An instance of CloudSolrServer is not able to handle consecutive request on different collections o.a.

CloudSolrServer saves urlList, leaderUrlList and replicasList on instance level, and only recalculates those lists in case of clusterState changes. The values calculated for the lists will be different for different target-collections. Therefore they also ought to recalculated for a request R, if the target-collection for R is different from the target-collection for the request handled just before R by the same CloudSolrServer instance.

Another problem with the implementation in CloudSolrServer is with the lastClusterStateHashCode. lastClusterStateHashCode is updated when the first request after a clusterState-change is handled. Before the lastClusterStateHashCode is updated one of the following two sets of lists are updated:

• In case sendToLeader==true for the request: leaderUrlList and replicasList are updated, but not urlList
• In case sendToLeader==false for the request: urlList is updated, but not leaderUrlList and replicasList
  But the lastClusterStateHashCode is always updated. So even though there was just one collection in the world there is a problem: If the first request after a clusterState-change is a sendToLeader==true-request urlList will (potentially) be wrong (and will not be recalculated) for the next sendToLeader==false-request to the same CloudSolrServer instance. If the first request after a clusterState-change is a sendToLeader==false-request leaderUrlList and replicasList will (potentially) be wrong (and will not be recalculated) for the next sendToLeader==true-request to the same CloudSolrServer instance.

Besides that it is a very bad idea to have instance- and local-method-variables with the same name. CloudSolrServer has an instance variable called urlList and method CloudSolrServer.request has a local-method-variable called urlList and the method also operates on instance variable urlList. This makes the code hard to read.

Havnt made a test in Apache Solr regi to reproduce the main error (the one mentioned at the top above) but I guess you can easily do it yourself:
Make a setup with two collections "collection1" and "collection2" - no default collection. Add some documents to "collection2" (without any autocommit). Then do cloudSolrServer.commit("collection1") and afterwards cloudSolrServer.commit("collection2") (use same instance of CloudSolrServer). Then try to search collection2 for the documents you inserted into it. They ought to be found, but are not, because the cloudSolrServer.commit("collection2") will not do a commit of collection2 - it will actually do a commit of collection1.
Well, actually you cant do cloudSolrServer.commit(<collection-name>) (the method doesnt exist), but that ought to be corrected too. But you can do the following instead:

UpdateRequest req = new UpdateRequest();
req.setAction(UpdateRequest.ACTION.COMMIT, true, true);
req.setParam(CoreAdminParams.COLLECTION, <collection-name>);
req.process(cloudSolrServer);

In general I think you should add misc tests to your test-suite - tests that run Solr-clusters with more than one collection and makes clever tests on that.