Java servlet knowledge summary

Ardun 2021-01-21 19:40:24
java servlet knowledge summary



Servlet summary

Why study Servlet

Java Web The evolution process can be divided into 4 Stages :

  • Servlet + jdbc + jsp
  • Spring + Struts2+ Hibernate(SSH)
  • Spring + SpringMVC + Mybatis(SSM)
  • Microservice stage

The first two stages are basically history , Today, Spring The family rules the world .

Now it is rarely used directly in actual development Servlet 了 , But the bottom layer of each framework is still heavily used Servlet, Study Servlet It is very helpful for the understanding and learning of the following frameworks .

What is? Servlet

Servlet Is running on the Web A program on a server or application server , It's as coming from Web Browser or something HTTP Client requests and HTTP The middle tier between databases or applications on the server .

Servlet It's really just a follow Servlet Developed java class .Serlvet yes Called by the server , Running on the server side .

Servlet The most important effect is to be able to Dealing with browsers brings HTTP request , And return a response to the browser , So as to realize the interaction between browser and server .

Workflow

  • Tomcat Encapsulate the request submitted by the browser as HttpServletRequest object , At the same time, the output stream is encapsulated as HttpServletResponse object

  • Tomcat hold request、response As a parameter , call Servlet The corresponding method of , for example doGet(request, response) etc.

  • Servlet It mainly deals with business logic


Life cycle

stay Web In the container ,Servlet The main experience 4 Stages , Here's the picture :
Servlet Life cycle

  1. load Servlet. When Tomcat First visit Servlet When ,Tomcat Will be responsible for creating Servlet Example
  2. initialization . When Servlet When instantiated ,Tomcat Meeting call init() Method to initialize this object
  3. Processing services . When browser visit Servlet When ,Servlet Meeting call service() Method to process the request
  4. The destruction . When Tomcat When closed or detected Servlet From you to Tomcat When you delete it, it will automatically call destroy() Method , Let the instance free up the occupied resources . One Servlet If it's not used for a long time , Will also be Tomcat Automatically destroy
  5. uninstall . When Servlet Call complete destroy() After the method , Waiting for garbage collection .

If It's necessary to use this again Servlet, Will recall init() Method to initialize .

Just visit Servlet,service() It will be called .init() Only the first visit Servlet When it is called .

destroy() Only in Tomcat It's called when it's closed .


How to handle the request

Servlet It is realized. Servlet Interface Class , Realization Servlet Interface When , Need to achieve 5 A way

public interface Servlet {
void init(ServletConfig var1) throws ServletException;
ServletConfig getServletConfig();
void service(ServletRequest var1, ServletResponse var2) throws ServletException, IOException;
String getServletInfo();
void destroy();
}

For ease of use, you can directly Inherit HttpServlet class , This class has been implemented by default Servlet All methods in the interface .

Writing Servlet When , Just rewrite the method you need .

And this class is still in the original Servlet The interface adds some and HTTP Protocol processing related methods .

  • Servlet There are three ways to handle requests :
    • Realization service() Method
    • rewrite doGet()
    • rewrite doPost()

HttpServletRequest and HttpServletResponse

about Each visit request ,Servlet engine Will create a new HttpServletRequest Request object And a new HttpServletResponse The response object , namely request and response object .

HttpServletRequest Common methods

  • String getContextPath()
    Get context path
  • String getHeader(String headName)
    Get the value of the corresponding request header according to the specified request header .
  • String getRequestURI()
    Returns the resource name of the current request . Context path / Resource name
  • StringBuffer getRequestURL()
    Return to the browser address bar
  • String getRemoteAddr()
    Return to the client of the requesting server IP

Method to get request parameters :

  • String getParameter(String name)
    According to the parameter name , Get the value of the corresponding parameter .
  • String[] getParameterValues(String name)
    According to the parameter name , Get multiple values of the parameter .
  • Enumeration getParameterNames()
    Get the names of all request parameters
  • Map<String,String[]> getParameterMap()
    Return the request parameter Map aggregate .

HttpServletResponse Common methods

  • OutputStream getOutputStream():
    Get byte output stream : File download

  • Writer getWriter()
    Get character output stream : Output content

  • resp.setContentType("text/html;charset=utf-8")

    Set the encoding format and content type of file output

  • resp.sendRedirect()

    302 Redirect , A temporary jump

    301 Use another method

Servlet Is a singleton

explain

The browser has done this many times Servlet Request , In general , The server only creates one Servlet object , in other words ,Servlet object Once created , will Resident in memory , Serve subsequent requests , Until the server shuts down .

Every time you access the request object and the response object are new

For each access request ,Servlet The engine will create a new HttpServletRequest Request object and a new HttpServletResponse The response object , And then we put these two objects As a parameter Passed to it to call Servlet Of service() Method ,service() Method and then call other methods according to the request mode .

Thread safety problem

When multiple users access Servlet When , The server creates a thread for each user . When multiple users access concurrently Servlet When sharing resources, there will be thread safety problems .

principle

  1. If a variable It needs to be shared by multiple users , When accessing the variable , It needs to be locked
  2. If a variable No need to share , be Directly in doGet() perhaps doPost() Definition , There will be no thread safety issues

Configure by annotation Servlet

In previous development work , Write one at a time Servlet Need to be in web.xml File to configure

<servlet>
<servlet-name>ActionServlet</servlet-name>
<servlet-class>com.web.controller.ActionServlet</servlet-class>
</servlet>
<servlet-mapping>
<servlet-name>ActionServlet</servlet-name>
<url-pattern>/servlet/ActionServlet</url-pattern>
</servlet-mapping>

And when there's a lot in a project Servlet, Then the configuration file will become very messy , stay Servlet 3.0 After launch , We can use annotations to configure Servlet

@WebServlet(name = "ActionServlet", urlPatterns = "/servlet/ActionServlet")

Web How to jump between components

Request forwarding (forward)

Also known as Direct forwarding , Clients and browsers Only one request ,Servlet、HTML、JSP Or other information resources , from The second information resource responds to the request , In the request object request in , Saved objects for Every information resource is shared .

such as : from AServlet Request forwarding to BServlet

img

  • grammar :
request.getRequestDispatcher(path).forward(request, response);

Parameters :path, The resource path to jump to : Context path / Resource path

  • characteristic :
    • The address in the address bar remains unchanged
    • There's only one request
    • Resources are shared
    • You can visit WEB-INF The resource
    • Request forwarding You can't Cross domain access

URl Redirect (redirect)

Also known as Indirect forwarding (Redirect) the truth is that two HTTP request , When the server responds to the first request , Let the browser go to another URL Request , So as to achieve the purpose of forwarding .

such as : from AServlet Redirect to BServlet

img

  • grammar :
response.sendRedirect(String location);

Parameters :location, Resource path to forward to

  • characteristic :
    • The address in the address bar 【 Meeting 】 Change
    • There are two requests
    • In two Servlet The data in the request cannot be shared in
    • The final response is given by BServlet To decide , and AServlet It doesn't matter.
    • No access WEB-INF The resource
    • Request forwarding Sure Cross domain access

Reference resources

版权声明
本文为[Ardun]所创,转载请带上原文链接,感谢
https://javamana.com/2021/01/20210121193951013s.html

  1. 【计算机网络 12(1),尚学堂马士兵Java视频教程
  2. 【程序猿历程,史上最全的Java面试题集锦在这里
  3. 【程序猿历程(1),Javaweb视频教程百度云
  4. Notes on MySQL 45 lectures (1-7)
  5. [computer network 12 (1), Shang Xuetang Ma soldier java video tutorial
  6. The most complete collection of Java interview questions in history is here
  7. [process of program ape (1), JavaWeb video tutorial, baidu cloud
  8. Notes on MySQL 45 lectures (1-7)
  9. 精进 Spring Boot 03:Spring Boot 的配置文件和配置管理,以及用三种方式读取配置文件
  10. Refined spring boot 03: spring boot configuration files and configuration management, and reading configuration files in three ways
  11. 精进 Spring Boot 03:Spring Boot 的配置文件和配置管理,以及用三种方式读取配置文件
  12. Refined spring boot 03: spring boot configuration files and configuration management, and reading configuration files in three ways
  13. 【递归,Java传智播客笔记
  14. [recursion, Java intelligence podcast notes
  15. [adhere to painting for 386 days] the beginning of spring of 24 solar terms
  16. K8S系列第八篇(Service、EndPoints以及高可用kubeadm部署)
  17. K8s Series Part 8 (service, endpoints and high availability kubeadm deployment)
  18. 【重识 HTML (3),350道Java面试真题分享
  19. 【重识 HTML (2),Java并发编程必会的多线程你竟然还不会
  20. 【重识 HTML (1),二本Java小菜鸟4面字节跳动被秒成渣渣
  21. [re recognize HTML (3) and share 350 real Java interview questions
  22. [re recognize HTML (2). Multithreading is a must for Java Concurrent Programming. How dare you not
  23. [re recognize HTML (1), two Java rookies' 4-sided bytes beat and become slag in seconds
  24. 造轮子系列之RPC 1:如何从零开始开发RPC框架
  25. RPC 1: how to develop RPC framework from scratch
  26. 造轮子系列之RPC 1:如何从零开始开发RPC框架
  27. RPC 1: how to develop RPC framework from scratch
  28. 一次性捋清楚吧,对乱糟糟的,Spring事务扩展机制
  29. 一文彻底弄懂如何选择抽象类还是接口,连续四年百度Java岗必问面试题
  30. Redis常用命令
  31. 一双拖鞋引发的血案,狂神说Java系列笔记
  32. 一、mysql基础安装
  33. 一位程序员的独白:尽管我一生坎坷,Java框架面试基础
  34. Clear it all at once. For the messy, spring transaction extension mechanism
  35. A thorough understanding of how to choose abstract classes or interfaces, baidu Java post must ask interview questions for four consecutive years
  36. Redis common commands
  37. A pair of slippers triggered the murder, crazy God said java series notes
  38. 1、 MySQL basic installation
  39. Monologue of a programmer: despite my ups and downs in my life, Java framework is the foundation of interview
  40. 【大厂面试】三面三问Spring循环依赖,请一定要把这篇看完(建议收藏)
  41. 一线互联网企业中,springboot入门项目
  42. 一篇文带你入门SSM框架Spring开发,帮你快速拿Offer
  43. 【面试资料】Java全集、微服务、大数据、数据结构与算法、机器学习知识最全总结,283页pdf
  44. 【leetcode刷题】24.数组中重复的数字——Java版
  45. 【leetcode刷题】23.对称二叉树——Java版
  46. 【leetcode刷题】22.二叉树的中序遍历——Java版
  47. 【leetcode刷题】21.三数之和——Java版
  48. 【leetcode刷题】20.最长回文子串——Java版
  49. 【leetcode刷题】19.回文链表——Java版
  50. 【leetcode刷题】18.反转链表——Java版
  51. 【leetcode刷题】17.相交链表——Java&python版
  52. 【leetcode刷题】16.环形链表——Java版
  53. 【leetcode刷题】15.汉明距离——Java版
  54. 【leetcode刷题】14.找到所有数组中消失的数字——Java版
  55. 【leetcode刷题】13.比特位计数——Java版
  56. oracle控制用户权限命令
  57. 三年Java开发,继阿里,鲁班二期Java架构师
  58. Oracle必须要启动的服务
  59. 万字长文!深入剖析HashMap,Java基础笔试题大全带答案
  60. 一问Kafka就心慌?我却凭着这份,图灵学院vip课程百度云