- 单例模式的线程安全
- 需要双重判空指针,降低锁冲突的概率,提高性能
- 原因1:
- 当第一次实例化单例时,可能有多个线程同时到来,并且svr指针为空
- 这时他们就会去竞争锁,但只有一个线程会最快拿到锁,并且成功实例化出单例对象
- 但此时如果不加双重判空指针,那些也进了第一层if判断的,仍然会去实例化出对象
- 原因2:
- 为了线程安全,必然要加锁,加锁之后再去判空
- 但每次调用
GetInstance()
都需要去获得锁,释放锁,效率低下 - 此时再加一层外层if判空,这样就会避免后续调用
GetInstance()
时没必要的锁竞争
static const uint16_t PORT = 8090;static const int BACKLOG = 128;class TcpServer{public: static TcpServer* GetInstance(uint16_t port = PORT) { static pthread_mutex_t lock = PTHREAD_MUTEX_INITIALIZER; if(svr == nullptr) { pthread_mutex_lock(&lock); if(svr == nullptr) { svr = new TcpServer(port); svr->Init(); } pthread_mutex_unlock(&lock); } return svr; } void Init() { Socket(); Bind(); Listen(); LOG(INFO, "TcpServer Init ... Success"); } void Socket() { _listenSock = socket(AF_INET, SOCK_STREAM, 0); if(_listenSock < 0) { LOG(FATAL, "Socket Error"); exit(1); } int opt = 1; setsockopt(_listenSock, SOL_SOCKET, SO_REUSEPORT, &opt, sizeof(opt)); LOG(INFO, "Create Listen Socket ... Success"); } void Bind() { struct sockaddr_in local; memset(&local, 0, sizeof(local)); local.sin_family = AF_INET; local.sin_addr.s_addr = INADDR_ANY; local.sin_port = htons(_port); if(bind(_listenSock, (struct sockaddr*)&local, sizeof(local)) < 0) { LOG(FATAL, "Bind Error"); exit(2); } LOG(INFO, "Bind Socket ... Success"); } void Listen() { if(listen(_listenSock, BACKLOG) < 0) { LOG(FATAL, "Listen Error"); exit(3); } LOG(INFO, "Listen Socket ... Success"); } int Sock() { return _listenSock; } ~TcpServer() { if(_listenSock >= 0) { close(_listenSock); } }private: TcpServer(uint16_t port) : _port(port) , _listenSock(-1) {} TcpServer(const TcpServer&) = delete;private: uint16_t _port; int _listenSock; static TcpServer* svr;};TcpServer* TcpServer::svr = nullptr;