bind/java: use the class loader cached during JNI_OnLoad call.

This allows the application class loader to be used when
the bind/java package or other part of JNI dynamically
loads java classes from a non-Java thread.

http://developer.android.com/training/articles/perf-jni.html#faq_FindClass

Fixes golang/go#10668.

Change-Id: I44df3a9362617fa6dd26ddf88247e4fdaee7c7e8
Reviewed-on: https://go-review.googlesource.com/9732
Reviewed-by: David Crawshaw <crawshaw@golang.org>

app/Go.java

@@ -34,5 +34,4 @@ public final class Go {
 	private static boolean running = false;
 
 	private static native void run(Context ctx);
-	private static native void waitForRun();
 }

app/android.c

@@ -19,19 +19,6 @@
 #define LOG_INFO(...) __android_log_print(ANDROID_LOG_INFO, "Go", __VA_ARGS__)
 #define LOG_FATAL(...) __android_log_print(ANDROID_LOG_FATAL, "Go", __VA_ARGS__)
 
-jint JNI_OnLoad(JavaVM* vm, void* reserved) {
-	current_vm = vm;
-	current_ctx = NULL;
-	current_native_activity = NULL;
-
-	JNIEnv* env;
-	if ((*vm)->GetEnv(vm, (void**)&env, JNI_VERSION_1_6) != JNI_OK) {
-		return -1;
-	}
-
-	return JNI_VERSION_1_6;
-}
-
 static jclass find_class(JNIEnv *env, const char *class_name) {
 	jclass clazz = (*env)->FindClass(env, class_name);
 	if (clazz == NULL) {
@@ -50,6 +37,44 @@ static jmethodID find_method(JNIEnv *env, jclass clazz, const char *name, const
 	return m;
 }
 
+static jclass app_class_loader;
+static jmethodID app_find_class_fn;
+
+// app_find_class finds the given class using the class loader cached
+// during JNI_OnLoad. This allows the bind/java package and other
+// part of JNI to dynamically load java classes from non-Java threads.
+//
+// http://developer.android.com/training/articles/perf-jni.html#faq_FindClass
+jclass app_find_class(JNIEnv* env, const char* name) {
+    jstring classname = (*env)->NewStringUTF(env, name);
+    return (*env)->CallObjectMethod(env, app_class_loader, app_find_class_fn, classname);
+}
+
+jint JNI_OnLoad(JavaVM* vm, void* reserved) {
+	JNIEnv* env;
+	if ((*vm)->GetEnv(vm, (void**)&env, JNI_VERSION_1_6) != JNI_OK) {
+		return -1;
+	}
+
+	current_vm = vm;
+	current_ctx = NULL;
+	current_native_activity = NULL;
+
+	// cache the class loader for use in JNI threads.
+	jclass a_clazz = (*env)->FindClass(env, "go/Go");
+	jclass clazz_clazz = (*env)->GetObjectClass(env, a_clazz);
+	jclass class_loader_clazz = (*env)->FindClass(env, "java/lang/ClassLoader");
+	jmethodID get_class_loader = (*env)->GetMethodID(env, clazz_clazz, "getClassLoader",
+			"()Ljava/lang/ClassLoader;");
+	jclass loader = (*env)->CallObjectMethod(env, a_clazz, get_class_loader);
+	app_class_loader = (*env)->NewGlobalRef(env, loader);
+
+	app_find_class_fn = (*env)->GetMethodID(env, class_loader_clazz, "findClass",
+		    "(Ljava/lang/String;)Ljava/lang/Class;");
+
+	return JNI_VERSION_1_6;
+}
+
 static void init_from_context() {
 	if (current_ctx == NULL) {
 		return;
@@ -163,6 +188,7 @@ Java_go_Go_run(JNIEnv* env, jclass clazz, jobject ctx) {
 		jclass context_clazz = find_class(env, "android/content/Context");
 		jmethodID getassets = find_method(
 			env, context_clazz, "getAssets", "()Landroid/content/res/AssetManager;");
+
 		// Prevent the java AssetManager from being GC'd
 		jobject asset_manager_ref = (*env)->NewGlobalRef(
 			env, (*env)->CallObjectMethod(env, current_ctx, getassets));

app/android.go

@@ -46,6 +46,8 @@ JavaVM* current_vm;
 // current_ctx is Android's android.context.Context. May be NULL.
 jobject current_ctx;
 
+jclass app_find_class(JNIEnv* env, const char* name);
+
 // current_native_activity is the Android ANativeActivity. May be NULL.
 ANativeActivity* current_native_activity;
 
@@ -192,6 +194,12 @@ func (androidState) JavaVM() unsafe.Pointer {
 	return unsafe.Pointer(C.current_vm)
 }
 
+// ClassFinder returns a C function pointer for finding a given class using
+// the app class loader. (jclass) (*fn)(JNIEnv*, const char*).
+func (androidState) ClassFinder() unsafe.Pointer {
+	return unsafe.Pointer(C.app_find_class)
+}
+
 func (androidState) AndroidContext() unsafe.Pointer {
 	return unsafe.Pointer(C.current_ctx)
 }

bind/java/seq_android.c

@@ -159,15 +159,28 @@ static void unpin_arrays(JNIEnv *env, mem *m) {
 	m->pinned = NULL;
 }
 
+static void describe_exception(JNIEnv* env) {
+	jthrowable exc = (*env)->ExceptionOccurred(env);
+	if (exc) {
+		(*env)->ExceptionDescribe(env);
+		(*env)->ExceptionClear(env);
+	}
+}
+
+// find_class_fn finds a class with the given name using the app class loader.
+// It is implemented in the app package and is initialized during the init_seq call.
+static jclass (*find_class_fn)(JNIEnv*, const char*);
 
 static jfieldID find_field(JNIEnv *env, const char *class_name, const char *field_name, const char *field_type) {
-	jclass clazz = (*env)->FindClass(env, class_name);
+	jclass clazz = find_class_fn(env, class_name);
 	if (clazz == NULL) {
+		describe_exception(env);
 		LOG_FATAL("cannot find %s", class_name);
 		return NULL;
 	}
 	jfieldID id = (*env)->GetFieldID(env, clazz, field_name , field_type);
 	if(id == NULL) {
+		describe_exception(env);
 		LOG_FATAL("no %s/%s field", field_name, field_type);
 		return NULL;
 	}
@@ -175,26 +188,28 @@ static jfieldID find_field(JNIEnv *env, const char *class_name, const char *fiel
 }
 
 static jclass find_class(JNIEnv *env, const char *class_name) {
-	jclass clazz = (*env)->FindClass(env, class_name);
+	jclass clazz = find_class_fn(env, class_name);
 	if (clazz == NULL) {
+		describe_exception(env);
 		LOG_FATAL("cannot find %s", class_name);
 		return NULL;
 	}
 	return (*env)->NewGlobalRef(env, clazz);
 }
 
-void init_seq(void *javavm) {
+void init_seq(void *javavm, void *classfinder) {
 	JavaVM *vm = (JavaVM*)javavm;
+	find_class_fn = (jclass (*)(JNIEnv*, const char*))classfinder;
+
 	JNIEnv *env;
 	int res = (*vm)->GetEnv(vm, (void**)&env, JNI_VERSION_1_6);
 	if (res == JNI_EDETACHED) {
-		if ((*vm)->AttachCurrentThread(vm, &env, NULL) != 0) {
+		if ((*vm)->AttachCurrentThread(vm, &env, NULL) != JNI_OK) {
 			LOG_FATAL("cannot attach to current_vm");
 		}
-	} else if (res != 0) {
+	} else if (res != JNI_OK) {
 		LOG_FATAL("bad vm env: %d", res);
 	}
-
 	memptr_id = find_field(env, "go/Seq", "memptr", "J");
 	receive_refnum_id = find_field(env, "go/Seq$Receive", "refnum", "I");
 	receive_handle_id = find_field(env, "go/Seq$Receive", "handle", "I");

bind/java/seq_android.go

@@ -82,7 +82,11 @@ func initSeq() {
 	vm := app.State.(interface {
 		JavaVM() unsafe.Pointer
 	}).JavaVM()
-	C.init_seq(vm)
+	classFinder := app.State.(interface {
+		ClassFinder() unsafe.Pointer
+	}).ClassFinder()
+
+	C.init_seq(vm, classFinder)
 }
 
 func seqToBuf(bufptr **C.uint8_t, lenptr *C.size_t, buf *seq.Buffer) {

bind/java/seq_android.h

@@ -2,4 +2,4 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-void init_seq(void* vm);
+void init_seq(void* vm, void* classfinder);