Add td::jni::get_jni_env wrapper.

GitOrigin-RevId: 5cc7cc927d0195bcdc7f8adbeea052266dff6af5

README.md

@@ -23,10 +23,10 @@ TDLib (Telegram Database library) is a cross-platform library for building [Tele
 * **Easy to use**: `TDLib` takes care of all network implementation details, encryption and local data storage.
 * **High-performance**: in the [Telegram Bot API](https://core.telegram.org/bots/api), each `TDLib` instance handles more than 18000 active bots simultaneously.
 * **Well-documented**: all `TDLib` API methods and public interfaces are fully documented.
-* **Consistent**: `TDLib` guarantees that all updates will be delivered in the right order.
+* **Consistent**: `TDLib` guarantees that all updates are delivered in the right order.
 * **Reliable**: `TDLib` remains stable on slow and unstable Internet connections.
 * **Secure**: all local data is encrypted using a user-provided encryption key.
-* **Fully-asynchronous**: requests to `TDLib` don't block each other or anything else, responses will be sent when they are available.
+* **Fully-asynchronous**: requests to `TDLib` don't block each other or anything else, responses are sent when they are available.
 
 <a name="usage"></a>
 ## Examples and documentation

td/tl/tl_jni_object.cpp

@@ -76,6 +76,22 @@ void register_native_method(JNIEnv *env, jclass clazz, std::string name, std::st
   }
 }
 
+std::unique_ptr<JNIEnv, JvmThreadDetacher> get_jni_env(JavaVM *java_vm, jint jni_version) {
+  JNIEnv *env = nullptr;
+  if (java_vm->GetEnv(reinterpret_cast<void **>(&env), jni_version) == JNI_EDETACHED) {
+#ifdef JDK1_2 // if not Android JNI
+    auto p_env = reinterpret_cast<void **>(&env);
+#else
+    auto p_env = &env;
+#endif
+    java_vm->AttachCurrentThread(p_env, nullptr);
+  } else {
+    java_vm = nullptr;
+  }
+
+  return std::unique_ptr<JNIEnv, JvmThreadDetacher>(env, JvmThreadDetacher(java_vm));
+}
+
 void init_vars(JNIEnv *env, const char *td_api_java_package) {
   BooleanClass = get_jclass(env, "java/lang/Boolean");
   IntegerClass = get_jclass(env, "java/lang/Integer");

td/tl/tl_jni_object.h

@@ -39,6 +39,37 @@ jfieldID get_field_id(JNIEnv *env, jclass clazz, const char *name, const char *s
 
 void register_native_method(JNIEnv *env, jclass clazz, std::string name, std::string signature, void *function_ptr);
 
+class JvmThreadDetacher {
+  JavaVM *java_vm_;
+
+  void detach() {
+    if (java_vm_ != nullptr) {
+      java_vm_->DetachCurrentThread();
+      java_vm_ = nullptr;
+    }
+  }
+
+ public:
+  explicit JvmThreadDetacher(JavaVM *java_vm): java_vm_(java_vm) {
+  }
+
+  JvmThreadDetacher(const JvmThreadDetacher &other) = delete;
+  JvmThreadDetacher &operator=(const JvmThreadDetacher &other) = delete;
+  JvmThreadDetacher(JvmThreadDetacher &&other): java_vm_(other.java_vm_) {
+    other.java_vm_ = nullptr;
+  }
+  JvmThreadDetacher &operator=(JvmThreadDetacher &&other) = delete;
+  ~JvmThreadDetacher() {
+    detach();
+  }
+
+  void operator()(JNIEnv *env) {
+    detach();
+  }
+};
+
+std::unique_ptr<JNIEnv, JvmThreadDetacher> get_jni_env(JavaVM *java_vm, jint jni_version);
+
 std::string fetch_string(JNIEnv *env, jobject o, jfieldID id);
 
 inline jobject fetch_object(JNIEnv *env, const jobject &o, const jfieldID &id) {