JNI, Win32, and EXE resources

[Corey Cole]

I'm attempting to create yet another Java service tool. After looking at the source code that comes with the Microsoft Java SDK, I found that their jexegen/jntsvc programs bind string params into a string table and raw classfile data into RT_RCDATA resources within an exe.
After some google searching, I found some good examples on how to update/read resources. However, I'm stuck when it comes to
actually instantiating the class. When I try the IBM JVM, I get to the point where I load the class but can't find the methodID for the static "main" method. When I use the Sun JVM, I don't even get that far, and my call to DefineClass returns a NULL.
Here's the source to an analog program (no EXE/DLL resource stuff, but it's pretty damn close):

#include <sys/stat.h>
#include <windows.h>
#include <iostream>
#include <fstream>
#include <jni.h>
using namespace std;
JNIEnv *env = 0;
JavaVM *jvm = 0;
BOOL initJVM() {
   #define numOptions 2
   JavaVMInitArgs vm_args;
   JavaVMOption options[numOptions];
   options[0].optionString = "-Djava.compiler=NONE";
   options[1].optionString = "-verbose:class";
   //options[2].optionString = "-Xcheck:jni";
   jint ret;
   vm_args.version = JNI_VERSION_1_2;
   vm_args.options = options;
   vm_args.nOptions = numOptions;
   vm_args.ignoreUnrecognized = TRUE;
   ret = JNI_CreateJavaVM(&jvm, (void**)&env, &vm_args);
   if (ret < 0) {
cout << "Can't create JVM!  Error: " << ret << endl;
return FALSE;
   }
   return TRUE;
}

int main(int argc, char* argv[])
{
BOOL isJVMCreated = FALSE;
isJVMCreated = initJVM();
if (isJVMCreated == FALSE) {
   cout << "Fatal error.  Exiting now." << endl;
   return 1;
}

// Check the file size
struct stat results;
jsize ResSize = 0;
if (stat("Hello.class", &results) == 0) {
   ResSize = (jsize) results.st_size;
   cout << "File size: " << results.st_size << endl;
   cout << "jsize: " << ResSize << endl;
} else {
   cout << "OMFG!  Zero length file" << endl;
   return 99;
}
// Open the file
ifstream myClass("Hello.class", ios_base::in|ios_base::binary);
jbyte* pRes;
pRes = new jbyte[results.st_size];
// Copy the file into pRes
myClass.read((char *)pRes, results.st_size);
myClass.close();
jclass loaderClass = env->FindClass("java/lang/ClassLoader");
if (loaderClass == NULL) {
   cout << "Invalid loader:  You're hosed" << endl;
   return 199;
}
jmethodID loaderMID = env->GetMethodID(loaderClass, "<init>", "()V");
jobject loaderObj = env->NewObject(loaderClass, loaderMID);

cout << "Loader loaded..." << endl;

cout << "Calling DefineClass..." << endl;
jclass mainClass = env->DefineClass("Hello", loaderObj, pRes, ResSize);
if (mainClass == NULL) {
           // This is where I die with the Sun JVM
   cout << "Couldn't load class" << endl;
   return 99;
}
cout << "Looking up the methodID" << endl;
jmethodID mid = env->GetStaticMethodID(mainClass, "main", "([Ljava/lang/String [img]http://www.javaranch.com/ubb/wink.gif[/img]V");
if (mid == NULL) {
           // This is where I die with the IBM JVM
   cout << "Got invalid 'main' MethodID" << endl;
   return 99;
}
cout << "Got the methodID" << endl;
env->CallStaticVoidMethod(mainClass, mid);
jvm->DestroyJavaVM();
delete[] pRes;
return 0;
}

(yes, it's getting to be a hack job, and yes, it's silly
to hardcode classnames in there...)
Finally, the "Hello" class it refers to is just a simple
Hello World.
Any pointers would be greatly appreciated. Thanks.

[Corey Cole]

Not to answer my own question (mostly because I don't have an answer), but I've narrowed the problem down.
I wasn't checking for exceptions after each JNI call. When I check for exceptions after each call, both the IBM and Sun JVMs
fail at the same point. I guess the IBM JVM is a little more lenient than the Sun JVM.
Anyways, it appears the issue is my creation of the ClassLoader object. I've never really played with ClassLoaders in pure code, so it's possible I'm not creating the loader object in the correct context...

[Corey Cole]

And now the circle is complete...
I was being naive in trying to instantiate a ClassLoader
object via it's constructor. Yes, javap shows the ctor
inside the classfile, but the _real_ method you want to call is
"getSystemClassLoader".
Here's the sample code in working form:

#include <sys/stat.h>
#include <windows.h>
#include <iostream>
#include <fstream>
#include <jni.h>
using namespace std;
JNIEnv *env = 0;
JavaVM *jvm = 0;
jmethodID getMsgID;
BOOL initJVM() {
   #define numOptions 2
   JavaVMInitArgs vm_args;
   JavaVMOption options[numOptions];
   options[0].optionString = "-Djava.compiler=NONE";
   //options[1].optionString = "-verbose:class";
   options[1].optionString = "-Xcheck:jni";
   jint ret;
   vm_args.version = JNI_VERSION_1_2;
   vm_args.options = options;
   vm_args.nOptions = numOptions;
   vm_args.ignoreUnrecognized = TRUE;
   ret = JNI_CreateJavaVM(&jvm, (void**)&env, &vm_args);
   if (ret < 0) {
cout << "Can't create JVM!  Error: " << ret << endl;
return FALSE;
   }
   return TRUE;
}
BOOL checkException() {
   jboolean isCopy;
   jthrowable throwObj = env->ExceptionOccurred();
   if (throwObj == NULL) {
// No exception
return FALSE;
   }
   // Get the jstring version of the message
   jstring msg = (jstring) env->CallObjectMethod(throwObj, getMsgID, NULL);
   const char * cmsg = env->GetStringUTFChars(msg, &isCopy);
   cout << "Exception Message: " << cmsg << endl;
   env->ExceptionClear();
   return TRUE;
}

int main(int argc, char* argv[])
{
BOOL isJVMCreated = FALSE;
isJVMCreated = initJVM();
if (isJVMCreated == FALSE) {
   cout << "Fatal error.  Exiting now." << endl;
   return 1;
}
// Set up an exception object for possible later use
jclass jException = env->FindClass("java/lang/Exception");
getMsgID = env->GetMethodID(jException, "getMessage", "()Ljava/lang/String;");

// Check the file size
struct stat results;
jsize ResSize = 0;
if (stat("Hello.class", &results) == 0) {
   ResSize = (jsize) results.st_size;
   cout << "File size: " << results.st_size << endl;
   cout << "jsize: " << ResSize << endl;
} else {
   cout << "OMFG!  Zero length file" << endl;
   return 99;
}
// Open the file
ifstream myClass("Hello.class", ios_base::in|ios_base::binary);
jbyte* pRes;
pRes = new jbyte[results.st_size];
// Copy the file into pRes
myClass.read((char *)pRes, results.st_size);
myClass.close();
jclass loaderClass = env->FindClass("java/lang/ClassLoader");
if ((checkException() == TRUE) | | loaderClass == NULL) {
   cout << "Invalid loader classname:  You're hosed" << endl;
   return 199;
}
jmethodID loaderMID = env->GetStaticMethodID(loaderClass, 
"getSystemClassLoader", "()Ljava/lang/ClassLoader;");
if ((checkException() == TRUE) | | loaderMID == NULL) {
   cout << "Invalid loader ctor method ID:  You're hosed" << endl;
   return 200;
}
jobject loaderObj = env->CallStaticObjectMethod(loaderClass, loaderMID);
if ((checkException() == TRUE) | | loaderObj == NULL) {
   cout << "Loader object creation failure" << endl;
   return 201;
}

jclass mainClass = env->DefineClass("Hello", loaderObj, pRes, ResSize);
if ((checkException() == TRUE) | | mainClass == NULL) {
   cout << "Couldn't load class" << endl;
   return 202;
}
cout << "Looking up the methodID" << endl;
jmethodID mid = env->GetStaticMethodID(mainClass, "main", "([Ljava/lang/String [img]http://www.javaranch.com/ubb/wink.gif[/img]V");
if ((checkException() == TRUE) | | mid == NULL) {
   cout << "Got invalid 'main' MethodID" << endl;
   return 99;
}
env->CallStaticVoidMethod(mainClass, mid);
jvm->DestroyJavaVM();
delete[] pRes;
return 0;
}

And please, no making fun. I've been working on this
for 12 hours straight. What finally clued me in is the
Delphi(!?) source found here: http://home.pacifier.com/~mmead/jni/delphi/javadpr/javadpr_dpr.html
Regards, and thanks to the JavaRanch for letting me talk to myself.