Develop Reference

Pass JNIEnv or Context from Java to Go

I've successfully written a go mobile library and used it from an Android Java app.
I've also successfully called a c function from JNI. This one has the JNIEnv pointer.
I'd like to pass the context/JNIEnv from Java to Go.
Here's where it seems to get complicated:
Go Mobile does not seem to use JNIEnv at all.
LoadJNI.java does have a context object and claims to pass it to the Go side. But there it isn't visible.
I've been able to include jni.h using the "C" directive
I can transfer the JNIEnv from JNI/C or store it there as a global variable but that's an ugly kludge.
https://github.com/golang/go/issues/16876 talks about a proposal to do reverse binding but it's unclear what the status is.
https://godoc.org/golang.org/x/mobile/cmd/gobind also talks about it but the following code snippet
import "Java/java/lang/System"
t := System.CurrentTimeMillis()
returns
cannot find package "Java/java/lang/System" in any of:
/usr/local/go/src/Java/java/lang/System (from $GOROOT)
/Users/----/go/src/Java/java/lang/System (from $GOPATH)
In C I solved it with a compiler directive: #cgo CFLAGS: -I...
But I'm not sure how to do it for Java
In short, I'm now stuck and would appreciate help. Ideally, I'd just like to pass the object as a parameter, if possible. However, I'd settle for any reverse binding to call Java from Go.

I hate to answer my own question, but I'd hate someone to go through this as well too. I'll update this answer as I go along.
Basic Setup
Edit build.gradle and add GOPATH and GO to gobind (additions in bold):
gobind {
pkg = ".../reverse/reverse"
GOPATH = "/Users/johndoe/go"
GO = "/usr/local/go/bin"
}
How to develop
Start from the reverse example. Get it to compile and run.
Make one modification and recompile.
Missing gobind
It turns out the installation process neglected to build gobind. Run go install golang.org/x/mobile/cmd/gobind
I'm Stuck
Trying to modify the reverse project to do anything other than what it's already doing is failing. Even stuff like passing an object and calling toString will fail.
More to come...

Android, import external library to NDK

i am making an Android Shapefile reader app on Android with the use of NDK. I want to use C++ to parse shape files. I found library "Shapefile C Library". Github: https://github.com/sw897/shapelib.
I am using Android studio and have no idea how to import it to my Android project so i could use functions described in: http://shapelib.maptools.org/shp_api.html
Any tips?

First, start hacking away at the Hello-JNI example from Google:
https://github.com/googlesamples/android-ndk/tree/master/hello-jni
Use that as a test bed.
Then, create a Java Class with public native methods that let you interact with your library. Something of a high level API, probably to pass a file name or buffer to SHPOpenLL and return a pointer to the ShapeFile context (ShapeHandle). Looks like your library is written in C, so you should be able to write a similar interface to query the shapefile passing a (jint) c-style cast of your ShapeHandle pointer.
Next, you need to play around with javah to generate the header for your shapefile interface. Once the header is generated, you can use it to implement your interface in a .cc file. From there you will basically make Java calls to your C interface and it will return jobjects. (jint, jbool, jstring, etc...)
I'm looking at the ShapeLib that you want to use and while it's easy enough, there will be some gotchas.
You will need to implement SAHooks for file I/O. I suggest looking at NVFile for an example how to access APK files (or downloaded files). You will need to init it using activity.context.assetmanager. Then use nv_file to wrap FRead/FSeek/etc...
Passing info back to java is going to be tough. Personally, I would build a Java class to hold the shape information and fill it out on the C side. However, you might be tempted to query these parameters from the SHPObject one at a time. Will be slow, but less error prone.
E.g.
// Java
MyJavaShapeObject obj = new MyJavaShapeObject();
_c_retrieveShapeObj((jint)pShapeFile, obj);
// C
java_blah_blah_c_retrieveShapeObj(JNIEnv* env, jclass activity, jint theShapeFile, jobject theObj){
ShapeHandle pShapeFileHandle = (ShapeHandle)theShapeFile; // c style conversion
// http://stackoverflow.com/questions/11647646/how-to-use-the-jni-to-change-the-fields-of-a-java-class
// http://docs.oracle.com/javase/7/docs/technotes/guides/jni/spec/functions.html#wp16613
// http://stackoverflow.com/a/36759159/7949696
}

Making sense of symbol table entry (when header function is not present)

I have a JNI function in a C++ library.
When I create the library using cmake (but forget to put function declaration) in the header file, the library is created successfully.
When I look for function name in library symbol table I get following output
nm libuserlibrary.so | grep printxx
00506e60 T _Z60Java_com_example_user_myapplication_userlibrary_printxxP7JNIEnv_P8_jobject
But when I give function declaration in the header file, I get following output
nm libuserlibrary.so | grep printxx
00506e50 T Java_com_example_user_myapplication_userlibrary_printxx
Why is there this difference between these two symbol table entries? What is the purpose of _Z60 and P7JNIEnv_P8_jobject around the function name?
I also noticed that in the first case, I cannot call the JNI funciton from Android java code (it says unsatisfied-linker-error, implementation not found).

C++ allows function overloads and namespaces like Java does. So, it annotated the function name with parameter information so the linker can bind to the correct overload.
JNI was designed for C which does not allow function overloads or namespaces. So it invented it's own annotation system and provides the javah tool to help you use it. The header can be used in C++ too. C++ was designed to allow some functions to be called as if they written in C. The header has code that indicates that to the compiler. So, put it all together and you can write Java-callable functions in C++.

calling jni method from other jni library

Is it possible to call jni method of 1 library from another method of different jni library ?
for eg: I have 2 libraries lib_1.so and lib_2.so.
I want to call a method get_interface() of lib_1.so from lib_2.so.
Is this possible?
If yes, please share the example of how this can be done.

When calling from one shared library to another, its really no longer jni, its just native code(c->c or c++ -> c++). Include the .h and invoke the function as you would normally, passing whatever parameters the function requires.

I think you can do dlopen("lib2.so") from lib1.so so you have a handle to your lib2.so library and then using that handle you can call the methods.

Android NDK and extern variables in C code

I have a library that I want to customize its work (I mean replacing the main of the library by my outside program so I can for example reorder the calls of the library's functions) and for that I had to change some static variable to extern variables so the library and my program can handle them both.
My program works fine when on run on my Linux desktop machine, however when I tried to port that to Android using NDK, I couldn't compile it because of the extern keyword .
So, I was wondering if there's a way concerning the Android.mk or alternative to the use of extern keyword, to make the things work.

You're getting an "undefined reference" because extern is not a variable definition, only a declaration. That means you must have an non-extern declation somewhere else in your code, otherwise the compiler doesn't allocate a symbol for your variable.
As for alternatives, no, an extern variable is the only way to have a global variable accross multiple object files in C.
If you only need a global variable in the same object file, you can use static so the symbol is not exported to other objects.