convert vector to jobject in C++/JNI?

Question

I am using Java native function -

public native ArrayList<String> parseXML();

In C++ my native function -

vector<string> resultList; JNIEXPORT jobject JNICALL Java_Sample1_parseXML (JNIEnv *env, jobject obj){ // logic return resultList; // here getting error }

My problem is that how to convert resultList (vector type) to jobject type?

+1: It a lot harder than you might imagine. ;)

Peter Lawrey
– Peter Lawrey

2011-10-15 09:12:57 +00:00
Commented Oct 15, 2011 at 9:12 — Peter Lawrey
– Peter Lawrey, Commented Oct 15, 2011 at 9:12

Dyonisos · Accepted Answer · 2011-10-15 10:20:48Z

15

You would have to create a wrapper for the ArrayList in C++. Something like:

vector <char*> vec; jclass clazz = (*env).FindClass("java/util/ArrayList"); jobject obj = (*env).NewObject(clazz, (*env).GetMethodID(clazz, "<init>", "()V")); for (int n=0;n<vec.size();n++) { char* str = (char*) static_cast<char*>(vec[n]); jstring _str = (*env).NewStringUTF(str); (*env).CallVoidMethod(object, (*env).GetMethodID(clazz, "add", "(java/lang/Object)V"), _str); } return obj;

for further information see:

http://download.oracle.com/javase/1.4.2/docs/guide/jni/spec/functions.html

answered Oct 15, 2011 at 10:20

Dyonisos

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5 Comments

dma_k Over a year ago

Nice answer. But it would be absolutely great, if someone writes an adapter for STL vector that implements java.util.List. Quite fascinating exercise!

Dyonisos Over a year ago

Right! This would be the solution of the solutions.

moshbear Over a year ago

Pardon my pedantry, but isn't (char*)static_cast<char*> redundant?

NeverJr Over a year ago

By the way, in CallVoidMethod, what is tha object parameter exactly? I mean as I found out, that should be a jobject. If I try to do the same as above, the compiler says, that object isn't declared in the scope.

Avijeet Over a year ago

what is the "object" in CallVoidMethod() and where are we using "jobject obj " declared at the top of code?

Mat · Accepted Answer · 2013-06-25 15:03:23Z

The method is:

bool add(Object);

So the signature is:

"(Ljava/lang/Object;)Z"

More at: http://docs.oracle.com/javase/1.5.0/docs/guide/jni/spec/types.html#wp276

vector <char*> vec; jclass clazz = (*env).FindClass("java/util/ArrayList"); jobject obj = (*env).NewObject(clazz, (*env).GetMethodID(clazz, "<init>", "()V")); for (int n=0;n<vec.size();n++) { char* str = (char*) static_cast<char*>(vec[n]); jstring _str = (*env).NewStringUTF(str); (*env).CallBooleanMethod(object, (*env).GetMethodID(clazz, "add", "(Ljava/lang/Object;)Z"), _str); } return obj;

This looks like an almost character-by-character copy of the first answer.
@moshbear: it's not. CallBooleanMethod here, CallVoidMethod there. The rest is rather mechanical, so similarities are expected.
Fair enough. My comment on the previous answer w.r.t. (char*)static_cast<char*> being redundant still stands, though.
what is the "object" in CallVoidMethod() and where are we using "jobject obj " declared at the top of code?

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