Attempt to call JNI_CreateJavaVM from libart.so fails

问题

I am working on a Xamarin.Android app with a C++ part. Now I need to call directly into Android Java interfaces from the C++ library.

I copied the code from Caleb Fenton's detailed and very helpful blog post which uses the JNI to call from C++ to Java. But I can't get the pointer to the JVM in the same way that he does it.

(By the way, I am mostly a C# programmer, so it's entirely possible that I've made an elementary mistake in C++).

In the header file:

 #pragma once
class MyJniClass
{
    //Create this once and cache it.
    JavaVM *m_jvm;                      // Pointer to the JVM (Java Virtual Machine)
    JNIEnv *m_env;                      // Pointer to native interface
    bool init_jvm();
}

In the .cpp file:

    #include <jni.h>
#include <dlfcn.h>
#include "MyJniClass.h"

typedef int(*JNI_CreateJavaVM_t)(void *, void *, void *);


/**Code is based on https://github.com/rednaga/native-shim/blob/master/vm.c  
*/
bool MyJniClass::init_jvm() 
{
    // https://android.googlesource.com/platform/frameworks/native/+/ce3a0a5/services/surfaceflinger/DdmConnection.cpp
    JavaVMOption opt[1];
    opt[0].optionString = "-Djava.class.path=."; // I added a small java class to the dll to which this C++ class is linked, 
                                                 //so that there would be a java class in the current directory.  

    //opt/*[1]*/.optionString = "-agentlib:jdwp=transport=dt_android_adb,suspend=n,server=y";


    JavaVMInitArgs args;
    args.version = JNI_VERSION_1_6;
    args.options = opt;
    args.nOptions = 1;
    args.ignoreUnrecognized = JNI_FALSE;

    void *libart_dso = dlopen("libart.so", RTLD_NOW); //libdvm.so is outdated,  libnativehelper.so doesn't work

    if (!libart_dso ) 
    {
        //Execution doesn't pass through here 
        return false;
    }

    //Try to get the JNI_CreateJavaVM function pointer
    JNI_CreateJavaVM_t JNI_CreateJavaVM;
    JNI_CreateJavaVM = (JNI_CreateJavaVM_t)dlsym(libart_dso, "JNI_CreateJavaVM");
    if (!JNI_CreateJavaVM) 
    {
        //Execution doesn't pass through here 
        return false;
    }

    signed int result = JNI_CreateJavaVM(&(m_jvm), &(m_env), &args);

    if ( result != 0)
    {
        ostringstream os;
        os << "Call to JNI_CreateJavaVM returned ";
        os << result;
        m_logger->writeEntry(Loglevel::debug, os.str()); // ===> Here, I can see that result is always -1
        return false;
    }

    return true;
}

I tried to find the function JNI_CreateJavaVM in the ART source code here, but I couldn't find it. But surely it should be there, so that dlsym can find the function? I think I have to look further to find the source code for libart.so.

What am I doing wrong, that I can't get a valid call to JNI_CreateJavaVM?

回答1:

the first modification here would be to add the diagnosis option Xcheck:jni. This will provide details in case of errors. Add another option by modifying JavaVMOption opt[1]; to JavaVMOption opt[2]; and then, add the following option: opt[1].optionString = "-Xcheck:jni".

Also, the dll must be loaded from it's original location (as other DLLs are involved) and not from your project directory. Mor details are provided in the following post: JNI_CreateJavaVM() terminates with exit code 1

Finally, you should cast the pointer to native interface JNIEnv by modifying:

signed int result = JNI_CreateJavaVM(&(m_jvm), &(m_env), &args);

signed int result = JNI_CreateJavaVM(&(m_jvm), (void**)&(m_env), &args);

This should solve the problem.

回答2:

It looks like issue is related to library that provides JNI_CreateJavaVM. I am referring to this line

void *libart_dso = dlopen("libart.so", RTLD_NOW);

If I strip your code from all the stuff that is not related to JVM, and if I use macOS based JDK it works just fine.

--- 8< CUT HERE 8< ----
#include <jni.h>
#include <dlfcn.h>
#include <iostream>
#include "MyJniClass.h"

using namespace std;

bool MyJniClass::init_jvm()
{
    JavaVM *jvm;
    JNIEnv *env = NULL;
    JavaVMOption opt[1];
    opt[0].optionString = "-Djava.class.path=.";

    JavaVMInitArgs args;
    args.version = JNI_VERSION_1_6;
    args.options = opt;
    args.nOptions = 1;
    args.ignoreUnrecognized = JNI_FALSE;

    int status = JNI_CreateJavaVM (&jvm, (void **) &env, &args);

    cout << status << endl;

    return true;
}

int main(int argc, char **argv) {
  MyJniClass jni;
  jni.init_jvm();
}
--- 8< CUT HERE 8< ----

--- 8< CUT HERE 8< ----
#pragma once
class MyJniClass
{
    //Create this once and cache it.
    JavaVM *m_jvm;                      // Pointer to the JVM (Java Virtual Machine)
    JNIEnv *m_env;                      // Pointer to native interface
    public: bool init_jvm();
};
--- 8< CUT HERE 8< ----

and then, I compile the code:

g++ -o lib/recipeNo027_main c/recipeNo027_main.c \
        -I/Library/Java/JavaVirtualMachines/jdk-12.0.1.jdk/Contents/Home/include \
        -I/Library/Java/JavaVirtualMachines/jdk-12.0.1.jdk/Contents/Home/include/darwin/ \
    -rpath -L/Library/Java/JavaVirtualMachines/jdk-12.0.1.jdk/Contents/Home/lib/server -ljvm

(it is based on recipeNo027 from here: http://jnicookbook.owsiak.org/recipe-no-027/)

I can run it without any issues