Android, import external library to NDK

Question

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?

Answer 1

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
}