Learning Solr API

Parse params in Solr config file
Made same error again, time to write it down:
<requestHandler ...>
<lst name="defaults">
<str name="dataFolder">data-folder</str>
</lst>
</requestHandler>
This code doesn't work, dataFolder would be null: the data in params is: defaults={dataFolder=data-folder}
public void init(NamedList args) {
SolrParams params = SolrParams.toSolrParams(args);
dataFolder = params.get(PARAM_DATA_FOLDER);
}

Should use:
super.init(args);
if (defaults != null) {
dataFolder = defaults.get(PARAM_DATA_FOLDER);
}

Be sure to close SolrQueryRequest
org.apache.solr.request.SolrQueryRequestBase
The close() method must be called on any instance of this class once it is no longer in use. 

org.apache.solr.core.SolrResourceLoader.normalizeDir(String)
org.apache.solr.core.CoreDescriptor.getDataDir()
if (new File(instanceDir).isAbsolute()) {
        result= SolrResourceLoader.normalizeDir(SolrResourceLoader.normalizeDir(instanceDir) + dataDir);
} else  {
result= SolrResourceLoader.normalizeDir(coreContainer.getSolrHome() +
SolrResourceLoader.normalizeDir(instanceDir) + dataDir);
}

Using QParser to parse query string
QueryResponseWriter responseWriter = core.getQueryResponseWriter(solrReq);
QParser qparser = QParser.getParser(tmpQuery, "lucene", req);
SolrQueryParser parser = new SolrQueryParser(qparser, req
.getSchema().getDefaultSearchFieldName());
Query query = parser.parse(tmpQuery);
DocSet docSet = searcher.getDocSet(query);
DocSet interSection = docs.intersection(docSet);

Run Query in Solr Server

final SolrQueryRequest newReq = new LocalSolrQueryRequest(core, newParams);
final SolrQueryResponse newResp = new SolrQueryResponse();
SolrRequestHandler handler = core.getRequestHandler("/select");
core.execute(handler, newReq, newResp);

SolrIndexSearcher searcher = newReq.getSearcher();
NamedList<Object> valuesNL = newResp.getValues();
Object rspObj = (Object) valuesNL.get("response");
if (rspObj instanceof ResultContext) {
 ResultContext rc = (ResultContext) rspObj;
 DocList docList = rc.docs;
 DocIterator dit = docList.iterator();
 while (dit.hasNext()) {
  int docid = dit.nextDoc();
  Document doc = searcher.doc(docid, (Set<String>) null);
  //
 }
} else if (rspObj instanceof SolrDocumentList) {
 SolrDocumentList docList = (SolrDocumentList) rspObj;
 long size = docList.size();
 Iterator<SolrDocument> docIt = docList.iterator();
 while (docIt.hasNext()) {
  SolrDocument doc = docIt.next();
  docIt.remove();
  //
 }
} else {
 throw new RuntimeException("Unkown response type: "
   + ((rspObj != null) ? rspObj.getClass() : "null"));
}

how solr returnFields defined
ReturnFields returnFields = new SolrReturnFields( req );
rsp.setReturnFields( returnFields );

Using getStatistics to add stats info
public NamedList<Object> getStatistics() {}
private final AtomicLong numTimeouts = new AtomicLong();
private final Timer requestTimes = new Timer();
numErrors.incrementAndGet();

Parse Queries in SolrConfig: org.apache.solr.core.QuerySenderListener.newSearcher(SolrIndexSearcher, SolrIndexSearcher)

try
{
  List<NamedList> allLists = (List<NamedList>)args.get("queries");  
  if (allLists == null) return;
  for (NamedList nlst : allLists) {
    NamedList result = new NamedList();
    if (params.get("distrib") == null) {
      params.add("distrib", false);
    } 
    SolrQueryRequest req = new LocalSolrQueryRequest(core,params);
    SolrQueryResponse rsp = new SolrQueryResponse();
    SolrRequestInfo.setRequestInfo(new SolrRequestInfo(req, rsp));
    core.execute(core.getRequestHandler(req.getParams().get(CommonParams.QT)), req, rsp);
  }
} finally {
  if (req != null) req.close();
  SolrRequestInfo.clearRequestInfo();
}

Using SolrPluginUtils.docListToSolrDocumentList to convert Lucene DocList to SolrDocumentList

SolrPluginUtils.docListToSolrDocumentList(docList,req.getSearcher(), fieldSet, null)

LocalParams
SolrParams localParams = QueryParsing.getLocalParams(param, req.getParams());
key = localParams.get(CommonParams.OUTPUT_KEY, key);
String excludeStr = localParams.get(CommonParams.EXCLUDE);

Map<?,?> tagMap = (Map<?,?>)req.getContext().get("tags");