78 lines
3.1 KiB
C++
78 lines
3.1 KiB
C++
/*
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* Copyright (c) 2015-present, Facebook, Inc.
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* All rights reserved.
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*
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* This source code is licensed under the BSD-style license found in the
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* LICENSE file in the root directory of this source tree. An additional grant
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* of patent rights can be found in the PATENTS file in the same directory.
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*/
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#pragma once
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#include <functional>
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#include <string>
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#include <jni.h>
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#include <fb/visibility.h>
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namespace facebook {
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namespace jni {
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// Keeps a thread-local reference to the current thread's JNIEnv.
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struct Environment {
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// May be null if this thread isn't attached to the JVM
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FBEXPORT static JNIEnv* current();
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static void initialize(JavaVM* vm);
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// There are subtle issues with calling the next functions directly. It is
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// much better to always use a ThreadScope to manage attaching/detaching for
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// you.
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FBEXPORT static JNIEnv* ensureCurrentThreadIsAttached();
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FBEXPORT static void detachCurrentThread();
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};
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/**
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* RAII Object that attaches a thread to the JVM. Failing to detach from a thread before it
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* exits will cause a crash, as will calling Detach an extra time, and this guard class helps
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* keep that straight. In addition, it remembers whether it performed the attach or not, so it
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* is safe to nest it with itself or with non-fbjni code that manages the attachment correctly.
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*
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* Potential concerns:
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* - Attaching to the JVM is fast (~100us on MotoG), but ideally you would attach while the
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* app is not busy.
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* - Having a thread detach at arbitrary points is not safe in Dalvik; you need to be sure that
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* there is no Java code on the current stack or you run the risk of a crash like:
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* ERROR: detaching thread with interp frames (count=18)
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* (More detail at https://groups.google.com/forum/#!topic/android-ndk/2H8z5grNqjo)
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* ThreadScope won't do a detach if the thread was already attached before the guard is
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* instantiated, but there's probably some usage that could trip this up.
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* - Newly attached C++ threads only get the bootstrap class loader -- i.e. java language
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* classes, not any of our application's classes. This will be different behavior than threads
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* that were initiated on the Java side. A workaround is to pass a global reference for a
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* class or instance to the new thread; this bypasses the need for the class loader.
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* (See http://docs.oracle.com/javase/7/docs/technotes/guides/jni/spec/invocation.html#attach_current_thread)
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* If you need access to the application's classes, you can use ThreadScope::WithClassLoader.
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*/
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class FBEXPORT ThreadScope {
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public:
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ThreadScope();
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ThreadScope(ThreadScope&) = delete;
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ThreadScope(ThreadScope&&) = default;
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ThreadScope& operator=(ThreadScope&) = delete;
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ThreadScope& operator=(ThreadScope&&) = delete;
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~ThreadScope();
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/**
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* This runs the closure in a scope with fbjni's classloader. This should be
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* the same classloader as the rest of the application and thus anything
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* running in the closure will have access to the same classes as in a normal
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* java-create thread.
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*/
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static void WithClassLoader(std::function<void()>&& runnable);
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static void OnLoad();
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private:
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bool attachedWithThisScope_;
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};
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}
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}
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