RamdonAccessFile 是个很好用的类，功能十分强大，可以利用它的
length()和seek()方法来轻松实现文件的追加，相信我下面这个例子是
很容易看懂的，先写入十行，用length()读出长度（以byte为单位），
在用seek()移动到文件末尾，继续添加，最后显示记录。

import java.io.*;
public class IOStreamDemo {
public static void main(String[] args) {
try{
RandomAccessFile rf1 = new RandomAccessFile("d:\\jeru.txt","rw");
for (int i = 0; i < 10; i ++ ){
rf1.writeBytes("xixi,this is line "+i+"\n");
}
rf1.close();

int i = 0;
String record = new String();
RandomAccessFile rf2 = new RandomAccessFile("d:\\jeru.txt","rw");
rf2.seek(rf2.length());
rf2.writeBytes("lala,append line"+"\n");
rf2.close();

RandomAccessFile rf3 = new RandomAccessFile("d:\\jeru.txt","r");
while ((record = rf3.readLine()) != null) {
i ++;
System.out.println("Value "+i+":"+record);
}
rf3.close();
 }catch(Exception e){}
}
}