CODEWITHSUNDEEP
javaandroidc++java-native-interface
nevsnirG
nevsnirG

Reputation: 19

Use prebuilt JNI library in Android Studio 3.1

I was given a shared object (*.so file) that I need to reference from my app. I know for a fact that the shared object uses JNI. Now, I have some experience with Android app development, but none with native code. I have looked at a lot of answers here on StackOverflow and on Google in general, but none seem to fit in my question.

Basically, this shared object was for another app, and I have the code for that app, and now I need to implement it in my app. I am lost as to where I should start or what I should do. If anyone can help guide me through this process that would be very nice.

Also, I do not have the source files nor the header files for the shared object. I do know the name of the native method.

Upvotes: 0

Views: 621

Answers (2)

Alex Cohn
Alex Cohn

Reputation: 57173

If you don't have the Java sources that defined these native methods originally, you can often reverse engineer the necessary native method definitions from the .so file itself. E.g., if the library exports Java_com_example_testjni_MainActivity_stringFromJni, then you must add a Java class

package com.example.thestjni;

class MainActivity {
    public native static String stringFromJni();
}

This does not give your the correct parameters or return types for the native methods. Also, you must guess whether the native method should be declared static or not.

To be on the safe side, you will add a static constructor to this class, to make sure the library is loaded:

package com.example.thestjni;

class MainActivity {
    static {
        System.loadLibrary("thirdparty");
    }
    public native static String stringFromJni();
}

You are free to rename the 3rd party library to your liking, but not the package and not the class. You don't care if the original Java class declared the native method private, you can safely declare it public in your project.

To find the names of JNI functions exported by prebuilt library libthirdparty.so, you can use the nm tool provided with Android NDK toolchains:

nm -D libthirdparty.so | grep Java_

Upvotes: 1

Gabe Sechan
Gabe Sechan

Reputation: 93542

If you have the Java code for another app that uses it- find the Java code in the other library that calls it. Those functions should be defined with the keyword native. Take that class(es), without renaming or changing the package, and put it in your project. Now you can call those native functions via the Java native function definitions.

Of course without source you can't compile for any other architecture. So hopefully you have the .so files for all the appropriate ones. Remember to load the library if those class(es) don't do it for you.

Upvotes: 0

Related Questions