I apologize, but there is no such class as "JsonStringEncoder" in the Apache Commons IO library. It seems that you might be confusing it with other class or package. Apache Commons IO focuses on providing utility classes for file and I/O operations, and it doesn't have a class specifically for encoding JSON strings.

If you are looking for a way to encode a String to be used in a JSON document, you can use the JsonStringEncoder class from the Jackson library. The JsonStringEncoder class provides methods for encoding a String so that it can be safely included in a JSON document.

Here is an example of using the JsonStringEncoder.getInstance().quoteAsString(String) method to encode a string:

import com.fasterxml.jackson.core.io.JsonStringEncoder;

public class JsonStringEncoderExample {
    public static void main(String[] args) {
        String input = "This is a test string with \"quotes\" and newlines\n";
        String encoded = JsonStringEncoder.getInstance().quoteAsString(input);
        System.out.println(encoded);
    }
}

In this example, we first create a string input that contains special characters like quotes and newlines. We then use the JsonStringEncoder.getInstance().quoteAsString(String) method to encode the string.

This example will output:

"This is a test string with \"quotes\" and newlines\n"

You can also use the JsonStringEncoder.getInstance().quoteAsUTF8(String) method to encode the string as UTF-8 bytes.