Summary: Use javaClassStatic() for class references stored in static … (pytorch#14744)

…variables - creates global references safe for persistence

findClassLocal() returns a local reference. Storing it in static auto
exceptionClass = ... could result in potential 'invalid local
reference:' as local references become invalid when the JNI frame ends

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### Summary
[PLEASE REMOVE] See [CONTRIBUTING.md's Pull
Requests](https://github.com/pytorch/executorch/blob/main/CONTRIBUTING.md#pull-requests)
for ExecuTorch PR guidelines.

[PLEASE REMOVE] If this PR closes an issue, please add a `Fixes
#<issue-id>` line.

[PLEASE REMOVE] If this PR introduces a fix or feature that should be
the upcoming release notes, please add a "Release notes: <area>" label.
For a list of available release notes labels, check out
[CONTRIBUTING.md's Pull
Requests](https://github.com/pytorch/executorch/blob/main/CONTRIBUTING.md#pull-requests).

### Test plan
[PLEASE REMOVE] How did you test this PR? Please write down any manual
commands you used and note down tests that you have written if
applicable.

Co-authored-by: Github Executorch <[email protected]>

Author: psiddh

extension/android/jni/jni_helper.cpp

@@ -13,10 +13,13 @@ namespace executorch::jni_helper {
 void throwExecutorchException(uint32_t errorCode, const std::string& details) {
   // Get the current JNI environment
   auto env = facebook::jni::Environment::current();
+  if (!env) {
+    return;
+  }
 
-  // Find the Java ExecutorchRuntimeException class
-  static auto exceptionClass = facebook::jni::findClassLocal(
-      "org/pytorch/executorch/ExecutorchRuntimeException");
+  // stable/global class ref — safe to cache
+  static const auto exceptionClass =
+      JExecutorchRuntimeException::javaClassStatic();
 
   // Find the static factory method: makeExecutorchException(int, String)
   static auto makeExceptionMethod =

extension/android/jni/jni_helper.h

@@ -23,4 +23,11 @@ namespace executorch::jni_helper {
  */
 void throwExecutorchException(uint32_t errorCode, const std::string& details);
 
+// Define the JavaClass wrapper
+struct JExecutorchRuntimeException
+    : public facebook::jni::JavaClass<JExecutorchRuntimeException> {
+  static constexpr auto kJavaDescriptor =
+      "Lorg/pytorch/executorch/ExecutorchRuntimeException;";
+};
+
 } // namespace executorch::jni_helper