[项目][WebServer][TcpServer]详细讲解

• 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

staticconstuint16_t PORT =8090;staticconstint BACKLOG =128;// 单例 -- 饿汉模式classTcpServer{public:static TcpServer*GetInstance(uint16_t port = PORT){static pthread_mutex_t lock = PTHREAD_MUTEX_INITIALIZER;if(svr ==nullptr)// 双重判空指针，降低锁冲突的概率，提高性能{// 注意线程安全pthread_mutex_lock(&lock);if(svr ==nullptr){
                svr =newTcpServer(port);
                svr->Init();}pthread_mutex_unlock(&lock);}return svr;}voidInit(){Socket();Bind();Listen();LOG(INFO,"TcpServer Init ... Success");}voidSocket(){
        _listenSock =socket(AF_INET, SOCK_STREAM,0);if(_listenSock <0){LOG(FATAL,"Socket Error");exit(1);}// 设置端口复用int opt =1;setsockopt(_listenSock, SOL_SOCKET, SO_REUSEPORT,&opt,sizeof(opt));LOG(INFO,"Create Listen Socket ... Success");}voidBind(){structsockaddr_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,(structsockaddr*)&local,sizeof(local))<0){LOG(FATAL,"Bind Error");exit(2);}LOG(INFO,"Bind Socket ... Success");}voidListen(){if(listen(_listenSock, BACKLOG)<0){LOG(FATAL,"Listen Error");exit(3);}LOG(INFO,"Listen Socket ... Success");}intSock(){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;