I was poking at the changes in SolrCLI in branch_5x to see what was done, and noticed a bunch of warnings. I'm using eclipse, with Java 8, but I'm hoping the source level setting of 1.7 makes everything relevant.
I started looking at what I could do to eliminate some warnings, and one of the first things I looked at was the argList variable in the main method. I changed this to a List<String> but still had a warning about unchecked cast ... so I went looking for the javadocs on the CommandLine object in commons-cli. That said it returned List<String>, so I was confused until I noticed that Solr is pulling in an older version of commons-cli.
I updated the ivy config to get the newest version of commons-cli, and that led to the discovery that we were using classes and methods that are deprecated in the newer version.
I changed all those to use the preferred classes/methods, and managed to eliminate every warning that eclipse showed for SolrCLI. I am seeing test failures, so I will try to fix those before I upload the patch.