[Java.Interop] Use Class.forName() as fallback to load Java classes
#1326
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Fixes: #23 Context: dotnet/android@aba2726 Context: b4d44e4 Context: xamarin/monodroid@ed984a3 Context: dotnet/android#7616 Commit b4d44e4 noted: > Android is..."special", in that not all threads get the same > ClassLoader behavior. Specifically, *managed* threads -- > System.Threading.Thread instances -- get a different ClassLoader than > the main/UI thread on Android. (Untested, but the ClassLoader *may* > behave sanely if you use a java.lang.Thread instance instead. But who > wants to use java.lang.Thread instances...?) dotnet/android#7616 provided additional context: `JNIEnv::FindClass()` behavior *is* tied to the thread that calls it, and one of the knock-on effects is that `Java.Lang.JavaSystem.LoadLibrary("MyLib")` doesn't work properly when invoked from a new `System.Threading.Thread` thread. Which brings us to xamarin/mondroid@ed984a3a, which updated then Xamarin.Android to use [`java.lang.Class.forName(String)`][0] instead of [`ClassLoader.loadClass(String)`][1] because the "real" JDK cannot use `ClassLoader.loadClass(String)` to load array types: Class c1 = Class.forName("[Ljava.lang.String;"); // works Class c2 = ClassLoader.getSystemClassLoader() .loadClass("[Ljava.lang.String;"); // throws java.lang.ClassNotFoundException Class c3 = Class.forName("[I"); // works; array of int Class c4 = ClassLoader.getSystemClassLoader() .loadClass("[I"); // throws java.lang.ClassNotFoundException Using `ClassLoader.loadClass(String)` to load array types works on Android, presumably as an undocumented implementation detail, but as xamarin/monodroid@ed984a3a was trying to get things working within the (now dead) Android Designer -- which ran using a Desktop JDK -- Android-specific extensions were not available. Update `JniEnvironment.Types` to use `Class.forName(String)` instead of `ClassLoader.loadClass(String)` to load Java classes, as a fallback for when `JNIEnv::FindClass()` fails to find the class. [0]: https://developer.android.com/reference/java/lang/Class#forName(java.lang.String) [1]: https://developer.android.com/reference/java/lang/ClassLoader#loadClass(java.lang.String)
jonpryor
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Mar 19, 2025
Context: dotnet/java-interop#1326 Does It Build™?
Context: dotnet/android#9931 Context: dotnet/android#9931 (comment) The attempted integration test crashed! F mono-rt : [ERROR] FATAL UNHANDLED EXCEPTION: Java.Lang.ClassNotFoundException: Didn't find class "crc641855b07eca6dcc03.MyCb" on path: DexPathList[[directory "."],nativeLibraryDirectories=[/system/lib64, /system/product/lib64, /system/lib64, /system/product/lib64]] F mono-rt : at Java.Interop.JniEnvironment.Types.TryFindClass(String classname, Boolean throwOnError) F mono-rt : at Java.Interop.JniEnvironment.Types.FindClass(String classname) F mono-rt : at Java.Interop.JniType..ctor(String classname) F mono-rt : at Java.Interop.JniPeerMembers.JniInstanceMethods..ctor(Type declaringType) F mono-rt : at Java.Interop.JniPeerMembers.JniInstanceMethods.GetConstructorsForType(Type declaringType) F mono-rt : at Java.Interop.JniPeerMembers.JniInstanceMethods.StartCreateInstance(String constructorSignature, Type declaringType, JniArgumentValue* parameters) F mono-rt : at Java.Lang.Object..ctor() F mono-rt : at Java.InteropTests.MyCb..ctor() F mono-rt : at Java.InteropTests.JnienvTest.<>c__DisplayClass29_0.<DoNotLeakWeakReferences>b__1() F mono-rt : at System.Threading.Thread.StartHelper.Callback(Object state) F mono-rt : at System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state) F mono-rt : --- End of stack trace from previous location --- F mono-rt : at System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state) F mono-rt : --- End of managed Java.Lang.ClassNotFoundException stack trace --- F mono-rt : java.lang.ClassNotFoundException: Didn't find class "crc641855b07eca6dcc03.MyCb" on path: DexPathList[[directory "."],nativeLibraryDirectories=[/system/lib64, /system/product/lib64, /system/lib64, /system/product/lib64]] F mono-rt : at dalvik.system.BaseDexClassLoader.findClass(BaseDexClassLoader.java:196) F mono-rt : at java.lang.ClassLoader.loadClass(ClassLoader.java:379) F mono-rt : at java.lang.ClassLoader.loadClass(ClassLoader.java:312) Perhaps unsurprisingly, we need to provide the system ClassLoader in order to load types from outside the default "classpath", which is why we used `ClassLoader.loadClass()` in the first place. Instead of `Class.forName()`, use the [`Class.forName(String name, boolean initialize, ClassLoader loader)`][0] overload, so that we can provide a `ClassLoader` instance. This *should* fix the crash. [0]: https://developer.android.com/reference/java/lang/Class#forName(java.lang.String,%20boolean,%20java.lang.ClassLoader)
jpobst
approved these changes
Mar 20, 2025
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Fixes: #23
Context: dotnet/android@aba2726
Context: b4d44e4
Context: https://github.com/xamarin/monodroid/commit/ed984a3a0bfbe71f6499a7855c647b5bc2b28466
Context: dotnet/android#7616
Commit b4d44e4 noted:
dotnet/android#7616 provided additional context:
JNIEnv::FindClass()behavior is tied to the thread that calls it, and one of the knock-on effects is thatJava.Lang.JavaSystem.LoadLibrary("MyLib")doesn't work properly when invoked from a newSystem.Threading.Threadthread.Which brings us to xamarin/mondroid@ed984a3a, which updated then Xamarin.Android to use
java.lang.Class.forName(String)instead ofClassLoader.loadClass(String)because the "real" JDK cannot useClassLoader.loadClass(String)to load array types:Using
ClassLoader.loadClass(String)to load array types works on Android, presumably as an undocumented implementation detail, but as xamarin/monodroid@ed984a3a was trying to get things working within the (now dead) Android Designer -- which ran using a Desktop JDK -- Android-specific extensions were not available.Update
JniEnvironment.Typesto useClass.forName(String)instead ofClassLoader.loadClass(String)to load Java classes, as a fallback for whenJNIEnv::FindClass()fails to find the class.