Can C++ Application load the .jar File using JNI?

Question

Thanks for having a look at question. I am trying to invoke a java method which is in class files using JNI interface. In turn the called class file should be executing the another .jar file which resides in the same direcotry ? I had hard time acheiving this and I am unsuccessful in executing the .jar file. I mean I am not able to get the results from the class fuile available in .jar file.

Can any one explain,whether it is possible to do that way or I should look for the another option ?

The code is like this:

class JNIInterface
{
private:
 JavaVMInitArgs vm_args;
 JavaVM *jvm;
 JNIEnv *env;
 long result;
 jmethodID mid;
 jfieldID fid;
 jobject jobj;
 jclass cls;
 int asize;
 char  JVMOptionString[20];
 char  className[20];
 char  methodName[20];
 JavaVMOption options[1];

public:
 JNIInterface(char* JVMOptionString)
 {
//  JavaVMOption options[1];
  options[0].optionString = JVMOptionString;
  vm_args.options = options;
  vm_args.nOptions = 1;

  vm_args.version = JNI_VERSION_1_6;
  vm_args.ignoreUnrecognized = JNI_FALSE;
 }
 void setClassName(char* className)
 {  
  result = JNI_CreateJavaVM( &jvm,(void **)&env, &vm_args);
  if(result == JNI_ERR ) 
  {
   printf("Error invoking the JVM\n");
   //return 0;
  }
  cls = env->FindClass("F2C");
  if( cls == NULL ) 
  {
   printf("can't find class F2C\n");
   //return 0;
  }

  env->ExceptionClear();
 }

 void setMethodName(char* methodName)
 {
  cout<<"----------  Function Name is "<<methodName<<endl;

  //----------  Integer Value with Parameter ----------------
  mid=env->GetStaticMethodID(cls, methodName, "([Ljava/lang/String;)V");
  if (mid != NULL)
  {
   env->CallStaticVoidMethod(cls,mid,"70");
  }
int main()
{
 JNIInterface JNIInterfaceObj("-Djava.class.path=C:\\MyPOC;C:\\MyPOC\\herong.jar");

    JNIInterfaceObj.setClassName("F2C");

 JNIInterfaceObj.setMethodName("main");
return 0;
}

.

//The java file which is calling jar files is - F2C.java

/**
 * F2C.java
 * Copyright (c) 2006 by Dr. Herong Yang, http://www.herongyang.com/
 */
import herong.TempratureConvertorBean;

public class F2C {

 public void test(String[] arg) {
  try {

   double f = 0.0;
   System.out.println("Inside test func:");
   TempratureConvertorBean b = new TempratureConvertorBean();

   if (arg.length>0) f = Double.parseDouble(arg[0]);
     b.setFahrenheit(f);
   double c = b.getCelsius();
   System.out.println("Fahrenheit = "+f);
   System.out.println("Celsius = "+c);
   System.out.println(b.getInfo());

  } 
}

public static void main(String[] arg) {
    F2C f2c = new F2C();
      f2c.test(arg);
  }
}  

this F2C.java uses the herong.jar file

Please suggest if you have any idea. Thanks, Asg

Answer 1

Your question isn't completely clear, but I will give a general answer...

In Java there is only two ways to get Java to look in a .jar file (and they really boil down to one way in the end), and that's to specify the .jar file in the classpath, or to create a classloader that will look in that jar file and add it to the list of classloaders Java will use.

And, of course, all the classpath is is a set of classloaders that Java instantiates and uses before your program even starts.

So a JNI program needs to make the Java calls (which I'm not looking up just now) to set up a new class loader and get Java to start using it if the JNI program needs Java to start looking in additional .jar files.