在写入文件时读取文件 [英] Read a file while it's being written

问题描述

我必须读取tomcat日志文件，过了一段时间（例如：一小时）我将再次读取该文件（仅用于新添加的内容），所以我创建了RandomAccessFile来记录我完成的最后一个位置，并使用BufferedReader.readLine（）方法。

I have to read the tomcat log file,and after some time(for example:one hour) I will read the file again(just for the new added content),so I create the RandomAccessFile to record the last postion I completed,and use the BufferedReader.readLine() method.

然而，我发现有时我无法读取整个文件行。

However,I found that sometime I can not read the whole line of the file.

例如，tomcat要写下面的内容（仅举例）：

For example,the tomcat is tring to write the following content(just example):

192.168.0.0 localhost /index.html .....

192.168.0.0 localhost /index.html .....

此时，我读到的时候可能会得到结果：

And at this moment,when I read I may get the result :

192.168.0 0 localhost /index.html .....

192.168.0 0 localhost /index.html .....

或

192.168.0.0 localhost /index.html .....

192.168.0.0 localhost /index.html .....

这就是说，如果正在编写此行，我的读者会读取不完整的行。

That's to say,my reader read a uncomplete line if this line is being written.

所以我想知道任何想法来决定是否是线阅读已经完成？

So I wonder any idea to decide if the line which is being reading has been completed?

这是核心代码：

raf = new RandomAccessFile(file, "r");
raf.seek(pos);
while ((line = raf.readLine()) != null) {
    System.out.println(line);
}
pos = raf.getFilePointer();
raf.close();

我试过这个（添加条件）：

I have tried this(add the contidtion):

raf = new RandomAccessFile(file, "r");
raf.seek(pos);
while ((line = raf.readLine()) != null) {
    if(line.chartAt(line.length()-1)=='\n'){
        System.out.println(line);
    } else
        raf.seek(pos); //roll back to the last position

}
pos = raf.getFilePointer();
raf.close();

但它不起作用..

任何想法？

推荐答案

readLine使用换行符。您不能使用它来确定您是否有完整的行。您需要一次读取一个字节并自行解析该行。

readLine consumes the newline character. You can't use it to determine if you have a complete line or not. You need to read one byte at a time and parse the line yourself.

当您获得有效行时，您也不会移动该位置。

Also you are not moving on the position when you get a valid line..

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