Implementing Native Methods

Using C++ in Native Method Implementations

The javah program generates C header and source files, which provide the glue for integrating C code with Java. You can use these C header and source files to integrate C++ code into Java as well.

To do so, you must ensure that C++ does not perform any name mangling on the C function names. If you are using the JDK 1.0.2 release, javah automatically does this for you by framing the function declarations in the generated header files within an extern "C" block. If you are using a release prior to the JDK 1.0.2 release you must do this by hand. Here's a version of the header file generated by javah for the NativeExample class used in various examples throughout this lesson that has been modified to work with C++:

/* DO NOT EDIT THIS FILE - it is machine generated */
#include 
/* Header for class NativeExample */

#ifndef _Included_NativeExample

typedef struct ClassNativeExample {
    long myValue;
} ClassNativeExample;
HandleTo(NativeExample);

extern "C" {
struct Hjava_lang_String;
extern struct Hjava_lang_String *NativeExample_quote(struct HNativeExample *,long);
extern long NativeExample_twoTimes(struct HNativeExample *);
struct HNativeExample;
extern struct HNativeExample *NativeExample_doubleUp(struct HNativeExample *);
}

The bold lines indicate code that was added to the header file after generation by javah. Note that if you run javah after you modify a header file, you will have to modify the file again.

The implementations for native methods may not be embedded in C++ classes. They must be global functions.

Implementing Native Methods