Java Native Interface idioms for C++ class hierarchies

SUMMARY The Java™ Native Interface (JNI) provides a set of mechanisms for implementing Java methods in C or C++. JNI is useful for reusing C and C++ code repositories within Java frameworks. JNI is also useful for real-time systems, where compiled C/C++ code executes performance-critical tasks, while Java code executes system control and feature tasks. Available JNI literature concentrates on creating Java proxy classes that allow Java clients to interact with C++ classes. Current JNI literature does not discuss Java proxies for entire C++ inheritance hierarchies; that is the topic of this paper. Our experience in reusing C++ class hierarchies within a Java framework has uncovered a set of useful techniques for constructing Java proxy class hierarchies that mirror their C++ counterparts. This report gives both high level design guidelines and specific programming idioms for constructing Java class hierarchies that serve as proxies for C++ counterparts. We begin by discussing opportunities for reuse within a proxy class hierarchy, as well as problems caused by differences between the Java and C++ approaches to inheritance. The two most significant differences are due to C++ support for invocation of a member function based on the static type of its class, and C++ support for multiple implementation inheritance. Two example C++ class hierarchies provide the basis for a set of sections that present the design guidelines and that codify the programming idioms. This work could serve as the basis for an automatic generator of Java proxy class hierarchies. The Java Native Interface [1] is a set of software mechanisms that support interaction between Java modules on one side of JNI and C/C++ modules on the other. A Java method declaration marked with the keyword native is identified as a native method written in C or C++. Within a Java source file a native method appears strictly as a method signature declaration; it contains no Java code. Invoking the generator program javah with a compiled Java class as an argument creates a C/C++ function declaration equivalent to each native method declaration. Each generated declaration serves as the starting point from which a programmer can write a C or C++ implementation of its method. Native methods can construct, delete and interact with C data structures and C++ objects. A Java system can dynamically load native methods. Conversely, a C or C++ system can construct a Java Virtual Machine (JVM) and interact with it via JNI. Many …