Is there a Groovy way of making a method synchronized?

If you can upgrade to Groovy 1.7.3 you can use the Synchronized AST transformation instead. You can use the annotation on instance and static methods. The annotation will create a lock variable in your class (or you can use an existing variable) and the code is synchronized on that lock variable.

The usage of a synchronized block should be preferred over adding the keyword to the method. If you use the synchronized keyword on the method you synchronize on this which means that all other threads that want to access any of the methods in your class have to wait until the lock is free again.

import groovy.transform.Synchronized

class YourClass {
    @Synchronized
    static Map getMap(def fileName) {
        ...
    }
}

Since Groovy 1.7.3 we have a new AST transformation: @Synchronized