A simple JNI demo was given earlier.

Let's review:

package diveinjvm;

public class Foo {
//        public static native void foo();
//        public native void bar(int i, long j);
        public native void bar(String s, Object o);

        int i = 0xDEADBEEF;

        public static void main(String[] args) {
            try {
                System.loadLibrary("foo");
            } catch (UnsatisfiedLinkError e) {
                e.printStackTrace();
                System.exit(1);
            }
            new Foo().bar("", "");
        }
    }

There is a problem here. How does the bar method in Foo.java know which c method to call?

We have generated a. h file through javac -h Foo.java command. Let's pay attention to the method name in this file: Java? Diveinjvm? Foo? Bar

As you can see here, the method name starts with Java + package name + class name + method name.

This is a case where the Java virtual machine automatically finds C functions that conform to the default naming specification.

One more thing:

Let's see that java.lang.Object has many native methods, such as:

public native int hashCode();

And the Object class begins with a piece of code

private static native void registerNatives();
    static {
        registerNatives();
    }

The registerNatives method should also correspond to a c method.

View the source code of openJdk, openjdk\openjdk8\openjdk\jdk\src\share\native\java\lang\Object.c

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]));
}

The hashCode is registered here. In this way, calling the hashCode in the Object class can call the JVM_IHashCode method in openjdk\openjdk8\openjdk\jdk\src\share\native\java\lang\Object.c.