JavaWeb study notes

getRemoteAddr() gets the client IP address

getContextPath() gets the root path of the application

getMethod() gets the request method

getRequestURI() gets the requested URI: /project name/…

getServletPath() gets the servlet path: without project name

13. Design a pure Servlet single page (oa project)

• 1. Design database:
• Use the dept table in scott

• 2. Design page:

• index.html main page

• list.html list page
• detail.html detail page
• modify.html modify page
• add.html new page
• 3. Function:

• delete

• New
• list
• Details
• Revise
• 4. Implement functions:

• EZ for me

14.Web application forwarding and redirection

• Forward:
• request.getRequestDispatcher(“/path”).forward(request,response);
• After the request is forwarded, the path in the browser address bar remains unchanged.
• Redirect:
• response.sentRedirect("/project name/path");
• The browser's address will change after forwarding.
• When redirecting, the browser sends two requests to the server, so the final path will change.
• Note: request.getContextPath() can obtain the root path of the application.

15. Develop Servlet based on annotations

• The first annotation: @WebServlet

import jakarta.servlet.annotation.WebServlet; 

1. @WebServlet attribute:

* name attribute: used to specify the servlet name, equivalent to <servlet-name>
* urlPatterns attribute: used to specify the mapping path of the Servlet, multiple strings can be specified<url-pattern>
* loadOnStartUp attribute: used to specify whether to load the Servlet when the server starts, equivalent to <load-on-startup>
* Important: value attribute: equivalent to the urlPatterns attribute, specifying the mapping path, but in the annotation, the attribute name equal to value can be omitted. In other words, we can write Servlet mapping like this:
* @WebServlet(“/url”)
2. How to use the annotation object: @annotation object (attribute name = attribute value, attribute name = attribute value)

16. Use template method design pattern to optimize oa project

• To solve the problem of class explosion, use a DeptServlet to complete all functions.
• Perform different functions according to the path returned by the front end, such as doList, doAdd, etc.

17. Analyze the shortcomings of using pure Servlet to develop web applications

High coupling requires writing front-end code in a java program.

• Unable to get timely prompts when errors occur.
• The code is not beautiful and error-prone.

solve:

• Let a program translate the front-end code to generate servlet code. This is the next technology: JSP

17.1 About the session mechanism of B/S structure

• Session, English name: session

• The principle of session:

• The browser stores a sessionid, and the server stores a sessionid. If the two IDs match, a session can be found.

• The first time request.getSession() is called, there is no sessionid. Create a new sessionid. This sessionid is stored in the browser cache, and then the sessionid is sent to the browser.
• Second call: The browser sends the sessionid to match with the server and finds a session.

• Why does the session expire when the browser is closed:

• Because the cache will disappear when the browser is closed, and when the browser is reopened, there will be no previous sessionid on the browser side.

• Browser cache status via cookie

• Is there any way to invalidate the session:

• 1. Timeout mechanism: available in web.xml file
• <session-config>
• <session-timeout>30</session-timeout>
• </session-config>
• The session setting will expire after more than 30 minutes.
• 2. Manual destruction: such as the safe exit button when exiting the online banking system.
• Use session.invalidate() method

• To disable cookies in your browser:

• The server sends the cookie normally, but the browser refuses to accept it, causing a new sessionid to be created every time.

17.2 About Cookies

• Each session corresponds to a sessionid
• For session-related cookies, this cookie is saved in the browser's "running memory".
• The cookie is ultimately stored in the browser client.
• Session saves the session state on the server side.
• Cookies can be saved to your hard drive.
• The role of cookies
• Save session state
• Cookie classic case:
• No login required for 126 mailboxes within 30 days. Implementation principle:
• The user enters the correct username and password and chooses not to log in for 30 days. The browser will save the cookie on the hard disk. When logging in within 30 days, the cookie will be loaded and the information will be automatically submitted to the server. The server will receive the cookie and obtain the username and password. , the login is successful after passing the verification.
• The cookie stores name and value.
• response.addCookie(Cookie cookie) The server sends cookies to the browser
• cookie.setMaxAge(int seconds) sets the cookie expiration time.
• If the cookie validity time is not set, it will be stored in the browser's running memory, the browser will be closed, and the cookie will disappear.
• As long as the cookie validity time is set to be greater than 0, it will be saved to the hard disk file.
• Set the cookie validity period equal to 0 and delete the cookie file. It is mainly used to delete cookies with the same name.
• Setting the cookie validity period to less than 0 means that the cookie will not be stored in the hard disk file, but will be placed in the browser's running memory.
• Use cookies to implement the login-free function for ten days:
• Add a checkbox to avoid logging in for ten days and set the name and value. This will be submitted to the server side with the submit button.
• After the server determines that the login is successful, it determines whether it has received data from the check box. If so, it creates two new cookies to store the user name and password.
• The welcome page is changed to a custom welcome servlet. Here, it is judged whether the cookie is empty. If it is empty, it jumps to the login page. If it is not empty, it traverses each cookie and obtains two cookies named username and userpwd as the user name and password. Save it.
• Obtain the username and password of the cookie and verify the database. If correct, it will jump directly to dept/list. If it is incorrect, it will jump to the index.jsp login page.
• **Important: Delete the cookie when exiting the system, use the same name overlay method, set a cookie with the same name, set the value to null, and finally respond.addCookie()
• Note: The path must be the same when overwriting! ! ! SetPath was used before, so the path must be consistent when deleting, otherwise it will be regarded as a new cookie! ! ! !

18.JSP

• My first JSP program
• There is an index.jsp file outside the WEB-INF directory. This file has no content.
• Open the browser to visit, a blank page.
• JSP is actually a Servlet class
• The first access of JSP is relatively inefficient because neither the java file nor the compiled class file is generated.
• Content written directly in JSP will be placed in out.print, which is equivalent to writing front-end code.
• JSP response garbled problem:
• Add <%@page contentType="UTF-8"%> at the beginning
• Indicates that the character set used is UTF-8
• How to write java program in JSP?
• <%javacode，What is written on this side is consideredjavaprogram，translated toserviceinternal;%>
• Comments for JSP:
• <%–Comment–%>
• How to translate to outside the service method
• <%!%>
• This syntax is rarely used and is not recommended because:
• Outside the method body, there are thread safety issues under high concurrency when modifying static variables and instance variables.
• JSP output format:
• Output a java variable to the browser:
• out.write() can only output character-related values.
• out.print() can output values ​​of numbers, characters, etc.
• Conveniently output the value of java variables
• Previously: <% out.write(n)%>
• Improvement: <%=n%>
• n will be translated into out.print() brackets
• n is a variable. Note: there is no semicolon after the variable.
• If you want to output multiple variables at one time, use the + sign to connect them, which is the same as Java syntax.
• JSP syntax summary:

• Written directly, it is output in html, translated into out.print(""); directly translated into double quotation marks, it is used as a string by the Java program and printed on the web page.

• <%%> is java code (remember to add; semicolon)

• The code in <%!%> is translated outside the service method and used to define member variables, methods, static code blocks, etc.

• The code in <%=%> is translated into out.print(), which is equivalent to outputting java variables. It is translated into out.print() without ""

• <%@page %>

• page directive, set the content of the response through contentType

• <%@page import="java.util.List"%>Import package in jsp
• <%@page errorPage="/error.jsp"%>Direct to error.jsp after error
• <%@page isErrorPage="true"%> allows the use of Exception, one of the nine built-in objects of JSP, and is used in conjunction with <%exception.printStackTrace()%>.

• JSP nine built-in objects

• HttpServletRequest request request domain

• HttpServletResponse response
• PageContext pageContext page request domain
• HttpSession session session domain
• exception = JspRuntimeLibrary.getThrowable(request)
• ServletContext application application domain
• ServletConfig config
• JspWriter out
• Object page
• 4 scope sizes:
• pageContext < request < session < application
• The above four scopes have methods: setAttribute(), getAttribute(), removeAttribute()
• Scope usage principle: Use as small a scope as possible.

19. Further optimization of the oa project

• Implement login function:
• Step 1. Design the database table with username and password.
• Step 2. Design a web page that requires user name and password. There is a login button. Click to submit the data to the background.
• Step 3. Write a servlet in the background to verify the user name and password.
• Login successful: jump to list and display information
• Login failed: Login failed page or return to login page

EL expression

Expression Language is a part of jsp. It has three major functions:

• 1. You can obtain data in the scope
• 2. Convert data into string
• 3. Output data to the browser

The format of EL expression: ${expression} The expression writes the name when setAttribute! ! Do not add "" double quotes

EL expressions need to output specific values ​​in a certain class and are connected with dots. What is connected here is the get method, which has nothing to do with member variables.

When an EL expression fetches data from a domain, it fetches data in ascending order of the domain.

pageContext < request < session < application

EL expressions have four implicit fields:

• pageScope
• requestScope
• sessionScope
• applicationScope

There are two methods for fetching data in EL expressions:

• .

• [“”]

• [] Entering the array element subscript between square brackets can also extract the element of the subscript. For example ${usernames[0]}

• If you store an array within an array:

• Then you can use ${depts[0].dname} to get the dname value in the array with subscript 0.

• How to locally ignore EL expressions? :

• \${} preceded by a backslash

• If you want to disable EL expressions globally, use ${@ page isELIgnore = “true”}

• Get the root of the application through an EL expression:

• There is no implicit request object in the EL expression, but there is pageContext, so:

• ${pageContext.request.contextPath}
• EL expression implicit objects:

1. pageContext
2. param
3. paramvalues
4. initParam

 # JSTLtag library

* what isJSTLtag library？

  * help us reducejavacode
* usage：use `<%@ taglib prefix="c" uri="http://java.sun.com/jstl/core_rt" %>Import core library`
* ```
  <c:forEach items="{list}" var="d">
      Department Number：{d.deptno}<br>
      Department name：{d.dname}<br>
  </c:forEach> 
```java

items is the collection to be traversed (EL expression can be used), var is the variable that stores the elements in the collection

</code></pre>

<p><c:if test="{empty param.username}">
    <h1>Username can not be empty！</h1>
</c:if> 

```java



The value in test must be of boolean type, and EL expressions can be used.


```
<c:forEach var="i" begin="0" end="10" step="1">
i:${i}<br>
</c:forEach>
```c


Equivalent tofor(int i = 0;i <= 10;i++){System.out.println(i)}


varStatusThe object hascountAttributes，from1Start incrementing，Mainly used for numbering/serial number。

```
<c:forEach var="i" begin="0" end="10" step="1" varStatus="v">
    serial number{v.count} i:{i}<br>
</c:forEach> 
```java



```
<c:choose>
<c:when test="{param.age<12}">
child
</c:when>
<c:when test="{param.age < 18}">
teenager
</c:when>
<c:when test="{param.age<30}">
youth
</c:when>
<c:when test="{param.age < 55}">
middle aged
</c:when>
<c:otherwise>
elderly
</c:otherwise>
</c:choose>
```xml


Similar toif else if else

Transformationoa

useELexpression andJSTLTag library transformationoaproject。
learn：
HTMLThere is one in <base >Label，You can set the root path of the project
Any path that does not begin with"/"started，will be added in frontbaseLabel。
Dynamic acquisitionbaseall paths in（useELexpression）

```
protocol：{pageContext.request.scheme} http 
server name：{pageContext.request.serverName} localhost
The port number：{pageContext.request.serverPort} 8080
Item name:{pageContext.request.contextPath} oa 
```xml



filter


Filter inherits Filter class
Configure the Filter class in xml:

```
<filter>
<filter-name>filter1</filter-name>
<filter-class>com.listener.Filter1</filter-class>
</filter>
<filter-mapping>
<filter-name>filter1</filter-name>
<url-pattern>*.do</url-pattern>
</filter-mapping>
        ```java


*   filter-mappingThe order ofFilterexecution priority


Configure using annotationsFilter：@WebFilter({“*.do”})

When using annotations, the order of execution is sorted by class name.。




chain.doFiltermethod can be executed nextFilter，if there is notFilter，Execute the same pathservlet。
Filterlife cycle？

Filterlife cycle andservletObject consistent。
andservletThe difference is that：It will be instantiated when the server startsFilterobject，andservletWon't。
Filterpriority ratioservlethigh。


rewriteoaproject

Design Patterns：chain of responsibility design pattern

Loop call，Works like a stack。
To keep changes closed，Expand the principle of openness。

listenerListener

The listener isServleta member of the norm
ServletWhat listeners are provided in the specification？

jakarta.servletBao Xia

ServletContextListener
ServletContextAttributeListener
ServletRequestListener
ServletRequestAttributeListener


jakarta.servlet.httpBao Xia：

HttpSessionListener
HttpSessionBindingListener
HttpSessionAttributeListener
HttpSessionIdListener
HttpSessionActivationListener


Steps to implement a listener：


<listener>
        <listener-class>com.listener.ListenerTest</listener-class>
    </listener> 







Or annotation: @WebListener

The listener is called at special times and is automatically called by the server without the intervention of the web programmer.
Classes that inherit ServletContextAttributeListener do not need @WebListener

Realize the function of counting online people