Detailed explanation of web.xml file
Preface: web.xml is used in general web projects. .xml is mainly used for configuration and can facilitate the development of web projects. web.xml is mainly used to configure Filter, Listener, Servlet, etc. But it should be noted that web.xml is not necessary. A web project does not need a web.xml file.
1. WEB project loading web.xml process
After personal testing, WEB The project loading order has nothing to do with the configuration order of element nodes in the file. That is, filter will not be loaded first because filter is written before listener. The loading sequence of the WEB container is: ServletContext -> context-param -> listener -> filter -> servlet. And these elements can be configured anywhere in the file.
The loading process sequence is as follows:
When starting a WEB project, the WEB container will read its The configuration file web.xml reads the two nodes
#Emergency, create a ServletContext (servlet context), all parts of this web project will share this context.
The container converts
The container creates a class instance in
##2. Detailed explanation of web.xml file elements
1. schema
The schema file of web.xml is defined by Sun. The root element of each web.xml file is
<?xml version="1.0" encoding="UTF-8"?><web-app version="2.4" xmlns="http://java.sun.com/xml/ns/j2ee" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://java.sun.com/xml/ns/j2ee http://java.sun.com/xml/ns/j2ee/web-app_2_4.xsd"></web-app>
2.
<icon>
<small-icon>/images/app_small.gif</small-icon>
<large-icon>/images/app_large.gif</large-icon></icon>
3.
<display-name>Tomcat Example</display-name>
<disciption>Tomcat Example servlets and JSP pages.</disciption>
Declaration application Initialization parameters within the range. It is used to provide key-value pairs, that is, application context information, to the ServletContext. Our listeners, filters, etc. will use the information in these contexts during initialization. In the servlet, it can be obtained through getServletContext().getInitParameter("context/param").
<context-param>
<param-name>ContextParameter</para-name>
<param-value>test</param-value>
<description>It is a test parameter.</description></context-param>
## Combine a name with an implementation of javaxs.servlet.Filter The interface is associated with the class.
<filter> <filter-name>setCharacterEncoding</filter-name> <filter-class>com.myTest.setCharacterEncodingFilter</filter-class> <init-param> <param-name>encoding</param-name> <param-value>UTF-8</param-value> </init-param></filter><filter-mapping> <filter-name>setCharacterEncoding</filter-name> <url-pattern>/*</url-pattern></filter-mapping>
<listener> <listerner-class>com.listener.SessionListener</listener-class> </listener>
##
#
##
<!-- 基本配置 --><servlet> <servlet-name>snoop</servlet-name> <servlet-class>SnoopServlet</servlet-class></servlet><servlet-mapping> <servlet-name>snoop</servlet-name> <url-pattern>/snoop</url-pattern></servlet-mapping><!-- 高级配置 --><servlet> <servlet-name>snoop</servlet-name> <servlet-class>SnoopServlet</servlet-class> <init-param> <param-name>foo</param-name> <param-value>bar</param-value> </init-param> <run-as> <description>Security role for anonymous access</description> <role-name>tomcat</role-name> </run-as></servlet><servlet-mapping> <servlet-name>snoop</servlet-name> <url-pattern>/snoop</url-pattern></servlet-mapping>
9、
单位为分钟。
<session-config> <session-timeout>120</session-timeout></session-config>
10、
<mime-mapping> <extension>htm</extension> <mime-type>text/html</mime-type></mime-mapping>
11、
<welcome-file-list> <welcome-file>index.jsp</welcome-file> <welcome-file>index.html</welcome-file> <welcome-file>index.htm</welcome-file></welcome-file-list>
12、
<!-- 1、通过错误码来配置error-page。当系统发生×××错误时,跳转到错误处理页面。 --><error-page> <error-code>404</error-code> <location>/NotFound.jsp</location></error-page><!-- 2、通过异常的类型配置error-page。当系统发生java.lang.NullException(即空指针异常)时,跳转到错误处理页面。 --><error-page> <exception-type>java.lang.NullException</exception-type> <location>/error.jsp</location></error-page>
13、
<jsp-config> <taglib> <taglib-uri>Taglib</taglib-uri> <taglib-location>/WEB-INF/tlds/MyTaglib.tld</taglib-location> </taglib> <jsp-property-group> <description>Special property group for JSP Configuration JSP example.</description> <display-name>JSPConfiguration</display-name> <url-pattern>/jsp/* </url-pattern> <el-ignored>true</el-ignored> <page-encoding>GB2312</page-encoding> <scripting-invalid>true</scripting-invalid> <include-prelude>/include/prelude.jspf</include-prelude> <include-coda>/include/coda.jspf</include-coda> </jsp-property-group></jsp-config>
对于Web 应用程式来说,Scriptlet 是个不乐意被见到的东西,因为它会使得HTML 与Java 程式码交相混杂,对于程式的维护来说相当的麻烦,必要的时候,可以在web.xml 中加上
3、Mapping规则
当一个请求发送到servlet容器的时候,容器先会将请求的url减去当前应用上下文的路径作为servlet的映射url,比如我访问的是http://localhost/test/aaa.html,我的应用上下文是test,容器会将http://localhost/test去掉,剩下的/aaa.html部分拿来做servlet的映射匹配。这个映射匹配过程是有顺序的,而且当有一个servlet匹配成功以后,就不会去理会剩下的servlet了。
其匹配规则和顺序如下:
精确路径匹配。例子:比如servletA 的url-pattern为 /test,servletB的url-pattern为 /* ,这个时候,如果我访问的url为http://localhost/test ,这个时候容器就会先 进行精确路径匹配,发现/test正好被servletA精确匹配,那么就去调用servletA,也不会去理会其他的servlet了。
Longest path matching. Example: The url-pattern of servletA is /test/*, and the url-pattern of servletB is /test/a/*. When accessing http://localhost/test/a, the container will select the servlet with the longest path. Match, which is servletB here.
#Extension matching, if the last segment of the URL contains an extension, the container will select the appropriate servlet based on the extension. Example: servletA's url-pattern: *.action
Starting with "/" and ending with "/*" are used for path mapping. Things starting with the prefix "*." are used for extended mapping. So, why is it wrong to define a seemingly normal match like "/*.action"? Because this match belongs to both path mapping and extended mapping. The container cannot determine
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