When the Object.h / .cpp compiled

Question

My question is: When is the Object.h/Object.cpp (the java Class "Object") compiled? Is it compiled along with JVM (when JVM is compiled) or it is included to the JVM dynamically (.dll)?

• my question concerns the implementation of JVM
• We assume that it was implemented in C / C ++

The philosophy of JNI is to create native methods and the communication between JVM and these classes are done via DLL files. But for native methods such a Object's native methods i cant understand if these methods communicate with the same way (via DLL files) or different.

Answer 1

It is not specified how JVM links Object's native methods; different JVM implementations may do it differently. As to OpenJDK and HotSpot JVM, there is no magic behind native methods of java.lang.Object. They are bound the same way as user-defined natives.

Note: it is not required that the native code for a Java class is loaded from one shared library - it can be scattered round multiple libraries. That's exactly what happen to Object's native methods: some of them are built into JVM, others are implemented in java.dll.

When java.lang.Object class is initialized, its registerNatives method is called. The native implementation of this method is a part of JDK class library. It is compiled from Object.c into java.dll at JDK build time.

static JNINativeMethod methods[] = {
    {"hashCode",    "()I",                    (void *)&JVM_IHashCode},
    {"wait",        "(J)V",                   (void *)&JVM_MonitorWait},
    {"notify",      "()V",                    (void *)&JVM_MonitorNotify},
    {"notifyAll",   "()V",                    (void *)&JVM_MonitorNotifyAll},
    {"clone",       "()Ljava/lang/Object;",   (void *)&JVM_Clone},
};

JNIEXPORT void JNICALL
Java_java_lang_Object_registerNatives(JNIEnv *env, jclass cls)
{
    (*env)->RegisterNatives(env, cls,
                            methods, sizeof(methods)/sizeof(methods[0]));
}

This method calls JNI RegisterNatives to bind other native methods of java.lang.Object: hashCode, wait, notify etc. The corresponding C++ functions JVM_IHashCode, JVM_MonitorWait and JVM_MonitorNotify are a part of JVM. They are compiled from jvm.cpp to jvm.dll at HotSpot build time.

However, one more method, Object.getClass is still implemented on JDK side using regular JNI interface:

JNIEXPORT jclass JNICALL
Java_java_lang_Object_getClass(JNIEnv *env, jobject this)
{
    if (this == NULL) {
        JNU_ThrowNullPointerException(env, NULL);
        return 0;
    } else {
        return (*env)->GetObjectClass(env, this);
    }
}