Few cents about my commits

Debugger: "Could not initialize class" when running debug build

08 Sep 2025 | debugger bug rollback

There was a report in MobiVM matrix channel for crash that is happening only during running under debugger:

java.lang.NoClassDefFoundError: Could not initialize class com.sample.mobile.ios.network.RoboVmNetzwerkStatusController
        at com.sample.mobile.ios.Main.didFinishLaunching(Main.java:208)
        at com.sample.mobile.ios.Main.$cb$application$didFinishLaunchingWithOptions$(Main.java)
        at org.robovm.apple.uikit.UIApplication.main(Native Method)
        at org.robovm.apple.uikit.UIApplication.main(UIApplication.java:442)
        at com.sample.mobile.ios.Main.main(Main.java:314)

TLTR: Fix and workaround

  • Rollback of #754;
  • Call Class.forName("org.robovm.objc.ObjCObject"); somewhere early in iOS code;

Background

it was happening in production-wide code base but luckily its was simplified to 10 file mini project what was reduced to following sample:

package com.example.mobile.ios;

import org.robovm.apple.foundation.NSObject;
import org.robovm.apple.foundation.NSString;

public class Main {
    public static void main(String[] args) throws Exception {
        new NSObject(); // cause Failed to preload inner ObjCClass host class com.example.mobile.ios.Foo: null
        new Foo();      // cause java.lang.NoClassDefFoundError: Could not initialize class com.example.mobile.ios.Foo
        new Foo.FooInner(); // to keep class from being tree shaked
    }
}

class Foo extends NSObject {
    static NSString npeHere = new NSString("NPE is here");

    public static class FooInner extends NSObject {
        public static void foo() { Foo.npeHere.toString(); }
    }
}

Once started it crashes with following output:

Failed to preload inner ObjCClass host class com.example.mobile.ios.Foo: null
java.lang.NoClassDefFoundError: Could not initialize class com.example.mobile.ios.Foo
	at com.example.mobile.ios.Main.main(Main.java:9)

From these messages it seems that:

  • Failed to preload inner ObjCClass host class comes from changes added in #754;
  • NoClassDefFoundError is from new Foo();

Both are related. As preload inner ObjCClass host class actually does Class.forName("com.example.mobile.ios.Foo"); and seems like it is failed during class initialization. Thus, any future operations with this class will fail.

Investigation

To get insights apps was started int verbose/trace mode by specifying -rvm:log=trace VM argument, in case of Simulator invocation looks as bellow:

xcrun simctl launch –console-pty D955FAB6-C55D-41F9-9893-2A4121AD0991 com.sample.mobile.ios -rvm:log=trace

It produces tons of output but related to class initialization looks as bellow:

Throwing a java/lang/NullPointerException. Call stack:
    org/robovm/apple/foundation/NSObject.alloc(Lorg/robovm/objc/ObjCClass;)J:185
    org/robovm/apple/foundation/NSObject.alloc()J:179
    org/robovm/objc/ObjCObject.<init>()V:86
    org/robovm/apple/foundation/NSObject.<init>(Lorg/robovm/apple/foundation/NSObject$SkipInit;)V:139
    org/robovm/apple/foundation/NSString.<init>(Ljava/lang/String;)V:108
    com/example/mobile/ios/Foo.<clinit>()V:27
    java/lang/Class.classForName(Ljava/lang/String;ZLjava/lang/ClassLoader;)Ljava/lang/Class;:-2
    java/lang/Class.forName(Ljava/lang/String;ZLjava/lang/ClassLoader;)Ljava/lang/Class;:218
    java/lang/Class.forName(Ljava/lang/String;)Ljava/lang/Class;:176
    org/robovm/objc/ObjCClass.<clinit>()V:78
    org/robovm/objc/ObjCObject.<clinit>()V:77
    com/example/mobile/ios/Main.testNSObject()V:9
    com/example/mobile/ios/Main.main([Ljava/lang/String;)V:14

NPE happens during Foo class static member initialization npeHere = new NSString("NPE is here") inside NSObjects code:

    private static final Selector alloc = Selector.register("alloc");
    static long alloc(ObjCClass c) {
        long h = ObjCRuntime.ptr_objc_msgSend(c.getHandle(), alloc.getHandle());
        if (h == 0L) {
            throw new OutOfMemoryError();
        }
        return h;
    }

At this line there are two options for NPU: either (or both) c to alloc is null. Long story short – it was alloc. But from code it can’t be normally null, but in reality can if NSString is not finished class initialization.

Back to trace log, to check sequence of class loading(simplified):

{
    "Loading com/example/mobile/ios/Main": { },
    "Initializing com/example/mobile/ios/Main": {},
    "Loading org/robovm/apple/foundation/NSObject": {},
    "Initializing org/robovm/apple/foundation/NSObject": {
        "Initializing org/robovm/objc/ObjCObject": {
            "Loading org/robovm/objc/ObjCClass": {},
            "Initializing org/robovm/objc/ObjCClass": {
                "Loading com/example/mobile/ios/Foo": {},
                "Initializing com/example/mobile/ios/Foo": {
                    "Loading org/robovm/apple/foundation/NSString": {},
                    "Initializing org/robovm/apple/foundation/NSString": {},
                    "Exception java/lang/NullPointerException": [
                        "org/robovm/apple/foundation/NSObject.alloc(Lorg/robovm/objc/ObjCClass;)J:185",
                        "org/robovm/apple/foundation/NSObject.alloc()J:179",
                        "org/robovm/objc/ObjCObject.<init>()V:86",
                        "org/robovm/apple/foundation/NSObject.<init>(Lorg/robovm/apple/foundation/NSObject$SkipInit;)V:139",
                        "org/robovm/apple/foundation/NSString.<init>(Ljava/lang/String;)V:108",
                        "com/example/mobile/ios/Foo.<clinit>()V:27",
                        "java/lang/Class.classForName(Ljava/lang/String;ZLjava/lang/ClassLoader;)Ljava/lang/Class;:-2",
                        "java/lang/Class.forName(Ljava/lang/String;ZLjava/lang/ClassLoader;)Ljava/lang/Class;:218",
                        "java/lang/Class.forName(Ljava/lang/String;)Ljava/lang/Class;:176",
                        "org/robovm/objc/ObjCClass.<clinit>()V:78",
                        "org/robovm/objc/ObjCObject.<clinit>()V:77",
                        "com/example/mobile/ios/Main.testNSObject()V:9",
                        "com/example/mobile/ios/Main.main([Ljava/lang/String;)V:14"
                    ]
                }
            }
        }
    }
}

From call tree: NSObject.alloc() was called while NSObject.<clinit> even not started. As before NSObject it’s super class has to be initialized first ObjCObject and before it – ObjCClass. And at last ObjCClass does class pre-load right before in middle of this.

Workaround

It shall work if ObjCClass is loaded in calm condition, but sadly preload of it with Class.forName("org.robovm.objc.ObjCClass") fails with:

java.lang.ExceptionInInitializerError
	at java.lang.Class.classForName(Native Method)
	at java.lang.Class.forName(Class.java:218)
	at java.lang.Class.forName(Class.java:176)
	at com.example.mobile.ios.Main.main(Main.java:8)
Caused by: java.lang.NullPointerException
	at org.robovm.objc.ObjCClass.getByType(ObjCClass.java:275)
	at org.robovm.objc.ObjCObject.<clinit>(ObjCObject.java:77)
	... 4 more

And here we have SAME cycle dependency:

ObjCClass.<clinit>() {
   super.<clinit>() {  // ObjCObject
      ObjCClass.getByType()  // <<<< NPE inside it as ObjCClass is not initialized 
   } 
}

As ObjCClass is final chances are low that it is going to be used something like this. As ObjCClass.getByType() inside ObjCObject is compiler generated it is better to leave things as they are.
As ObjCObject has references to ObjCClass it might cause class being initialized, and this does work:

Following code being added at very beginning of iOS code will fix the issue.

Class.forName(“org.robovm.objc.ObjCObject”); // FIXME: workaround for Failed to preload inner ObjCClass xxxx

The fix. Reverts MobiVM/robovm#754

Reasons to revert:

  • Crash itself;
  • Another moment to revert: Jetbrains has fixed https://youtrack.jetbrains.com/issue/IDEA-332794 and delivered fix in 2025.2. (bad moment – it fixes big Java case but there is still issue with Robo)
  • These changes were applied only to NSObject subclasses but any inner Java classes affected.

Fix

Revert was proposed as #815

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