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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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Figure 3. The RoboVM toolchain

The RoboVM toolchain

RoboVM was designed as a path for Android developers to reuse their business logic code on iOS. You can either create the UI programmatically in Java, or you can use Apple's Interface Builder and Xcode to develop the UI and then copy the Nib file into your RoboVM project and link it up to your Java classes. It includes some nifty annotations that enable you to register Java classes with the Objective-C runtime and register Java methods to respond to Objective-C messages, so that you can define outlets for a Nib file inside a Java class. Figure 4 shows RoboVM's application architecture.

Figure 4. RoboVM application architecture

RoboVM application architecture

RoboVM is still very young, so if you decide to try it out you should expect to get your hands dirty with its API. You will likely run into missing methods for which you'll need to generate bindings. All of the tools are present but documentation at this early stage is still scarce, so you may have to go through some cycles of trial and error.

Avian

Avian is a bit of a misfit in this group because it wasn't written to solve the Java iOS problem specifically. Instead, Avian is a lightweight JVM that includes a tool (bootimage-generator) that compiles Java bytecode into native binaries that can be run on ARM devices like the iPhone. Avian's creator, Joel Dice, has published a sample iOS application that serves as a proof-of-concept for building iOS applications in Avian.

Figure 5 shows a typical path from Java to iOS using Avian.

Figure 5. The Avian toolchain

The Avian toolchain

Avian doesn't provide specific bindings for iOS native APIs and it doesn't provide special APIs for building mobile applications. Any interaction between Java and the native environment must take place using JNI. Avian includes its own class library that is a more modular and portable subset of JavaSE, but it also allows you to build applications against OpenJDK 7. In order to reduce the size of the resulting binary, Avian uses Proguard to strip out dead code. Figure 6 shows Avian's application architecture.

Figure 6. Avian application architecture

Avian application architecture

XMLVM

XMLVM has a much broader focus than most of the other tools discussed here because it aims to allow translation between many different languages. One such translation path is from Java bytecode to C source code, which is used to build iOS applications in Java. The typical XMLVM build process is to write some code in Java, compile it using javac, then use XMLVM to convert the .class files into .h and .c files. These C source files are then added to an Xcode project and built into an iOS application, as shown in Figure 7.

Figure 7. The XMLVM toolchain

The XMLVM toolchain

XMLVM can be invoked from the command-line to perform conversion directly, or it can be invoked to create a skeleton NetBeans project that includes the appropriate libraries and build scripts to compile a Java application for iOS. The "Run" option actually creates an Xcode project with the translated C files, and opens it in Xcode. There you can debug the project, run it on a connected device (iPhone or iPad), or run it in Apple's simulator.

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