How to set classpath of a running JVM in C++(JNI)?

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In C++ (JNI), I get an already running JVM using the following JNI function:

JNI_GetCreatedJavaVMs(&jvm,1,&vmnb);

Then I attach the current thread using the following code:

jvm->AttachCurrentThread((void**)&env, NULL);

Question: How can I set classpath in that case? Thanks.

Note: Creating a new JVM and passing the classpath in vm_args to the new JVM is not an option for me.

jvm java-native-interface c++ java
2 Answers

I ran into this issue, and I did not have the option to call back into the JVM, so I implemented the whole add_path busniess on the JNI side.

void add_path(JNIEnv* env, const std::string& path)
{
    const std::string urlPath = "file:/" + path;
    jclass classLoaderCls = env->FindClass("java/lang/ClassLoader");
    jmethodID getSystemClassLoaderMethod = env->GetStaticMethodID(classLoaderCls, "getSystemClassLoader", "()Ljava/lang/ClassLoader;");
    jobject classLoaderInstance = env->CallStaticObjectMethod(classLoaderCls, getSystemClassLoaderMethod);
    jclass urlClassLoaderCls = env->FindClass("java/net/URLClassLoader");
    jmethodID addUrlMethod = env->GetMethodID(urlClassLoaderCls, "addURL", "(Ljava/net/URL;)V");
    jclass urlCls = env->FindClass("java/net/URL");
    jmethodID urlConstructor = env->GetMethodID(urlCls, "<init>", "(Ljava/lang/String;)V");
    jobject urlInstance = env->NewObject(urlCls, urlConstructor, env->NewStringUTF(urlPath.c_str()));
    env->CallVoidMethod(classLoaderInstance, addUrlMethod, urlInstance);
    std::cout << "Added " << urlPath << " to the classpath." << std::endl;
}
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