Android NDK：从C ++调用Java函数 [英] Android NDK: Calling Java functions from C++

问题描述

我对JNI非常新，我想弄清楚某些东西在我将C ++ iOS代码移植到它之前如何工作。我成功地得到一个NDK样本在Android studio中工作，我可以看到Java是如何能够调用C ++函数。

我一直在搜索并获取大块代码，但是我无法在我的具体实现中使用它。

只是为了测试如何工作我在java中设置了一个简单的文本日志功能，我试图从我的本机代码调用它，但我遇到了问题。

这是我的Java函数：

  public static void log（String s）{
 Log.d（Native，s）; 
} 
  

和C ++：

  void Log（std :: string s）{
 
 JNIEnv * env; 
 g_JavaVM-> GetEnv（（void **）& env，JNI_VERSION_1_6）; 
 
 jstring jstr1 = env-> NewStringUTF（s.c_str（））; 
 
 jclass clazz = env-> FindClass（com / android / gl2jni / GL2JNILib）; 
 jmethodID mid = env-> GetStaticMethodID（clazz，log，（Ljava / lang / String;）V）; 
 
 jobject obj = env-> CallStaticObjectMethod（clazz，mid，jstr1）; 
} 
  

从我看到的不同的例子，这应该工作，错误：

  29835-29849 / com.android.gl2jni A / libc：致命信号11（SIGSEGV）0x00000000 = 1），thread 29849（Thread-17371）
  

我错过了什么？

编辑：

我将它改为GetStaticMethodID。但在记录函数的进度后，我发现失败的行是：

  g_JavaVM-> GetEnv （void **）& env，JNI_VERSION_1_6）; 
  

这是因为g_JavaVM设置为 static JavaVM * g_JavaVM = NULL ; ，然后再次触摸。我想我需要设置这个变量，但如何？

解决方案

我的问题的一部分是，我没有初始化JavaVM。另一部分是我正在使用C ++，但我正在尝试使用C函数。

工作代码是：

Java：

  public static void log（String s）{
 Log.d ，s）; 
} 
  

C ++：

  void Log（std :: string s）{
 
 JNIEnv * env; 
 g_JavaVM-> GetEnv（（void **）& env，JNI_VERSION_1_6）; 
 
 jstring jstr1 = env-> NewStringUTF（s.c_str（））; 
 
 jclass clazz = env-> FindClass（com / android / gl2jni / GL2JNILib）; 
 jmethodID mid = env-> GetStaticMethodID（clazz，log，（Ljava / lang / String;）V）; 
 
 jobject obj = env-> CallStaticObjectMethod（clazz，mid，jstr1）; 
} 
 
 //在一些初始化函数中使用环境变量
 
 env-> GetJavaVM（& g_JavaVM）;希望这可以帮助有同样问题的其他人。
 
 
I am very new to JNI and I am trying to figure out how certain things work before I port my C++ iOS code to it. I was successful in getting one of the NDK samples working in Android studio and I can see how Java is able to call C++ functions. 

I've been searching around and taking chunks of code, but I haven't been able to get it to work in my specific implementation.

Just to test how things worked I set up a simple text log function in java, and I am trying to call it from my native code but I've run into issues.

Here is my Java function:
public static void log(String s){
        Log.d("Native", s);
}
And C++:
void Log(std::string s){

    JNIEnv *env;
    g_JavaVM->GetEnv((void**)&env, JNI_VERSION_1_6);

    jstring jstr1 = env->NewStringUTF(s.c_str());

    jclass clazz = env->FindClass("com/android/gl2jni/GL2JNILib");
    jmethodID mid = env->GetStaticMethodID(clazz, "log", "(Ljava/lang/String;)V");

    jobject obj = env->CallStaticObjectMethod(clazz, mid, jstr1);
}
From what I've seen with different examples this should work, but it throws an error:
29835-29849/com.android.gl2jni A/libc﹕ Fatal signal 11 (SIGSEGV) at 0x00000000 (code=1), thread 29849 (Thread-17371)
Am I missing something?

EDIT:

I've changed it to GetStaticMethodID. But after logging the progress of the function I've found out that the line that fails is:
g_JavaVM->GetEnv((void**)&env, JNI_VERSION_1_6);
Which I figure is because g_JavaVM is set as static JavaVM* g_JavaVM = NULL; and then never touched again. I'm guessing I need to set this variable, but how?
解决方案
Part of my problem was that I didn't initialize The JavaVM. The other part was that I was using C++, but I was trying to use the C functions.

The working code is:

Java:
public static void log(String s){
        Log.d("Native", s);
}
C++:
void Log(std::string s){

    JNIEnv *env;
    g_JavaVM->GetEnv((void**)&env, JNI_VERSION_1_6);

    jstring jstr1 = env->NewStringUTF(s.c_str());

    jclass clazz = env->FindClass("com/android/gl2jni/GL2JNILib");
    jmethodID mid = env->GetStaticMethodID(clazz, "log", "(Ljava/lang/String;)V");

    jobject obj = env->CallStaticObjectMethod(clazz, mid, jstr1);
}

//In some initialization function with Environment variable

env->GetJavaVM(&g_JavaVM);
Hopefully this can help other people with the same problem.

                        
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