I'm starting to learn java's JNI to use with an android device.
As I read somewhere, you must have some "glue" for the C++ part in order to be loaded through JNI.
My question is: Is it posible to have a run() function in C with the glue for JNI having that running the real app and having java only for the entry point?
Because I don't know if when invoking that run() function through JNI it may cause problems if that function calls another functions and so on.
Thanks for the tip!
EDIT: I want to code in C++ using ndk and trying to avoid coding in java. thus, I wanted to know if a) if I can compile and run in native with ndk or b) if i can use java only to invoke my app, example: calling woth jni something like app->run() and let it do all stuff instead of java. then, java will act only as an entry point.
If you want to develop for Android in C/C++ (no Java) AND you target newer devices (Gingerbread, android-9 app platform onwards) consider using NativeActivity.
See http://developer.android.com/reference/android/app/NativeActivity.html and folder inside NDK package $NDK/docs/NATIVE-ACTIVITY.HTML together with the sample code $NDK/samples/native-activity.
Good luck!
I made a step-by-step howto in the following post: How to create dll using android You can read it and put questions if you don't understand something.
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There seems to be "jni.h" header in the android toolchain but no library to link to.
Can somebody guide me how can I invoke Java functions from C code in my app?
Android does not export a JNI library. This means that there is no way in the system to call JNI_CreateJavaVM() and her ilk from user-space C code. You can create your own JVM, but it will not be the same Java, and won't have access to Android SDK classes and methods.
In Android, a Zigote process is started with a special JVM (Dalvik or ART), and then Java can load your C code (in form of .so files), not vice versa.
Your C code can use the standard JNI techniques like CallVoidMethod() and her kin to invoke Java methods. Note that Android SDK Java methods often need some 'handles', like context, to have their work done; usually, you must rely on some calls from Java to C that will give you these handles.
I am facing a critical issue trying to export my opencv project to android.
Since all of my code is in opencv (C++ version) under visual studio 2013, i started off looking into JNI. I ended up realizing that JNI works good when we export C stuff as name mangling is turned off. Does this affect my C++ code? I think it should as name mangling has to be there for C++ class functions.
Then I found out about JavaCPP. This seemed more promising but the extra overhead its Pointer logic seems strange to get my head around and extra build steps to create .so files.
And finally, read about Visualgdb. Though seems to be in trial, but with my liking for Visual studio, looks reasonable.
Can someone guide my which way I have to follow to translate/bind my code in android so that I can export my c++ classes? Does plain JNI have a real issue with C++ code and no problems with C? OR do I go for JavaCPP or visualgdb?
You valuable comments will be highly appreciated.
Thanks
AFAIK you can use JNI on your c++ code, I'm using it. But you should only use JNI if you want to write all your code in c++ and want java to use that exact c++ code in java side. And for that you should export all functions in your public API. And I must say, passing through custom objects such as Mat, Rect, etc is not a very easy problem, also debugging is not also straight forward. I suggest you to use OpenCV Java and port your code to Java.
Good luck!
I have an image processing code in matlab containing its inbuilt functions like graythresh(), bwareaopen(),etc. I need to make that code processable on a smartphone(preferably with android os).The code also contains methods of image segmentation like Otsu. It would be of great help if you could suggest me the best and easy way to do that. I know elementary Java, but I do not know much about android programming.
You have several options:
The best things to do is to port your code in java or port your code to use openCV.
Use Matlab coder to generate C code. This C code doesn't require the MCR, and could be compiled to run directly on Android
You could try using the very handy Octave for Android:
https://play.google.com/store/apps/details?id=com.octave&hl=en
It may not have all of the functions you need (as it is Octave, and a port) but it will be a good start.
I have a general question according an android app, I need to use some pcap functionality in my android app. Because java does not give the possibility in raw packet injections and low layer programming (as far as I know, pls correct me if I'm wrong) so I was looking for an alternative. So far I found the following:
ANDROID NDK
JNETPCAP
Any suggestions which one I should use or does anyone have other suggestions?
The JNI Solution
You need to wrap the calls and the logic you need out of libpcap in C or C++ and expose the underlying functions through JNI (Java Native Interface) so your application can call native code in Java.
The documentation on JNI is pretty complete on internet, a lot of tutorials exists on this subject such as this one.
If you want to easily wrap native code in JNI you can use Swig which allow you to automatically generate JNI code based on your C/C++ native headers.
The obtained JNI code should be compiled using the Android NDK as a dynamic library (.so). This library is to be placed in your application package under libs/. You can then invoke System.loadLibrary(path_to_you_dynamic_library) to load all the symbols contained in the library and use them in Java.
Using a third-party library
If you're afraid of getting headaches while figuring out how to use JNI, you can look at this library which does the hard work for you, and provides an API to manipulate raw sockets in Java.
http://www.savarese.com/software/rocksaw/
You need to wrap the calls and the logic you need out of libpcap in C or C++ and expose the underlying functions through JNI (Java Native Interface) so your application can call native code in Java.
Or you need to get a library that's already done that, such as, err, umm, jNetPcap.
One problem you may have with any attempt to do packet capture on Android - or any other OS using the Linux kernel - is that, by default, the underlying kernel mechanism used by libpcap (PF_PACKET sockets) requires root privileges. If there's a way to run your code as root, or to give it CAP_NET_RAW and possibly CAP_NET_ADMIN privileges, it might be possible to make it work.
I want to create native Mac OS X application using Cocoa + Objective C but I need to connect to proprietary data source, and for this, owner of the data source only provides Java library. So I need to somehow import this Java library into my project and call functions on its Java classes.(Or create java wrapper around this library and then call my wrapper from objective-C).
Now, how can I do this? Quick google search leads me to JNI but I haven't found any good and actual(current) article/tutorial. I would really need some HOW TO article, how to load this java library, start VM if needed, and how to create java objects and call functions on them. Really something simple and I can move from there. Thanks.
Just to clarify, I repeat: I WANT to call Java functions from Objective-C, I do NOT want to call native functions from Java.
You're probably looking for the Invocation API, a little-known corner of Java Native Interface (JNI) which allows you to load the Java runtime in-process.
That said, you might have an easier time of it with a Java service application that communicates with your Objective-C application over network sockets.
You're looking for the Java-Objective C bridge, try looking at this article or on Apple's developer site. Be aware it is deprecated, that is it isn't being kept up to date with changes to Cocoa. But if you're just using it for an API passing standard Java datatypes you should be OK.