Few cents about my commits

Framework improved -- exposing ObjectiveC classes to natively create instances


Previous improvement simplified way Framework target was created right out the box without need to write native code. But it didn’t allow creating objects native way and workaround was to use single point singleton and consider objects as protocols. RoboVM creates objects runtime using objc/runtime API so there is no information available during linkage phase. And a try to use it will cause symbol not found exception like this:

Undefined symbols for architecture x8664: “_OBJC_CLASS$_RoboClass”, referenced from: objc-class-ref in ViewController.o

The goal of this Improvement:

  • hide any need to initialize JVM from user;
  • allow natively creating and using of java classes as native.

In general it should be enough:

  • drop framework in Xcode project;
  • instantiate and use classes like bellow:
    let demo = MyFrameworkDemo(text: "demo")!
    // or
    MyFrameworkDemo *demo = [[MyFrameworkDemo alloc] initWithText:@"demo"];
    NSLog(@"%@", [demo roboVmVersion]);

(skip long read and start creating framework)

Step one. OBJC_CLASS_$_${CLASSNAME} structure

OBJC_CLASS_$_${CLASSNAME} represent structure of class, one that is being returned by [MyClass class]. For ObjectiveC v2 its definition is not available in SDK header files but it can be found in Apple/Objc4 objc-runtime-new.h and it looks as bellow:

typedef struct class_t {
    struct class_t *isa;
    struct class_t *superclass;
    Cache cache;
    IMP *vtable;
    class_rw_t *data;
} class_t;

In general it points to 5 different structures that describes superclass, methods, members and so on. Compiler is responsible to generate and export all these structure. So proper way would be to teach RoboVM compiler generating these.

But its to much to be done. RoboVM creates backing ObjectiveC classes runtime during loading matching Java class.

Solution is to hack:

  • compiler will generate OBJC_CLASS_$_${CLASSNAME} for each custom java class and framework will expose it. This allows to write native initialization of object and binary binary will be linked without Undefined symbols error;
  • runtime will create backing ObjC class using objc/runtime as usual;
  • and THE TRICK: code will copy class_t created using objc/runtime api into OBJC_CLASS_$_${CLASSNAME} structure and will make it valid.

The moment here – is to fill OBJC_CLASS_$_${CLASSNAME} with valid information before class is used on native side. In other word – corresponding Java class has to be loaded before its ObjectiveC counter part is used.

Step two. Starting JVM automatically and preloading ObjC classes

Previous improvement framework support introduced api that allows to initiate JVM and preload java class (singleton of framework) manually:

// static function that returns instance of Framework's main class. On first access it also instantiate RoboVM
static MyFramework* MyFrameworkInstance() {
    extern MyFramework* rvmInstantiateFramework(const char *className);
    return rvmInstantiateFramework("com.mycompany.myframework.MyFrameworkImpl");

And in general all classes might be preloaded by com.mycompany.myframework.MyFrameworkImpl java class. But it is not fluent in 2020. RoboVM framework target support was changed to do this automatically when Application loads framework(magic!). Following was done:

  • compiler makes a list of all classes annotated with @CustomClass annotation and available in class path (and not dropped by tree shaker);
  • frameworksupport.m extended with __attribute__((constructor)) module initializer function that is triggered, when framework is loaded, details at Apple;
  • module inializer starts JVM (so you don’t have to bother about) and loads all @CustomClass classes exposed by RoboVM compiler.

At this moment java classes can be natively used in ObjectiveC/Swift native code.

end of long read

Tutorial: Implementing Framework new way

Extended from NSObject, annotate with @CustomClass(“ClassName”)

Exposed classes HAVE to be extended from NSObject (or another class that extended from it) and annotated with @CustomClass annotations:

public class MyFrameworkDemo extends NSObject {
// ...

@CustomClass annotation should specify a name this class will be used in native part (it might don’t match Java one but it is better to keep these same).

Annotate constructors/methods with @Method annotation

@Method annotation connects a method with ObjectiveC selector it going to bind to. +/- prefix of selector should not be specified. Constructor have to be bind to -init initializer:

     * mapping constructor to -(void)init selector
     * IMPORTANT: no minus sign !
    @Method(selector = "init")
    public MyFrameworkDemo() {
        System.out.println("Default constructor was called!");

     * secondary constructor with parameter.
     * mapped to -(void)initWithText:(NSString*)test selector
     * IMPORTANT: no minus sign in @Method annotation
    @Method(selector = "initWithText:")
    public MyFrameworkDemo(String text) {
        System.out.println("Secondary constructor with " + text + " called!");

Writing headers/MyFramework.h

This file is specifies API that native code will see and use. It content should match class names specified in @CustomClass and selectors should match ones in @Method(selector = ) annotations:

// MyFrameworkDemo class with basic API demonstration
@interface MyFrameworkDemo

Tell treeshaker what classes to keep

Framework target uses aggressive tree shaker by default which will drop all not referenced stuff, so configure robovm.xml to have exposed classes force linked:

    <!-- Force link all classes in the SDK packages. -->

Natively use your Java code in ObjectiveC/Swift application

At this point project should be compiled into framework. Included in Xcode project and natively used like bellow:

import MyFramework
// ...
let demo = MyFrameworkDemo(text: "demo")!
let calc = Calculator(value: 1000)!

Framework template

Framework template was updated to these changes and its good idea to start with it. Available in Idea plugin.


Code was delivered as PR457