1, Servlet overview

Element 'web-app' cannot have character [children]...

Find the reason:

Finally, after continuous testing, the problem is found: < servlet > < / servlet > and < servlet mapping > < servlet mapping >

There can be no comments between the two groups of labels, that is, you need to remove <-- Mapping -- > labels

After removing it, I typed the label again:

Type "demo1" at the website

At this time, there is no response at the web page, but the direct call at java is successful. , every time you refresh, a hello servlet appears

Execution steps:

1.tomcat loads the bytecode file corresponding to the full class name into memory, class.forName()

2. Create object   class.newInstance()

3. Call the method -- service(), the method in the interface

Execution principle:

1. After receiving the request from the client browser, the server receiver will parse the request URL path and obtain the resource path of the accessed servlet.

2. Find the Web.xml file and check whether there is corresponding < URL pattern > tag body content

3. If yes, find the corresponding < servlet class > full class name in the

4.tomcat will load the bytecode file into memory and create its object

5. Call its method

Next, let's introduce the servlet() method

Rebuild a servletDemo2, Don't forget to rewrite the tag in web.xml

 

package cn.itcast.web.servlet;
import javax.servlet.*;
import java.io.IOException;
public class servletDemo2 implements Servlet {
/*
servlet Method, don't forget to configure it in web.xml
*/
/*
*Initialization method
* When the servlet is created, it is executed only once
*/
@Override
public void init(ServletConfig servletConfig) throws ServletException {
System.out.println("init.....");
}

/*
*Get servletConfig object
* servletConfig: Servlet Configuration object for
*/
@Override
public ServletConfig getServletConfig() {
return null;
}
/*
*Methods of providing services
* Execute every time the servlet is accessed
*/
@Override
public void service(ServletRequest servletRequest, ServletResponse servletResponse) throws ServletException, IOException {
System.out.println("service");
}
/*
*Get Servlet information, version, author, etc
*/
@Override
public String getServletInfo() {
return null;
}
/*
*Destruction method
* When the server shuts down normally, it is executed only once
*/
@Override
public void destroy() {
System.out.println("destroy");
}
}

Lifecycle analysis of servlet s:

1. Created: execute the init method only once (indicating that a Servlet has only one object in memory, which is a singleton)

init executes only once in the log

2. Provide service: the service() method will be executed many times

3. Destroyed: execute the destroy() method once

The above three cycles have the following problems:

When is the servlet created during the creation phase?

By default, the servlet is created the first time it is accessed

sure Configure the creation time of the execution servlet

The steps are as follows:

Setting up web.xml

be careful:

Because servlets are singleton, there may be thread safety problems when multiple users access them.

Solution: try not to define member variables in the servlet. Even if you define member variables, do not modify the values

The execution service method of the service phase is provided and executed multiple times

This method is executed once every time it is called

Destroyed: destroy is executed once and only when the server is shut down normally.

Note: the destroy() method executes before the servlet is destroyed

The < load on startup > tag can be used to set the creation time of servlet in web.xml. The specific methods are as follows:

<servlet>
<servlet-name>demo2</servlet-name>
<servlet-class>cn.itcast.web.servlet.servletDemo1</servlet-class>
<!--appoint Servlet Time to create
1.When accessed for the first time, it is created at this time <load-on-startup>The value of is negative, and the default is-1
2.When the server starts, create a <load-on-startup>The value of is positive, usually at 0-10 between
-->
<load-on-startup>5</load-on-startup>
</servlet>
<servlet-mapping>
<servlet-name>demo2</servlet-name>
<url-pattern>/demo2</url-pattern>
</servlet-mapping>

3, Annotation configuration for servlet version 3.0

Benefit: you don't need to configure it in web.xml

Steps:

1. Create a Java EE project and select servlet3.0 or above

2. Define a class to implement servlet interface

3. Review methods

4. Use @ WebServlet annotation on the class for configuration

When creating a module, do not check the Web.xml under Version.

Check the WebServlet and select the WebServlet after @ instead of @, crtl+b

The virtual path needs to be configured. Write it after @ WebServlet

@WebServlet(urlPatterns = "/demo")

Test:

You can also write the resource path of the servlet directly, which can also be accessed

@WebServlet("/demo2")

get!!!