Abstract

The two language approach to software development has been investigated by several language designers. The primary hypothesis of such an approach being that both strong compile-time type checking and loose run-time type checking are desirable in evolutionary software development. Java is a static type-checked language which offers performance, robustness and modularity as such, while Python is a run-time type-checked language which offers rapid prototyping, dynamic run-time modification, and delayed evaluation. The premise of this work is that evolutionary software development can be more efficient using both languages together than using either language alone.

The Java Python Interface (JPI) is an interface between Java and Python which allows the two languages to interoperate through dynamic message lookup and the conversion and exchange of objects and exceptions. Through the JPI, Python can be used as a scripting language for Java-from a Python interpreter one can prototype AWT components and even create bindings which call Python code. In addition, the JPI can be used to add user level Python scripting to Java programs.

1 Introduction

Over the last decade the use and value of prototypes in design, both in software and other engineering disciplines, has been investigated and discussed by researchers [Budde92] [Krogh96]. Evolutionary software development, which consists of the rapid prototyping of new components and their evolution into hardened components [Budde92], offers many benefits to the object-oriented software development process [Cox91]. Included among these benefits is the ability to address rapidly changing software requirements. In addition, the efficiency gained from user-centered design, which consists of constant user feedback impacting the software design, has received significant attention recently [Landauer95].

The requirements of user-centered and evolutionary software development cycles share many similarities but also include conflicting aspects. User-centered development demands the ability to rapidly add features, modify behavior, and create new user interface components. Evolutionary software development can have these same requirements but also requires the creation of well-defined modules and stable, efficient code.

Clean module interfaces, compile-time error checking, and performance make statically typed languages such as C++ [Stroustrup87] and Java [Arnold96] useful for the creation of hardened components. Meanwhile, rapid prototyping, fast compile-test-debug cycles, and high programming flexibility make dynamically typed languages such as Python [Watters96] and Tcl [Ousterhout94] appropriate for the rapid creation of prototype components.

Ousterhout argues that a two language approach proves both feasible and practical for software development [Ousterhout97]. For example, languages such as Python and Tcl provide consistent interfaces to C to allow developers to move performance critical methods and procedures into C without modification of the calling code. In addition, the dynamic language can be used to make calls to and exchange data between existing C or C++ modules.

The Java Python Interface (JPI) attempts to merge the strengths of two languages which are very similar in syntax and function. The purpose of the JPI is two fold: 1) allow for the simultaneous development of prototype components in Python and hardened components in Java, and 2) allow for the addition of user-level Python scripting to Java programs.

This paper will describe the motivation of such an approach, and then focus on the technical details of the JPI.

2 Motivation

Design practices in software engineering vary widely from usage of the waterfall model to the evolutionary development model. In addition, explorations into user- centered design in software engineering have increased over the last few years [Karat96]. Although a unified definition of user-centered design has not been reached, Karat offers elements of one. Karat draws on Gould's four high level principles of good design as a partial definition of user-centered design.

The principles include early and continual focus on users (suggesting that this involves direct user involvement in the design process), early and continual evaluation, iterative design and development, and integrated (whole system) design. [Karat96]

In order to carry out design in this fashion the development environment must allow for the co- existence of stable modules with well-specified interfaces and newly prototyped modules with still- changing interfaces. In addition, the ability to quickly prototype new features, particularly in a running system, with a fast compile-test-debug cycle is required, while at the same time, the ability to statically determine code validity is necessary.

2.1 Two Languages

Supporting the above requirements well requires a language which can support both static and dynamic components at the same time. Currently, the most widely used development languages do not allow for this type of flexibility. Instead, developers are often left to choose a language which does not satisfy all of the necessary requirements. Unfortunately, choosing a single language leaves many pitfalls in the development process.

If developers choose to use a statically typed language they often encounter problems in the early prototyping and development phases. First, they may begin to define programming module interfaces prematurely which can cause delays for other developers when modules are redefined and require recompilation of part or all of the system. Second, they are forced to engage in long compile-test-debug cycles due to the longer compilation times of static languages and the need to restart the program and recreate the state under which the error occurred. Third, determining the cause of an error can be difficult due to the inability to directly query the running program for information other than through the use of a debugger. Fourth, it is difficult to debug part of an incomplete program since the compiler will not allow the program to run until all of the compile-time errors are corrected. While a static language offers many benefits, these limitations often make it difficult to rapidly prototype all or part of an application.

On the other hand, if developers choose to use a dynamically typed language they often encounter problems in the latter phases of development when performance, scalability, and maintainability are crucial. First, they may end up not defining module interfaces well since the language often will not require the strict specification of the interface to a module. Second, they often encounter performance problems when writing computationally intensive code and as a result spend a great deal of time optimizing code. Third, they suffer scale problems related to CPU and memory usage when a system becomes large. Fourth, the detection of all coding errors, except syntactical, is delayed until the actual line of code containing the error is executed - meaning completely exhaustive tests must be written in order to check the validity of code. Despite the benefits of a highly dynamic language, these limitations make it difficult to maintain a large piece of software.

Fortunately, alternatives exist to choosing either a static or dynamic language. Languages such as Python and Tcl, offer the option of using both a dynamic component, the language itself, and a static component, C. Therefore, a developer can use a language such as Python to prototype and debug new code and then reimplement that code in C for performance and scalability. This two language approach to software development offers many benefits over using a single language. Programmers can prototype new features, work out their logic, and fix errors without the need to engage in a long compile-test-debug cycle. Once code logic has been established and verified, the code can then be moved into the static language to gain speed and robustness and force the creation of a well-defined interface.

2.2 Java and Python

The use of two languages does not immediately solve the challenges facing a development team. Often, using two languages can have as many problems as using a single language.

For several years, the n-dim project at Carnegie Mellon University has investigated the two language approach with an object system called BOS [Dutoit96]. BOS, standing for Basic Object System, allows for the prototyping of code in an interpreted language called stitch and the hardening of code in C, much like Python and Tcl. Our experiences have shown that the main drawback of this approach is that moving code into C is a costly effort since it requires a complete reimplementation of the code and often the logic. As a result, developers typically take the first step of prototyping code in the dynamic language and then neglect the needed step of reimplementing the code in the static language despite the performance and scalability gains. Recently, tools such as SWIG [Beazley97] make this process easier for languages including Perl, Python, and Tcl, but they still require significant work on the part of the programmer.

Python and Java share many common features [Masse96] - they are both object-oriented, support garbage collection, provide object-based exception handling, guarantee memory safety, and have a similar syntax. As a result, using them together in the two language approach offers a potential solution to the barrier of converting prototype code to hardened code. Given the ability to seamlessly communicate between the two languages, one could maintain different parts of a program in each language. Classes prototyped in Python could easily be converted to Java classes, but at the same time, particular methods, such as those undergoing a lot of rapid change, could be kept in Python. The similarities of Python and Java allow for the transition of code from one language to the other to be extremely easy and even offer the possibility of complete automation.

The Java Python Interface (JPI), provides a mechanism for this type of communication. Messages can be sent to Java objects from Python and vice-versa, argument conversions are performed between the two languages, objects from one language can `exist' in the memory space of the other, and exceptions thrown from one language can be caught in the other. The following section describes the JPI in more detail and gives examples of how it can be used to satisfy the requirements of both rapid prototyping and code hardening in software development.

3 The JPI Overview

The JPI is an interface between Java and Python. It allows for the dynamic manipulation of Java objects through the use of Python and vice-versa. By writing Python code one can program using existing Java classes. In addition, the syntactical similarities between Python and Java are great enough that the conversion of prototyped Python code to hardened Java code is very easy and can even be largely automated.

The JPI consists of a simple interface between the two languages, but at the same time, it is quite powerful. An example of the use of the JPI is the Python class shown in Example 1 which creates a Java AWT button and prints `Hello World!' when it is clicked. This section describes the implementation of the JPI.

# HelloWorld.py 

import java 
Frame = java.findClass("java.awt.Frame") 
Button = java.findClass("java.awt.Button") 
PyEventListener = \ 
java.findClass("PyEventListener")

class HelloWorld:

  def __init__(this): 
    # Create a frame and a button 
    frame = Frame.new("Hello World Test") 
    button = Button.new("Click Here") 
    frame.add(button) 
    frame.pack() 
    frame.setVisible(1) 

    # Create a listener object which will 
    # call this object back when an event 
    # occurs and add it to the button 
    listener = PyEventListener.new(this) 
    button.addActionListener(listener) 

    # Tell the listener what event to 
    # listen for and what message to send 
    # when the event occurs 
    listener.listenFor("actionPerformed", "printMessage")

  def printMessage(this, event): 
    print "Hello World!" 

% java Python 
>>> from HelloWorld import * 
>>> hw = HelloWorld() # Button appears 
>>> # Click away! 
Hello World! 
Hello World! 
Hello World! 
Hello World! 
Hello World!

3.1 Basics

The JPI consists of a Python C module (java), a Python C type (JavaObject), and three simple Java classes (Python, PyObject, and PyEventListener).

• The `java' module implements the simple functionality of looking up Java classes through a `findClass' method.
• The `JavaObject' type wraps an actual Java object in a Python C type. Any messages sent to a `JavaObject' are answered by dynamically finding the appropriate method defined for the Java object.
• The `Python' class implements the functionality of sending code to Python to be evaluated and allows the getting and setting of attributes in Python's main module. This class also implements the method for dynamically finding and invoking Java methods.
• The PyObject class wraps an actual Python object in a Java object. Messages can be sent to the Python object by invoking the `send' method with a message name and arguments.
• The `PyEventListener' class is provided to listen for AWT events.

In addition, the `java' module and `Python' class handle the object conversions described below.

3.2 Java Classes

To create a Java instance a program must first obtain a class to instantiate. This is done using the `findClass' method of the `java' module. It is useful to assign classes to variables for more convenient use. Once a class is obtained, the `new' message will invoke a constructor matching the arguments provided. Example 2 demonstrates this.

# Get the Java AWT Frame class 
>>> Frame = java.findClass("java.awt.Frame")

# Create a Frame instance 
>>> f = Frame.new("My Frame") 

Frame f = new Frame("My Frame"); 

In addition, the static fields and methods of a class can be accessed through this handle to a Java class. As Example 3 shows, sending a message to a Java class from Python looks almost exactly as it does in Java.

# Get the Java System class
>>> System = java.findClass("java.lang.System")

# Invoke the static currentTimeMillis 
# method on the System class 
>>> t = System.currentTimeMillis() 

long t = System.currentTimeMillis() 

3.3 Java Instance Methods and Variables

Messages are sent to Java through a dynamic message lookup scheme. The `JavaObject' Python C type, which wraps a Java object, contains a global reference, provided by the JNI, to the actual Java object. When a message is sent to a `JavaObject' a dynamic message lookup ensues in order to find out if the actual Java object has a method with the correct name and argument types. If an appropriate method is found it is invoked. If no method is found, then the lookup proceeds to search for an instance variable with the correct name and argument type. If a matching variable is found, then the variable is either set (if one argument was provided) or its contents retrieved (if no argument was provided). If no variable is found or more than one argument was sent, then an exception is thrown.

Example 4 shows Python code which both invokes a method and gets the value of a variable. Unfortunately, as the example demonstrates, accessing a variable is not as seamless as sending messages since it requires an extraneous set of parenthesis. The reason for this is that the `JavaObject' type uses the `getattr' function to answer any messages sent to the object. Since the `getattr' function is not given the information of whether the user is attempting to invoke a method or get the contents of a variable, it is not possible to know whether a method or variable lookup should be done. Presumably this information is available to Python, but it is not passed to `getattr'. An alternative solution to forcing the use of parentheses is to have the variable lookup take precedence over the method lookup, but the behavior would still not be ideal.

# `o' is a Java object with the method 
# `length' and variable `elems' 

# Send the `length' message with no 
# arguments to invoke the `length' method 
>>> num = o.length() 

# Send the `elems' message to get the 
# contents of the `elems' variable 
>>> el = o.elems() 

Likewise, the Java class, PyObject, wraps an actual Python object. An instance of PyObject contains a pointer to the actual Python object and a `send' method is provided to send a message to the Python object. Clearly, the code to send a message to Python is not as clean. This is because Java does not provide any mechanism for answering any message sent to an object such as Python's `getattr'. Example 5 gives the implementation of a Java method called `reverseRange' which takes a Python object as an argument and sends it the `reverse' message.

// This method sends the `reverse' 
// message to a Python range 
public void reverseRange(PyObject r) throws Exception { 
  // If the object passed in does 
  // not answer `reverse' then an 
  // exception will be thrown 
  r.send("reverse"); 
}

3.4 Object Conversions

Sending messages without arguments or without receiving results would be quite limiting. Therefore, an integral aspect of the JPI is the passing of objects between Java and Python through type conversions. The supported type conversions from Java to Python are listed in Table 1, and the type conversions from Python to Java are listed in Table 2.

The argument conversion code will convert objects in both directions when used in an argument list or when returning results. If an object can not be converted to a primitive type it will be converted to either a PyObject or a JavaObject depending on the direction of conversion. An exception to this rule is the `new' primitive of the `java' C module which always returns a JavaObject. The code in Example 6 demonstrates several object conversions.

# Vector class and new instance are 
# converted to JavaObject type 
>>> Vector = java.findClass("java.util.Vector") 
>>> v = Vector.new() 

# Create Python range - no conversions here 
>>> r = range(5) 

# Python range is converted to a PyObject 
>>> v.addElement(r) 

# int 0 is converted to Java int 0 and 
# element found is converted from PyObject 
# back to Python range 
>>> print v.elementAt(0) 
[0, 1, 2, 3, 4] 

The JPI attempts to support object conversions well in both directions, but there are no guarantees that a problematic situation can not arise. As illustrated in Example 7, a Java class could define two methods with the same name and arguments similar enough that a message sent from Python could potentially be answered by both methods. Currently, the choice is arbitrary - the first method it finds will be the one selected. There is, of course a way around this as the example also shows.

// Java class Test implements two 
// different `set' methods 
public class Test() { 
  public void set(int i) { } 
  public void set(boolean b) { } 
} 

>>> o = java.findClass("Test").new() 
>>> o.set(1234) 

>>> Integer = java.findClass("java.lang.Integer") 
>>> o = java.findClass("Test").new() 
>>> i = Integer.new(1234) 
>>> o.set(i) 

3.5 Exception Handling

Exceptions are handled by the JPI by converting an exception back and forth between a Python exception and a Java exception as it passes up the call stack. The programmer can catch the exception in either of the languages. Currently though, the exception handling mechanism loses information about the original exception when it is converted between languages. So an exception occurring when sending a message to Java will result in a general Exception object being thrown. Due to this drawback, the JPI prints out the exception information just before conversion so that the programmer can see the actual exception that occurred. Future work will improve upon this such that in either language the actual exception which was thrown will be passed back and forth. The current and future behavior are shown in Example 8.

>>> v = Vector.new() 
>>> v.elementAt(0) 
java.lang.ArrayIndexOutOfBoundsException: 0 >= 0 
  at java.util.Vector.elementAt 
      (Vector.java) 
  at Python.dynamicMessageSend 
      (Python.java:241) 
  at Python.runInterpreter 
      (Python.java:444) 
  at Python.main 
      (Python.java:467) 
Traceback (innermost last): 
  File "<string>", line 1, in ? 
java.error: Exception thrown from Java 
java.lang.Exception: Exception thrown from Python 
  at Python.runInterpreter 
      (Python.java:444) 
  at Python.main 
      (Python.java:467) 

>>> v = Vector.new() 
>>> v.elementAt(0) 
java.lang.ArrayIndexOutOfBoundsException: 0 >= 0 
  at java.util.Vector.elementAt 
      (Vector.java) 
  at Python.dynamicMessageSend 
      (Python.java:241) 
  at Python.runInterpreter 
      (Python.java:444) 
  at Python.main 
      (Python.java:467) 

3.6 Other Implementation Details

Classes and Interfaces. In order to have a Python class implement a Java interface or subclass a specific class, a Java class doing so must be created which then forwards messages to a Python object. The PyEventListener class is an example of this. The PyEventListener class implements all of the AWT event listener interfaces. Its constructor requires a PyObject to which it will forward calls to the interface methods. The PyEventListener serves merely as a custom wrapper for a PyObject to satisfy the interface requirements of methods which demand some type of event listener. Example 1 gives an example of how the PyEventListener can be used.

The (Fake) Interpreter. The JPI provides its own interpreter because difficulties were encountered when trying to run the Python interpreter from C. This fake interpreter merely reads input and sends it to Python for the real interpretation, but as a result, the behavior of the interpreter does not exactly match that of Python. Multi- line expressions are finished as soon as a blank line is entered, and results are not printed automatically, the `print' command must be used.

Reserved Words. There is also a problem when sending messages which correspond to Python reserved words, e.g., print. This is because the Python parser looks for the reserved words and immediately generates an error if the reserved word is not being used properly. Therefore, trying to send a message to a Java object which matches a reserved word will result in a syntax error. The work around for this is to use the `send' method defined on `JavaObject' to by-pass the Python parser. The `send' method takes the message to be sent as its first argument.

4 Other Approaches

Other approaches to building software out of both static and dynamic components exist-a language supporting both static and dynamic modules could be written, different languages could be chosen for integration, and the languages could be integrated in different ways.

As described in Section 2, the n-dim group has experimented with a prototype-based object-oriented system, BOS, which satisfies many of the requirements outlined above [Dutoit96]. For example, BOS allows for method and variable declarations to be typed or untyped. Although BOS currently does not perform any static type-checking, this feature could be added to BOS to verify code validity when types are provided. In this way, developers could opt to leave out typing information when in the prototyping phase and later add typing information when in the code- hardening phase.

One of the goals of the JPI was to have a system which consisted of commercially supported, or near commercially supported, languages. Section 2 described the reasons for choosing Python and Java. The similarities were great enough that an integration of the two could be much cleaner than any other alternatives. For example, if the attempt was to integrate Python and C++, then garbage collection issues would certainly be an obstacle. Python could not maintain a handle to a C++ object without concern of when the C++ object is destroyed. With Java, Python can simply call a JNI function which establishes a global reference to the Java object. This prevents the Java object from being garbage collected even when Python is the only reference holder. Likewise, when Java holds a reference to a Python object the object's reference counter can be incremented to prevent garbage collection. In both cases, the referring wrapper object can release its hold on the object when the wrapper object is itself garbage collected.

Another approach to building a system with both dynamic and static components is through the use of an interfacing tool such as CORBA or ILU. These tools allow the programmer to specify the interface of a module such that the module can be used from a different language, by another program, or even by a program running on a different machine. While this offers flexibility with regards to the number of possible ways in which the module can be used, it requires the rigid specification of a module interface. In fact, CORBA and ILU are orthogonal to the JPI and do not conflict with, but instead augment, its usefulness.

Finally, this is not the only project attempting to integrate Python and Java. Kevin Butler from Brigham Young University has implemented a package called PyJava which also provides an integration between Java and Python. Although the functionality is quite similar, the development methods employed are very different. While a full comparison of the two approaches is beyond the scope of this paper, the most notable difference in functionality is that PyJava currently only allows for Python to call Java and not vice-versa.

5 Future Directions

This integration of Java and Python is not yet complete and still reveals many seams. Improvements can be made to the JPI and changes to Java and Python could also improve the quality of the integration.

First, the JPI needs more work in the areas of exception handling and threads. Improvements to these two aspects of the JPI are currently being investigated.

Second, if Python provided a way of finding out the pretenses under which `getattr' was being called, then the extraneous set of parenthesis, mentioned in Section 3, could be disposed of when accessing Java variables. Also, if the Python interpreter could be modified such that it does not generate an error when a reserved word is used as a message name, then code calling a Java method would not need to use the `send' primitive described above. Both of these changes would make code written in Python closer to its Java equivalent.

Third, there are changes to Java that would allow for better access to Java objects and a symmetric way of calling Python. A privileged code status would allow certain code modules (e.g. a debugger, JPI, etc.) to access private and protected methods and variables. This feature is mentioned in the Java documentation but has yet to be included in a release. Also, a Java feature similar to Python's `getattr' would allow for a symmetric way of calling Python. For example, Java could reserve a method which will get invoked when no other method can answer the message. This feature could be explicitly turned on so as not to damage the benefits of compilation. Hence, this method could be added to the PyObject class so that any message sent to it from Java could be forwarded to Python.

The JPI is still under development. Features to tighten the match between Java and Python are being investigated, while bugs and porting issues are being addressed as well.

6 Conclusions

The JPI has been used extensively by the author. Despite the described shortcomings it has proven very valuable. Code can be prototyped very quickly in Python and, once it works properly, moved to Java. Programs can be interactively debugged such that the state of a running program can be queried and modified to find the cause of a bug and the potential corrections for it. Graphical interfaces can be prototyped using the Java AWT and modifications can be made directly from the Python command line such that the programmer can experiment with layout and appearance without needing to recompile and rerun the program. User level scripts can be added to programs such that run-time customizations can be made to programs.

Finally, due to the many similarities between Java and Python, the JPI is able to support the two language approach to software development in an intuitive manner. For example, in one particular case a LoginDialog class was prototyped in Python while establishing the look and feel and then reimplemented in Java once completed. The dynamic nature of Python allowed for many iterations of the look and feel to be experimented with over the course of a day. Once the final design was determined, the LoginDialog class was reimplemented in Java. The reimplementation consisted primarily of a syntax conversion and was completed in less than an hour.

7 References

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