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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

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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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