call_jvm_from_c.cc

#include<stdio.h>
#include<jni.h>
#include<string.h>

/*
 * here we expect Hoge class in my.hoge package
 *
 */

int main(int argc, char** argv)
{
	JNIEnv *jnienv;
	JavaVM *javavm;
	JavaVMInitArgs vm_args;

	JavaVMOption options[1];
	options[0].optionString = "-Djava.class.path=.";
	vm_args.version = JNI_VERSION_1_2;
	vm_args.options = options;
	vm_args.nOptions = 1;
	vm_args.ignoreUnrecognized = true;

	printf("Java VM generate\n");

	int result = JNI_CreateJavaVM(&javavm, (void **)&jnienv, &vm_args);
	puts("create jvm");

	if(result != 0){
		printf("jvm failed to create : %d\n", result);
		return 1;
	}

	puts("searching class");
	jclass cls = jnienv->FindClass("my/hoge/Hoge");
	if(cls == 0){
		puts("failed to find class");
		return 1;
	}


	puts("find constructor method");
	jmethodID cns = jnienv->GetMethodID(cls, "<init>", "()V");
	if(cns == NULL){
		puts("constructor method find failed");
		return 1;
	}

	puts("instantialize");
	jobject obj = jnienv->NewObject(cls, cns);

	if(!obj){
		puts("failed to new");
		return 1;
	}

	// printHoge
	puts("searching method: printHoge");
	jmethodID printHogeId = jnienv->GetMethodID(cls, "printHoge", "()V");
	if(printHogeId == NULL){
		puts("failed to find printHoget");
		return 1;
	}

	puts("----method calling");
	jnienv->CallVoidMethod(obj, printHogeId);
	puts("----method called");


	// add
	puts("searching method: add");
	jmethodID addId = jnienv->GetMethodID(cls, "add", "(II)I");
	if(!addId){
		puts("failed to find add");
		return 1;
	}

	puts("----call");
	int addResult = (int)jnienv->CallIntMethod(obj, addId, 10, 100);
	printf("----called : result = %d\n", addResult);

	// getHoge
	puts("searching method: getHoge");
	jmethodID getHogeId = jnienv->GetMethodID(cls, "getHoge", "()Ljava/lang/String;");
	if(!getHogeId){
		puts("failed to find getHoge");
		return 1;
	}
	
	puts("----call");
	jstring jstr1 = (jstring)jnienv->CallObjectMethod(obj, getHogeId);
	const char* str1 = jnienv->GetStringUTFChars(jstr1, 0);
	char* resStr1 = strdup(str1);
	jnienv->ReleaseStringUTFChars(jstr1, str1);
	printf("----called : result = %s\n", resStr1); 
	

	// getMsg
	puts("searching method: geMsg");
	jmethodID getMsgId = jnienv->GetMethodID(cls, "getMsg", "(Ljava/lang/String;)Ljava/lang/String;");
	if(!getMsgId){
		puts("failed to find getMsg");
		return 1;
	}

	puts("----call");
	jstring jstr2 = (jstring)jnienv->CallObjectMethod(obj, getMsgId, jnienv->NewStringUTF("Hell to the Sack code!"));
	const char* str2 = jnienv->GetStringUTFChars(jstr2, 0);
	char* resStr2 = strdup(str2);
	jnienv->ReleaseStringUTFChars(jstr2, str2);
	printf("----called : result = %s\n", resStr2);


	// destroy JVM
	result = javavm->DestroyJavaVM();
	if(result){
		puts("Failed to destroy jvm");
		return 1;
	}

	puts("Safely destroyed jvm");
	return 0;

}

Hoge.java

package my.hoge;

public class Hoge {
	public Hoge()
	{
	}

	public void printHoge(){
		System.out.println("Hoge!");
	}

	public String getHoge(){
		return new String("Hoge");
	}

	public int add(int i, int j)
	{
		return i + j;
	}

	public String getMsg(String msg){
		return new String("Hoget is " + msg);
	}
}