I am using CommonsHttpSolrServer in a Web Application. Is it safe to reuse the CommonsHttpSolrServer over multiple requests or should I instantiate a new object for each request? Could not find the answer in the API docs.
According to the documentation and the source comments, SolrJ is thread safe.
However, be careful when you update solr. According to this post, the transactions are implemented per instance, not per queue. This means that each thread does not have it's own isolated transaction to work with. Rollback will rollback every call (regardless of originating thread) to the last commit.
Overall, this means that you should be safe to query (using the same CommonsHttpSolrServer) with as many threads as you like. However, if you wish to take advantage of rollback, you will need to ensure only one thread is updating your solr instance at a time (regardless of object distribution).
来源：https://stackoverflow.com/questions/3371101/solrj-thread-safety