Java IO学习笔记四：Socket基础

作者：Grey

原文地址：Java IO学习笔记四：Socket基础

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。

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

服务端可以显示客户端的数据

[root@io socket]# java SocketServerBIOTest
server started , port : 9090

client 56944 connected!!!
client read some data is :30 val :asdfasdfasdfasfdfasdfasdfasdas

服务端执行nestat -ntap,

[root@io ~]# 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 2266/java 
...

可以看到已经分配了一个PID，通过:

lsof -p 2266

查看这个java进程相关的文件描述符

[root@io ~]# lsof -p 2266
COMMAND  PID USER   FD   TYPE             DEVICE  SIZE/OFF      NODE NAME
...
java    2266 root    6u  IPv6              26479       0t0       TCP 192.168.205.138:websm->192.168.205.149:56944 (ESTABLISHED)
...

文件描述符6u就对应了服务端和客户端连接的一个Socket。

在创建服务端的时候，我们指定了一个参数：backlog=2

// 指定了BACK_LOG = 2
server.bind(new InetSocketAddress(PORT), BACK_LOG);

backlog的解释是

requested maximum length of the queue of incoming connections.

重新启动我们的服务端

[root@io socket]# javac SocketServerBIOTest.java && java SocketServerBIOTest
server started , port : 9090

启动三个客户端连接这个服务端, 然后再服务端执行netstat -ntap

[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    192.168.205.149:50532   ESTABLISHED -                   
tcp6       0      0 192.168.205.138:9090    192.168.205.149:50536   ESTABLISHED -                   
tcp6       0      0 192.168.205.138:9090    192.168.205.149:50534   ESTABLISHED -    
...

可以看到，服务端创建了三个连接，但是，当我们再启动一个客户端连接进来的时候，新增的这个连接的状态为：SYN_RECV

[root@io socket]# netstat -ntap
Active Internet connections (servers and established)
Proto Recv-Q Send-Q Local Address           Foreign Address         State       PID/Program name    
....
tcp        0      0 192.168.205.138:9090    192.168.205.149:50538   SYN_RECV    -                  
....
tcp6       0      0 192.168.205.138:9090    192.168.205.149:50532   ESTABLISHED -                   
tcp6       0      0 192.168.205.138:9090    192.168.205.149:50536   ESTABLISHED -                   
tcp6       0      0 192.168.205.138:9090    192.168.205.149:50534   ESTABLISHED -               

SYN_RECV表示：

服务端收到报文后，向客户端发送确认的报文，服务端进入SYNC_RECV状态，但是因为设置了backlog=2
超过了服务端设置的最大连接数，服务端就不再继续向客户端发送报文。

在具体的编程中，服务端和客户端都有很多配置的参数。

详见：

ServerSocket

socketOpt

java socket 参数

关于MTU和MSS: MTU TCP-MSS详解

源码：Github