Jython Course Outline

Jython Course Outline

Jython Course Outline

Author:	Dave Kuhlman
Address:	dkuhlman@rexx.com http://www.rexx.com/~dkuhlman
Revision:	1.1b
Date:	May 9, 2008
Copyright:	Copyright (c) 2006 Dave Kuhlman. All Rights Reserved. This software is subject to the provisions of the MIT License http://www.opensource.org/licenses/mit-license.php.

Abstract

This document provides an outline of an introductory
course on programming in Jython and connecting Jython to Java

Contents

• 1   How-to Write Jython Code
• 2   Installing and Running Jython
  
  • 2.1   Install Jython
  • 2.2   Configuration
    
    • 2.2.1   Command-line options
    • 2.2.2   Jython configuration files
    • 2.2.3   Checking configuration values
    • 2.2.4   Classpath and python path
  • 2.3   Running Jython
  • 2.4   Installing Jython/Python packages
• 3   Integrating Java into Jython/Python
  
  • 3.1   Calling existing Java code
  • 3.2   Extending a Java class in Jython
  • 3.3   Emulating Jython classes in Java
  • 3.4   Preparing Java code to be called from Jython
    
    • 3.4.1   Adding doc strings to a Java class
    • 3.4.2   Working with Jython arguments
    • 3.4.3   Sub-classing a Java class
    • 3.4.4   Emulating Jython Dictionaries, Sequences, Etc.
      
      • 3.4.4.1   Solution #1 -- Emulating a dictionary
      • 3.4.4.2   Solution #2 -- Extending PyDictionary
    • 3.4.5   Emulating Jython object attribute access
  • 3.5   Extending a built-in Jython class in Java
  • 3.6   jarray -- Creating and passing Java arrays to Java
• 4   Integrating Jython/Python into Java
  
  • 4.1   Calling Jython from Java
    
    • 4.1.1   Run Jython code on an interpreter embedded in Java
    • 4.1.2   How to run Jython code on an interpreter embedded in Java
      
      • 4.1.2.1   Example -- a possible organization on disk
      • 4.1.2.2   Example -- the Jython classes
      • 4.1.2.3   Example -- A Java interface class
      • 4.1.2.4   Example -- another interface class
      • 4.1.2.5   Example -- the factory class
      • 4.1.2.6   Example -- the Java consumer code
    • 4.1.3   Additional support for running Jython code on an interpreter embedded in Java
  • 4.2   Embedding Jython into Java
    
    • 4.2.1   Why embedded Jython
    • 4.2.2   Embedding Jython into Java is simple
    • 4.2.3   Passing values into a Jython script
    • 4.2.4   Initializing the interpreter
      
      • 4.2.4.1   An interpreter with an initial system state
      • 4.2.4.2   A system state and a custom class loader
    • 4.2.5   Retrieving values from a Jython script
    • 4.2.6   There are also a few complexities
    • 4.2.7   Exposing transparent objects
    • 4.2.8   Exposing opaque objects
    • 4.2.9   Type conversion
    • 4.2.10   Using a custom class loader
    • 4.2.11   Embedding a Jython console
  • 4.3   Embedding Jython with the Java Scripting Engine
    
    • 4.3.1   Installing script engine support
    • 4.3.2   Using script engine support
  • 4.4   Compiling Jython code with jythonc
    
    • 4.4.1   Introduction to jythonc
    • 4.4.2   How to run jythonc
    • 4.4.3   Calling Jython from Java using jythonc
• 5   Deployment and Distribution
  
  • 5.1   Building jars - some samples
    
    • 5.1.1   Add Jython install stuff to our jar
    • 5.1.2   Add modules and paths to the jar file
    • 5.1.3   Run the script/jar
    • 5.1.4   A more self-contained jar file
    • 5.1.5   A summary
• 6   Integrating, Embedding, and Extending -- A Summary
• 7   Jython+Java -- Other Advanced Topics
  
  • 7.1   Event handling
  • 7.2   XML
    
    • 7.2.1   jaxp
    • 7.2.2   Xerces
    • 7.2.3   dom4j
      
      • 7.2.3.1   Installation and setup
      • 7.2.3.2   Examples etc
    • 7.2.4   XMLBeans
      
      • 7.2.4.1   Installation
      • 7.2.4.2   An example
  • 7.3   Database access
    
    • 7.3.1   JDBC
    • 7.3.2   zxJDBC
• 8   Additional Exercises
• 9   References and Sources

1   How-to Write Jython Code

Jython is Python. That's one of its big advantages: you get two for
the price of one. If your learn Python, then you have also learned
Jython, and vice versa. If you already know Python, then you know
Jython.

But, if you do know know Python or Jython, then here are good
training aids:

• Python documentation --
  http://python.org/doc/
• Python tutorial --
  http://docs.python.org/tut/tut.html
• BeginnersGuide/NonProgrammers --
  http://wiki.python.org/moin/BeginnersGuide/NonProgrammers
• BeginnersGuide/Programmers --
  http://wiki.python.org/moin/BeginnersGuide/Programmers

2   Installing and Running Jython

2.1   Install Jython

You will need Java installed, of course. And, since you are likely
to want to use Jython class libraries from Jython, it is also likely
that you will want the Java SDK. Important: If more than one
version of Java is installed on your machine, make sure that when
you install Jython, you use the version of Java for which the SDK is
installed and the version of Java that you will be using when you
run Jython.

Download the Jython installation jar file -- You can file the Jython
distribution here: Jython downloads --
http://jython.org/Project/download.html.

Install Jython -- Follow the instructions at:
Jython installation --
http://jython.org/Project/installation.html:

$ java -jar jython_installer-2.2.1.jar

Command line history -- On MS Windows, command line history for the
Jython interactive interpreter comes built-in. On Linux,
to get command line history, command line editing, and readline
support, follow the instructions here:
ReadlineSetup --
http://wiki.python.org/jython/ReadlineSetup.

Standalone mode -- You can also create a self-contained Jython jar
file. Run the standard installer (above), then you come to the
"Installation type" page, select "Standalone". For more on this, see:
Standalone mode --
http://www.jython.org/Project/installation.html#standalone-mode

2.2   Configuration

There are several places to configure Jython.

2.2.1   Command-line options

To display the options for jython, type:

$ jython --help

2.2.2   Jython configuration files

For explanation of configuration options and values, see:

• The comments in the (default) registry file.
• The Jython Registry --
  http://www.jython.org/docs/registry.html.

2.2.3   Checking configuration values

From within the Jython interactive interpreter or from within your
Jython application, you can display the values of configuration
properties.

To get the system properties as a dictionary-like object, do:

>>> from java.lang import System
>>> props = System.getProperties()

Of particular interest are the following:

• props['java.class.path'] -- Location of the Jython jar file.
• props['java.library.path'] -- Locations of Java class libraries.

Other properties are in sys.registry:

>>> import sys
>>> r = sys.registry
>>> for k in r:
...   print k, r[k]

Here is a script that you may find useful when interactively
inspecting system properties:

>>> from java.lang import System
>>> props = System.getProperties()
>>> names = []
>>> for name in props.keys():
...     names.append(name)
...
>>> names.sort()    # now you can list the keys in alpha order
>>> for val in props['java.class.path'].split(':'):
...     print val
...
/home/dkuhlman/a1/Python/Jython/Tmp1/Jython-2.1/jython.jar
/usr/share/jython/jython.jar

2.2.4   Classpath and python path

Jython can pick up Java class files from locations on either the
Jython/Python path (see sys.path) or the Java classpath. Set
these with the following:

• The Python/Jython path can be set in your registry file. See
  registry variable python.path.

  Or, at runtime, you could do:

  >>> import sys
  >>> sys.path.append('/path/to/module')
  

  But, you must do the above before trying to import the module.

• Set the classpath by setting the CLASSPATH environment variable.
  Note that (on my Linux machine, at least) the CLASSPATH environment
  variable is picked up and added to the Java -classpath flag.

A few rules about CLASSPATH and python.path:

• sys.path in the registry file -- Add here to enable importing
  from Java classes (.java), Java class libraries (.jar), and
  Jython/Python (.py).
• CLASSPATH -- Add paths to this environment variable in order to
  enable importing from Java classes (.java) and Java class
  libraries (.jar), but not Jython/Python (.py).

2.3   Running Jython

The Jython interactive, command-line interpreter: jython.

Jython IDEs (interactive development environments) -- There is a
Jython plug-in for Eclipse. See: http://pydev.sourceforge.net/.

Exercises -- Start the Jython interpreter. Then do each of the
following:

• Print "hello".
• Define an empty class.
• Import a Python/Jython file containing a class definition.
  Create an instance of that class.
• Import a module from the standard Python/Jython library, for
  example, re or os.path. Use a method from that module.
• Import a Java class, for example, java.util.Vector. Create
  and use an instance of that class.

Running Jython scripts:

• From the command line, run a script with jython. For
  example:

  $ jython myscript.py
  

• For help, run:

  $ jython --help
  

• For debugging, use something similar to the following:

  import pdb
  pdb.run('main()')
  

  Or:

  import pdb
  pdb.set_trace()
  

  For example:

  def main():
      util101()
  
  if __name__ == '__main__':
      import pdb; pdb.set_trace()
      main()
  

• To "set a breakpoint" in your code so that it will drop into
  debugger, either (1) use the b command at the pdb prompt
  or (2) add the following to your code at the location where you
  wish to drop into the debugger:

  import pdb; pdb.set_trace()
  

  For more information on the Python debugger, see The Python
  Debugger in the
  Python standard documentation, or while in the debugger, type
  help.

• To make a script both "run-able" and "import-able", use the
  following idiom:

  if __name__ == '__main__':
      main()
  

Don't forget to include a doc string at the top of your module for
documentation.

Exercise -- Create a small Jython script:

• Include a class in your script that creates an instance of
  java.util.Vector.
• Make the script both "run-able" and "import-able".
• From the Jython interpreter, import the script and create an instance
  of the class.
• From the command line, use jython to run the script.
• Add pdb debugging to your script. Run the script again from
  the command line. Step through several lines of code.

2.4   Installing Jython/Python packages

Some Jython packages will be distributed as a Java jar file. If that
is the case, add the jar file to your classpath.

If the package is distributed as a standard Python package with a
setup.py installer file and if there are no C/C++ files in the
package, then you might try something like the following:

$ python setup.py install --prefix /path/to/install/directory

And, then put that install directory on your classpath.

3   Integrating Java into Jython/Python

3.1   Calling existing Java code

In order to call Java code from Jython do the following:

1. Import the Java module.
2. Use the Java module to create an instance/object.
3. Call functions and objects in it.

It works the way you would hope and expect it to. Here is an
example:

>>> from java.util import Vector
>>> v = Vector()
>>> dir(v)
['__init__', 'add', 'addAll', 'addElement', 'capacity', 'class', 'clear', 'clone', 'contains', 'containsAll', 'copyInto', 'elementAt', 'elements', 'empty', 'ensureCapacity', 'equals', 'firstElement', 'get', 'getClass', 'hashCode', 'indexOf', 'insertElementAt', 'isEmpty', 'iterator', 'lastElement', 'lastIndexOf', 'listIterator', 'notify', 'notifyAll', 'remove', 'removeAll', 'removeAllElements', 'removeElement', 'removeElementAt', 'retainAll', 'set', 'setElementAt', 'setSize', 'size', 'subList', 'toArray', 'toString', 'trimToSize', 'wait']
>>>
>>> v.add('aaa')
1
>>> v.add('bbb')
1
>>> for val in v:
...     print val
...
aaa
bbb

In some cases you will need to pass Java objects to Java methods.

Special treatment for some overloaded Java methods -- Explicitly
create and pass Jython objects. For more on this, see:
Overloaded Java Method Signatures
-- http://www.jython.org/Project/userguide.html#overloaded-java-method-signatures.

Often you can use Python/Jython style and idioms to process Java
objects. For example: the Jython for statement can be applied
to Java collection objects.

Exercise -- Use the class java.util.Hashtable to create a
dictionary with several keys and values, then print out the keys
and their values. Solution:

>>> from java.util import Hashtable
>>> impl_language = Hashtable()
>>> impl_language.put('jython', 'java')
>>> impl_language.put('python', 'c')
>>> for key in impl_language.keys():
...     print '%s is implemented in %s' % (key, impl_language[key])
...
python is implemented in c
jython is implemented in java

3.2   Extending a Java class in Jython

You can import and then extend (sub-class) a Java class.

Example -- This sample extends the Java filestream class by adding a
method that converts all characters to upper case::

import sys
from java.io import FileOutputStream

class UppercaseFileOutputStream(FileOutputStream):
    def write_upper(self, text):
        text = text.upper()
        self.write(text)

def test(outfilename):
    fos = UppercaseFileOutputStream(outfilename)
    for idx in range(10):
        fos.write_upper('This is line # %d\n' % idx)
    fos.close()
    infile = open(outfilename, 'r')
    for line in infile:
        line = line.rstrip()
        print 'Line: %s' % line

def main():
    args = sys.argv[1:]
    if len(args) != 1:
        print 'usage: extend_fileoutputstream.py <infilename>'
        sys.exit(1)
    test(args[0])

if __name__ == '__main__':
    main()

3.3   Emulating Jython classes in Java

You can make a Java class "act like" one of the built-in Jython
classes. In order to do so, you would implement one or more of
Jython's special methods. You can find descriptions of the special
methods in the "Python Reference Manual":
3.4 Special method names --
http://docs.python.org/ref/specialnames.html.

Example: This module implements a class that acts like a sequence in
certain ways, specifically (1) it responds to the len() operator
by returning a length; (2) it supports an append method; and (3)
it supports the use of the [] operator to get a value:

import java.util.Vector;