When the Servlet container initializes a Servlet, it will create a ServletConfig object for this Servlet and pass the ServletConfig object as a parameter to the Servlet. The initialization parameter information of the current Servlet can be obtained through the ServletConfig object.
There can be multiple ServletConfig objects in a Web application, and a Servlet can only correspond to one ServletConfig object, that is, the initialization parameters of a Servlet are only valid for the current Servlet.
Get ServletConfig object
There are generally two ways to obtain ServletConfig objects:
1. Extract directly from the init() method with parameters
public class ServletConfigDemo extends HttpServlet {
private ServletConfig servletConfig;
protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
//Get the name of the Servlet
this.servletConfig.getServletName();
}
@Override
public void init(ServletConfig config) throws ServletException {
//Extract the ServletConfig object from the init method with parameters
this.servletConfig = config;
}
}2. Call the getServletConfig() method provided by GenericServlet to obtain
//Call the getServletConfig method provided by GenericServlet to obtain the ServletConfig object ServletConfig servletConfig = this.getServletConfig();
ServletConfig interface
javax. The servlet package provides a ServletConfig interface, which provides the following methods.
| return type | method | Function description |
|---|---|---|
| String | getInitParameter(String name) | Return the corresponding initialization parameter value according to the initialization parameter name name. |
| Enumeration<String> | getInitParameterNames() | Returns an enumerated set of all initialization parameter names of the Servlet. If the Servlet has no initialization parameters, an empty set is returned. |
| ServletContext | getServletContext() | Returns a ServletContext object representing the current Web application. |
| String | getServletName() | Returns the name of the Servlet, that is, web The value of the <Servlet name> element in XML. |
Configure Servlet initialization parameters
There are two ways to configure Servlet initialization parameters:
- Use web XML configuration initialization parameters;
- Use @WebServlet to configure initialization parameters.
1. Use web XML configuration initialization parameters
On the web One or more <init param> elements can be used in XML to configure initialization parameters for servlets. The code is as follows.
<?xml version="1.0" encoding="UTF-8"?> <web-app xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns="http://xmlns.jcp.org/xml/ns/javaee" xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/javaee http://xmlns.jcp.org/xml/ns/javaee/web-app_4_0.xsd" id="WebApp_ID" metadata-complete="false" version="4.0"> <servlet> <servlet-name>MyServlet</servlet-name> <servlet-class>net.biancheng.www.MyServlet</servlet-class> <!-- Servlet Initialization parameters --> <init-param> <param-name>name</param-name> <param-value>Programming help</param-value> </init-param> <!-- Servlet Initialization parameters --> <init-param> <param-name>URL</param-name> <param-value>www.biancheng.net</param-value> </init-param> </servlet> </web-app>
The above configuration is described as follows:
- The <init param> element is a child element of <Servlet>, which needs to be used in the <Servlet> element, indicating that it is only valid for the current Servlet.
- The <param name> sub element represents the name of the parameter.
- The <param value> sub element represents the value of the parameter.
2. Use @WebServlet to configure initialization parameters
Initialization parameters can also be set for the Servlet through the initParams attribute of the @WebServlet. The code is as follows.
package net.biancheng.www;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebInitParam;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;
@WebServlet(urlPatterns = {"/MyServlet"}, initParams = {@WebInitParam(name = "name", value = "Programming help"),
@WebInitParam(name = "URL", value = "www.biancheng.net")})
public class MyServlet extends HttpServlet {
private static final long serialVersionUID = 1L;
@Override
protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
}
@Override
protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
doGet(req, resp);
}
}Get Servlet initialization parameters
Next, we use an example to demonstrate how to read the initialization parameters of a Servlet through the ServletConfig object.
Take servletDemo project as an example, in net.biancheng Under www package, create a class named ReadConfigServlet with the following code.
package net.biancheng.www;
import java.io.IOException;
import java.io.PrintWriter;
import java.util.Enumeration;
import javax.servlet.ServletConfig;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebInitParam;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
/**
*
* @author Programming help www.biancheng.net
* Get initialization parameters of Servlet
*
*/
@WebServlet(urlPatterns = { "/ReadConfigServlet" }, initParams = { @WebInitParam(name = "name", value = "Programming help"),
@WebInitParam(name = "URL", value = "www.biancheng.net") })
public class ReadConfigServlet extends HttpServlet {
private static final long serialVersionUID = 1L;
protected void doGet(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
response.setContentType("text/html;charset=UTF-8");
PrintWriter writer = response.getWriter();
// Get ServletConfig object
ServletConfig config = getServletConfig();
// Get servletName
String servletName = config.getServletName();
// Returns a collection of the names of the initialization parameters of the servlet
Enumeration<String> initParameterNames = config.getInitParameterNames();
// Traverse the collection to get the initialization parameter name
while (initParameterNames.hasMoreElements()) {
// Get initialization parameter name
String initParamName = initParameterNames.nextElement();
// Get the value of the corresponding initial parameter
String initParamValue = config.getInitParameter(initParamName);
// Output to page
writer.write(initParamName + " : " + initParamValue + "<br/>");
}
// Close flow
writer.close();
}
protected void doPost(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
doGet(request, response);
}
}
Start the Tomcat server and enter in the address bar“ http://localhost:8080/servletDemo/ReadConfigServlet ”, access the readconfigservlet, and the results are shown in the following figure.
