Servlet overview

1. A servlet is a small Java program (that is, a server-side applet) running in a Web server. Servlets typically receive and respond to requests from Web clients over HTTP (Hypertext Transfer Protocol).
2. Preparation process:
   1. Write a java class to implement the servlet interface
   2. Modify the web.xml file to provide the servlet with an accessible URI address

<!--Create a servlet Example-->
<servlet>
	<servlet-name>servletdemo1</servlet-name>
	<servlet-class>day08_00_servlet.servletdemo</servlet-class>
</servlet>
<!--to servlet Provide a url address-->
<servlet-mapping>
	<servlet-name>servletdemo1</servlet-name>
	<url-pattern>/demo1</url-pattern>
</servlet-mapping>

<!--among/demo1 Equals http://localhost:8080/.../demo1，If not'/'，Will become http://localhost:8080/demo1-->

1. Deploy application to tomcat server
2. test

servlet life cycle

1. Construct the servlet and initialize it using the init method
2. Handles all calls to the service method from the client
3. Take the servlet out of the service, destroy it with destroy method, recycle it and terminate it

• Instantiation initialization service destroy
• ps: how to create a servlet when the server starts:

 <servlet>
	<servlet-name>servletdemo1</servlet-name>
	<servlet-class>day08_00_servlet.servletdemo</servlet-class>
	<load-on-startup>2</load-on-startup>
</servlet>

Three ways to create a Servlet

1. Implement the javax.servlet.Servlet interface (the first is the method shown above)
2. Inherit javax.servlet.GenericServlet class (adapter mode)
3. Inherit javax.servlet.http.HttpServlet class (template design pattern) (commonly used in development) (do not override the service method of the parent class)

public class ServletDemo3 extends HttpServlet{

	@Override
	protected void doGet(HttpServletRequest req, HttpServletResponse resp)
			throws ServletException, IOException {
		System.out.println("*******doGet *******");
		System.out.println(req.getRemoteAddr());
	}

	@Override
	protected void doPost(HttpServletRequest req, HttpServletResponse resp)
			throws ServletException, IOException {
		System.out.println("**********doPost**********");
	}
	
}

Servlet mapping:

1. You can configure multiple mapping paths:

<servlet-mapping>
	<servlet-name>servletdemo1</servlet-name>
	<url-pattern>/demo1</url-pattern>
</servlet-mapping>
<servlet-mapping>
	<servlet-name>servletdemo1</servlet-name>
	<url-pattern>/*</url-pattern>
</servlet-mapping>
<servlet-mapping>
	<servlet-name>servletdemo1</servlet-name>
	<url-pattern>*.do</url-pattern>
</servlet-mapping>

1. Wildcard * can represent any string

<!--/* Any string can be accessed-->
<servlet-mapping>
	<servlet-name>servletdemo1</servlet-name>
	<url-pattern> /* </url-pattern>
</servlet-mapping>
<!--with*.String requests can be accessed, but do not add/-->
<servlet-mapping>
	<servlet-name>servletdemo1</servlet-name>
	<url-pattern>*.do</url-pattern>
</servlet-mapping>
<!--with /action The first requests are accessible-->
<servlet-mapping>
	<servlet-name>servletdemo1</servlet-name>
	<url-pattern>/action/*</url-pattern>
</servlet-mapping>

1. Match rule: priority: absolute match > / start match > extension method match

Thread safety of Servlet

Single instance: multiple threads per access
Problem: the following code example like this, because it is multithreaded, will have the same number, resulting in inaccurate data

public class ServletDemo3 extends HttpServlet {
	int num = 1;
	protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
		num++;
		System.out.println(num);
	}
	protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
		doGet(request, response);
	}

}

Solution: do not write global variables, but local variables

public class ServletDemo3 extends HttpServlet {
	protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
		int num = 1;
		num++;
		System.out.println(num);
	}
	protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
		doGet(request, response);
	}

}

Servlet getting configuration information

Using ServletConfig

1. Method 1:

public class ServletDemo3 extends HttpServlet {
	//1. Define local variables
	private ServletConfig config;
	public void init(ServletConfig config) throws ServletException {
	//2. Get the config of the parent class
		this.config = config;
	}
	protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
	//3. Get by getInitParameter method
		String name = config.getInitParameter("");
	}
	protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
		doGet(request, response);
	}
}

1. Mode two:

public class ServletDemo3 extends HttpServlet {
	
	protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
	//Obtained by using the method of inheriting the parent class
		String name = this.getServletConfig().getInitParameter("");
	}
	protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
		doGet(request, response);
	}
}

1. Mode three:

public class ServletDemo3 extends HttpServlet {
	
	protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
		String name = this.getInitParameter("");
	}
	protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
		doGet(request, response);
	}
}

ServletContext(*)

1. Overview: ServletContext: represents the whole application. An application has only one ServletContext object, single instance.
2. Effect:
   1. Domain object: within a certain range (in the current application), multiple servlets can share data
   * 1.1
   *1.2 common methods:
   *1.2.1 void setAttribute (string name, object value); / / add data to the map attribute of the ServletContext object
   *1.2.2 Object getAttribute(String name); / / get data from the map of ServletContext object
   *1.2.3 void rmoveAttribute(String name); / / remove data according to name
   2. Global configuration information can be obtained
   *1. String getInitParameter(String name) / / get value according to the key in the configuration file
   3. Access to resources
   *String getRealPath(String path); / / get the absolute path of the resource according to the resource name
   4. It can realize Servlet forwarding

//Implement request forwarding
ServletContext sc = this.getServletContext();
RequestDispatcher rd = sc.getRequestDispatcher(String path);//path means to jump there
rd.forward(request,response);