The Motivation behind J2ObjC
Several approaches to solving this problem were investigated. XMLVM looked very promising, but at the time its iOS translator page stated that the project was suspended (it's now active again, and is a good alternative to this project). Other translation tools did one-time code conversion, requiring additional editing before their output would successfully build and run.
A New Project is Born
From the start, many engineers thought a translator like J2ObjC wasn't possible. Creating a tool that can accurately translate all Java application code to iOS while perfectly preserving its semantics is indeed impossible! That's because iOS has rigorous user interface design standards, and its users are very aware when an app doesn't adhere to them. In our opinion, the only way to get a world-class, fast iOS UI is to write it in Objective-C using Apple's iOS SDK frameworks.
As most engineers learned from limits in differential calculus, however, it can be very useful to just get close to the impossible. We therefore set out with a set of limits that would improve the chances of J2ObjC succeeding:
- Only support client-side development. Command-line tools and server code in theory could be translated, but that use case is likely to have problems not addressed by J2ObjC.
- Only support business logic code, and stay far, far away from user interface APIs (as the old maps used to have in their outlying corners, "here there be monsters").
- Require the iOS Foundation Framework, not a more general base.
- Use Xcode's Instruments to verify acceptable performance and memory use, after implementing Apple's best practices for memory management.
- Only focus on what's needed by the application developers, rather than what is needed for completeness. Real applications' needs drive project requirements.
We Find It Useful, Maybe You Will, Too
We open-sourced J2ObjC as some internal projects were finding it solves the problem of sharing Java business logic with their iOS apps. Several teams rely on the translator now, and we're busily adding new functionality and fixing lots of bugs. We welcome other mobile app teams to give it a try, and let us know what works and what needs improving.
We also find working on the project rewarding. The two hardest tasks for any Java translator are 1) correctly parsing, type checking and resolving Java source, and 2) providing a compliant Java runtime environment. The first task is handled well by the javac compiler, and the runtime environment (including its unit tests) is based on the Android libcore library. That leaves the fun stuff for us: mutating abstract syntax trees and generating generally easy to debug source output. If you have an interest in Java tools or compilers, join us! There's still lots to do, and we would love the help.