes compiler. Specifically, the Microsoft SDK jvc compiler is not recommended.

Invoke jythonc like this:

jythonc [options] [module]*

Options are given in the table below. Note that jythonc now uses the standard Python getopt module, meaning that both long and short forms of the options are available. The old option names still work, and are described below, but noted as deprecated and will be removed in a future version.

--package package
-p package
-package package      Put all compiled code into the named Java package.
--jar jarfile
-j jarfile
-jar jarfile  Specifies a .jar file to create and put the results of
the freeze into. Implies the --deep option.
-deep         Compile all Python dependencies of the module. This is
used for creating applets.
-core         Include the core Jython libraries (about 130K). Needed for
applets since Netscape doesn't yet support multiple archives. Implies
the --deep option.
-all  Include all of the Jython libraries (everything in core +
compiler and parser). Implies the --deep option.
--bean jarfile
-b jarfile
-bean jarfile         Compile into jarfile, including the correct
manifest for the bean.
--addpackages pkgs
-A pkgs
-addpackages pkgs     Include Java dependencies from this list of
packages. Default is org.python.modules and org.apache.oro.text.regex.
--workdir directory
-w directory
-workdir directory    Specify the working directory where the generated
Java source code is placed. Default is "./jpywork"
--skip modules
-s modules
-skip modules         Don't include any of these modules in
compilation. This is a comma-separated list of modules.
--compiler path
-C path
-compiler path        Use a different compiler than `standard'
javac. If this is set to "NONE" then compile ends with the generation
of the Java source file. Alternatively, you can set the property
python.jythonc.compiler in the registry.
--compileropts options
-J options    Options passed directly to the Java
compiler. Alternatively, you can set the property
python.jythonc.compileropts in the registry.
--falsenames names
-f names
-falsenames names     A comma-separated list of names that are always
false. Can be used to short-circuit if clauses.
-h    Print a usage message and exit.
[module]*     A list of Python modules to freeze. Can be either modules
names that are on the python.path or .py files

In order to create real java class, the Python module must contain a Python class with the same name as the module, and that Python class must subclass exactly one Java class or interface. The real Java class created will be a subclass of whatever Java class is subclassed.

Compiling python methods to java methods

Normally, compiled python methods are not directly callable from java. There is simple not sufficient information in a python method definition to create a useable java method. There are however two ways a python method be made into real java method. Both ways require that the python class have a java class or a java interface as its base classes.

  • Overriding / Implementing a java method.

    By overriding a java method, the overridden method's signature is used for the real java class. That includes return type, privacy modifier, arguments, argument types and throws clauses.

  • Adding java method signature to python methods docstrings.

    Using a special convention in the method docstring allow for the creation of a corresponding java method. The signature must be written on one line and must start with the text "@sig". All class names (except those ion java.lang) must be specified as fully qualified class names. Example:

    def setExpression(self, e):
        "@sig public void setExpression(java.lang.String e)"


On my Windows NT machine, I used the following command to create the file appletdemo.jar (containing all of the applet demos on the web site, as well as the core Jython libraries):

c:\jython\Demo\applet> jythonc --core --deep --jar appletdemo.jar *.py

Note: One special thing to consider when building applets is the java VM version used when compiling the sources must usually be the same as the VM version used by the browser. If a newer JVM is used when compiling the applets, the compiled code may have references to classes that does not exists in older JVM's.

To generate a skeleton class to allow a Python class to be used as java.awt.Component in a Java gui program, I used the following command:

c:\jython\Demo\javaclasses> jythonc Graph.py

To generate a Jython-based bean I do the following:

c:\jython\Demo\bean> jythonc --deep --bean f2c.jar

To use this with SUN's BDK, I do the following (the important steps are to make the bean and the Jython libraries known to the tool).

  1. modify BDKbeanboxrun.bat (or run.sh) to include the jythonJavaCode in the CLASSPATH
  2. copy f2c.jar to BDKjars

You should now be able to access the FahrenheitToCelsius bean from SUN's BeanBox (or other Bean-aware tool).

Freezing modules

jythonc can also be used to freeze a python application. The frozen application can then be distributed and deployed as any other java application.

Some diffrences between an interpreted application and a frozen application exists:

  • Properties are diffrent
  • sys.argv[0]
  • loading of python classes