java 文件的读写,追加

写入读出
String num = request.getParameter("num");
String sessionId = request.getParameter("s") + ";" + num  + "\n";
File writefile;
try{
    String path = "tt.txt";
    writefile = new File(path);
    if (writefile.exists() == false) {
        writefile.createNewFile();
        writefile = new File(path); // 重新实例化
    }
    FileOutputStream fos = new FileOutputStream(writefile);
    out.println("###content:" + sessionId);
    fos.write(sessionId.getBytes());
    fos.flush();
    fos.close();
    byte[] b = new byte[17*1024];
    FileInputStream fis = new FileInputStream(writefile);
    fis.read(b);
    String s = new String(b);
    out.println(s);
    fis.close();
}catch(Exception ex){
    System.out.println(ex.getMessage());
}
追加:
String path=request.getRealPath(".");
RandomAccessFile rf=new RandomAccessFile("WriteData.txt","rw");//定义一个类RandomAccessFile的对象，并实例化
rf.seek(rf.length());//将指针移动到文件末尾
rf.writeBytes("\nAppend a line to the file!");
rf.close();//关闭文件流
out.println("写入文件内容为：＜br>");
FileReader fr=new FileReader(path + "\\WriteData.txt");
BufferedReader br=new BufferedReader(fr);//读取文件的BufferedRead对象
String Line=br.readLine();
while(Line!=null){
out.println(Line + "＜br>");
Line=br.readLine();
}
fr.close();//关闭文件