Is it possible to reinitialize static mutable fields in a class?

I'm trying to automate the testing process for customly written programs designed to solve competitive programming challenges. Below is a dummy sample implementation of Solution:

public class Solution {
    private static String dummyField = "initial";


    public static int initialize(InputStream in) {
        //competitive programmer custom code
        System.out.println(dummyField);
        dummyField = "changed";
        return subCaseCount;
    }

    public void processSingleSubCase(InputStream in) {
        //competitive programmer custom code
    }
}

Prewritten test code for solution regardless of its implementation:

public void testSolution() throws FileNotFoundException {
        for(File testResource : testResources) {
            InputStream in = new FileInputStream(testResource);
            int subCaseCount = Foo.initialize(in);
            for (int subCase = 0; subCase < subCaseCount; subCase++) {
                new Foo().processSingleSubCase(in);
            }

            //magic call to re-init all static fields without knowing their number/names in advance goes here
        }

        //console current output:
        //initial
        //changed
        //changed
        //...

        //desired:
        //initial
        //initial
        //initial
        //....
}

The static fields can be mutable, so mapping the initial values to field names using reflection, then reassigning them at each iteration wouldn't do.

I did manage to come up with a working solution which basically reloads the class using a different class loader at each iteration, it did work but was quite slow: it took about 50 seconds just to reload classes 300 times (test resources are auto generated and I'd like to have the flexibility to auto generate as many as tolerable).

Is there a faster alternative?