Java IO學習筆記四:Socket基礎

Grey Zeng 發表於 2021-06-14
Java

作者:Grey

原文地址:Java IO學習筆記四:Socket基礎

準備兩個Linux例項(安裝好jdk1.8),我準備的兩個例項的ip地址分別為:

io1例項:192.168.205.138
io2例項:192.168.205.149

安裝必要工具:

yum install -y strace lsof  pmap tcpdump

準備服務端程式碼

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 {
            // 接受客戶端連線
            while (true) {
                // 先阻塞,這樣客戶端暫時無法連線進來
                System.in.read();

                // 這個方法也是阻塞的,如果沒有客戶端連線進來,會一直阻塞在這裡,除非設定了超時時間
                Socket client = server.accept();

                System.out.println("client " + client.getPort() + " connected!!!");
                // 客戶端連線進來後,開闢一個新的執行緒去接收並處理
                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();
            }
        }

    }
}

死迴圈中,由於第一句:

System.in.read();

導致

 Socket client = server.accept();

無法執行,即:服務端此時是無法接收客戶端的。

準備客戶端程式碼:

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();
        }
    }
}

在例項io1中啟動服務端程式碼:

javac SocketServerBIOTest.java && java SocketServerBIOTest

在io1中開啟抓包工具:

tcpdump -nn -i ens33 port 9090

在io2中執行客戶端程式碼:

javac SocketClientTest.java && java SocketClientTest

由於我們在服務端加了一段:

System.in.read()

方法,導致服務端其實沒辦法執行accept()

但是在服務端檢視抓包資訊:

[[email protected] 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

核心已經為客戶端和服務端建立了連線並完成了三次握手,在服務端使用netstat檢視:

[[email protected] 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 -          
...

顯示已經建立了連線,只不過還沒有分配:PID/Program name。

在客戶端輸入一些資訊,

[[email protected] socket]# javac SocketClientTest.java && java SocketClientTest
asdfasdfasdfasf

在服務端再次執行netstat

[[email protected] 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 -        
...

也顯示出接收到了資料。

再服務端再次開啟抓包:

tcpdump -nn -i ens33 port 9090

再次在客戶端輸入一些資料:

[[email protected] socket]# javac SocketClientTest.java && java SocketClientTest
asdfasdfasdfasf
dfasdfasdfasdas

可以看到抓包資訊:

[[email protected] ~]# 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

以上實驗主要說明了一個問題:

雖然在應用層面,服務端沒有呼叫accept() 去接收客戶端,但是,核心其實已經完成了客戶端和服務端的三次握手以及資料傳輸。

接下來,我們觸發服務端:

Socket client = server.accept();

這段邏輯

服務端可以顯示客戶端的資料

^C[[email protected] socket]# java SocketServerBIOTest
server started , port : 9090

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

服務端執行nestat -ntap,

[[email protected] ~]# 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 
...

可以看到已經分配了一個PID,通過:

lsof -p 2266

檢視這個java程式相關的檔案描述符

[[email protected] ~]# 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)
...

6u就對應了服務端和客戶端連線的一個Socket。

在建立服務端的時候,我們指定了一個引數:backlog

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

backlog的解釋是

requested maximum length of the queue of incoming connections.

重新啟動我們的服務端

[[email protected] socket]# javac SocketServerBIOTest.java && java SocketServerBIOTest
server started , port : 9090

啟動三個客戶端連線這個服務端, 然後再服務端執行netstat -ntap

[[email protected] 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,服務端建立了三個連線,但是,當我們再啟動一個客戶端連線進來的時候,新增的這個連線的狀態為:SYN_RECV

[[email protected] 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:

服務端收到報文後,向客戶端傳送確認的報文,服務端進入SYNC_RECV狀態,但是因為設定了backlog=2
超過了服務端設定的最大連線數,服務端就不再繼續向客戶端傳送報文。

在具體的程式設計中,服務端和客戶端都有很多配置的引數。
詳見:

ServerSocket

socketOpt

java socket 引數

關於MTU和MSS: MTU TCP-MSS詳解

原始碼:Github