Solr是作为一个Servlet运行在Tomcat里面的，可以查看Solr的web.xml。
一. web.xml配置
<filter><filter-name>SolrRequestFilter</filter-name><filter-class>org.apache.solr.servlet.SolrDispatchFilter</filter-class><init-param><param-name>path-prefix</param-name><param-value>/xxx</param-value></init-param></filter><filter-mapping><filter-name>SolrRequestFilter</filter-name><url-pattern>/*</url-pattern></filter-mapping> <servlet><servlet-name>RedirectOldAdminUI</servlet-name><servlet-class>org.apache.solr.servlet.RedirectServlet</servlet-class><init-param><param-name>destination</param-name><param-value>${context}/#/</param-value></init-param></servlet><servlet-mapping><servlet-name>RedirectOldAdminUI</servlet-name><url-pattern>/admin/</url-pattern></servlet-mapping>由web.xml可以看出，基本上所有Solr的操作都是在 SolrDispatchFilter中实现的。当输入http://localhost:8080/solr/前缀的URL就会触发SolrDispatchFilter.
二. SolrDispatchFilter的实现
SolrDispatchFilter继承了Filter，实现主要分为三个接口：init，dofilter，destory。其中init和destory分别在tomcat的启动和关闭时候进行。
/***初始化，当tomcat启动时候开始初始化，其中主要调用createCoreContainer来实现Solr的初始化*/
public void init(FilterConfig config) throws ServletException{log.info("SolrDispatchFilter.init()");try {// web.xml configurationthis.pathPrefix = config.getInitParameter( "path-prefix" );this.cores = createCoreContainer();log.info("user.dir=" + System.getProperty("user.dir"));}catch( Throwable t ) {// catch this so our filter still workslog.error( "Could not start Solr. Check solr/home property and the logs");SolrCore.log( t );if (t instanceof Error) {throw (Error) t;}}log.info("SolrDispatchFilter.init() done");}/*** filter接口的具体实现，这里主要实现了主要的Solr功能*/@Overridepublic void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws IOException, ServletException {doFilter(request, response, chain, false);}/*** 关闭Solr*/@Overridepublic void destroy() {if (cores != null) {cores.shutdown();cores = null;} }三. Servlet的实现
通过查看web.xml以及源码可以看到，虽然Solr继承并实现了Servlet接口，但是Solr的主要操作却是主要集中在dofilter里面。以RedictServlet为例，它主要实现了http的重定向功能。从web.xml配置中可以看到，当url为solr/admin时候就会重定向为solr/#/
/*** A Simple redirection servlet to help us deprecate old UI elements*/
public class RedirectServlet extends BaseSolrServlet {static final String CONTEXT_KEY = "${context}";String destination;int code = HttpServletResponse.SC_MOVED_PERMANENTLY;@Overridepublic void init(ServletConfig config) throws ServletException {super.init(config);destination = config.getInitParameter("destination");if(destination==null) {throw new ServletException("RedirectServlet missing destination configuration");}if( "false".equals(config.getInitParameter("permanent") )) {code = HttpServletResponse.SC_MOVED_TEMPORARILY;} // 获取重定向的url 解析init-param 获取destination值// Replace the context keyif(destination.startsWith(CONTEXT_KEY)) {destination = config.getServletContext().getContextPath()+destination.substring(CONTEXT_KEY.length());}}@Overridepublic void doGet(HttpServletRequest req, HttpServletResponse res)throws ServletException,IOException {res.setStatus(code);res.setHeader("Location", destination);}}