服务器启动时Webapp的web.xml中配置的加载顺序
??<user username="admin" password="admin" roles="admin"/>
</tomcat-users>
此XML片段包括一个tomcat-users根元素，它包含一个或多个role和user元素。
然后在Web应用程序的web.xml中创建security-constraint、login-config和security-role元素。
<security-constraint>
??????<web-resource-collection>
??????????<web-resource-name>HelloServlet</web-resource-name>
??????????<url-pattern>/HelloServlet</url-pattern>
??????????<http-method>GET</http-method>
??????????<http-method>POST</http-method>
??????</web-resource-collection>
??????<auth-constraint>
??????????<description>This applies only to the "tomcat" security role</description>
??????????<role-name>admin</role-name>
??????</auth-constraint>
??????<user-data-constraint>
??????????<transport-guarantee>NONE</transport-guarantee>
??????</user-data-constraint>
??</security-constraint>
??
??<login-config>
??????<auth-method>BASIC</auth-method>
??</login-config>
??<security-role>
??????<role-name>admin</role-name>
??</security-role>
其中security-constraint元素包含一个或多个web-resource-collection元素，它是描述Web应用程序中的哪些web资源受到指定安全限制的保护。http-method元素指定安全限制覆盖的HTTP方法。上面的例子中，当我们对/HelloServlet的GET或POST请求时将触发配置的安全机制。
auth-constraint元素用于描述允许访问Web组件的安全角色。此例中安全角色的例子有tomcat、manager、admin。而只有当作为admin角色的用户才可以访问HelloServlet。
Web应用程序通过login-config元素来认证用户，并确认该用户是否为正确的角色。
longin-config包含的transport-guarantee子元素用来指定认证方法，BASIC是一种常见的Web认证方式，浏览器给用户提示一个对话框，要求输入用户名和密码，随后Tomcat将给出的用户名和密码与tomcat-users.xml中的用户名和密码进行比较，然后使用前面的security-constraint配置来确定用户是否可访问受保护的servlet。
（除BASIC外，还可以是FORM、CLIENT-CERT、DIGEST等）
其实这种认证方法实际上有两个步骤：
1、检查提供的用户名和密码是否正确。
2、判断用户是否映射到特定的安全角色。例如，用户可能提供了正确的用户名和密码，但没有映射到特定的安全角色，也将被禁止访问特定的Web资源。
五相应元素配置????
1、Web应用图标：指出IDE和GUI工具用来表示Web应用的大图标和小图标????
<icon>????
<small-icon>/images/app_small.gif</small-icon>????
<large-icon>/images/app_large.gif</large-icon>????
</icon>????
2、Web 应用名称：提供GUI工具可能会用来标记这个特定的Web应用的一个名称????
<display-name>Tomcat Example</display-name>????
3、Web 应用描述：给出于此相关的说明性文本????
<disciption>Tomcat Example servlets and JSP pages.</disciption>????
4、上下文参数：声明应用范围内的初始化参数。????
<context-param>????
????<param-name>ContextParameter</para-name>????
????<param-value>test</param-value>????
????<description>It is a test parameter.</description>????
</context-param>????
在servlet里面可以通过getServletContext().getInitParameter("context/param")得到????
5、过滤器配置：将一个名字与一个实现javaxs.servlet.Filter接口的类相关联。????
<filter>????
????????<filter-name>setCharacterEncoding</filter-name>????
????????<filter-class>com.myTest.setCharacterEncodingFilter</filter-class>????
????????<init-param>????
????????????<param-name>encoding</param-name>????
????????????<param-value>GB2312</param-value>????
????????</init-param>????
</filter>????
<filter-mapping>????
????????<filter-name>setCharacterEncoding</filter-name>????
????????<url-pattern>/*</url-pattern>????
</filter-mapping>????
6、监听器配置????
<listener>????
??????<listerner-class>listener.SessionListener</listener-class>????
</listener>????
7、Servlet配置????
???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>????
???元素说明????
?????<servlet></servlet> 用来声明一个servlet的数据，主要有以下子元素：????
?????<servlet-name></servlet-name> 指定servlet的名称????
?????<servlet-class></servlet-class> 指定servlet的类名称????
?????<jsp-file></jsp-file> 指定web站台中的某个JSP网页的完整路径????
?????<init-param></init-param> 用来定义参数，可有多个init-param。在servlet类中通过getInitParamenter(String name)方法访问初始化参数????
?????<load-on-startup></load-on-startup>指定当Web应用启动时，装载Servlet的次序。????
?????????????????????????????????当值为正数或零时：Servlet容器先加载数值小的servlet，再依次加载其他数值大的servlet.????
?????????????????????????????????当值为负或未定义：Servlet容器将在Web客户首次访问这个servlet时加载它????
?????<servlet-mapping></servlet-mapping> 用来定义servlet所对应的URL，包含两个子元素????
???????<servlet-name></servlet-name> 指定servlet的名称????
???????<url-pattern></url-pattern> 指定servlet所对应的URL????
8、会话超时配置（单位为分钟）????
???<session-config>????
??????<session-timeout>120</session-timeout>????
???</session-config>????
9、MIME类型配置????
???<mime-mapping>????
??????<extension>htm</extension>????
??????<mime-type>text/html</mime-type>????
???</mime-mapping>????
10、指定欢迎文件页配置????
???<welcome-file-list>????
??????<welcome-file>index.jsp</welcome-file>????
??????<welcome-file>index.html</welcome-file>????
??????<welcome-file>index.htm</welcome-file>????
???</welcome-file-list>????
11、配置错误页面????
一、通过错误码来配置error-page????
???<error-page>????
??????<error-code>404</error-code>????
??????<location>/NotFound.jsp</location>????
???</error-page>????
上面配置了当系统发生404错误时，跳转到错误处理页面NotFound.jsp。????
二、通过异常的类型配置error-page????
???<error-page>????
???????<exception-type>java.lang.NullException</exception-type>????
???????<location>/error.jsp</location>????
???</error-page>????
上面配置了当系统发生java.lang.NullException（即空指针异常）时，跳转到错误处理页面error.jsp????
12、TLD配置????
???<taglib>????
???????<taglib-uri>http://jakarta.apache.org/tomcat/debug-taglib</taglib-uri>????
???????<taglib-location>/WEB-INF/jsp/debug-taglib.tld</taglib-location>????
???</taglib>????
???如果MyEclipse一直在报错,应该把<taglib> 放到 <jsp-config>中????
???<jsp-config>????
??????<taglib>????
??????????<taglib-uri>http://jakarta.apache.org/tomcat/debug-taglib</taglib-uri>????
??????????<taglib-location>/WEB-INF/pager-taglib.tld</taglib-location>????
??????</taglib>????
???</jsp-config>????
13、资源管理对象配置????
???<resource-env-ref>????
???????<resource-env-ref-name>jms/StockQueue</resource-env-ref-name>????
???</resource-env-ref>????
14、资源工厂配置????
???<resource-ref>????
???????<res-ref-name>mail/Session</res-ref-name>????
???????<res-type>javax.mail.Session</res-type>????
???????<res-auth>Container</res-auth>????
???</resource-ref>????
???配置数据库连接池就可在此配置：????
???<resource-ref>????
???????<description>JNDI JDBC DataSource of shop</description>????
???????<res-ref-name>jdbc/sample_db</res-ref-name>????
???????<res-type>javax.sql.DataSource</res-type>????
???????<res-auth>Container</res-auth>????
???</resource-ref>????
15、安全限制配置????
???<security-constraint>????
??????<display-name>Example Security Constraint</display-name>????
??????<web-resource-collection>????
?????????<web-resource-name>Protected Area</web-resource-name>????
?????????<url-pattern>/jsp/security/protected/*</url-pattern>????
?????????<http-method>DELETE</http-method>????
?????????<http-method>GET</http-method>????
?????????<http-method>POST</http-method>????
?????????<http-method>PUT</http-method>????
??????</web-resource-collection>????
??????<auth-constraint>????
????????<role-name>tomcat</role-name>????
????????<role-name>role1</role-name>????
??????</auth-constraint>????
???</security-constraint>????
16、登陆验证配置????
???<login-config>????
?????<auth-method>FORM</auth-method>????
?????<realm-name>Example-Based Authentiation Area</realm-name>????
?????<form-login-config>????
????????<form-login-page>/jsp/security/protected/login.jsp</form-login-page>????
????????<form-error-page>/jsp/security/protected/error.jsp</form-error-page>????
?????</form-login-config>????
???</login-config>????
17、安全角色：security-role元素给出安全角色的一个列表，这些角色将出现在servlet元素内的security-role-ref元素的role-name子元素中。????
????分别地声明角色可使高级IDE处理安全信息更为容易。????
<security-role>????
?????<role-name>tomcat</role-name>????
</security-role>????
18、Web环境参数：env-entry元素声明Web应用的环境项????
<env-entry>????
?????<env-entry-name>minExemptions</env-entry-name>????
?????<env-entry-value>1</env-entry-value>????
?????<env-entry-type>java.lang.Integer</env-entry-type>????
</env-entry>????
19、EJB 声明????
<ejb-ref>????
?????<description>Example EJB reference</decription>????
?????<ejb-ref-name>ejb/Account</ejb-ref-name>????
?????<ejb-ref-type>Entity</ejb-ref-type>????
?????<home>com.mycompany.mypackage.AccountHome</home>????
?????<remote>com.mycompany.mypackage.Account</remote>????
</ejb-ref>????
20、本地EJB声明????
<ejb-local-ref>????
?????<description>Example Loacal EJB reference</decription>????
?????<ejb-ref-name>ejb/ProcessOrder</ejb-ref-name>????
?????<ejb-ref-type>Session</ejb-ref-type>????
?????<local-home>com.mycompany.mypackage.ProcessOrderHome</local-home>????
?????<local>com.mycompany.mypackage.ProcessOrder</local>????
</ejb-local-ref>????
21、配置DWR????
<servlet>????
??????<servlet-name>dwr-invoker</servlet-name>????
??????<servlet-class>uk.ltd.getahead.dwr.DWRServlet</servlet-class>????
</servlet>????
<servlet-mapping>????
??????<servlet-name>dwr-invoker</servlet-name>????
??????<url-pattern>/dwr/*</url-pattern>????
</servlet-mapping>????
22、配置Struts????
????<display-name>Struts Blank Application</display-name>????
????<servlet>????
????????<servlet-name>action</servlet-name>????
????????<servlet-class>????
????????????org.apache.struts.action.ActionServlet????
????????</servlet-class>????
????????<init-param>????
????????????<param-name>detail</param-name>????
????????????<param-value>2</param-value>????
????????</init-param>????
????????<init-param>????
????????????<param-name>debug</param-name>????
????????????<param-value>2</param-value>????
????????</init-param>????
????????<init-param>????
????????????<param-name>config</param-name>????
????????????<param-value>/WEB-INF/struts-config.xml</param-value>????
????????</init-param>????
????????<init-param>????
????????????<param-name>application</param-name>????
????????????<param-value>ApplicationResources</param-value>????
????????</init-param>????
????????<load-on-startup>2</load-on-startup>????
????</servlet>????
????<servlet-mapping>????
????????<servlet-name>action</servlet-name>????
????????<url-pattern>*.do</url-pattern>????
????</servlet-mapping>????
????<welcome-file-list>????
????????<welcome-file>index.jsp</welcome-file>????
????</welcome-file-list>????
????<!-- Struts Tag Library Descriptors -->????
????<taglib>????
????????<taglib-uri>struts-bean</taglib-uri>????
????????<taglib-location>/WEB-INF/tld/struts-bean.tld</taglib-location>????
????</taglib>????
????<taglib>????
????????<taglib-uri>struts-html</taglib-uri>????
????????<taglib-location>/WEB-INF/tld/struts-html.tld</taglib-location>????
????</taglib>????
????<taglib>????
????<taglib-uri>struts-nested</taglib-uri>????
????<taglib-location>/WEB-INF/tld/struts-nested.tld</taglib-location>????
????</taglib>????
????<taglib>????
????????<taglib-uri>struts-logic</taglib-uri>????
????????<taglib-location>/WEB-INF/tld/struts-logic.tld</taglib-location>????
????</taglib>????
????<taglib>????
????????<taglib-uri>struts-tiles</taglib-uri>????
????????<taglib-location>/WEB-INF/tld/struts-tiles.tld</taglib-location>????
????</taglib>????
23、配置Spring（基本上都是在Struts中配置的）????
???<!-- 指定spring配置文件位置 -->????
???<context-param>????
??????<param-name>contextConfigLocation</param-name>????
??????<param-value>????
???????<!--加载多个spring配置文件 -->????
????????/WEB-INF/applicationContext.xml, /WEB-INF/action-servlet.xml????
??????</param-value>????
???</context-param>????
???<!-- 定义SPRING监听器，加载spring -->????
<listener>????
?????<listener-class>org.springframework.web.context.ContextLoaderListener</listener-class>????
</listener>????
<listener>????
?????<listener-class>????
???????org.springframework.web.context.request.RequestContextListener????
?????</listener-class>????
</listener>?
?
其他说明：
web.xml 文件中一般包括 servlet, spring, filter, listenr的配置。那么他们是按照一个什么顺序加载呢？
加载顺序会影响对spring bean 的调用。
????比如filter 需要用到 bean ，但是加载顺序是 先加载filter 后加载spring，则filter中初始化操作中的bean为null；
首先可以肯定 加载顺序与他们在web.xml 文件中的先后顺序无关。
web.xml 中 listener 和 serverlet 的加载顺序为 先 listener 后serverlet
最终得出结果：先 listener >> filter >> servlet >>??spring
?所以，如果过滤器中要使用到 bean，可以将spring 的加载 改成 Listener的方式
<listener>
????????<listener-class>
????????????org.springframework.web.context.ContextLoaderListener
????????</listener-class>
????</listener>
?搞定！
?关于他们的内部执行顺序，也需要注意，如下面文章中遇到的问题
web.xml的filter执行顺序导致的乱码，切记！
2008-05-02 01:38
发现引起bug的原因是web.xml的下面几行：
????<filter-mapping>
????????<filter-name>SecurityFilter</filter-name>
????????<url-pattern>*.do</url-pattern>
????</filter-mapping>
????<filter-mapping>
????????<filter-name>CharacterEncoding</filter-name>
????????<url-pattern>*.do</url-pattern>
????</filter-mapping>
????<filter-mapping>
????????<filter-name>CharacterEncoding</filter-name>
????????<url-pattern>*.jsp</url-pattern>
????</filter-mapping>
根据servlet2.3规范filter执行是按照web.xml配置的filter-mapping先后顺序进行执行，所以上面的配置会导致遇见*.do的url请求，先进行SecurityFilter的过滤器处理，这时候没有做编码处理，已经是乱码，到下面的filter处理时已经时乱码，再做编码处理已经没有用处。
修正方式，调整filter-mapping顺序，如下：
????<filter-mapping>
????????<filter-name>CharacterEncoding</filter-name>
????????<url-pattern>*.do</url-pattern>
????</filter-mapping>
????<filter-mapping>
????????<filter-name>CharacterEncoding</filter-name>
????????<url-pattern>*.jsp</url-pattern>
????</filter-mapping>
????<filter-mapping>
????????<filter-name>SecurityFilter</filter-name>
????????<url-pattern>*.do</url-pattern>
????</filter-mapping>
?