Wizard API updated!
Tim Boudreau has released a new version of the Swing Wizard library (version 0.997) that fixes the WizardException bug reported in JavaWorld's recent Open Source Java Project profile. The article's examples have been reworked to test out the new, improved WizardException. Thanks, Tim, for this helpful fix!
Open Source Java Projects: The Wizard API

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Java Tip 17: Integrating Java with C++

<H3>Learn how to use C++ code from within a Java application and how to call from C++ to a Java object</H3></CENTER>

By Bill Foote, JavaWorld.com, 09/01/96

In this article, I'll discuss some of the issues involved in integrating C++ code with a Java application. After a word about why one would want to do this and what some of the hurdles are, I'll build up a working Java program that uses objects written in C++. Along the way, I'll discuss some of the implications of doing this (such as interaction with garbage collection), and I'll present a glimpse of what we can expect in this area in the future.

Why integrate C++ and Java?

Why would you want to integrate C++ code into a Java program in the first place? After all, the Java language was created, in part, to address some of the shortcomings of C++. Actually, there are several reasons why you might want to integrate C++ with Java:

  • Performance. Even if you're developing for a platform with a just-in-time (JIT) compiler, odds are that the code generated by the JIT runtime is significantly slower than the equivalent C++ code. As JIT technology improves, this should become less of a factor. (In fact, in the near future, good JIT technology may well mean that Java runs faster than the equivalent C++ code.)
  • For reuse of legacy code and integration into legacy systems.
  • To directly access hardware or do other low-level activities.
  • To leverage tools that are not yet available for Java (mature OODBMSes, ANTLR, and so on).


If you take the plunge and decide to integrate Java and C++, you do give up some of the important advantages of a Java-only application. Here are the downsides:

  • A mixed C++/Java application cannot run as an applet.
  • You give up pointer safety. Your C++ code is free to miscast objects, access a deleted object, or corrupt memory in any of the other ways that are so easy in C++.
  • Your code may not be portable.
  • Your built environment definitely won't be portable -- you'll have to figure out how to put C++ code in a shared library on all platforms of interest.
  • The APIs for integrating C and Java are works in progress and will very likely change with the move from JDK 1.0.2 to JDK 1.1.


As you can see, integrating Java and C++ is not for the faint of heart! However, if you wish to proceed, read on.

We'll start with a simple example showing how to call C++ methods from Java. We'll then extend this example to show how to support the observer pattern. The observer pattern, in addition to being one of the cornerstones of object-oriented programming, serves as a nice example of the more involved aspects of integrating C++ and Java code. We'll then build a small program to test our Java-wrapped C++ object, and we'll end with a discussion of future directions for Java.

Calling C++ from Java

What's so hard about integrating Java and C++, you ask? After all, SunSoft's Java Tutorial has a section on "Integrating Native Methods into Java Programs" (see Resources). As we'll see, this is adequate for calling C++ methods from Java, but it doesn't give us enough to call Java methods from C++. To do that, we'll need to do a little more work.

As an example, we'll take a simple C++ class that we'd like to use from within Java. We'll assume that this class already exists and that we're not allowed to change it. This class is called "C++::NumberList" (for clarity, I'll prefix all C++ class names with "C++::"). This class implements a simple list of numbers, with methods to add a number to the list, query the size of the list, and get an element from the list. We'll make a Java class whose job it is to represent the C++ class. This Java class, which we'll call NumberListProxy, will have the same three methods, but the implementation of these methods will be to call the C++ equivalents. This is pictured in the following object modeling technique (OMT) diagram:

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