This tutorial will show you how to fix eclipse missing tools.jar issue. Eclipse is configured to use JRE instead of JDK. If you're using newer Java versions, it could happen that during the start of STS/Eclipse you get error saying that tools.jar could not be found. Since this tools.jar library could not be found in JRE, what you need to do is make your STS/Eclipse use JDK since it contains tools.jar.
So, how to fix missing tools.jar in STS? First, go to Window -> Preferences.
You'll see many options there, but what we're interested in is submenu Java and it's item Installed JREs.
In the list you'll see JDKs and JREs installed on your system. What you need to do is to uncheck all things that you would not be using, and leave only one JDK from JAVA version you prefer. As you can see on the picture above, I unchecked the JRE and left only JDK. This solved my missing tools.jar issue, hope it did the same for you.