- 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;