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 .


  • 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


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 .


  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


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


  • 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


  • 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


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