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How to Call Java Functions from C Using JNI

, 13 Jan 2008 CPOL
This article covers calling Java functions from C using JNI. It also covers passing/returning simple parameters, arrays, and structure arrays in Java functions.
#include "StdAfx.h"
#include <stdio.h>
#include <jni.h>
#include <string.h>

 #define PATH_SEPARATOR ';' /* define it to be ':' on Solaris */
 #define USER_CLASSPATH "." /* where Prog.class is */

struct ControlDetail
{
	int ID;
	char Name[100];
	char IP[100];
	int Port;
};

struct WorkOrder
{
	char		sumSerialId[20];	
	char		accessNumber[18];
	char		actionType[4];
	char		effectiveDate[24];
	char		fetchFlag[2];
	char		reason[456];
	char		accessSource[100];	
};


JNIEnv* create_vm(JavaVM ** jvm) {
	
    JNIEnv *env;
    JavaVMInitArgs vm_args;
    JavaVMOption options;
    options.optionString = "-Djava.class.path=D:\\Java Src\\TestStruct"; //Path to the java source code
    vm_args.version = JNI_VERSION_1_6; //JDK version. This indicates version 1.6
    vm_args.nOptions = 1;
    vm_args.options = &options;
    vm_args.ignoreUnrecognized = 0;
    
    int ret = JNI_CreateJavaVM(jvm, (void**)&env, &vm_args);
    if(ret < 0)
    	printf("\nUnable to Launch JVM\n");   	
	return env;
}

int main(int argc, char* argv[])
{
	JNIEnv *env;
	JavaVM * jvm;
	env = create_vm(&jvm);
	if (env == NULL)
		return 1;
		
	struct ControlDetail ctrlDetail;	
	ctrlDetail.ID = 11;
	strcpy(ctrlDetail.Name,"HR-HW");
	strcpy(ctrlDetail.IP,"10.32.164.133");
	ctrlDetail.Port = 9099;
	
	printf("Struct Created in C has values:\nID:%d\nName:%s\n IP:%s\nPort:%d\n",ctrlDetail.ID,ctrlDetail.Name,ctrlDetail.IP,ctrlDetail.Port);

	/********************************************************/
	struct WorkOrder WO[2];
	strcpy(WO[0].sumSerialId,"2000");
	strcpy(WO[0].accessNumber,"2878430");
	strcpy(WO[0].actionType,"04");
	strcpy(WO[0].effectiveDate,"25-12-2007 12:20:30 PM");
	strcpy(WO[0].fetchFlag, "0");
	strcpy(WO[0].reason,"Executed Successfully");
	strcpy(WO[0].accessSource,"PMS");
	strcpy(WO[1].sumSerialId,"1000");
	strcpy(WO[1].accessNumber,"2878000");
	strcpy(WO[1].actionType,"T4");
	strcpy(WO[1].effectiveDate,"25-12-2007 11:20:30 PM");
	strcpy(WO[1].fetchFlag,"0");
	strcpy(WO[1].reason,"");
	strcpy(WO[1].accessSource,"RMS");

	
    jclass clsH=NULL;
    jclass clsC = NULL;
    jclass clsW = NULL;
	jclass clsR = NULL;
    jmethodID midMain = NULL;
    jmethodID midCalling = NULL;
    jmethodID midDispStruct = NULL;
    jmethodID midDispStructArr = NULL;
	jmethodID midRetObjFunc = NULL;
    jmethodID midCtrlDetConst = NULL;
    jmethodID midWoConst = NULL;
    
    jobject jobjDet = NULL;
	jobject jobjRetData = NULL;
    jobjectArray jobjWOArr = NULL;
    
    //Obtaining Classes
    clsH = env->FindClass("HelloWorld");
    clsC = env->FindClass("ControlDetail");
    clsW = env->FindClass("WorkOrder");
	
	//Obtaining Method IDs
    if (clsH != NULL)
    {
		midMain       = env->GetStaticMethodID(clsH, "main", "([Ljava/lang/String;)V");
		midCalling    = env->GetStaticMethodID(clsH,"TestCall","(Ljava/lang/String;)V");
		midDispStruct = env->GetStaticMethodID(clsH,"DisplayStruct","(LControlDetail;)I");
		midDispStructArr = env->GetStaticMethodID(clsH,"DisplayStructArray","([LWorkOrder;)V");
		midRetObjFunc = env->GetStaticMethodID(clsH,"ReturnObjFunc","()Ljava/lang/Object;");
	}
	else
    {
    	printf("\nUnable to find the requested class\n");    	
    }
	if(clsC != NULL)
	{
		//Get constructor ID for ControlDetail
		midCtrlDetConst = env->GetMethodID(clsC, "<init>", "(ILjava/lang/String;Ljava/lang/String;I)V");		
	}
	else
    {
    	printf("\nUnable to find the requested class\n");    	
    }	

	if(clsW != NULL)
	{
		//Get Constructor ID for WorkOrder
		midWoConst = env->GetMethodID(clsW, "<init>", "(Ljava/lang/String;Ljava/lang/String;Ljava/lang/String;Ljava/lang/String;Ljava/lang/String;Ljava/lang/String;Ljava/lang/String;)V");		
	}
	else
    {
    	printf("\nUnable to find the requested class\n");    	
    }

	/************************************************************************/
	/* Now we will call the functions using the their method IDs			*/
	/************************************************************************/
	if(midMain != NULL)
		env->CallStaticVoidMethod(clsH, midMain, NULL); //Calling the main method.
	
	if (midCalling!=NULL)
	{
		jstring StringArg = env->NewStringUTF("\nTestCall:Called from the C Program\n");
		//Calling another static method and passing string type parameter
		env->CallStaticVoidMethod(clsH,midCalling,StringArg);
	}
	
	printf("\nGoing to Call DisplayStruct\n");
	if (midDispStruct!=NULL)
	{
		if(clsC != NULL && midCtrlDetConst != NULL)
		{
			jstring StringArgName = env->NewStringUTF(ctrlDetail.Name);
			jstring StringArgIP = env->NewStringUTF(ctrlDetail.IP);
			
			//Creating the Object of ControlDetail.
			jobjDet = env->NewObject(clsC, midCtrlDetConst, (jint)ctrlDetail.ID, StringArgName, StringArgIP, (jint)ctrlDetail.Port);
		}
		
		if(jobjDet != NULL && midDispStruct != NULL)
			env->CallStaticIntMethod(clsH,midDispStruct,jobjDet); //Calling the method and passing ControlDetail Object as parameter
	}
	//Calling a function from java and passing Structure array to it.
	printf("\n\nGoing to call DisplayStructArray From C\n\n");
	if (midDispStructArr!=NULL)
	{
		if(clsW != NULL && midWoConst != NULL)
		{
			//Creating the Object Array that will contain 2 structures.
			jobjWOArr = (jobjectArray)env->NewObjectArray(2,clsW,env->NewObject(clsW, midWoConst,env->NewStringUTF(""),env->NewStringUTF(""),env->NewStringUTF(""),
														  env->NewStringUTF(""),env->NewStringUTF(""),env->NewStringUTF(""),env->NewStringUTF("")));         	
			//Initializing the Array
			for(int i=0;i<2;i++)
			{
				env->SetObjectArrayElement(jobjWOArr,i,env->NewObject(clsW, midWoConst,env->NewStringUTF(WO[i].sumSerialId),
														env->NewStringUTF(WO[i].accessNumber),
														env->NewStringUTF(WO[i].actionType),
														env->NewStringUTF(WO[i].effectiveDate),
														env->NewStringUTF(WO[i].fetchFlag),
														env->NewStringUTF(WO[i].reason),
														env->NewStringUTF(WO[i].accessSource)));	
			}
		}
		//Calling the Static method and passing the Structure array to it.
		if(jobjWOArr != NULL && midDispStructArr != NULL)
			env->CallStaticVoidMethod(clsW,midDispStructArr,jobjWOArr);
	}
	//Calling a Static function that return an Object
	if (midRetObjFunc != NULL)
	{
		//Calling the function and storing the return object into jobject type variable
		//Returned object is basically a structure having two fields (string and integer)
		jobjRetData = (jobject)env->CallStaticObjectMethod(clsH,midRetObjFunc,NULL);
		//Get the class of object
		clsR = env->GetObjectClass(jobjRetData);
		//Obtaining the Fields data from the returned object
		jint nRet = env->GetIntField(jobjRetData,env->GetFieldID(clsR,"returnValue","I"));
		jstring jstrLog = (jstring)env->GetObjectField(jobjRetData,env->GetFieldID(clsR,"Log","Ljava/lang/String;"));
		const char *pLog = env->GetStringUTFChars(jstrLog,0);
		
		printf("\n\nValues Returned from Object are:\nreturnValue=%d\nLog=%s",nRet,pLog);
		//After using the String type data release it.
		env->ReleaseStringUTFChars(jstrLog,pLog);
	}
	//Release resources.
	int n = jvm->DestroyJavaVM();
    return 0;
}

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This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)

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About the Author

ajalilqarshi
Software Developer (Senior)
United Kingdom United Kingdom
Ahmad Qarshi is an IT Consultant and have good expertise in different development technologies. He loves to code in .NET, JavaFX, Delphi, XSLT/Schematron. Above all very keen to learn new stuff Smile | :)

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