fix(droidplug): Resolve Adapter through LoaderContext in init

README.md

@@ -122,6 +122,8 @@ by the consuming application.
 
 There is now a build script at `./scripts/build-java.sh` for building the java portion of the library on linux or macOS. This can also be used as a guide for manual building if need be.
 
+Call `btleplug::platform::init` from a thread whose context class loader sees the application's classes. Threads that entered native code from Java already do. A thread attached from native code must first set its context class loader to the application's, such as `Context.getClassLoader()`.
+
 If your app uses Proguard/R8 with `minifyEnabled true`, you must add keep rules for btleplug's
 Java classes. All of btleplug's Java code (including vendored jni-utils classes) is only accessed
 via JNI from native code, so R8 will treat it as dead code and strip it without these rules.

src/droidplug/jni/mod.rs

@@ -28,8 +28,12 @@ fn init_inner(env: &mut Env) -> crate::Result<()> {
     // Seed the JavaVM singleton so JavaVM::singleton() works from any thread.
     env.get_java_vm()?;
     {
-        let adapter_class =
-            env.find_class(jni_str!("com/nonpolynomial/btleplug/android/impl/Adapter"))?;
+        let loader = jni::objects::LoaderContext::default();
+        let adapter_class = loader.load_class(
+            env,
+            jni_str!("com.nonpolynomial.btleplug.android.impl.Adapter"),
+            false,
+        )?;
         unsafe {
             env.register_native_methods(
                 &adapter_class,
@@ -57,7 +61,6 @@ fn init_inner(env: &mut Env) -> crate::Result<()> {
         };
         use objects::*;
 
-        let loader = jni::objects::LoaderContext::default();
         <JPeripheral as Reference>::lookup_class(env, &loader)?;
         <JScanFilterClass as Reference>::lookup_class(env, &loader)?;
         <JNotConnectedException as Reference>::lookup_class(env, &loader)?;

tests/android/rust/src/lib.rs

@@ -40,7 +40,7 @@ pub fn find_descriptor(
     );
 }
 
-use jni::objects::JClass;
+use jni::objects::{JClass, JObject, JThread};
 use jni::{Env, EnvUnowned, jni_str};
 use jni::errors::ThrowRuntimeExAndDefault;
 use std::sync::OnceLock;
@@ -85,19 +85,31 @@ fn run_test(env: &mut Env, test_name: &str, f: impl std::future::Future<Output =
     }
 }
 
-/// Initialize btleplug's Android/JNI layer. Must be called once before any tests.
+/// Runs `platform::init` on a natively attached thread with the app's context class loader.
 #[unsafe(no_mangle)]
 pub extern "system" fn Java_com_nonpolynomial_btleplug_test_NativeTests_initBtleplug(
     mut env: EnvUnowned,
-    _class: JClass,
+    this: JObject,
 ) {
     android_logger::init_once(
         android_logger::Config::default()
             .with_max_level(log::LevelFilter::Debug)
             .with_tag("btleplug-test"),
     );
-    env.with_env(|env| btleplug::platform::init(env))
-        .resolve::<ThrowRuntimeExAndDefault>();
+    env.with_env(|env| -> btleplug::Result<()> {
+        let loader = env.get_object_class(&this)?.get_class_loader(env)?;
+        let loader = env.new_global_ref(loader)?;
+        let vm = env.get_java_vm()?;
+        std::thread::spawn(move || {
+            vm.attach_current_thread(|env| -> btleplug::Result<()> {
+                JThread::current_thread(env)?.set_context_class_loader(env, &loader)?;
+                btleplug::platform::init(env)
+            })
+        })
+        .join()
+        .expect("btleplug init thread panicked")
+    })
+    .resolve::<ThrowRuntimeExAndDefault>();
 }
 
 // ── Test JNI exports ────────────────────────────────────────────────