单例模式的标准写法、注意事项、作用及测试

public static void main(String[] args) {
    final CountDownLatch latch = new CountDownLatch(1);
    int threadCount = 1000;

    for (int i = 0; i < threadCount; i++) {
        new Thread() {

            @Override
            public void run() {
                try {
                    // all thread to wait
                    latch.await();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }

                // test get instance
                System.out.println(Singleton.getInstance().hashCode());
            }
        }.start();
    }

    // release lock, let all thread excute Singleton.getInstance() at the same time
    latch.countDown();
}