java - java nio select 和 linux epoll 有什么区别？

Question

java nio select 的代码和 linux epoll 的代码看起来是一样的。不存在循环获取socket fd这样的东西，linux select代码片段确实使用了循环获取socket fd。

所以我的问题是，java nio select 和 linux epoll 是一样的吗？

java nio选择

        while (true) {
            try {
                selector.select();
                Set<SelectionKey> selectionKeys = selector.selectedKeys();
                selectionKeys.forEach((selectionKey) -> {
                    final SocketChannel client;
                    try {
                        if (selectionKey.isAcceptable()) {
                            ServerSocketChannel server = (ServerSocketChannel) selectionKey.channel();
                            client = server.accept();
                            client.configureBlocking(false);
                            client.register(selector, SelectionKey.OP_READ);

                            String key = "[" + UUID.randomUUID().toString() + "]";

                            clientMap.put(key, client);
                        } else if (selectionKey.isReadable()) {
                            client = (SocketChannel) selectionKey.channel();

                            ByteBuffer readBuffer = ByteBuffer.allocate(1024);

                            int count = client.read(readBuffer);
                            if (count > 0) {
                                //...
                            }

                        }
                    } catch (Exception ex) {
                        ex.printStackTrace();
                    }
                });

                selectionKeys.clear();

            } catch (Exception e) {
                e.printStackTrace();
            }
        }

Linux 电子投票

   for (;;) {
        nfds = epoll_wait(epfd, events, 20, 500);

        for (i = 0; i < nfds; ++i) {
            if (events[i].data.fd == listenfd)

            {
                connfd = accept(listenfd, (sockaddr *) &clientaddr, &clilen);
                if (connfd < 0) {
                    perror("connfd<0");
                    exit(1);
                }
                //setnonblocking(connfd);

                char *str = inet_ntoa(clientaddr.sin_addr);
                ev.data.fd = connfd;

                ev.events = EPOLLIN | EPOLLET;

                epoll_ctl(epfd, EPOLL_CTL_ADD, connfd, &ev);
            } else if (events[i].events & EPOLLIN)

            {
                cout << "EPOLLIN" << endl;
                if ((sockfd = events[i].data.fd) < 0)
                    continue;
                if ((n = read(sockfd, line, MAXLINE)) < 0) {
                    if (errno == ECONNRESET) {
                        close(sockfd);
                        events[i].data.fd = -1;
                    } else
                        std::cout << "readline error" << std::endl;
                } else if (n == 0) {
                    close(sockfd);
                    events[i].data.fd = -1;
                }
                line[n] = '/0';
                cout << "read " << line << endl;

                ev.data.fd = sockfd;

                ev.events = EPOLLOUT | EPOLLET;

                //epoll_ctl(epfd,EPOLL_CTL_MOD,sockfd,&ev);

            } else if (events[i].events & EPOLLOUT) 
            {
                sockfd = events[i].data.fd;
                write(sockfd, line, n);

                ev.data.fd = sockfd;

                ev.events = EPOLLIN | EPOLLET;

                epoll_ctl(epfd, EPOLL_CTL_MOD, sockfd, &ev);
            }
        }
    }

Linux选择

    for (;;) {
        rset = allset;        /* structure assignment */
        nready = select(maxfd + 1, &rset, NULL, NULL, NULL);

        if (FD_ISSET(listenfd, &rset)) /* new client connection */
        {
            clilen = sizeof(cliaddr);
            connfd = accept(listenfd, (struct sockaddr *) &cliaddr, &clilen);
#ifdef    NOTDEF
            printf("new client: %s, port %d\n",
                    inet_ntop(AF_INET, &cliaddr.sin_addr, 4, NULL),
                    ntohs(cliaddr.sin_port));
#endif

            for (i = 0; i < FD_SETSIZE; i++)
                if (client[i] < 0) {
                    client[i] = connfd;    /* save descriptor */
                    break;
                }
            if (i == FD_SETSIZE) {
                printf("too many clients");
                exit(0);
            }

            FD_SET(connfd, &allset);    /* add new descriptor to set */
            if (connfd > maxfd)
                maxfd = connfd;            /* for select */
            if (i > maxi)
                maxi = i;                /* max index in client[] array */

            if (--nready <= 0)
                continue;                /* no more readable descriptors */
        }

        for (i = 0; i <= maxi; i++)    /* check all clients for data */
        {
            if ((sockfd = client[i]) < 0)
                continue;
            if (FD_ISSET(sockfd, &rset)) {
                if ((n = read(sockfd, buf, MAXLINE)) == 0)/* connection closed by client */
                {
                    close(sockfd);
                    FD_CLR(sockfd, &allset);
                    client[i] = -1;
                } else
                    write(sockfd, buf, n);

                if (--nready <= 0)
                    break;                /* no more readable descriptors */
            }
        }
    }

Answer 1

JAVA NIO是Java对非阻塞io的抽象。不同平台上会有不同的底层实现。在Linux上，它是使用epool实现的。在其他平台上，使用其他技术，例如kqueue。

这导致NIO与epoll非常相似，但是java希望你学习它的nio抽象，而不必关心底层实现。