shared_counter.java

Context c1 = Context.create("js");
Context c2 = Context.create("js");

// create and share a thread-safe counter
AtomicInteger counter = new AtomicInteger(0);
c1.getBindings("js").putMember("counter", counter);
c2.getBindings("js").putMember("counter", counter);

Thread thread = new Thread(new Runnable() {
  @Override
  public void run() {
    while (counter.get() < 1000) {
      c1.eval("js", "counter.incrementAndGet()");
    }
  }
});
thread.start();

while (counter.get() < 1000) {
  c2.eval("js", "counter.incrementAndGet()");
}