Servlet virtual path mapping

Posted by kpulatsu on Wed, 19 Jan 2022 05:31:05 +0100

The client accesses the resources in the Web server through the URL address. If the Servlet program wants to be accessed by the outside world, it must be mapped to a URL address. Often, the URL address is inconsistent with the physical path (storage location on the hard disk) of the Servlet program, so it is called a virtual path. The corresponding relationship between Servlet and virtual path is called Servlet virtual path mapping.

Servlet virtual path mapping can be divided into two categories:

Single mapping
Multiple mapping

The following describes how to implement single mapping and multiple mapping.

Servlet single mapping

Servlet single mapping means that a servlet is mapped to only one virtual path.

There are two ways to implement Servlet single mapping:

Using web XML to achieve a single mapping;
Use @ WebServlet to implement a single mapping.

1. web.xml implements a single mapping

On the web In the XML file, the < Servlet > and < Servlet mapping > elements are used to realize a single Servlet mapping. 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>
    <servlet-mapping>
        <servlet-name>MyServlet</servlet-name>
        <url-pattern>/myServlet</url-pattern>
    </servlet-mapping>
</web-app>

The above labels are described as follows:

The < Servlet > element is used to register the Servlet, that is, to give the Servlet a unique name.
< Servlet > contains two main sub elements < Servlet name > and < Servlet class >, which are respectively used to specify the name of the Servlet and the fully qualified name of the Servlet (package name + class name).
The < Servlet mapping > element is used to define the mapping between the Servlet and the virtual path.
< Servlet mapping > contains two sub elements < Servlet name > and < URL pattern >, which are used to specify the name and virtual path of the Servlet respectively.

2. @WebServlet implements a single mapping

In the @ WebServlet annotation, the value attribute is generally used to realize a single Servlet mapping. The code is as follows.

package net.biancheng.www;
import java.io.IOException;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
@WebServlet("/MyServlet")
public class MyServlet extends HttpServlet {
    private static final long serialVersionUID = 1L;
    public void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
    }
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        doGet(req, resp);
    }
}

You can also use the urlPatterns attribute to implement a single Servlet mapping. The code is as follows.

package net.biancheng.www;
import java.io.IOException;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
@WebServlet(urlPatterns = "/myServlet")
public class MyServlet extends HttpServlet {
    private static final long serialVersionUID = 1L;
    public void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
    }
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        doGet(req, resp);
    }
}

Servlet multiple mapping

Multiple mapping of servlets means that a Servlet can be mapped to multiple virtual paths. At this time, the client can access the same Servlet through multiple paths.

There are three ways to implement Servlet multi mapping:

Configure multiple < servlet mapping > elements.
Configure multiple < URL pattern > child elements.
Use a string array in the urlPatterns property of the @ WebServlet

1. Configure multiple < servlet mapping > elements

Before the Servlet 2.5 specification, < servlet mapping > elements were allowed to contain only one < URL pattern > sub element. To realize multiple mapping of Servet, you can only configure multiple < servlet mapping > elements.

Take serveltDemo as an example, on the web Two < servlet mapping > elements are configured in XML, and 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>
        <load-on-startup>1</load-on-startup>
    </servlet>
    <servlet-mapping>
        <servlet-name>MyServlet</servlet-name>
        <url-pattern>/myServlet</url-pattern>
    </servlet-mapping>
    <servlet-mapping>
        <servlet-name>MyServlet</servlet-name>
        <url-pattern>/myServlet2</url-pattern>
    </servlet-mapping>
</web-app>

2. Configure multiple < URL pattern > sub elements

Starting from Servlet 2.5, < Servlet mapping > elements can contain multiple < URL pattern > sub elements, and each < URL pattern > represents the mapping rule of a virtual path. Therefore, multiple mapping of servlets can also be realized by configuring multiple < URL pattern > sub elements in a < Servlet mapping > element.

Take servletDemo as an example, add two < URL pattern > sub elements under the < servlet mapping > element. 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>
        <load-on-startup>1</load-on-startup>
    </servlet>
    <servlet-mapping>
        <servlet-name>MyServlet</servlet-name>
        <url-pattern>/myServlet</url-pattern>
        <url-pattern>/myServlet3</url-pattern>
    </servlet-mapping>
</web-app>

3. @WebServlet realizes multiple mapping

Servlet 3.0 adds support for @ WebServlet annotation. We can specify a set of mapping rules in the form of string array in urlPatterns attribute to realize multiple mapping of servlets.

Take servletDemo as an example, add a group of virtual paths in the urlPatterns attribute of the @ WebServlet annotation. The code is as follows.

@WebServlet(
        urlPatterns = { "/myServlet", "/myServlet4" })

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