准备两个Linux实例(安装好jdk1.8)，我准备的两个实例的ip地址分别为：
io1实例：192.168.205.138
io2实例：192.168.205.149
安装必要工具：
yum install -y strace lsof pmap tcpdump准备服务端代码
import java.io.*; import java.net.InetSocketAddress; import java.net.ServerSocket; import java.net.Socket; /** * BIO Socket Server */ public class SocketServerBIOTest { private static final int PORT = 9090; private static final int BACK_LOG = 2; public static void main(String[] args) { ServerSocket server = null; try { server = new ServerSocket(); server.bind(new InetSocketAddress(PORT), BACK_LOG); System.out.println("server started , port : " + PORT); } catch (IOException e) { e.printStackTrace(); } try { // 接受客户端连接 while (true) { // 先阻塞，这样客户端暂时无法连接进来 System.in.read(); // 这个方法也是阻塞的，如果没有客户端连接进来，会一直阻塞在这里，除非设置了超时时间 Socket client = server.accept(); System.out.println("client " + client.getPort() + " connected!!!"); // 客户端连接进来后，开辟一个新的线程去接收并处理 new Thread(() -> { try { InputStream inputStream = client.getInputStream(); BufferedReader reader = new BufferedReader(new InputStreamReader(inputStream)); char[] data = new char[1024]; while (true) { int num = reader.read(data); if (num > 0) { System.out.println("client read some data is :" + num + " val :" + new String(data, 0, num)); } else if (num == 0) { System.out.println("client read nothing!"); continue; } else { System.out.println("client read -1..."); System.in.read(); client.close(); break; } } } catch (Exception e) { e.printStackTrace(); } }).start(); } } catch (Exception e) { e.printStackTrace(); } finally { try { server.close(); } catch (IOException e) { e.printStackTrace(); } } } }死循环中，由于第一句：
System.in.read();导致
Socket client = server.accept();无法执行，即：服务端此时是无法接收客户端的。
准备客户端代码：
import java.io.*; import java.net.Socket; /** * Socket Client */ public class SocketClientTest { public static void main(String[] args) { try { Socket client = new Socket("192.168.205.138", 9090); OutputStream out = client.getOutputStream(); InputStream in = System.in; BufferedReader reader = new BufferedReader(new InputStreamReader(in)); while (true) { String line = reader.readLine(); if (line != null) { byte[] bb = line.getBytes(); for (byte b : bb) { out.write(b); } } } } catch (IOException e) { e.printStackTrace(); } } }在实例io1中启动服务端代码：
javac SocketServerBIOTest.java && java SocketServerBIOTest在io1中开启抓包工具：
tcpdump -nn -i ens33 port 9090在io2中执行客户端代码：
javac SocketClientTest.java && java SocketClientTest由于我们在服务端加了一段：
System.in.read()方法，导致服务端其实没办法执行accept()
但是在服务端查看抓包信息：
[root@io socket]# tcpdump -nn -i ens33 port 9090 tcpdump: verbose output suppressed, use -v or -vv for full protocol decode listening on ens33, link-type EN10MB (Ethernet), capture size 262144 bytes 15:19:56.021974 IP 192.168.205.149.56944 > 192.168.205.138.9090: Flags [S], seq 391962776, win 29200, options [mss 1460,sackOK,TS val 16515471 ecr 0,nop,wscale 7], length 0 15:19:56.022035 IP 192.168.205.138.9090 > 192.168.205.149.56944: Flags [S.], seq 2744580571, ack 391962777, win 28960, options [mss 1460,sackOK,TS val 16517545 ecr 16515471,nop,wscale 7], length 0 15:19:56.022349 IP 192.168.205.149.56944 > 192.168.205.138.9090: Flags [.], ack 1, win 229, options [nop,nop,TS val 16515472 ecr 16517545], length 0内核已经为客户端和服务端建立了连接并完成了三次握手，在服务端使用netstat查看：
[root@io socket]# netstat -ntap Active Internet connections (servers and established) Proto Recv-Q Send-Q Local Address Foreign Address State PID/Program name ... tcp6 0 0 192.168.205.138:9090 192.168.205.149:56944 ESTABLISHED - ...显示已经建立了连接，只不过还没有分配：PID/Program name。
在客户端输入一些信息，
[root@io2 socket]# javac SocketClientTest.java && java SocketClientTest asdfasdfasdfasf在服务端再次执行netstat
[root@io socket]# netstat -ntap Active Internet connections (servers and established) Proto Recv-Q Send-Q Local Address Foreign Address State PID/Program name ... tcp6 15 0 192.168.205.138:9090 192.168.205.149:56944 ESTABLISHED - ...也显示出接收到了数据。
再服务端再次开启抓包：
tcpdump -nn -i ens33 port 9090再次在客户端输入一些数据：
[root@io2 socket]# javac SocketClientTest.java && java SocketClientTest asdfasdfasdfasf dfasdfasdfasdas可以看到抓包信息：
[root@io ~]# tcpdump -nn -i ens33 port 9090 tcpdump: verbose output suppressed, use -v or -vv for full protocol decode listening on ens33, link-type EN10MB (Ethernet), capture size 262144 bytes 15:26:48.632564 IP 192.168.205.149.56944 > 192.168.205.138.9090: Flags [P.], seq 391962792:391962793, ack 2744580572, win 229, options [nop,nop,TS val 16928082 ecr 16757410], length 1 15:26:48.632609 IP 192.168.205.138.9090 > 192.168.205.149.56944: Flags [.], ack 1, win 227, options [nop,nop,TS val 16930156 ecr 16928082], length 0 15:26:48.632791 IP 192.168.205.149.56944 > 192.168.205.138.9090: Flags [P.], seq 1:15, ack 1, win 229, options [nop,nop,TS val 16928082 ecr 16930156], length 14 15:26:48.632825 IP 192.168.205.138.9090 > 192.168.205.149.56944: Flags [.], ack 15, win 227, options [nop,nop,TS val 16930156 ecr 16928082], length 0以上实验主要说明了一个问题：
虽然在应用层面，服务端没有调用accept() 去接收客户端，但是，内核其实已经完成了客户端和服务端的三次握手以及数据传输。
接下来，我们触发服务端：
Socket client = server.accept();这段逻辑