带锁单例实例化

//在特殊场合才能用的到,在多线程里并发时确保对象只实例化了一次
private static volatile WindowUtils windowUtils;

public static WindowUtils getInstance() {
    if (windowUtils == null) {
        synchronized (WindowUtils.class) {
            if (windowUtils == null) {
                windowUtils = new WindowUtils();
            }
        }
    }
    return windowUtils;
}