单例写法

 private volatile static HttpManager INSTANCE;

    //构造方法私有
    private HttpManager() {
    }

    //获取单例
    public static HttpManager getInstance() {
        if (INSTANCE == null) {
            synchronized (HttpManager.class) {
                if (INSTANCE == null) {
                    INSTANCE = new HttpManager();
                }
            }
        }
        return INSTANCE;
    }