The first acquaintance with the HTTP protocol by writing the simplest Web server in Java

I think that it would not be an exaggeration to assert that knowledge and understanding of the essence of the HTTP protocol is necessary for anyone who has decided to seriously address any of the areas of modern Web development. My personal experience suggests that this understanding does not come immediately. I am ashamed to say that there were times when the words GET and POST were for me akin to magic spells, and I did not even suspect the existence of PUT, PATCH and DELETE.

A few months ago, in addition to the actual development, I also began teaching, and the question arose of how to communicate the essence of the HTTP protocol to future Java developers in a simpler and clearer way. After several days of fussing and a number of unsuccessful attempts to make a presentation, an idea arose, and why not write a simple HTTP server in Java, because nothing so well explains the essence of the protocol as its simplest, but working implementation.

As it turned out, this is not difficult at all. Below is the code that will be enough for correct interaction with any browser! All we need is ServerSocket and some standard I / O.

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.PrintWriter;
import java.net.ServerSocket;
import java.net.Socket;
import java.nio.charset.StandardCharsets;
publicclassHttpServer{
    publicstaticvoidmain(String[] args){
        try (ServerSocket serverSocket = new ServerSocket(8080)) {
            System.out.println("Server started!");
            while (true) {
                // ожидаем подключения
                Socket socket = serverSocket.accept();
                System.out.println("Client connected!");
                // для подключившегося клиента открываем потоки // чтения и записиtry (BufferedReader input = new BufferedReader(new InputStreamReader(socket.getInputStream(), StandardCharsets.UTF_8));
                     PrintWriter output = new PrintWriter(socket.getOutputStream())) {
                    // ждем первой строки запросаwhile (!input.ready()) ;
                    // считываем и печатаем все что было отправлено клиентом
                    System.out.println();
                    while (input.ready()) {
                        System.out.println(input.readLine());
                    }
                    // отправляем ответ
                    output.println("HTTP/1.1 200 OK");
                    output.println("Content-Type: text/html; charset=utf-8");
                    output.println();
                    output.println("<p>Привет всем!</p>");
                    output.flush();
                    // по окончанию выполнения блока try-with-resources потоки, // а вместе с ними и соединение будут закрыты
                    System.out.println("Client disconnected!");
                }
            }
        } catch (IOException ex) {
            ex.printStackTrace();
        }
    }
}

We try to run this code. It is worth noting that the port for which the ServerSocket is created should be free. If the specified port is busy, you need to either release it or use another free port.

After running this code, go to the browser window and type in the address bar http://localhost:8080/. If everything went well, then in the browser window we will see the text “Hello to all”, and in the server log there is a text similar to the one below:

Server started!
Client connected!
GET / HTTP/1.1
Host: localhost:8080
Connection: keep-alive
Cache-Control: max-age=0
Upgrade-Insecure-Requests: 1
User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_13_6) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/72.0.3626.109 Safari/537.36
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,image/apng,*/*;q=0.8
Accept-Encoding: gzip, deflate, br
Accept-Language: ru-RU,ru;q=0.9,en-US;q=0.8,en;q=0.7,he;q=0.6,de;q=0.5,cs;q=0.4
Cookie: _ga=GA1.1.1849608036.1549463927; portainer.pagination_containers=100; _gid=GA1.1.80775985.1550669456;
If-Modified-Since: Sat, 05 Jan 2019 12:10:16 GMT
Client disconnected!

Every time we enter something into the address bar of the browser and press Enter, nothing else happens, like sending text that starts with the word GET and ends with a line break. After the word GET, a space follows the path to the requested document on the server. Try typing in the browser http://localhost:8080/somethingand see how the query text in the log changes.

In the query strings, starting from the second, there are so-called. headers with the help of which the client’s client settings are transmitted to the server. Each header line has a format [имя заголовка] : [значение]; [значение]; ... [значение].

After the request text is fully read by the server, we send it the simplest answer, the structure of which is quite simple and similar to the request structure. The first line contains the HTTP protocol version and the 200 OK code .which will inform the browser that the request has been successfully processed (everyone is better acquainted with the code 404, isn’t it;)). Then there is only one Content-Type header in which information about the format of the transmitted document (text / html) and its encoding (charset = utf-8) is transmitted. The header is followed by a newline (a mandatory requirement of the HTTP protocol) and the actual text that will be displayed in the browser.

That's all! Of course, this is not all that you need to know about the HTTP protocol and the principles of developing Web servers, but I would not like to complicate this example, since its main task is to demonstrate the simplest HTTP communication. In one of my following materials I will try to develop the topic of studying the HTTP protocol through its implementation.

UPD. A much more advanced example of such a server can be found in How To Tomcat Works , Chapter 1 - Simple Web Server .