使用哪一个handler：
1. 可以通过在url中追加有名字的handler(以 ' / ' 开头命名)的名称来指定使用哪一个handler. 如：
<requestHandler name="/foo" class="my.package.CustomRequestHandler" />
http://localhost:8983/solr/foo?...
2. 旧的 handleSelect=true 方式(现已不推荐使用 qt指定handler, 现在select已经被显示命名为一个handler，而不是requestDispatcher: <requestHandler name="/select" class="solr.SearchHandler"> -v4.7.0 )
The <requestDispatcher> element has a handleSelect attribute which defaults to false as of Solr 3.6. Before then(v3.6) it was true.
When it is true, there is an additional dispatch rule that comes into place if the request uses "/select" but there is no request handler by that name. Instead of it being an error, Solr uses the "qt" parameter to lookup the handler by name. If there is no 'qt' parameter then the default handler is chosen.
http://localhost:8983/solr/select?qt=mysearch&...
'qt' can be used to choose a request handler in other cases aside from a new HTTP request. Two others are for the warming queries and also for the ping query, both configured in solrconfig.xml. For these other cases, there is no restrictions on a leading '/'. And in these cases, when 'qt' is not specified, the default handler is chosen.
The default handler is normally the handler named "/select". However another specific handler can be anointed as such with a default="true" attribute on it, if desired. "standard" was the old default name and will still work but it's legacy/deprecated. If there is no default handler, then most/all cases in which a default one is chosen will instead trigger an error.
<!-- Request Dispatcher
This section contains instructions for how the SolrDispatchFilter
should behave when processing requests for this SolrCore.
handleSelect is a legacy option that affects the behavior of requests
such as /select?qt=XXX
handleSelect="true" will cause the SolrDispatchFilter to process
the request and dispatch the query to a handler specified by the
"qt" param, assuming "/select" isn't already registered.
handleSelect="false" will cause the SolrDispatchFilter to
ignore "/select" requests, resulting in a 404 unless a handler
is explicitly registered with the name "/select"
handleSelect="true" is not recommended for new users, but is the default
for backwards compatibility
-->
<requestDispatcher handleSelect="false" >
http://wiki.apache.org/solr/SolrRequestHandler