C++ to Java interface

Question

I am implementing a C++ to Java interface using JNI. I have a C++ function as follows:

static int testhandler(void *arg, uint32_t stream, uint32_t func, const char* name, uint32_t funcgroup, uint32_t source);

When I have to call this in Java using eclipse, I do it as follows:

public void handle(int stream, int func,char name, int group, int token);

But I am not able to read the const char part in Java using Eclipse. Does anybody know what the problem might be? Should I call the method in Java in some other manner?

In Java, char is a UTF-16 encoded character. In C++, char is a byte, and char * is most likely a nul-terminated string. — Angew is no longer proud of SO
– Angew is no longer proud of SO, Commented Jun 4, 2013 at 9:39
At least testhandler doesn't look like a JNI function. Did you miss something we should know? — BlueWanderer
– BlueWanderer, Commented Jun 4, 2013 at 9:48
@Angew I use a Java function signature from C++ to Java as follows: "(IICII)V" Should I change the C to B(Byte) or String? I am not sure how to set signature for string though. — user2358330
– user2358330, Commented Jun 4, 2013 at 9:49
@BlueWanderer the JNI part is like this:JNIEXPORT void JNICALL Java_otf_OtfJni_otfjni_1set_1handler (JNIEnv *env, jobject obj, jint type, jobject handler) { struct callback *cb; jclass handler_class = (*env)->GetObjectClass(env, handler); cb = (struct callback *)malloc(sizeof(struct callback)); cb->env = env; cb->handler = (*env)->NewGlobalRef(env, handler); cb->id = (*env)->GetMethodID(env, handler_class, "handle", "(IICII)V"); printf("adding a handler\n"); OTF_HandlerArray_setHandler(handlers, testhandler, type); OTF_HandlerArray_setFirstHandlerArg(handlers, (void *)cb, type); — user2358330
– user2358330, Commented Jun 4, 2013 at 9:53

Tsar Ioann · Accepted Answer · 2013-06-04 09:54:05Z

1

You can't just pass const char* string through JNI.

Create a jstring from it using function NewStringUTF (it is member-function of class JNIEnv). Then pass it to Java's String.

answered Jun 4, 2013 at 9:54

Tsar Ioann

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6 Comments

user2358330 Over a year ago

I understand that I have to use NewStringUTF, but in my case, the const char* is part of a function call as seen in testhandler. So shud I use the NewStringUTF inside the function call or inside the JNIExport function?

Tsar Ioann Over a year ago

@user2358330 Could you please give some code parts here for me to understand what do you mean?

user2358330 Over a year ago

The JNIExport function makes a call to testhandler which has the const char*. The JNI code is something like this:

JNIEXPORT void JNICALL Java_otf_OtfJni_otfjni_1set_1handler (JNIEnv *env, jobject obj, jint type, jobject handler){  OTF_HandlerArray_setHandler(handlers, testhandler, type);  }

And the testhandler function is:

{ struct callback *cb = (struct callback *)arg;  JNIEnv *env = cb->env;  (*env)->CallVoidMethod(env, cb->handler, cb->id, stream, func, name, funcgroup, source);  return (*env)->NewStringUTF(env, name); }

Tsar Ioann Over a year ago

@user2358330 I can't get the logic. Why int testhandler is returning jstring? When I was answering the main question, I meant that parameter name of type char in java is not equivalent to const char* from C++. Here you need NewStringUTF to convert this parameter to jstring and give to java function, which gets String.

Tsar Ioann Over a year ago

@user2358330 I thought of something like this: env->CallVoidMethod(......., env->NewStringUTF(name), .....).

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