You could create a string of total file content and replace all the occurrence in the string and write to that file again.
You could something like this:
File log= new File("log.txt");
String search = "textFiles/a.txt";
String replace = "replaceText/b.txt";
try{
FileReader fr = new FileReader(log);
String s;
String totalStr = "";
try (BufferedReader br = new BufferedReader(fr)) {
while ((s = br.readLine()) != null) {
totalStr += s;
}
totalStr = totalStr.replaceAll(search, replace);
FileWriter fw = new FileWriter(log);
fw.write(totalStr);
fw.close();
}
}catch(Exception e){
e.printStackTrace();
}