Calling Java method from c

813103 · Apr 15, 2011 5:44PM

Hi. THis is my problem:

I want to write applet that draws amplitude of a line-out, meaning it draws what you hear. I have the c++ part that reads samples and stores them in a buffer alredy written and the java part that calculate FFT writen as well. Now when the buffer is full c++ dispatche the event and i grab those samples and now i my problem. How to write function that passes that array to the java class. For clarity.

Java code:

public class applet extends Applet
{
init(){...}
public native void start();
public native void stop();

public void setSamples(byte[] array)  //<-- this is the method that i need to call from c++
}

C++ code:

void fullBuffer(char *array)//<-- this function is run when buffor is filled
{
...
lEnv->CallVoidMethod(class_to_be_called,mid_byteAray,jbyteArray); //<-- this of cousre doesn't work but i need somthing with the same effect
}

813103 · Apr 15, 2011 8:21PM

OK i figure it out i'm posting the solution for any one who ecounter simmilar problem:

C++ code:

JavaVM *g_vm ;
JNIEnv *localEnv;
JNIEXPORT jint JNICALL JNI_OnLoad(JavaVM *vm,void *reserved)
{
	g_vm = vm;
	g_vm->AttachCurrentThread( (void**) &localEnv, NULL);
	return JNI_VERSION_1_2;
}
JNIEXPORT void JNICALL Java_jni_callWave_setClass(JNIEnv *env, jobject obj, jobject classref)//we call this function first to set up global reference to our variable
{
	class_to_be_called = env->NewGlobalRef(classref);
}
void sendByteArrayToJavaApplet(std::string str)
{
	int result = g_vm->GetEnv((void **)&localEnv,JNI_VERSION_1_2);
	jclass jc = localEnv->GetObjectClass(class_to_be_called);
	jmethodID mid_bytear; = localEnv->GetMethodID(jc,"setByteArray","([B)V");;
	localEnv->CallVoidMethod(class_to_be_called,mid_bytear,jByteArray);
}

jschellSomeoneStoleMyAlias · Apr 16, 2011 5:48PM

810100 wrote:
{
class_to_be_called = env-> NewGlobalRef (classref);
}

I don't see anything that suggests that is a good idea.

EJP · Apr 16, 2011 8:35PM

It's a C++callback. It has no JNI context when it executes. It needs a class ref. Classic case for using a global ref.

jschellSomeoneStoleMyAlias · Apr 18, 2011 3:21PM

EJP wrote:
It's a C++callback. It has no JNI context when it executes. It needs a class ref. Classic case for using a global ref.

Ok.

I do note however that the OPs method name says in part 'JavaApplet'.

The Sun VM uses class loaders for Applets. So I would suspect it is possible that problems might arise.

And one can do a callback without doing that of course.

EJP · Apr 19, 2011 6:32PM

There's a lot of FUD there and not much in the way of fact. All JVMs use class loaders for all Java applications. He has the class. It is already loaded. And a callback requires either a class reference, if it is static, or an object reference if it isn't, and the context demands that either be held as a GlobalRef.

jschellSomeoneStoleMyAlias · Apr 20, 2011 2:19PM

EJP wrote:
There's a lot of FUD there and not much in the way of fact. All JVMs use class loaders for all Java applications.

In the console VMs that I use the class loaders used by the VM itself are not unloaded while the VM is running.

Browser applets run in in a VM where the class loader will be unloaded.

He has the class. It is already loaded. And a callback requires either a class reference, if it is static, or an object reference if it isn't, and the context demands that either be held as a GlobalRef.

Not sure what you mean.

A callback requires that one accesses something in java.
In that context it is no different than any code used in JNI except that one cannot expect that java entity will be passed in via a parameter.

And in JNI code, callback or not, I can write code to get a class and from that either interact with it or interact with other entities. It doesn't require any parameters to do that.

And I have done lots of callbacks and there was never a "demand" that it be held as a GlobalRef.

It certainly is possible as a choice however.

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