Invocation API

Question

I am working with the invocation api from the tutorial. I do not want to set the path to the jvm.dll before running the exe. How can I dynamically load the dll. When I start the exe, immediately it popsup a message box saying that jvm.dll not found. Can anyone please help me out as to how to load the dll dynamically. All help is appreciated.
Razi

Answer 1

Try this:
// runjava.cpp
#include <windows.h>
#include <jni.h>
#define DOTRELEASE  "1.4" /* Same for 1.3.1, 1.3.2 etc. */
#define JRE_KEY         "Software\\JavaSoft\\Java Runtime Environment"
// typedefs for calls to jvm.dll
typedef jint (JNICALL *CreateJavaVM_t)(JavaVM **pvm, void **env, void *args);
typedef jint (JNICALL *GetDefaultJavaVMInitArgs_t)(void *args);
// globals
JavaVMInitArgs vmArgs;
JavaVM* jvm;
JNIEnv* env;
bool running=false;
void LogError(const char * format,...)
    char    message[256];
    va_list args;
    va_start(args, format);
    vsprintf(message, format, args);
    va_end(args);
     printf("%s\n",message);          
void LogWin32Error(const char * pTitle)
    PCHAR pBuffer;
    LONG  lError = GetLastError ( );
    FormatMessage ( FORMAT_MESSAGE_ALLOCATE_BUFFER | FORMAT_MESSAGE_FROM_SYSTEM,
                    NULL,
                    lError,
                    MAKELANGID(LANG_NEUTRAL, SUBLANG_DEFAULT),
                    (char *)&pBuffer,
                    0,
                    NULL );
     LogError ("Win32 error: %s %s\n",pBuffer,pTitle);
    LocalFree ( pBuffer );
bool GetStringFromRegistry(HKEY key, const char *name, unsigned char *buf, int bufsize)
    DWORD type, size;
    if (RegQueryValueEx(key, name, 0, &type, 0, &size) == 0
     && type == REG_SZ
     && (size < (unsigned int)bufsize))
          if (RegQueryValueEx(key, name, 0, 0, buf, &size) == 0)
               return true;
    return false;
bool GetPublicJREHome(char *buf, int bufsize)
    HKEY key, subkey;
    char version[MAX_PATH];
    /* Find the current version of the JRE */
    if(RegOpenKeyEx(HKEY_LOCAL_MACHINE, JRE_KEY,0,KEY_READ,&key)!=0)
          LogError("Error opening registry key '" JRE_KEY "'\n");
          return false;
    if(!GetStringFromRegistry(key,"CurrentVersion",(unsigned char *)version, sizeof(version)))
          LogError("Failed reading value of registry key:\n\t"JRE_KEY "\\CurrentVersion\n");
          RegCloseKey(key);
          return false;
    if(strcmp(version, DOTRELEASE)!= 0)
          LogError("Registry key '" JRE_KEY "\\CurrentVersion'\nhas value '%s', but '" DOTRELEASE "' is required.\n", version);
          RegCloseKey(key);
          return false;
    /* Find directory where the current version is installed. */
    if(RegOpenKeyEx(key,version,0,KEY_READ, &subkey)!= 0)
          LogError("Error opening registry key '"JRE_KEY "\\%s'\n", version);
          RegCloseKey(key);
          return false;
    if(!GetStringFromRegistry(subkey, "JavaHome", (unsigned char *)buf, bufsize))
          LogError("Failed reading value of registry key:\n\t"JRE_KEY "\\%s\\JavaHome\n", version);
          RegCloseKey(key);
          RegCloseKey(subkey);
          return false;
    RegCloseKey(key);
    RegCloseKey(subkey);
    return true;
jint JNICALL _vfprintf_(FILE *fp, const char *format, va_list args)
     LogError(format,args);
     return 0;
void JNICALL _exit_(jint code)
     LogError("VM exited");
     exit(code);
void JNICALL _abort_(void)
     LogError("VM aborted");
     abort();
void LoadJVM(char * dir)
     HINSTANCE handle;
     JavaVMOption options[5];
     char JREHome[MAX_PATH];
     char JVMPath[MAX_PATH];                                        
     char classpathOption[MAX_PATH];
     char librarypathOption[MAX_PATH];
     if(!GetPublicJREHome(JREHome, MAX_PATH))
          LogError("Could not locate JRE");
          abort();
     strcpy(JVMPath,JREHome);
     strcat(JVMPath,"\\bin\\client\\jvm.dll");
    if ((handle=LoadLibrary(JVMPath))==0)
          LogError("Error loading: %s", JVMPath);
          abort();
    CreateJavaVM_t pfnCreateJavaVM=(CreateJavaVM_t)GetProcAddress(handle,"JNI_CreateJavaVM");
    if (pfnCreateJavaVM==0)
          LogError("Error: can't find JNI interfaces in: %s",JVMPath);
          abort();
     strcpy(classpathOption,"-Djava.class.path=");
     strcat(classpathOption,dir);
     strcat(classpathOption,";");
     strcat(classpathOption,JREHome);
     strcat(classpathOption,"\\lib");
     strcat(classpathOption,";");
     strcat(classpathOption,JREHome);
     strcat(classpathOption,"\\lib\\comm.jar");
     strcpy(librarypathOption,"-Djava.library.path=");
     strcat(librarypathOption,JREHome);
     strcat(librarypathOption,"\\lib");
     OutputDebugString("classpath option=");
     OutputDebugString(classpathOption);
     OutputDebugString("\n");
     OutputDebugString("librarypath option=");
     OutputDebugString(librarypathOption);
     OutputDebugString("\n");
     options[0].optionString=classpathOption;     
     options[1].optionString=librarypathOption;     
     options[2].optionString="vfprintf";
     options[2].extraInfo=_vfprintf_;
     options[3].optionString="exit";
     options[3].extraInfo=_exit_;
     options[4].optionString="abort";
     options[4].extraInfo=_abort_;
    vmArgs.version  = JNI_VERSION_1_2;
    vmArgs.nOptions = 5;
    vmArgs.options  = options;
    vmArgs.ignoreUnrecognized = false;
     //if(JNI_CreateJavaVM(&jvm, (void**)&env, &vmArgs)<0)
    if(pfnCreateJavaVM(&jvm,(void**)&env, &vmArgs)!=0)
          LogError("Could not create VM");
          abort();
void main(int argc, char *argv[])
    char path[MAX_PATH];
    char drive[MAX_PATH];
    char file[MAX_PATH];
    char dir[MAX_PATH];
    char ext[MAX_PATH];
     jclass  cls;
     jmethodID mid;
     jobjectArray args;
     if(argc>=2)
          _splitpath(path,drive,dir,file,ext);
          LoadJVM(dir);
          if(env==NULL)
               LogError("Could not load VM");
               abort();
          if(GetModuleFileName(NULL,path,MAX_PATH)==0)
               LogWin32Error("Getting module filename");
               abort();
          cls=env->FindClass(argv[1]);
          if(cls==NULL)
               LogError("Could not find class %s (or its superclass)",argv[1]);
               exit(-1);
          mid=env->GetStaticMethodID(cls,"main","([Ljava/lang/String;)V");
          if(mid==NULL)
               LogError("Could not find method 'main'");
               exit(-1);
          args=env->NewObjectArray(argc-2,env->FindClass("java/lang/String"),NULL);
          if(args==NULL)
               LogError("Could not create args array");
          for(int arg=0;arg<argc-2;arg++)
               env->SetObjectArrayElement(args,arg,env->NewStringUTF(argv[arg+2]));
          env->CallStaticVoidMethod(cls,mid,args);
          jvm->DestroyJavaVM();
     else
          printf("Usage: runjava classname\n");
          exit(-1);
//*****************************************************************************