Related concepts of cookies and sessions

Cookie、Session

1. conversation

conversation ： The user opens a browser , Click a lot of hyperlinks , Visit multiple web Dimension , Close the browser , This process can be called conversation

Stateful conversation ： Sessions with access records

1. The server will give the client a cookie, When the client accesses next time, it will carry cookie Just visit cookie

2. The server registers that the client has accessed , Match to the client on the next access ; session

2. Two techniques for saving a session

cookie

• Client technology （ Respond to , request ）

session

• Server technology , Using this technology , Can save user session information , Information or data can be stored in Session in .

Common scenes ： After the website logs in , Don't log in again , The second visit went straight up ！

3.Cookie

1. Get... From the request cookie Information

2. The server responds to the client cookie

cookie Related methods ：

Cookie[] cookies = req.getCookies(); // get cookie
cookie.getName() // get cookie The key 
cookie.getValue() // get cookie The value in  
new Cookie("LastLoginTime",System.currentTimeMillis()+"") // Create a new one cookie
cookie.setMaxAge(24*60*60); // Set up cookie The period of validity 
resp.addCookie(cookie); // The response gives the client a cookie

Case study ：

@Override
protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
    
    // Solve the Chinese garbled code 
    req.setCharacterEncoding("utf-8");
    resp.setCharacterEncoding("utf-8");
    resp.setContentType("text/html; charset=utf-8");
    PrintWriter out = resp.getWriter();
    Cookie[] cookies = req.getCookies();
    // Judge cookie Whether there is 
    if(cookies == null){
    
        out.println(" First visit to the website ");
    }else{
    
        out.write(" Your last visit was ：");
        for (Cookie cookie : cookies) {
    
            if("LastLoginTime".equals(cookie.getName())){
    
                // obtain cookie The value in 
                long time = Long.parseLong(cookie.getValue());
                Date date = new Date(time);
                DateFormat dfd = DateFormat.getDateInstance(DateFormat.MEDIUM,Locale.CHINA);
                DateFormat dft = DateFormat.getTimeInstance(DateFormat.MEDIUM, Locale.CHINA);
                out.write(dfd.format(date)+dft.format(date));
            }
        }
    }
    Cookie cookie = new Cookie("LastLoginTime",System.currentTimeMillis()+"");
    // Set up cookie Valid for one day 
    cookie.setMaxAge(24*60*60);
    resp.addCookie(cookie);
}

cookie： It is usually saved in the local user directory appdate;

A website cookie Whether there is an upper limit ？

• One cookie Only one message can be saved ;
• One web Websites can send multiple messages to browsers cookie, At most 20 individual cookie;
• cookie There's a limit to the size 4kb
• 300 individual cookie Browser limit

Delete cookie：

• Do not set the validity period , Close the browser , Automatic failure ;
• Set the validity period to 0;

Be careful ： stay cookie Is best used when the value of is in Chinese URLEncoder.encode() To code , Prevent Chinese miscoding . When taking value URLDecoder.decode() To decode .

4.Session( a key )

What's up? session：

• The server will create one for each user session object
• One session Monopolize a browser , As long as the browser is not closed , This Session There is a ;
• After the user logs in , The entire website is accessible --> Save user information ;
  
  Session Common methods
  

Session and cookie The difference between ：

• Cookie It is to write the user's data to the user's browser , Browser save （ You can save multiple ）
• session Write the user's data to the user's exclusive Session in , Server side save （ Keep important information , Reduce the waste of server resources ）
• session Objects are created by the server ;

Use scenarios ：

• Save the information of a login user ;
• Shopping cart information ;
• Data often used throughout the website , We save it to Session in ;

Use session：

@Override
protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
    
    // Solve the mess 
    req.setCharacterEncoding("utf-8");
    resp.setCharacterEncoding("utf-8");
    resp.setContentType("text/html; charset=utf-8");
    PrintWriter out = resp.getWriter();
    // obtain Session
    HttpSession session = req.getSession();
    // to session There's something in it 
    session.setAttribute("name",new Person(" Zhang San ",18));
    // obtain session Of id
    String id = session.getId();
    // Judge session Is it newly created 
    if(session.isNew()){
    
        out.write("session Create success ,ID:"+id);
    }else{
    
        out.write("session Already exist ,id："+id);
    }
    //Session What did you do when you created it 
    // Cookie jsessionid = new Cookie("JSESSIONID", id);
    // resp.addCookie(jsessionid);
}

//Person class 
public class Person {
    
    private String name;
    private int age;

    @Override
    public String toString() {
    
        return "Person{" +
                "name='" + name + '\'' +
                ", age=" + age +
                '}';
    }

    public Person() {
    
    }

    public Person(String name, int age) {
    
        this.name = name;
        this.age = age;
    }

    public String getName() {
    
        return name;
    }

    public void setName(String name) {
    
        this.name = name;
    }

    public int getAge() {
    
        return age;
    }

    public void setAge(int age) {
    
        this.age = age;
    }
}

obtain session Information in

// obtain Session
HttpSession session = req.getSession();

Person name = (Person) session.getAttribute("name");
System.out.println(name);

Cancellation session

// obtain Session
HttpSession session = req.getSession();
session.removeAttribute("name");
// Cancellation session
session.invalidate();

Session expires automatically ：web.xml To configure

<!--  Set up Session Default expiration time for -->
    <session-config>
<!-- 15 Minutes later session Automatic failure , In minutes -->
        <session-timeout>15</session-timeout>
    </session-config>