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Compiling Python Source to Real Java Classes

The previous section describes how Python classes can be created that subclass from Java classes. This works very well when you want to pass a Python class into Java from Jython. This is not adequate for building real Java ".class" files that implement a Java class and can be passed directly to Java programs. This sort of functionality is required to build applets, servlets, or JavaBeans in Jython that can be used exactly like their Java equivalents. It is also very valuable for users who wish to subclass Python classes in Java.

In order to build a real java class from a Python class, you will use the jythonc tool. 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.
Compile all Python dependencies of the module. This is used for creating applets.
Include the core Jython libraries (about 130K). Needed for applets since Netscape doesn't yet support multiple archives. Implies the --deep option.
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.
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 conversion.FahrenheitToCelsius

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 BDK\beanbox\run.bat (or run.sh) to include the jython\JavaCode in the CLASSPATH
  2. copy f2c.jar to BDK\jars

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. XXX
  • sys.argv[0]
  • loading of python classes.