Solr4.8.0源码分析(1)之Solr的Servlet

Solr是作为一个Servlet运行在Tomcat里面的，可以查看Solr的web.xml。

1.web.xml配置

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.

   <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>
13   </filter-mapping>
14   <servlet>
15     <servlet-name>RedirectOldAdminUI</servlet-name>
16     <servlet-class>org.apache.solr.servlet.RedirectServlet</servlet-class>
17     <init-param>
18       <param-name>destination</param-name>
19       <param-value>${context}/#/</param-value>
20     </init-param>
21   </servlet>
22   <servlet-mapping>
23     <servlet-name>RedirectOldAdminUI</servlet-name>
24     <url-pattern>/admin/</url-pattern>
25   </servlet-mapping>

2. 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 configuration
       this.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 works
       log.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
    */
   @Override
   public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws IOException, ServletException {
     doFilter(request, response, chain, false);
   }

   /**
    * 关闭Solr
    */
   @Override
   public void destroy() {
     if (cores != null) {
       cores.shutdown();
       cores = null;
     }
   }
 }

3.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;

   @Override
   public 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 key
     if(destination.startsWith(CONTEXT_KEY)) {
       destination = config.getServletContext().getContextPath()
           +destination.substring(CONTEXT_KEY.length());
     }
   }

   @Override
   public void doGet(HttpServletRequest req, HttpServletResponse res)
           throws ServletException,IOException {

     res.setStatus(code);
     res.setHeader("Location", destination);
   }

 }