What is correct way to use a static StringWriter in a web application?

I have a static StringWriter variable in a web application's Handler class, used by multiple private methods in the class. Each method appends a String to this variable, and finally the StringWriter writes the concatenated String to file. But while testing the web app I realized that the StringWriter was still holding the values from the all the previous tests. I used the answer from this question (How do you "empty" a StringWriter in Java?) as a workaround, but I feel this isn't correct in terms of design pattern and security.

Is it correct? Is there a better way?

public class BaseHandler {
    private static StringWriter sw = new StringWriter();

    public static void writeToFile(){
        firstMethod();
        secondMethod();
        finalMethod(); 
    }
    private static void firstMethod(){
        sw.append("Pandora's");
    }

    private static void secondMethod(){
        sw.append("Box");
    }

    private static void finalMethod(){
        sw.append("!");
        //sw writes value to file
        ...
        sw.getBuffer().setLength(0);
    }
}

I would ask myself, do I need a BaseHandler which hold state? Now your handler is holding a state in the sw field, but if you don't need this state then you don't create a field.

For example you can do this:

public class BaseHandler {


    public static void writeToFile(){
        StringWriter sw = new StringWriter();
        firstMethod(sw);
        secondMethod(sw);
        finalMethod(sw); 
    }
    private static void firstMethod(StringWriter sw){
        sw.append("Pandora's");
    }

    private static void secondMethod(StringWriter sw){
        sw.append("Box");
    }

    private static void finalMethod(StringWriter sw){
        sw.append("!");
        //sw writes value to file
        ...
    }
}

Exiting writeToFile the StringWriter is marked for garbage collection.