What's the best way to pass/return Input/Output streams from Java to JNI?

[Dan Bizman]

I want to pass in an inputstream to JNI and then manipulate that data, or use it in C++. As well, I want to grab a stream of data in C++ and be able to pass back an InputStream (from a JNI method) which a C++ thread will continue streaming data to.

What's the best way to do this?

[Henry Wong]

Originally posted by Dan Bizman:
I want to pass in an inputstream to JNI and then manipulate that data, or use it in C++. As well, I want to grab a stream of data in C++ and be able to pass back an InputStream (from a JNI method) which a C++ thread will continue streaming data to.

What's the best way to do this?

IMHO, there is no "best way" to do this. Unless you have the option of doing all the stream processing in Java and only pass the data to C++.

C++ can call methods of Java objects. It can even contruct and return Java objects to Java. It is just incredibly clumsy.

For example... here is how you would create an array list and call the add method to add a Java object.

  jclass clsArrayList = jenv->FindClass("java/util/ArrayList");
  jmethodID construct = jenv->GetMethodID(clsArrayList, "<init>", "()V");
  jobject list = jenv->NewObject(clsArrayList, construct);
 
  jmethodID addmethod = jenv->GetMethodID
                  (clsArrayList, "add", "(Ljava/lang/Object;)Z");
 
  jenv->CallBooleanMethod(list, addmethod, someJStringToAdd);

Henry

[Dan Bizman]

No, I know how to use Java objects from C++, but here's what I'm confused about.

I'm writing to a modem's outline (via C++'s waveOutWrite, etc). The problem is I don't know how to stream to that method from Java. Every call to wavOutWrite requires a header along with a pointer to the data (and the data length) to be written. The problem is I can't see anywhere that I can know it's read a byte and then write another byte before it reads the next one. I.e. be streaming into the LPSTR buffer that it uses. Can I do this? From java?

[Dan Bizman]

For some reason, I can't get it to work where I stream the bytes in (512 ata a time). The only way I can get it to work is write it into a ByteArrayOutputStream and then pass in stream.toByteArray() and read it in one shot. Any idea why?

[Henry Wong]

Originally posted by Dan Bizman:
For some reason, I can't get it to work where I stream the bytes in (512 ata a time). The only way I can get it to work is write it into a ByteArrayOutputStream and then pass in stream.toByteArray() and read it in one shot. Any idea why?

Oh, I see what you are trying to do... you want to write to a stream on the Java side. And on the C++ side, you want to read from the stream to write the data out to the modem. Is this correct?

If this is true, I guess you can create a pair of piped streams (PipedInputStream and PipedOutputStreams) pass one end to the C++ side to read, and have Java write to the other end. You'll need to have separate thread as the C++ side should not return until the operation has completed.

Henry

[Dan Bizman]

Originally posted by Henry Wong:


Oh, I see what you are trying to do... you want to write to a stream on the Java side. And on the C++ side, you want to read from the stream to write the data out to the modem. Is this correct?

If this is true, I guess you can create a pair of piped streams (PipedInputStream and PipedOutputStreams) pass one end to the C++ side to read, and have Java write to the other end. You'll need to have separate thread as the C++ side should not return until the operation has completed.

Henry

That's not a bad idea but I wonder, performance-wise, how will it be? When you're talking about a Piped stream on the C++ side, I assume you mean doing the "env->GetMethodId(...) etc, stuff. Isn't that basically reflection? So I'd be taking a reflection hit on top of a JNI hit. Not saying that I have any alternative, but if there is one...

Thanks again. I'll be taking a look at that option and see if it works.