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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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Like RoboVM, XMLVM contains Cocoa bindings that enable you to use native iOS APIs directly from Java, but they have many missing pieces. If you intend to use these bindings you should be prepared to pop the hood and possibly fill in some gaps yourself. Figure 8 shows an XMLVM application architecture using Cocoa wrapper classes for UI.

Figure 8. XMLVM application architecture using Cocoa

XMLVM Architecture using Cocoa

If you want to include source that has been translated by XMLVM into an existing Xcode project, you can do so, but you will need to take care to add the appropriate build rules and initialization code for the garbage collector. You should also understand the implications of mixing garbage-collected code with reference-counted code. Figure 9 shows an XMLVM application architecture where the Java business logic has been compiled to C and included in an existing Xcode project.

Figure 9. A Java app compiled to C and ported to an Xcode project

XMLVM compiles a Java app and ports it to an Xcode project

J2ObjC

J2ObjC has the narrowest focus of the tools introduced in this article. Open sourced late last year, it was designed by Google specifically to allow code to be shared between Java (Android and GWT) and Objective-C (iOS) projects. Figure 10 shows a typical path from Java to iOS using J2ObjC.

Figure 10. The J2ObjC toolchain

The J2ObjC toolchain

J2ObjC includes a command-line tool for converting Java source code to Objective-C source code. It also includes some Java annotations and a Java compatibility library for Objective-C. J2ObjC's narrow focus makes it an ideal choice if your goal is to share business logic between apps written inJava and Objective-C. Integrating code generated with J2ObjC with an existing Objective-C project is actually painless, and the close correspondence between the original Java class structure and the resulting Objective-C class structure makes it easy to use the APIs from Objective-C.

Figure 11. J2ObjC application architecture

J2ObjC Application Architecture

A key differentiator between J2ObjC and the other solutions is its preference for reference counting over garbage collection (that is, while it can use GC, it isn't supported on iOS). In addition to "transpile-time" management for reference-counting, and its support for manual reference counting and ARC via command-line flags, J2ObjC provides two mechanisms to help you deal with memory management in your Java code:

  1. Annotations to provide memory management hints (e.g. @Weak)
  2. A memory profiling tool to help detect memory cycles, called MemDebug

Garbage collection versus reference counting

While garbage collection makes our lives easier inside the Java world, it complicates them in the native world, where reference counting is the rule of the day. For instance, if you set a property of an Objective-C object to be a Java object, the garbage collector will know nothing about this reference. As a result, it may free the memory of the object if it is no longer accessible inside the Java world, even if the native world still needs it. The solution is to introduce your own form of reference counting for objects that you wish to maintain references to inside non garbage-collected structures.

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