import java.text.ParseException;
import java.text.SimpleDateFormat;
 
public class Main {
    public static void main(String[] args) throws ParseException {
        String strDateTime = "2021-05-31T14:12:44.8020000";
        strDateTime = strDateTime.substring(0, strDateTime.indexOf('.') + 4);
 
        String formatIn = "yyyy-MM-dd'T'HH:mm:ss.SSS";
        String formatOut = "yyyy-MM-dd HH:mm:ss.SSS";
 
        SimpleDateFormat in = new SimpleDateFormat(formatIn);
        SimpleDateFormat out = new SimpleDateFormat(formatOut);
 
        System.out.println(out.format(in.parse(strDateTime)));
    }
}

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