Invoking java methods from C/C++ on the machine with different JREs

Question

I implemented Windows NT Service instantiating JVM and invoking several java methods. Everything works but I have an issue with running the service on the machine where multiple different versions of JRE have been installed. The service is calling java methods that require JRE 1.3 or later so I wrote the code that is setting system PATH from within the service based on the configuration stored in the external file. The problem is that the service requires jvm.dll to be in the PATH prior lunching it since this library is instantiated through the implicit linking. When I put jvm.dll in the same path as the service binary I can lunch it but JNI_CreateJavaVM fails and returns -1. This happens even if JRE 1.3 is in the system PATH prior lunching the service.
Everything works if the system PATH contains references to JRE 1.3 and jvm.dll is removed from the service's directory.
I am looking for an advice on what is the proper way to deal with invoking java methods from the C/C++ executable in the environment with different versions of JRE.
Thanks, Kris.

Answer 1

Here's a way I have done what you are asking about:
What you want to do is make all of your linking happen at runtime, rather than at compile time. This way, you can edit the PATH variable before the jvm.dll gets loaded.
Following is some code that I used to handle a dll of my own in this manner. You can decide if you want to write a "wrapper" dll, or if you find it simpler to approach the jvm.dll in this way.
// Define pointer type for DLL entry point.
     typedef void JREPDLL_API (*EXECUTEREQUEST)(char*, Arguments&);
// Set up path, load dll, and pass everything off to it.
HINSTANCE javaServer = javaServer = LoadLibrary("jrepdll.dll");
if (javaServer != NULL) {
EXECUTEREQUEST executeRequest = (EXECUTEREQUEST)GetProcAddress(javaServer, "ExecuteRequest");
if (executeRequest != NULL) {
if (argc == 1)
     // Execute the request.
     executeRequest("-run", args);
else
     // Execute the request.
     executeRequest("-console", args);
Here's some code for how to edit the PATH:
          // Edit the PATH environment variable so that we use our private java.
char *appendPt;
char *newPath;
char *path;
          char tmp[_MAX_PATH];
          // Get the current PATH variable setting.
path = getenv("PATH");
          // Allocate space for an edited path setting.
          if (path != NULL)
               newPath = (char*)malloc((_MAX_PATH * 2) + strlen(path));
          else
               newPath = (char*)malloc((_MAX_PATH * 2));
          // Get upper part of path to desired directories.
          strcpy(tmp, filepath);
          appendPt = strstr(tmp, "dbin\\jreplicator");
          appendPt[0] = '\0';
sprintf(newPath, "PATH=%sjava\\jre1.2.2\\bin;%sjava\\jre1.2.2\\bin\\classic", tmp, tmp);
// Append the value of the existing PATH environment variable.
// If there is anything, append it.
if (path != NULL) {
     strcat(newPath, ";");
     strncat(newPath, path, (sizeof(newPath) - strlen(newPath) - 2));
// Set new PATH value.
_putenv(newPath);
          free(newPath);