Fix #7536: Android build fails to start (#8554)

This was a regression from #7435 which introduced threads and caused JNI
to misbehave and fail to load our expected classes. Provide a workaround
based on the description in https://stackoverflow.com/a/16302771 which
stores a main thread's class loader and uses that in neighbouring
threads.

distribution/changelog.txt

@@ -21,6 +21,7 @@
 - Fix: [#6191] OpenRCT2 fails to run when the path has an emoji in it.
 - Fix: [#7439] Placement messages have mixed strings
 - Fix: [#7473] Disabling sound effects also disables "Disable audio on focus loss".
+- Fix: [#7536] Android builds fail to start.
 - Fix: [#7689] Deleting 0-tile maze gives a MONEY32_UNDEFINED (negative) refund.
 - Fix: [#7828] Copied entrances and exits stay when demolishing ride.
 - Fix: [#7945] Client IP address is logged as `(null)` in server logs.

src/openrct2-ui/UiContext.Android.cpp

@@ -13,8 +13,10 @@
 
 #    include <SDL.h>
 #    include <dlfcn.h>
+#    include <jni.h>
 #    include <openrct2/common.h>
 #    include <openrct2/core/String.hpp>
+#    include <openrct2/platform/platform.h>
 #    include <openrct2/ui/UiContext.h>
 #    include <sstream>
 #    include <stdexcept>

src/openrct2/core/ZipAndroid.cpp

@@ -9,6 +9,7 @@
 
 #ifdef __ANDROID__
 
+#    include "../platform/platform.h"
 #    include "IStream.hpp"
 #    include "Zip.h"
 
@@ -26,7 +27,7 @@ public:
         // retrieve the JNI environment.
         JNIEnv* env = (JNIEnv*)SDL_AndroidGetJNIEnv();
 
-        jclass jniClass = env->FindClass("website/openrct2/ZipArchive");
+        jclass jniClass = platform_android_find_class(env, "website/openrct2/ZipArchive");
         jmethodID constructor = env->GetMethodID(jniClass, "<init>", "(Ljava/lang/String;)V");
 
         jstring jniPath = env->NewStringUTF(path.data());

src/openrct2/platform/Android.cpp

@@ -62,4 +62,60 @@ bool platform_get_steam_path(utf8* outPath, size_t outSize)
     return false;
 }
 
+AndroidClassLoader::AndroidClassLoader()
+{
+    log_info("Obtaining JNI class loader");
+
+    // This is a workaround to be able to call JNI's ClassLoader from non-main
+    // thread, based on https://stackoverflow.com/a/16302771
+
+    // Apparently it's OK to use it from across different thread, but JNI
+    // only looks for ClassLoader in the _current_ thread and fails to find
+    // it when searched for from a native library's non-main thread.
+
+    // The solution below works by obtaining a ClassLoader reference in main
+    // thread and caching it for future use from any thread, instead of using
+    // it via env->FindClass(). ClassLoader itself is abstract, so we cannot
+    // create it directly; instead we take an arbitrary class and call
+    // getClassLoader() on it to create a reference that way.
+
+    // If we're here, SDL's JNI_OnLoad has already been called and set env
+    JNIEnv* env = (JNIEnv*)SDL_AndroidGetJNIEnv();
+
+    // Take an arbitrary class. While the class does not really matter, it
+    // makes sense to use one that's most likely already loaded and is unlikely
+    // to be removed from code.
+    auto randomClass = env->FindClass("website/openrct2/MainActivity");
+    jclass classClass = env->GetObjectClass(randomClass);
+
+    // Get its class loader
+    auto classLoaderClass = env->FindClass("java/lang/ClassLoader");
+    auto getClassLoaderMethod = env->GetMethodID(classClass, "getClassLoader", "()Ljava/lang/ClassLoader;");
+
+    // Store the class loader and its findClass method for future use
+    _classLoader = env->NewGlobalRef(env->CallObjectMethod(randomClass, getClassLoaderMethod));
+    _findClassMethod = env->GetMethodID(classLoaderClass, "findClass", "(Ljava/lang/String;)Ljava/lang/Class;");
+}
+
+AndroidClassLoader::~AndroidClassLoader()
+{
+    JNIEnv* env = (JNIEnv*)SDL_AndroidGetJNIEnv();
+    env->DeleteGlobalRef(_classLoader);
+}
+
+jobject AndroidClassLoader::_classLoader;
+jmethodID AndroidClassLoader::_findClassMethod;
+
+static std::shared_ptr<AndroidClassLoader> acl;
+
+void platform_android_init_class_loader()
+{
+    acl = std::make_shared<AndroidClassLoader>();
+}
+
+jclass platform_android_find_class(JNIEnv* env, const char* name)
+{
+    return static_cast<jclass>(
+        env->CallObjectMethod(AndroidClassLoader::_classLoader, AndroidClassLoader::_findClassMethod, env->NewStringUTF(name)));
+}
 #endif

src/openrct2/platform/Shared.cpp

@@ -212,6 +212,10 @@ void core_init()
     {
         initialised = true;
 
+#ifdef __ANDROID__
+        platform_android_init_class_loader();
+#endif // __ANDROID__
+
         platform_ticks_init();
         bitcount_init();
         mask_init();

src/openrct2/platform/platform.h

@@ -15,6 +15,10 @@
 #include <string>
 #include <time.h>
 
+#ifdef __ANDROID__
+#    include <jni.h>
+#endif // __ANDROID__
+
 struct TTFFontDescriptor;
 struct rct2_install_info;
 
@@ -160,4 +164,18 @@ void macos_disallow_automatic_window_tabbing();
 utf8* macos_str_decomp_to_precomp(utf8* input);
 #endif
 
+#ifdef __ANDROID__
+class AndroidClassLoader
+{
+public:
+    AndroidClassLoader();
+    ~AndroidClassLoader();
+    static jobject _classLoader;
+    static jmethodID _findClassMethod;
+};
+
+void platform_android_init_class_loader();
+jclass platform_android_find_class(JNIEnv* env, const char* name);
+#endif // __ANDROID__
+
 #endif