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Sometimes it can be hard to determine if a plugin and the underlying libraries are thread-safe, so when adding @threadSafe the following checklist can be used:

• Check all static fields/variables in plugin/plugin code are not subject to threading problems.
  You might want to pay special attention to static member variables of the subclasses of "java.text.Format" (NumeberFormat, DateFormat etc), most of which
  are not threadsafe and cannot be shared as static variables.
• Check any plexus components.xml; if the components defined are singletons they need to be threadsafe.
• Check for presence of known tainted libraries.
• Check thread safety of any other third party libraries. This last item can be a bit hard, but inquiries on mailing lists can get you a long way.

This checklist qualifies for a "simple thread safety" review of a mojo.

If you need to learn more about multithreading and the java memory model it is probably wise to start off with a book like "Java Concurrency In Practice" or similar.

If a mojo uses a known-non-threadsafe external dependency, you may want to do something like this:

public class MyMojo
    extends AbstractMojo
{

  private static final Object lock = new Object();

  public void execute()
  {
     synchronized( lock) 
     {
         // Main mojo code
     }
  }
}