bind/java: initialize seq machinery from Java

This ensures that the java bindings are ready before any calls are
made by user code. As a bonus, the JNIEnv* is from the Seq class so I
believe no tricks are required to find the right class loader.

Fixes golang/go#10903.

Change-Id: I33b3b39cef6cc2da36e271de882ba8d26610ea34
Reviewed-on: https://go-review.googlesource.com/10296
Reviewed-by: Elias Naur <elias.naur@gmail.com>
Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>

bind/java/Seq.java

@@ -66,6 +66,12 @@ public class Seq {
 		return tracker.get(refnum);
 	}
 
+	static {
+		initSeq();
+	}
+
+	static native void initSeq();
+
 	// Informs the Go ref tracker that Java is done with this ref.
 	static native void destroyRef(int refnum);
 

bind/java/doc.go

@@ -12,5 +12,7 @@ package java
 // Init initializes communication with Java.
 // Typically called from the Start callback in app.Run.
 func Init() {
-	initSeq()
+	close(running)
 }
+
+var running = make(chan struct{})

bind/java/noimpl.go

@@ -1,9 +0,0 @@
-// Copyright 2015 The Go Authors. All rights reserved.
-// Use of this source code is governed by a BSD-style
-// license that can be found in the LICENSE file.
-
-// +build !android
-
-package java
-
-func initSeq() {}

bind/java/seq_android.c

@@ -167,12 +167,8 @@ static void describe_exception(JNIEnv* 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 = find_class_fn(env, class_name);
+	jclass clazz = (*env)->FindClass(env, class_name);
 	if (clazz == NULL) {
 		describe_exception(env);
 		LOG_FATAL("cannot find %s", class_name);
@@ -188,7 +184,7 @@ 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 = find_class_fn(env, class_name);
+	jclass clazz = (*env)->FindClass(env, class_name);
 	if (clazz == NULL) {
 		describe_exception(env);
 		LOG_FATAL("cannot find %s", class_name);
@@ -197,19 +193,8 @@ static jclass find_class(JNIEnv *env, const char *class_name) {
 	return (*env)->NewGlobalRef(env, clazz);
 }
 
-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) != JNI_OK) {
-			LOG_FATAL("cannot attach to current_vm");
-		}
-	} else if (res != JNI_OK) {
-		LOG_FATAL("bad vm env: %d", res);
-	}
+JNIEXPORT void JNICALL
+Java_go_Seq_initSeq(JNIEnv *env, jclass clazz) {
 	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");
@@ -219,14 +204,10 @@ void init_seq(void *javavm, void *classfinder) {
 	// its class info, because finding the jbyteArray class ("[B") using
 	// find_class_fn or JNIEnv's FindClass does not work on android-L.
 	jbyteArray a = (*env)->NewByteArray(env, 0);
-	jclass clazz = (*env)->GetObjectClass(env, a);
-	jbytearray_clazz = (*env)->NewGlobalRef(env, clazz);
+	jclass bclazz = (*env)->GetObjectClass(env, a);
+	jbytearray_clazz = (*env)->NewGlobalRef(env, bclazz);
 
 	LOG_INFO("loaded go/Seq");
-
-	if (res == JNI_EDETACHED) {
-		(*vm)->DetachCurrentThread(vm);
-	}
 }
 
 JNIEXPORT void JNICALL

bind/java/seq_android.go

@@ -15,7 +15,6 @@ import (
 	"sync"
 	"unsafe"
 
-	"golang.org/x/mobile/app"
 	"golang.org/x/mobile/bind/seq"
 )
 
@@ -26,6 +25,7 @@ const debug = false
 // Send is called by Java to send a request to run a Go function.
 //export Send
 func Send(descriptor string, code int, req *C.uint8_t, reqlen C.size_t, res **C.uint8_t, reslen *C.size_t) {
+	<-running
 	fn := seq.Registry[descriptor][code]
 	if fn == nil {
 		panic(fmt.Sprintf("invalid descriptor(%s) and code(0x%x)", descriptor, code))
@@ -78,13 +78,6 @@ func init() {
 	res.out = make(map[int32]*seq.Buffer)
 }
 
-func initSeq() {
-	vm := app.GetConfig().JavaVM()
-	classFinder := app.GetConfig().ClassFinder()
-
-	C.init_seq(vm, classFinder)
-}
-
 func seqToBuf(bufptr **C.uint8_t, lenptr *C.size_t, buf *seq.Buffer) {
 	if debug {
 		fmt.Printf("seqToBuf tag 1, len(buf.Data)=%d, *lenptr=%d\n", len(buf.Data), *lenptr)