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Open source Java iOS tools compared

Five free tools for developing iPhone and iPad applications in Java

By Steven Hannah, JavaWorld.com, 03/27/13

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Java programming for Apple's iOS devices is not only possible but it's getting easier all the time. Steve Hannah surveys the recent evolution of the Java iOS landscape, then introduces five open source Java iOS tools. Find out how Avian, Codename One, J2ObjC, RoboVM, and XMLVM resolve the challenges of Java iOS native client development for developers who are ready to go mobile. (Includes an introductory section on garbage collection vs reference counting in Java iOS application development.)

As a Java developer I have felt somewhat excluded by the wall around Apple's iOS garden. Until recently, if I wanted to write an application for an Apple device such as the iPhone or iPad, it meant leaving the Java ecosystem behind and coding in Objective-C. The barriers to entry were both political and technical, but Apple's decision in September 2010 to relax the restrictions on its development tools effectively dissolved the political constraints. Since that announcement, a handful of open source solutions have emerged that allow developers to write iOS applications in Java. I briefly introduce five of those tools in this article:

  1. Codename One
  2. RoboVM
  3. Avian
  4. XMLVM
  5. J2ObjC

Each of these tools can be used to write Java code that can be run directly on an iOS device natively. I omitted the myriad of server-side Java EE solutions that publish HTML5/CSS/JavaScript applications to a Web browser or similar client on the device. There's enough to say about tools in this category for another complete review. Note, too, that Oracle has recently announced its intention to open source the iOS versions of JavaSE Embedded and Java FX. Both of these platforms are potentially viable pathways from Java to iOS, especially when combined with the recently announced proposal to enhance the JNI specification with support for statically-linked native libraries.

Strategies for running Java on iOS

Three categories of tools allow Java developers to write to iOS devices:

  1. Server-side technologies (such as Vaadin, Tabris, and ICEmobile) run Java code on a server and publish either HTML5/CSS/JavaScript or JSON to be rendered on the device via a web browser.
  2. Client-side technologies, discussed in this article, run Java code directly on the device using a compile-time conversion to a native binary.
  3. Hybrid technologies like Oracle ADF deploy both server-side and client-side Java components.

The client-side tools profiled in this article are especially geared to creating native Java apps that run on iOS devices such as the iPhone and iPad. They enable developers to write code in Java that is then compiled into a native iOS binary. Although their user interface and integration strategies vary, each tool provides a native method mechanism that allows you to call Objective-C and C APIs from Java code.

Avian, for instance, uses standard JNI, whereas XMLVM uses its own native method system, which is similar to JNI but provides stronger compile-time validation. RoboVM and XMLVM also provide Java wrappers for many of the iOS APIs (such as UIKit), so that you can develop an entire iOS application in Java without once delving into Objective-C. Codename One solves the user-interface puzzle by providing its own pure-Java lightweight UI toolkit, but it also supports native interfaces that allow you to talk to Objective-C from Java. J2ObjC's answer to native interfaces is similar to GWT's, in that it allows you to define native Objective-C implementations for Java methods inline using OCNI.

Note that this article assumes that you are familiar with the basic architecture of iOS tooling and applications.

Codename One

Codename One provides a full-development toolchain for writing iOS applications in Java. Applications written in Codename One can be deployed to a variety of devices, including Android, iOS, BlackBerry, Windows Phone, and J2ME (although some features may not be available on lower powered devices). Codename One comes the closest of any Java mobile toolkit to supporting Java's "write once run anywhere" premise. Two components are key to enabling it to work across multiple platforms:

  1. Lightweight UI toolkit: The cross-platform UI toolkit is similar to Swing. Codename One UIs are pluggable and themeable, enabling them to take on the look-and-feel of a native device. A Codename One app running on an Android device will match the Android look and feel; on iOS it will match the iOS look and feel.
  2. Pluggable implementation: All of Codename One's platform-dependent code is contained in a single class that can be easily overridden to support another platform. This makes the framework extremely portable. The default iOS port is is currently XMLVM, but it isn't difficult to change it to Avian or RoboVM. (In fact, I created an Avian port as a research exercise without too much difficulty.)

Figure 1 shows a typical path from Java to iOS using Codename One.

Figure 1. The Codename One toolchain (click to enlarge)

The Codename One toolchain

Codename One comprises a Java API, the Codename One Designer tool (a WYSIWYG GUI builder), a simulator that allows you to test your application, and a cloud build server that allows you to build your application for any device. One nice thing about the cloud build server is that you don't need to have proprietary tools such as the Apple developer tools installed. Codename One's build server frees you from the platform dependency typically associated with iOS development. With Codename One, you don't need to have a Mac to develop apps for iOS anymore.

Figure 2. Codename One application architecture

Codename One application architecture

It's also possible to set up your own build environment and not depend on the Codename One cloud server. Doing that would entail creating a multi-step build process involving javac, a build tool such as Ant, XMLVM (see below), and Xcode.

RoboVM

RoboVM is brand new (in version 0.0.1 at time of writing) and much of the Cocoa/Objective-C bridge is unfinished, but it appears to be full of potential. RoboVM is a command-line tool that you can use to compile Java .class files into native machine code. It provides an AOT (ahead-of-time) compiler based on LLVM, C bindings, and an Objective-C bridge, which lays the foundation for pure-Java applications to be deployed on iOS. It also includes a full set of Cocoa bindings (which should enable you to use the iOS native APIs directly from Java) and an Eclipse plugin that allows you to compile and run on the iOS simulator directly from Eclipse.

Figure 3 shows a typical toolchain path from Java to iOS using RoboVM.

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