在另一个线程同时记录日志的同时，如何安全地读取Log4j日志？

Question

If I have multiple threads that use log4j to write to a single log file, and I want another thread to read it back out, is there a way to safely read(line by line) those logs such that I always read a full line?

EDIT: Reason for this is I need to upload all logs to a central location and it might be logs that are days old or those that are just being written

Answer 1

You should use a read write lock.

Read locks can be held by multiple users if there is no one writing to the file, but a write lock can only be held by 1 thread at a time no matter what.

Just make sure that as your writing thread is done writing, it releases the readwritelock to allow the reading threads to read. Likewise, always release the read lock when the reader are done reading so log4j can continue to write

Check out

http://docs.oracle.com/javase/1.5.0/docs/api/java/util/concurrent/locks/ReadWriteLock.html

However, coming to think of it, what is your purpose for this? If you simply want to monitor your logs, you should use a different solution rather than having a monitor thread within the same application. Seems to not make sense. If the data is available within the application / service, why pass it off to a file and read it right back in?

Answer 2

It is going to be a pain if you need to implement what you are doing, especially you have to deal with file rolling.

For your specific requirement, there are better choices:

1. If the location you are going to be backed up can be directly written (ie mounted in your file system), it is better to simply set your file rolling to write to that backup directory; or

2. Make use of log management tools like Splunk to monitor and manage your log files (so that you don't even need to copy to that backup directory); or

3. Even you need to do the backup all by yourself, you don't need to (and have no reason to) do it in a separate thread. Trying to write a shell script monitoring your log directory, and make use of tools like rsync or write similar logic by yourself, to do the upload only for files that are not matching in local and remote location.