Cookies and sessions

Cookie and Session

Go to the registration office of the hospital , You will get a doctor card , The card contains key information about the current patient （ amount to cookie）.
You can swipe your doctor card in every department , When you swipe your doctor card , You can use the server of the hospital , To get a series of information about the current patient .（ It's not just identity information , There are also previous cases ）
Visit card is equivalent to Cookie
On the data server of the hospital , It saves the user's information , That is, through session The way to save .

HttpServletRequest Related methods in class

getSession

HttpServletRequest Related methods in class .

Calling getSession The specific things to do when

1. Create a session
   First, get the request cookie Inside sessionId Field （ It is equivalent to the identity of the session ）
   Judge this sessionId Whether there is..., on the current server
   If it doesn't exist , Enter the create session logic .
   Create a session ： Will create a HttpSession object , And generate a sessionId, Then we'll put this sessionId As key, Put this HttpSession Object as value, Put this key value right , Give a to save to the server memory “ Hashtable ” In such a structure .
   After that , The server will return a HTTP Respond to , hold sessionId adopt Set-Cookie Field is returned to the browser .
   The browser can save this sessionId To Cookie It's in .

It doesn't have to be a hash table , But it must be a similar structure that can store key value pairs , And the data is in memory .

2. Get session
   Get the... In the request first cookie Inside sessionId Field （ It is also the identity of the session ）
   Judge this sessionId Whether there is..., on the current server （ That is, whether there is any in the hash table ）
   If there is , Just query this directly HttpSession object , And return by return value .

HttpSession
This object is also a “ Key value pair ” Structure .
HttpSession Each key value pair inside , be called “ attribute ”（Attribute）.
HttpSession There are two ways
（1）getAttribute（ Take key value pairs ）
（2）setAttribute（ Save key value pairs ）

getCookies

Cookie[] getCookies(): Get... In the request Cookie data .
The return value is Cookie An array of types , Each element is a Cookie object , Every Cookie The object contains two more properties ,name and value（ Or key value pairs ）
HTTP In the request cookie Fields are organized in key value pairs , The key value pairs here , General format , Use ; To split multiple key value pairs , Use = To split keys and values , These key value pairs will pass in the request cookie Field is passed to the server . After the server receives the request , It will be parsed , It can be interpreted as Cookie[] Form like this .

Cookie Here you can save any customized key value pairs
If it is a general key value pair , Use it directly getCookies To get
If it is a special key value pair （ Express sessionId The key/value pair ）, No need to use getCookies, Use it directly getSession, Just help us automatically from Cookie To take sessionId 了 .

HttpServletResponse Related methods in class

The response can be based on addCookie This method , To add a Cookie Information into the response message .
The key value pairs added here , Will act as HTTP In response set-Cookie Fields to represent .

Website login

Click the login button , Will send a login request to the server .

@WebServlet("/login")
public class LoginServlet extends HttpServlet {
    
    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
    
        // Handling user requests 
        String username=req.getParameter("username");
        String password=req.getParameter("password");
        // Determine whether the user name and password are correct 
        // Normally , This decision operation should be placed in the database for access 
        // Here for simplicity , Just write it in the code , Suppose the user name is "zhangsan", The password is "123"
        if("zhangsan".equals(username) && "123".equals(password)) {
    
            // Login successful 
            resp.sendRedirect("index");
        }else {
    
            // Login failed 
            resp.getWriter().write("login failed");
        }
    }
}

In the following jump to index When , The server should be able to obtain the identity information of the current user , So you need to create a session For later use . And in session Fill in some necessary identity information in the , For later use .

@WebServlet("/login")
public class LoginServlet extends HttpServlet {
    
    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
    
        // Handling user requests 
        String username=req.getParameter("username");
        String password=req.getParameter("password");
        // Determine whether the user name and password are correct 
        // Normally , This decision operation should be placed in the database for access 
        // Here for simplicity , Just write it in the code , Suppose the user name is "zhangsan", The password is "123"
        if("zhangsan".equals(username) && "123".equals(password)) {
    
            // Login successful 
            // Create a session , Save the necessary identity information 
           HttpSession httpSession= req.getSession(true);
           // Store key value pairs into the session , Necessary identification information 
            httpSession.setAttribute("username",username);

            resp.sendRedirect("index");
        }else {
    
            // Login failed 
            resp.getWriter().write("login failed");
        }
    }
}

@WebServlet("/index")
public class IndexServlet extends HttpServlet {
    
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
    
        // Return to a home page ( The home page is a simple html fragment )
        // You need to get the user name , from httpsession You can get it in the middle 
        // here getSession The parameter of must be false, In the previous login process , The session has been created , Here is to get the previous session directly 

        HttpSession session=req.getSession(false);
        String username=(String)session.getAttribute("username");
        resp.setContentType("text/html;charset=utf8");
        resp.getWriter().write("<h3> Welcome "+username+"</h3>");
    }
}

<body>
    <form action="login" method="post">
        <input type="text" name="username">
        <input type="password" name="password">
        <input type="submit" value=" Sign in ">

    </form>
    
</body>

In conversation , Can save any specified user information
Here, , You can realize a function , Record the number of times the current user has visited the home page .
After logging in , First visit home , On the home page , Currently accessed 1 Time , When you refresh the page , The times will add up .

Upload files

It is also a typical requirement in development

When uploading files , At the front end, you need form Forms
form In the form , Need to use a special type form-data
This is the time to submit documents , The browser will put the contents of the file as form-data The format of is constructed to HTTP In request . The server can use getPart To get .

One HTTP request , You can submit multiple files at once
Each file is called a part, Every part There is one. name（ Identification ）, Server code can be based on name Find the corresponding part.
Based on this part You can further obtain the file information , And proceed to the next stage
Part Class method as follows ：

@MultipartConfig
@WebServlet("/upload")
public class UpLoadServlet extends HttpServlet {
    
    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
    
       Part part= req.getPart("MyImage");
       System.out.println(part.getSubmittedFileName());
       System.out.println(part.getContentType());
       System.out.println(part.getSize());
       part.write("e:/ Drawing board /aaa.jpg");
       resp.setContentType("text/html;charset=utf8");
       resp.getWriter().write(" Upload successful ");

    }
}

<body>
    <form action="upload" method="post" enctype="multipart/form-data">
        <input type="file" name="MyImage">
        <input type="submit" value=" Submit ">
    
    </form>

</body>