容器中的default servlet

每个servlet容器都有一个默认的servlet，一般都叫做default，在容器的默认配置中式这样写的（以tomcat为例）：

 

 

    <servlet>
        <servlet-name>default</servlet-name>
        <servlet-class>org.apache.catalina.servlets.DefaultServlet</servlet-class>
        <init-param>
            <param-name>debug</param-name>
            <param-value>0</param-value>
        </init-param>
        <init-param>
            <param-name>listings</param-name>
            <param-value>false</param-value>
        </init-param>
        <load-on-startup>1</load-on-startup>
    </servlet>

    <servlet-mapping>
        <servlet-name>default</servlet-name>
        <url-pattern>/</url-pattern>
    </servlet-mapping>

 

 

 

配置文件时在${tomcat_home}/conf/web.xml中，如果想隐藏静态文件，或者希望静态文件（如图片，HTML文件等）不用其他servelt来解析，浪费资源呀，可以这样设置：

 

<!--
	 - Map static resources to the default servlet
	 - examples:
	 -     http://localhost:8080/static/images/pets.png
	 -     http://localhost:8080/static/styles/petclinic.css
	-->	
	
	<servlet-mapping>
		<servlet-name>default</servlet-name>
		<url-pattern>/static/*</url-pattern>
	</servlet-mapping>

 

 

 

如此配置后，所有在/html、/images下的静态文件都必须使用/static/html和/static/images来访问，并且不会再被其他的servlet解析，也减轻了服务器的压力。这个方法可以说是apache的替代品，非常好用，但是……

 

Default servlet并不是servlet api的规范，也就是说它只是一个行规，大部分的serlvet容器都有一个default servlet，但是没有标准，这才是要老命的问题。拿jetty的default servle来做比较，先看tomcat的配置说明：

 

 

 

  <!-- The default servlet for all web applications, that serves static     -->
  <!-- resources.  It processes all requests that are not mapped to other   -->
  <!-- servlets with servlet mappings (defined either here or in your own   -->
  <!-- web.xml file.  This servlet supports the following initialization    -->
  <!-- parameters (default values are in square brackets):                  -->
  <!--                                                                      -->
  <!--   debug               Debugging detail level for messages logged     -->
  <!--                       by this servlet.  [0]                          -->
  <!--                                                                      -->
  <!--   fileEncoding        Encoding to be used to read static resources   -->
  <!--                       [platform default]                             -->
  <!--                                                                      -->
  <!--   input               Input buffer size (in bytes) when reading      -->
  <!--                       resources to be served.  [2048]                -->
  <!--                                                                      -->
  <!--   listings            Should directory listings be produced if there -->
  <!--                       is no welcome file in this directory?  [false] -->
  <!--                       WARNING: Listings for directories with many    -->
  <!--                       entries can be slow and may consume            -->
  <!--                       significant proportions of server resources.   -->
  <!--                                                                      -->
  <!--   output              Output buffer size (in bytes) when writing     -->
  <!--                       resources to be served.  [2048]                -->
  <!--                                                                      -->
  <!--   readonly            Is this context "read only", so HTTP           -->
  <!--                       commands like PUT and DELETE are               -->
  <!--                       rejected?  [true]                              -->
  <!--                                                                      -->
  <!--   readmeFile          File name to display with the directory        -->
  <!--                       contents. [null]                               -->
  <!--                                                                      -->
  <!--   sendfileSize        If the connector used supports sendfile, this  -->
  <!--                       represents the minimal file size in KB for     -->
  <!--                       which sendfile will be used. Use a negative    -->
  <!--                       value to always disable sendfile.  [48]        -->
  <!--                                                                      -->
  <!--   useAcceptRanges     Should the Accept-Ranges header be included    -->
  <!--                       in responses where appropriate? [true]         -->
  <!--                                                                      -->
  <!--  For directory listing customization. Checks localXsltFile, then     -->
  <!--  globalXsltFile, then defaults to original behavior.                 -->
  <!--                                                                      -->
  <!--   localXsltFile       Make directory listings an XML doc and         -->
  <!--                       pass the result to this style sheet residing   -->
  <!--                       in that directory. This overrides              -->
  <!--                        globalXsltFile[null]                          -->
  <!--                                                                      -->
  <!--   globalXsltFile      Site wide configuration version of             -->
  <!--                       localXsltFile This argument is expected        -->
  <!--                       to be a physical file. [null]                  -->
  <!--                                                                      -->
  <!--          

 

 

 

再看看jetty的default servlet:

 

 

 

/** The default servlet.                                                 
 * This servlet, normally mapped to /, provides the handling for static 
 * content, OPTION and TRACE methods for the context.                   
 * The following initParameters are supported, these can be set either
 * on the servlet itself or as ServletContext initParameters with a prefix
 * of org.mortbay.jetty.servlet.Default. :                          
 * <PRE>                                                                      
 *   acceptRanges     If true, range requests and responses are         
 *                    supported                                         
 *                                                                      
 *   dirAllowed       If true, directory listings are returned if no    
 *                    welcome file is found. Else 403 Forbidden.        
 *
 *   welcomeServlets  If true, attempt to dispatch to welcome files 
 *                    that are servlets, but only after no matching static 
 *                    resources could be found. 
 *                    
 *                    This must be false if you want directory listings,
 *                    but have index.jsp in your welcome file list.
 *
 *   redirectWelcome  If true, welcome files are redirected rather than
 *                    forwarded to.
 *
 *   gzip             If set to true, then static content will be served as 
 *                    gzip content encoded if a matching resource is 
 *                    found ending with ".gz"
 *
 *  resourceBase      Set to replace the context resource base
 *
 *  relativeResourceBase    
 *                    Set with a pathname relative to the base of the
 *                    servlet context root. Useful for only serving static content out
 *                    of only specific subdirectories.
 * 
 *  aliases           If True, aliases of resources are allowed (eg. symbolic
 *                    links and caps variations). May bypass security constraints.
 *                    
 *  maxCacheSize      The maximum total size of the cache or 0 for no cache.
 *  maxCachedFileSize The maximum size of a file to cache
 *  maxCachedFiles    The maximum number of files to cache
 *  cacheType         Set to "bio", "nio" or "both" to determine the type resource cache. 
 *                    A bio cached buffer may be used by nio but is not as efficient as an
 *                    nio buffer.  An nio cached buffer may not be used by bio.    
 *  
 *  useFileMappedBuffer 
 *                    If set to true, it will use mapped file buffer to serve static content
 *                    when using NIO connector. Setting this value to false means that
 *                    a direct buffer will be used instead of a mapped file buffer. 
 *                    By default, this is set to true.
 *                    
 *  cacheControl      If set, all static content will have this value set as the cache-control
 *                    header.
 *                    
 *

 两个的差别很大，甚至是默认的设置都不一样，同样是设置:

 

	<servlet-mapping>
		<servlet-name>default</servlet-name>
		<url-pattern>/static/*</url-pattern>
	</servlet-mapping>

 

在jetty就run不起来，很正常，jetty必须要求一个resourceBase或者relativeResourceBase，它需要映射到一个目录，或者相对目录，这点比tomcat要好很多。  

 

结论就是： 如果要在Servlet容器中使用default Servlet，一定要考虑到容器迁移的事项。