Java backend development: API client state management using Java Servlet Cookies

With the continuous development of the Internet, the development of network applications has become more and more important. In this process, state management is extremely important for applications. Whether it is user login, shopping cart, browsing history or other status, they need to be managed.

Under the architecture of separation of front and back ends, RESTful API has become the mainstream server-side development method. In API clients, cookies are the most common state storage method. This article will introduce how to use Java Servlet Cookie for API client state management.

1. What is Cookie

Cookie is a small text file that can be stored on the client (usually a browser) to store application-related state information . When a user accesses a web application, the application can send a cookie to the user's browser through an HTTP response header, and the browser saves the cookie locally. The next time the user visits the application, the browser will automatically send the cookie associated with it, and the application can read state information from the cookie.

Cookies have the following characteristics:

• Cookies are stored in the form of key-value pairs;
• Each Cookie has a name and corresponding value;
• The data stored in the cookie will be sent to the server along with the HTTP request;
• You can set which domain name and path to send the cookie to;
• You can set the expiration time and security of the cookie Properties etc.

Therefore, Cookies have become a very convenient way to manage state. Applications can use Cookies to store state information such as session IDs and user key values.

2. Use of Cookies in Java Servlet

In Java Servlet, the Cookie object is used to process Cookies. Using cookies to store state information requires the following steps:

1. Creating a Cookie object

You can use the following code to create a Cookie object:

Cookie cookie = new Cookie("cookie_name", "cookie_value");

Among them, "cookie_name" is the name of the cookie, and "cookie_value" is the value of the cookie.

2. Add Cookie to HTTP response header

response.addCookie(cookie);

3. Get Cookie from HTTP request header

Cookie[] cookies = request.getCookies();

4. Reading data from Cookie

for (Cookie cookie : cookies) {
    if (cookie.getName().equals("cookie_name")) {
        String cookieValue = cookie.getValue();
        // Do something with cookieValue
    }
}

When the client sends an HTTP request, the browser sends the locally stored Cookie to the server. Use Servlet's response.addCookie(cookie) to add the cookie to the response header. When the browser receives the response, it will store the cookie locally and send the cookie to the server through the header on the next request.

The following is an example of specific implementation.

3. Example of implementing a session management

The following is an example of a simple Java Servlet session management. In this example, we use Cookie to store the user session ID, To achieve the effect of client state management.

import javax.servlet.ServletException;
import javax.servlet.http.Cookie;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;
import java.util.Optional;
import java.util.UUID;

public class SessionServlet extends javax.servlet.http.HttpServlet {
    private static final long serialVersionUID = -3436700273785948283L;
    private static final String SESSION_COOKIE_NAME = "session_id";

    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        resp.setContentType("text/html");
        resp.setCharacterEncoding("UTF-8");

        Optional<Cookie> sessionCookie = getSessionCookie(req);
        sessionCookie.ifPresent(cookie -> {
            resp.getWriter().write(String.format("Found session cookie with value %s
", cookie.getValue()));
        });

        if (!sessionCookie.isPresent()) {
            String sessionId = generateSessionId();
            Cookie newSessionCookie = new Cookie(SESSION_COOKIE_NAME, sessionId);
            resp.addCookie(newSessionCookie);
            resp.getWriter().write(String.format("No session cookie found, created session with id %s
", sessionId));
        }
    }

    private String generateSessionId() {
        return UUID.randomUUID().toString();
    }

    private Optional<Cookie> getSessionCookie(HttpServletRequest req) {
        Cookie[] cookies = req.getCookies();
        if (cookies != null) {
            for (Cookie cookie : cookies) {
                if (cookie.getName().equals(SESSION_COOKIE_NAME)) {
                    return Optional.of(cookie);
                }
            }
        }
        return Optional.empty();
    }
}

In the above code, we first check whether the client already has a Session ID. If not, generate one, save it in Cookie and return it to the client. If the Session ID already exists, it is output to the client.

This example demonstrates how to use Java Servlet Cookie for state management.

4. Conclusion

This article introduces how to use Java Servlet Cookie to manage API client status. In web applications, state management is very important, and cookies are a very common way of state management. In Java Servlet, using cookies requires creating a Cookie object, adding it to the HTTP response header, obtaining the Cookie from the HTTP request header, and reading data from the Cookie. By implementing a simple session management example, we show how to use Servlet Cookies for state management.

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