JNI (Java Native Interface)

JNI is Sun's standard programming interface for calling "native methods" (compiled, platform-specific, object code) from Java applications, and for embedding and calling the Java Virtual Machine from native applications. The primary goal is binary compatibility of native method libraries across all Java Virtual Machine implementations on a given platform.

By programming to the JNI, you can:

Call non-Java object modules
Pass and return primitive and user-defined data types
Throw and catch exceptions
Pass Java objects and invoke methods on those objects
Load Java classes and obtain class information

You can also use the JNI with the Invocation API to enable an arbitrary native application to embed the Java VM. This allows programmers to easily make their existing applications Java-enabled without having to link with the JVM source code.

JNI should not be confused with other competing "native method" APIs:

JDK 1.0's NMI (Native Method Interface)
Netscape's JRI (Java Runtime Interface ) [http://www17.netscape.com/eng/jri/jri-intro.html]
Microsoft's RNI (Raw Native Interface)

These other APIs do not offer the future and/or the portability guaranteed by Sun's official JNI. Native method programmers should program to Sun's JNI. This insulates the code from unknowns, such as the JVM implementation that the end user is running, and, provides the assurance of active support (or a migration path) in the future. By conforming to the JNI standard, the native method library produced will have the best chance of running in any Java VM - now, and in the future.

This chapter will demonstrate JNI's C binding. "You can also use C++ instead of C to write native methods. There are a few advantages - type checking is slightly stricter, and accessing the JNI functions is a bit more convenient. However, JNI does not support any direct correspondence between classes in Java and C++. Even when programming with C++, you must access the fields and methods of Java classes through C function calls." [Horstmann98, p581]

There are several implementation possibilities not covered in this chapter: accessing Java static members, mapping Java native methods to C++ member functions, passing array arguments, the va_list type, and throwing exceptions from Java to C++.