JSP uses application object to implement simple counting of websites

Description: JSP uses the application object to implement simple counting of websites

Code: Note that web development is a multi-threaded operation, so synchronization is required

<%@ page language="java" contentType="text/html" pageEncoding="UTF-8"%>
<!DOCTYPE html>
<html>
<head>
    <meta charset="UTF-8">
    <title>JSP</title>
</head>
<body>
    <%
        // Add a synchronization method to ensure +1
        synchronized(application) {
            // Get the last count, note that the application returns the Object type so it needs to be converted
            String data = (String)application.getAttribute("count");
            if (data == null) {
                application.setAttribute("count", "1"); // first visit
            }
            else {
                int i = Integer.parseInt(data); // Get the last data
                i++;                                        // +1
                application.setAttribute("count", Integer.toString(i)); // rewrite
            }
        }
    %>
    <%-- Use EL for input, you can also use application.getAttribute("count") input--%>
    <h2>You are the ${count} visitor! </h2>
</body>
</html>

Disadvantage: The count value will be +1 every time it is refreshed, so it needs to be improved

Improvement: Use the isNew method of the session object to determine whether it is a new user, if it is, +1, otherwise it will not be added

<%@ page language="java" contentType="text/html" pageEncoding="UTF-8"%>
<!DOCTYPE html>
<html>
<head>
    <meta charset="UTF-8">
    <title>JSP</title>
</head>
<body>
    <%
        // Join the session to determine whether it is a new user
        if (session.isNew()) {
            
            // Add a synchronization method to ensure +1
            synchronized(application) {
                // Get the last count, note that the application returns the Object type so it needs to be converted
                String data = (String)application.getAttribute("count");
                if (data == null) {
                    application.setAttribute("count", "1"); // first visit
                }
                else {
                    int i = Integer.parseInt(data); // Get the last data
                    i++;                                        // +1
                    application.setAttribute("count", Integer.toString(i)); // rewrite
                }
            }
        }
    %>
    <%-- Use EL for input, you can also use application.getAttribute("count") input--%>
    <h2>You are the ${count} visitor! </h2>
</body>
</html>

Tip: There are still shortcomings, because the application object will disappear after restarting the server, so if you develop counters, you should use the method of saving to a file. When the user accesses and is a new user, +1 is added and the data is written to the file, so that As a result, even if the server is changed, it will not affect the previous visitor statistics.

Final improvements:

<%@ page language="java" contentType="text/html" pageEncoding="UTF-8"%>

<%-- import package--%>
<%@ page import="java.io.* "%>
<%@ page import="java.math.* "%>
<%@ page import="java.util.* "%>
<!DOCTYPE html>
<html>
<head>
    <meta charset="UTF-8">
    <title>JSP</title>
</head>
<body>
    <%-- BigInteger can be used if the amount of data is large --%>
    <%-- Define global count for counting--%>
    <%!
        BigInteger count = null;
    %>
    <%-- define global method--%>
    <%!
        // define file operation method
        public BigInteger getCount(File file) {
            BigInteger count = null;
            
            // The operation of the file will be abnormal, so it needs to be caught
            try{
                if (file.exists()) { // must determine if the file exists
                    Scanner scan = new Scanner(new FileInputStream(file));
                    // There is only one data in the file, so just if not while
                    if (scan.hasNext()) {
                        count = new BigInteger(scan.next()); // get data
                    }
                    scan.close(); // close the input stream
                }
                else{
                    count = new BigInteger("0"); // new
                    save(file, count);                      // 保存
                }
            }
            catch(Exception e){
                e.printStackTrace ();
            }
            return count; // return value
        }

        // save document
        public void save(File file, BigInteger count) {
            try{
                PrintStream out = null; // instantiate the print stream
                out = new PrintStream(new FileOutputStream(file));
                out.print(count); // write data
                out.close(); // close the print stream
            }
            catch(Exception e){
                e.printStackTrace ();
            }
        }
    %>
    <%
        // Get the real path of the virtual path and create a new count.txt file in the root directory
        String path = this.getServletContext().getRealPath("/") + "count.txt";
        File file = new File(path);
        if (session.isNew()) {
            synchronized(this){
                count = getCount(file); // get the value
                count = count.add(new BigInteger("1")); // count value + 1
                System.out.println(count);
                save(file, count);                               // 保存
            }
        }
    %>

    <%-- Consider the case of count==null--%>
    <h2>You are the <%=(count==null)? 0 : count%> visitor! </h2>
</body>
</html>

Conclusion: The last improved version can be applied to any website, no matter how big the data is

JSP uses application object to implement simple counting of websites

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