fix(android): stop crash when USB is first used off the main thread

src/android/fd_bridge.rs

@@ -77,20 +77,21 @@ fn map_exception(env: &mut JNIEnv, fallback: &str) -> Result<(), Error> {
 }
 
 #[cfg(target_os = "android")]
-fn cache(env: &mut JNIEnv) -> Result<&'static FdJniCache, Error> {
-    if let Some(c) = CACHE.get() {
-        return Ok(c);
-    }
-    let class = env
-        .find_class("app/tauri/serialplugin/UsbNative")
-        .map_err(|e| Error::new(e.to_string()))?;
-    let global = env
-        .new_global_ref(class)
-        .map_err(|e| Error::new(e.to_string()))?;
-    let _ = CACHE.set(FdJniCache { class: global });
+fn cache(_env: &mut JNIEnv) -> Result<&'static FdJniCache, Error> {
     CACHE.get().ok_or_else(not_init)
 }
 
+/// Called from `UsbNative.nativeInit` on a Java thread, where the app class loader is in scope.
+#[cfg(target_os = "android")]
+pub fn init_class(env: &mut JNIEnv, class: &JObject) {
+    if CACHE.get().is_some() {
+        return;
+    }
+    if let Ok(global) = env.new_global_ref(class) {
+        let _ = CACHE.set(FdJniCache { class: global });
+    }
+}
+
 #[cfg(target_os = "android")]
 pub fn init_java_vm(vm: JavaVM) {
     let _ = JVM.set(vm);

src/android/jni.rs

@@ -32,9 +32,12 @@ pub extern "system" fn Java_app_tauri_serialplugin_MobileBridge_onDeviceDetached
 #[cfg(target_os = "android")]
 #[no_mangle]
 pub extern "system" fn Java_app_tauri_serialplugin_UsbNative_nativeInit(
-    env: JNIEnv,
-    _class: JClass,
+    mut env: JNIEnv,
+    class: JClass,
 ) {
+    // `class` is UsbNative itself. Cache it here: FindClass on a Rust-attached
+    // thread uses the system class loader and cannot see app classes.
+    crate::android::fd_bridge::init_class(&mut env, &class);
     if let Ok(vm) = env.get_java_vm() {
         crate::android::fd_bridge::init_java_vm(vm);
     }