JNI - Why rt.jar System.load not working but wrapped method working?

And here is a demo (I omit utils, They just check if exception and print message):

First try, It should work:

C++ part:

jclass jClass_java_lang_System = env->FindClass("java/lang/System");
jmethodID jMethodID_java_lang_System_load = env->GetStaticMethodID(jClass_java_lang_System, "load", "(Ljava/lang/String;)V");
env->CallStaticVoidMethod(jClass_java_lang_System, jMethodID_java_lang_System_load, library_path);

No exception but after that, Call a native method cause UnsatisfiedLinkError.

Second try, Write a wrapper method:

public static void load(String path) {
    System.load(path);
}

And call it from C++

jmethodID jMethodID_Driver_load = env->GetStaticMethodID(jClass_Driver, "load", "(Ljava/lang/String;)V");
env->CallStaticVoidMethod(jClass_Driver, jMethodID_Driver_load, library_path);
checkException(env);

It just a wrapper for System.load and nothing else, It working fine. The native call working properly.

Then for more test but not make any sense - Use both of them:

jclass jClass_java_lang_System = env->FindClass("java/lang/System");
jmethodID jMethodID_java_lang_System_load = env->GetStaticMethodID(jClass_java_lang_System, "load", "(Ljava/lang/String;)V");
env->CallStaticVoidMethod(jClass_java_lang_System, jMethodID_java_lang_System_load, library_path);
if (!checkException(env)) std::cout << "Load by rt.jar no Exception" << std::endl;

jclass jClass_Driver = env->FindClass("Driver");
jmethodID jMethodID_Driver_load = env->GetStaticMethodID(jClass_Driver, "load", "(Ljava/lang/String;)V");
env->CallStaticVoidMethod(jClass_Driver, jMethodID_Driver_load, library_path);
checkException(env); // The first UnsatisfiedLinkError print by this util

// Second UnsatisfiedLinkError print by native method call, I omit it.

Got this result:

Load by rt.jar no Exception
java.lang.UnsatisfiedLinkError: Native Library XXXXX already loaded in another classloader
java.lang.UnsatisfiedLinkError: XXXXXXX

That make it more confuse, The first try show load by java.lang.System-load() not working but in fact the library is loaded. Then throw a duplicate load exception.

And reverse the order:

jclass jClass_Driver = env->FindClass("Driver");
jmethodID jMethodID_Driver_load = env->GetStaticMethodID(jClass_Driver, "load", "(Ljava/lang/String;)V");
env->CallStaticVoidMethod(jClass_Driver, jMethodID_Driver_load, library_path);
checkException(env);

jclass jClass_java_lang_System = env->FindClass("java/lang/System");
jmethodID jMethodID_java_lang_System_load = env->GetStaticMethodID(jClass_java_lang_System, "load", "(Ljava/lang/String;)V");
env->CallStaticVoidMethod(jClass_java_lang_System, jMethodID_java_lang_System_load, library_path);
if (!checkException(env)) std::cout << "Load by rt.jar no Exception" << std::endl;

And got this result:

Load by wrapper no Exception
java.lang.UnsatisfiedLinkError: Native Library XXXX already loaded in another classloader
Result is - 2468
Result is - 2468

Even throw a duplicate load exception, The native call working propproperly.

Question: What happen? How to solve?

When you load a native library with System.load() the VM will try to bind any JNI functions it finds to their Java counterparts, i.e. the class that declares the native methods. It can only do that when that class is already loaded. If you load the class afterwards you will have unbound native methods, and when you call them you get an UnsatisfiedLinkError.

To be able to call your wrapper method, you do load the class and therefore the VM can bind the native methods. To make this work with only a call to System.load(), make sure the VM already has the class. That said, it would probably be better to use the usual way of loading the native library from a static initializer in the class itself. loadLibrary will also find statically linked libraries. So if you separate the JNI functions from the rest of your code, and put them into their own library, you can statically link that and use loadLibrary with a simple name.