摘要：在Java SDK中，對於Socket原生提供了支持，它分爲ServerSocket和Socket。
本文分享自華爲雲社區《Java Socket 如何實現服務器和客戶端數據交互》，作者： jackwangcumt 。
1 Socket概述
根據百度百科的定義，Socket 譯爲套接字，它是對網絡中不同主機上的應用進程之間進行雙向通信的端點的抽象。一個Socket實例就是網絡上進程通信的一端，提供了應用層進程利用網絡協議交換數據的機制。Socket向上連接各種應用進程，向下連接各種網絡協議，是應用程序通過網絡協議進行通信的接口。其示意圖如下下圖所示：
（來自《Java TCP/IP Socket編程》）
從上圖可知，套接字Socket在OSI七層模型中處於應用層和傳輸層之間，是一個重要的接口。一般來說，傳輸層協議有TCP協議和UDP協議。這兩個協議底層都基於IP網絡層協議。
2 Java Socket 實現
在Java SDK中，對於Socket原生提供了支持，它分爲ServerSocket和Socket，其中ServerSocket發起一個服務端的Socket，其中需要提供一個端口號，如果給定0，則自動申請可用的端口。當然了，也可以指定具體的端口號（有一定的範圍，不超過 65535 ）。不過這裏需要注意，傳入的端口不能被其他應用佔用，否則啓動服務失敗。下面給出一個簡單的ServerSocket示例，代碼如下：
package com.example.demo.network; import java.io.IOException; import java.io.PrintWriter; import java.net.ServerSocket; import java.net.Socket; import java.util.Scanner; import java.util.concurrent.ExecutorService; import java.util.concurrent.Executors; public class MyServer { //<= 65535 private static final int PORT = 65535; public static void main(String[] args) throws Exception { try (ServerSocket listener = new ServerSocket(PORT)) { System.out.println("Server Started"); //線程池大小，它根據客戶端數來決定，當大於10時，則之前的10個線程仍然可以工作， // 而超過的線程則進入隊列中，等待。 //當之前的客戶端釋放量後，則在隊列中的線程仍然可以工作。 ExecutorService pool = Executors.newFixedThreadPool(10); while (true) { //多線程 pool.execute(new MyServerDemo01(listener.accept())); System.out.println(pool); } } } //Runnable接口的實現對象可以被線程Thread調用 private static class MyServerDemo01 implements Runnable { private Socket socket; MyServerDemo01(Socket socket) { this.socket = socket; } @Override public void run() { System.out.println("Client [" + socket.getRemoteSocketAddress().toString()+" ] Connected"); try { //輸入 Scanner in = new Scanner(socket.getInputStream()); //輸出 PrintWriter out = new PrintWriter(socket.getOutputStream(), true); while (in.hasNextLine()) { String msg = in.nextLine(); System.out.println("Client [" + socket.getRemoteSocketAddress().toString()+" ] : " + msg); out.println(msg.toUpperCase()); } } catch (Exception e) { System.out.println("Error:" + socket+ e.getMessage()); } finally { try { //關閉socket socket.close(); } catch (IOException e) { e.printStackTrace(); } System.out.println("Closed: " + socket); } } } }
啓動該Server端，並開始監聽客戶端的連接。當客戶端沒有連接時，服務器線程池pool並未啓動單獨的線程。下面給出客戶端的Java Socket實現，具體的示例代碼如下：
package com.example.demo.network; import java.io.PrintWriter; import java.net.Socket; import java.util.Scanner; public class MyClient { //<= 65535 private static final int PORT = 65535; //服務器地址 private static final String IP = "127.0.0.1"; public static void main(String[] args) throws Exception { try (Socket socket = new Socket(IP, PORT)) { System.out.println("Client ["+socket.getRemoteSocketAddress().toString()+" ] Started"); Scanner scanner = new Scanner(System.in); Scanner in = new Scanner(socket.getInputStream()); PrintWriter out = new PrintWriter(socket.getOutputStream(), true); while (scanner.hasNextLine()) { out.println(scanner.nextLine()); System.out.println("Server Response:"+ in.nextLine()); } }catch (Exception ex){ System.out.println("Error : "+ ex.getMessage()); } } }
從代碼可知，try (Socket socket = new Socket(IP, PORT)) 是一種包含資源釋放的try-with-resource機制，它會自動進行資源釋放，而不需要手動進行釋放。Socket對象要想和服務器通信，必須要明確服務器的IP地址和端口，否則不能正確通信，Socket啓動時，也會在主機上佔用自己的端口。我們首先啓動Server端，然後可以同時啓動10個以上的Client端，比如13個，那麼超過Executors.newFixedThreadPool(10)限定的數量10後，將進入queued tasks隊列中進行排隊等待。通過打印pool對象，可以看出當前的狀態，比如[Running, pool size = 4, active threads = 4, queued tasks = 0, completed tasks = 0]說明當前在運行狀態，線程池大小爲10，激活的線程爲10，等待的任務線程queued tasks爲0。
下面給出Server端相關輸出示例：
Server Started java.util.concurrent.ThreadPoolExecutor@256216b3[Running, pool size = 1, active threads = 1, queued tasks = 0, completed tasks = 0] Client [/127.0.0.1:64590 ] Connected java.util.concurrent.ThreadPoolExecutor@256216b3[Running, pool size = 2, active threads = 2, queued tasks = 0, completed tasks = 0] Client [/127.0.0.1:64597 ] Connected java.util.concurrent.ThreadPoolExecutor@256216b3[Running, pool size = 3, active threads = 3, queued tasks = 0, completed tasks = 0] Client [/127.0.0.1:64603 ] Connected Client [/127.0.0.1:64590 ] : hello Client [/127.0.0.1:64590 ] : hello Client [/127.0.0.1:64597 ] : world Client [/127.0.0.1:64597 ] : world Client [/127.0.0.1:64603 ] : python Client [/127.0.0.1:64597 ] : python02 java.util.concurrent.ThreadPoolExecutor@256216b3[Running, pool size = 4, active threads = 4, queued tasks = 0, completed tasks = 0] Client [/127.0.0.1:57806 ] Connected java.util.concurrent.ThreadPoolExecutor@256216b3[Running, pool size = 5, active threads = 5, queued tasks = 0, completed tasks = 0] Client [/127.0.0.1:57814 ] Connected java.util.concurrent.ThreadPoolExecutor@256216b3[Running, pool size = 6, active threads = 6, queued tasks = 0, completed tasks = 0] Client [/127.0.0.1:57820 ] Connected java.util.concurrent.ThreadPoolExecutor@256216b3[Running, pool size = 7, active threads = 7, queued tasks = 0, completed tasks = 0] Client [/127.0.0.1:57827 ] Connected java.util.concurrent.ThreadPoolExecutor@256216b3[Running, pool size = 8, active threads = 8, queued tasks = 0, completed tasks = 0] Client [/127.0.0.1:57833 ] Connected java.util.concurrent.ThreadPoolExecutor@256216b3[Running, pool size = 9, active threads = 9, queued tasks = 0, completed tasks = 0] Client [/127.0.0.1:57839 ] Connected java.util.concurrent.ThreadPoolExecutor@256216b3[Running, pool size = 10, active threads = 10, queued tasks = 0, completed tasks = 0] Client [/127.0.0.1:57845 ] Connected java.util.concurrent.ThreadPoolExecutor@256216b3[Running, pool size = 10, active threads = 10, queued tasks = 1, completed tasks = 0] java.util.concurrent.ThreadPoolExecutor@256216b3[Running, pool size = 10, active threads = 10, queued tasks = 2, completed tasks = 0] java.util.concurrent.ThreadPoolExecutor@256216b3[Running, pool size = 10, active threads = 10, queued tasks = 3, completed tasks = 0] Closed: Socket[addr=/127.0.0.1,port=64590,localport=65535] Client [/127.0.0.1:57854 ] Connected Client [/127.0.0.1:57814 ] : t2 Client [/127.0.0.1:57814 ] : tw2
客戶端相關輸入輸出界面如下：
Client [/127.0.0.1:65535 ] Started world Server Response:WORLD world Server Response:WORLD python02 Server Response:PYTHON02