Using Java library in iOS project

Question

I am developing a project that requires business logic calculations based on a relative complex rule set. The calucations are to be executed on the following platforms:

• Android app
• iOS app
• Java EE application container

The business logic does not depend on any platform specific details but is strictly about "number crunching".

To avoid double implementation for Java and Objective C, which would require elaborate testing and so forth, I would like to create a Java library cotaining the implementation and use it within the iOS app.

What would be the best way to do this? Would it be better to do a native port for Objective C? Would it be better to do it the other way around and write the library in Objective C and use it on Android/Java?

Answer 1

There is no way how you can use a Java library in an iOS application. Java requires runtime to interpret (JRE) and you don't have that on iOS.

Technically, you could compile a Java library into native code (there are tools for that) but I don't recommend it.

For sharing bussiness code between Android and iOS the best solution is to use pure C libraries. You can call C libraries from Java using JNI (NDK for Android).

Since Obj-C is only a small object layer over pure C, you can use C libraries easily from Obj-C.

You can compile Obj-C for NDK, too, but it is much more complicated than just using C.

Answer 2

You could try parsing the java library to Objective C using j2objc https://code.google.com/p/j2objc/

Update: This Swift Java bridge is in development https://github.com/SwiftJava/SwiftJava/blob/master/README.md

Answer 3

Kotlin Native now has a Swift integration. So with some build process overhead you could share business logic written in Kotlin with iOS and Android.

https://blog.jetbrains.com/kotlin/2017/12/kotlinnative-v0-5-released-calling-kotlin-from-swift-and-c-llvm-5-and-more/