英文:
JNI 8 C++ : Thread attach and detach And async callback
问题
如何从std::thread异步调用Java方法？
假设这是一个即时通讯机器人SDK，因为它的逻辑基本上是一个即时通讯机器人SDK。
最重要的是：如何异步调用Java方法并回调本地方法。
底部有逻辑流程，可能有帮助。
例如：
接收消息A "backup"，然后使用MsgA调用Java插件，插件处理此事件需要10秒，并调用5次本地方法以满足其需求。
同时，接收消息B "echo"，仅需要10毫秒处理，并通过调用本地方法发送消息。
因此，MsgB在MsgA之后接收，但在MsgA之前完成。
如果只使用纯C、C++、Java或其他任何语言，实现起来将非常简单。但我在这里遇到了一个头痛的问题：JNI线程附加。
※ 第一个问题：奇怪的JNI附加
我阅读了文档找到了答案，但其中没有一个适用于我的情况。
我正在使用Zulu JDK8（zulu8.48.0.53-ca-fx-jdk8.0.265-win_x64）和MinGW64 C++，演示代码如下：
public class ThreadTest {private static int count = 0;private static final Random random = new Random();public static int count() {try {Thread.sleep(random.nextInt(2000));} catch (InterruptedException e) {e.printStackTrace();}return count++;}}
以下是C++中的worker函数：
void worker(JNIEnv* localEnv) {jclass clazz = localEnv->FindClass("ThreadTest");jmethodID method = localEnv->GetStaticMethodID(clazz, "count", "()I");jchar result = localEnv->CallStaticCharMethod(clazz, method);int tid = std::hash<std::thread::id>{}(std::this_thread::get_id());printf("[Worker Done] %d => %d\n", tid, result);}
不附加情况下，我们会得到以下结果，这是预期的：
worker(env);// 这里是主线程的第一次调用，正常工作；// [Worker Done] -1444639049 => 0jvm->DetachCurrentThread();std::thread t1(worker, env);t1.join();// 进程崩溃，因为未附加JNI// Process finished with exit code -1073741819 (0xC0000005)
然后为t1添加tWorker函数：
void tWorker(JavaVM* gJvm) {int tid = std::hash<std::thread::id>{}(std::this_thread::get_id());printf("[Thread Run] %d\n", tid);JavaVMAttachArgs* args;args->version = JNI_VERSION_1_8;args->name = nullptr;args->group = nullptr;JNIEnv* lEnv;printf("[Attach for] %d\n", tid);int attachResult = gJvm->AttachCurrentThread(reinterpret_cast<void**>(&lEnv), &args);printf("[Attach Done] %d => %d\n", tid, attachResult);delete args;worker(lEnv);gJvm->DetachCurrentThread();}
我得到了这个结果：
[Worker Done] -1444639049 => 0[Thread Run] 1709724944Process finished with exit code -1073741819 (0xC0000005)
一些答案说你应该使用GetEnv：
void tWorker02(JavaVM* gJvm, JNIEnv* gEnv) {int tid = std::hash<std::thread::id>{}(std::this_thread::get_id());printf("[Thread Run] %d\n", tid);JavaVMAttachArgs* args;args->version = JNI_VERSION_1_8;args->name = nullptr;args->group = nullptr;JNIEnv* lEnv;printf("[GetEnv for] %d\n", tid);int getEnvResult = gJvm->GetEnv(reinterpret_cast<void**>(&lEnv), JNI_VERSION_1_8);printf("[GetEnv Done] %d => %d\n", tid, getEnvResult);printf("[Attach for] %d\n", tid);int attachResult = gJvm->AttachCurrentThread(reinterpret_cast<void**>(&lEnv), &args);printf("[Attach Done] %d => %d\n", tid, attachResult);delete args;worker(gEnv);gJvm->DetachCurrentThread();}
得到相同的结果：
[Worker Done] -1444639049 => 0[Thread Run] 1709724944Process finished with exit code -1073741819 (0xC0000005)
在更多的帖子中，有人建议将局部变量替换为全局变量（这在逻辑和文档上都没有意义，但在他们的问题中问题得到了解决）：
//JNIEnv* lEnv;printf("[GetEnv for] %d\n", tid);int getEnvResult = gJvm->GetEnv(reinterpret_cast<void**>(&gEnv), JNI_VERSION_1_8);printf("[GetEnv Done] %d => %d\n", tid, getEnvResult);printf("[Attach for] %d\n", tid);int attachResult = gJvm->AttachCurrentThread(reinterpret_cast<void**>(&gEnv), &args);printf("[Attach Done] %d => %d\n", tid, attachResult);
这是无效的，即使我尝试了所有16种组合，也不适用于我的情况。
[Worker Done] -1444639049 => 0[Thread Run] 1709724944Process finished with exit code -1073741819 (0xC0000005)
问题一：这是怎么回事？
※ 第二个问题：如何实现这一点：
（以下内容中未提供翻译，请参考原文进行理解）
Update 1:
问题1已解决。
void tWorker02(JavaVM* gJvm, JNIEnv* gEnv) {int tid = std::hash<std::thread::id>{}(std::this_thread::get_id());printf("[Thread Run<details><summary>英文:</summary>How to async call Java method from std::thread ?Let's assuming this is a IM bot sdk, Because it's logic basicly a IM bot sdk.The most importtant is: How to async call java method and callback native.There is logic flow at the bottom, Maybe helpful.For example:Receive message A "backup", Then call the java plugin with MsgA, The plugin porcess this event need 10second, And call 5 time native method for what ever it need.Mean while, Receive message B "echo", That only take 10ms to process, And send an message by invoke native method.So, MsgB recived after MsgA, But finish befor MsgA.If using pure C C++ java or what ever, That will be so easy to achive. But here I found a headache problem: JNI thread Attach.※ First question: Wired JNI attachI have read doc find answer, None of them working and my condition different with everyoneI'm using Zulu JDK8 (zulu8.48.0.53-ca-fx-jdk8.0.265-win_x64) and MinGW64 C++, For demo:```javapublic class ThreadTest {private static int count = 0;private static final Random random = new Random();public static int count() {try {Thread.sleep(random.nextInt(2000));} catch (InterruptedException e) {e.printStackTrace();}return count++;}}
Here is the worker function in C++
void worker(JNIEnv* localEnv) {jclass clazz = localEnv->FindClass("ThreadTest");jmethodID method = localEnv->GetStaticMethodID(clazz, "count", "()I");jchar result = localEnv->CallStaticCharMethod(clazz, method);int tid = std::hash<std::thread::id>{}(std::this_thread::get_id());printf("[Worker Done] %d =>> %d\n", tid, result);}
And without attach we will get, That is expected:
worker(env);// Here the first call from main thread, Working find;// [Worker Done] -1444639049 =>> 0jvm->DetachCurrentThread();std::thread t1(worker, env);t1.join();// Process crashed because not attach jni// Process finished with exit code -1073741819 (0xC0000005)
And add the tWorker function for t1:
void tWorker (JavaVM* gJvm) {int tid = std::hash<std::thread::id>{}(std::this_thread::get_id());printf("[Thread Run] %d\n", tid);JavaVMAttachArgs* args;args->version = JNI_VERSION_1_8;args->name = nullptr;args->group = nullptr;JNIEnv* lEnv;printf("[Attach for] %d\n", tid);int attachResult = gJvm->AttachCurrentThread(reinterpret_cast<void**>(lEnv), &args);printf("[Attach Done] %d =>> %d\n", tid, attachResult);delete args;worker(lEnv);gJvm->DetachCurrentThread();}
I got this:
[Worker Done] -1444639049 =>> 0[Thread Run] 1709724944Process finished with exit code -1073741819 (0xC0000005)
Some answer say you should use GetEnv:
void tWorker02(JavaVM* gJvm, JNIEnv* gEnv) {int tid = std::hash<std::thread::id>{}(std::this_thread::get_id());printf("[Thread Run] %d\n", tid);JavaVMAttachArgs* args;args->version = JNI_VERSION_1_8;args->name = nullptr;args->group = nullptr;JNIEnv* lEnv;printf("[GetEnv for] %d\n", tid);int getEnvResult = gJvm->GetEnv(reinterpret_cast<void**>(lEnv), JNI_VERSION_1_8);printf("[GetEnv Done] %d =>> %d\n", tid, getEnvResult);printf("[Attach for] %d\n", tid);int attachResult = gJvm->AttachCurrentThread(reinterpret_cast<void**>(lEnv), &args);printf("[Attach Done] %d =>> %d\n", tid, attachResult);delete args;worker(gEnv);gJvm->DetachCurrentThread();}
Got same result:
[Worker Done] -1444639049 =>> 0[Thread Run] 1709724944Process finished with exit code -1073741819 (0xC0000005)
For more post I found, Replace Local to Global (That dosen't make any sense for logic and Document but in their question problem solved)
//JNIEnv* lEnv;printf("[GetEnv for] %d\n", tid);int getEnvResult = gJvm->GetEnv(reinterpret_cast<void**>(gEnv), JNI_VERSION_1_8);printf("[GetEnv Done] %d =>> %d\n", tid, getEnvResult);printf("[Attach for] %d\n", tid);int attachResult = gJvm->AttachCurrentThread(reinterpret_cast<void**>(gEnv), &args);printf("[Attach Done] %d =>> %d\n", tid, attachResult);
That is usless, Even I try all 16 combination, That not work for me.
[Worker Done] -1444639049 =>> 0[Thread Run] 1709724944Process finished with exit code -1073741819 (0xC0000005)
Question one: What happen in there?
※ Second Question: How to achive that:
Update 1:
Question 1 solved.
void tWorker02(JavaVM* gJvm, JNIEnv* gEnv) {int tid = std::hash<std::thread::id>{}(std::this_thread::get_id());printf("[Thread Run] %d\n", tid);auto* args = new JavaVMAttachArgs{};args->version = JNI_VERSION_1_8;args->name = nullptr;args->group = nullptr;JNIEnv* lEnv;printf("[GetEnv for] %d\n", tid);int getEnvResult = gJvm->GetEnv(reinterpret_cast<void**>(&args, JNI_VERSION_1_8);printf("[GetEnv Done] %d =>> %d\n", tid, getEnvResult);if (getEnvResult == JNI_EDETACHED) {printf("[Attach for] %d\n", tid);int attachResult = gJvm->AttachCurrentThread(reinterpret_cast<void**>(&lEnv), &args);printf("[Attach Done] %d =>> %d\n", tid, attachResult);}delete args;worker(gEnv);gJvm->DetachCurrentThread();}
Without cast will cause a complie error error: invalid conversion from 'JNIEnv**' {aka 'JNIEnv_**'} to 'void**' [-fpermissive]
答案1
得分: 2
看起来你的问题不在于JVM的使用，而在于C++代码。看一下这段代码：
void tWorker02(JavaVM* gJvm, JNIEnv* gEnv) {int tid = std::hash<std::thread::id>{}(std::this_thread::get_id());printf("[Thread Run] %d\n", tid);JavaVMAttachArgs* args;args->version = JNI_VERSION_1_8;args->name = nullptr;args->group = nullptr;
注意这里：
JavaVMAttachArgs* args;args->version = JNI_VERSION_1_8;
你的args是一个指针，并且没有被初始化。这会引发未定义的行为，很可能会导致崩溃。
而且你尝试删除了未初始化的指针：
delete args;
此外，我不理解这段代码的用意：
JNIEnv* lEnv;...int getEnvResult = gJvm->GetEnv(reinterpret_cast<void**>(lEnv), ...
在这里使用reinterpret_cast的意义是什么？根据函数的定义，需要传递一个指向指针的指针，而不是一个转换：
JNIEnv* lEnv;...int getEnvResult = gJvm->GetEnv(&lEnv, ...
好吧，你可以进行类型转换，但是在这里你应该传递一个指向指针的指针static_cast<void**>(&lEnv)，但这可能并不是必需的。
英文:
Looks like the your problems are not in usage of JVM but in C++ code. Looking at this piece of code:
void tWorker02(JavaVM* gJvm, JNIEnv* gEnv) {int tid = std::hash<std::thread::id>{}(std::this_thread::get_id());printf("[Thread Run] %d\n", tid);JavaVMAttachArgs* args;args->version = JNI_VERSION_1_8;args->name = nullptr;args->group = nullptr;
Pay attention here:
JavaVMAttachArgs* args;args->version = JNI_VERSION_1_8;
Your args is a pointer and is not initialized. It invokes undefined behavior, is most likely to crash.
Also you are trying to delete it uninitialized:
delete args;
Also I don't understand this piece of code:
JNIEnv* lEnv;...int getEnvResult = gJvm->GetEnv(reinterpret_cast<void**>(lEnv), ...
What is the sense of reinterpret_cast here? By definition of the function there is required a pointer to pointer, not a cast:
JNIEnv* lEnv;...int getEnvResult = gJvm->GetEnv(&lEnv, ...
Ok, you can cast it, but you should pass pointer to pointer here, so cast a pointer to pointer static_cast<void**>(&lEnv), but it is probably not required.
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