jni from c

jni_test.c

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

int main(){
 JNIEnv *env;
 JavaVM *jvm;
 int res;
 jclass clazz;
 jmethodID mid;
 jmethodID mid_to_str;
 jobject obj;
jstring result;
 const char *bytes;
 jsize len;
 int i = 0;
 JavaVMOption options[1];
 options[0].optionString = "-verbose:gc";
 
 JavaVMInitArgs vm_args;
 vm_args.version = JNI_VERSION_1_4;
 vm_args.options = options;
 vm_args.nOptions = 1;

 res = JNI_CreateJavaVM(&jvm, (void **)&env, &vm_args);
 if(res < 0){
	printf("Error on JNI_CreateJavaVM\n");
	return -1;
 }

clazz = (*env)->FindClass(env, "Ljava/util/Date;");
if(clazz == 0){
 printf("error on FindClass\n");
 return -1;
}
// コンストラクタは"<init>"で取得
mid = (*env)->GetMethodID(env,clazz, "<init>","()V"); 
if(mid == NULL){
 printf("error on GetMethodId(<init>);\n");
 return -1;
}
obj = (*env)->NewObject(env,clazz, mid);
if(obj == NULL){
 printf("error on NewObject\n");
 return -1;
}
mid_to_str = (*env)->GetMethodID(env,clazz, "toString", "()Ljava/lang/String;");
if(mid_to_str == NULL){
 printf("error on GetMethodID(toString)\n");
return -1;
}
result = (jstring)(*env)->CallNonvirtualObjectMethod(env,obj, clazz, mid_to_str);
if(result == NULL){
 printf("error on CallNonvirtualObjectMethod\n");
 return -1;
}
bytes = (*env)->GetStringUTFChars(env,result,NULL);
printf("bytes = %p\n", bytes);
len = (*env)->GetStringLength(env,result);
printf("len = %d\n", len);
for(i = 0; i < len; i++){
  printf("%d,", bytes[i]);
} 
printf("\n");
(*env)->ReleaseStringUTFChars(env,result, bytes);
 return 0;
}

make.sh

gcc -lverify -ljava -ljvm test.c