多个Java线程使用Java Native Access进行多个方法调用

Question

I am calling for eg getServer() method implemented in C++ library using JNA which takes input parameter and connects to server and return the server object. I am calling above native method using different Java threads. And this native method uses some global variables where native method is implemented.

I have declared native method in Native.h file as extern "C" and implementing native method in Native.cpp file. This method is standalone method ie not part of any class or structure.

So my query here is, if there are some global variables present in Native.cpp, are their value being changed from different Java threads which calls same native method?

Answer 1

Are their value being changed from different Java threads which calls same native method?

Sure, the global variables will be shared by all virtual machine then by threads as well, let's write a very simple example to proof that:

counter.c

#include<stdio.h>

int counter = 0;
int incrementAndGet(){
    counter++;
    return counter;
}

Calling it in java using JNA

final ExecutorService executorService = Executors.newFixedThreadPool(5);
for (int i = 0; i < 20; i++) {
    executorService.submit(() -> {
        try {
            TimeUnit.MILLISECONDS.sleep((long) (Math.random() * 1000));
        } catch (InterruptedException e) {}
        System.out.printf("thread=%s, counter=%d%n", Thread.currentThread().getName(), CounterLib.INSTANCE.incrementAndGet());
    });
}
executorService.awaitTermination(10, TimeUnit.SECONDS);
executorService.shutdown();

output

thread=pool-1-thread-2, counter=1
thread=pool-1-thread-4, counter=2
thread=pool-1-thread-1, counter=3
thread=pool-1-thread-5, counter=4
thread=pool-1-thread-2, counter=5
thread=pool-1-thread-3, counter=6
thread=pool-1-thread-2, counter=7
thread=pool-1-thread-4, counter=8
thread=pool-1-thread-1, counter=9
thread=pool-1-thread-2, counter=10
thread=pool-1-thread-5, counter=11
thread=pool-1-thread-3, counter=12
thread=pool-1-thread-5, counter=13
....

You also must worry about concurrency at this global variables, or at java or at c, in the two is not necessary, then if you are just reading values from the global variables then you must not worry about concurrency.