Is FileOutStream.write(byte[]) always blocking?

I wondered if FileOu开发者_JS百科tputStream.write(byte[]) is always blocking the current thread, leading to a ThreadContext switch, or can it be that this operation does not block if the OS buffers are large enought to handle the bytes.

The reason for these thoughts are, I wondered if the logging I do with log4j in my application is a real performance hit, and if it would be faster to use a Queue of logging messages which is read by a separate thread and written to the logfiles (I know the disadvantages of swallowed logging statement if the app quits and the statements in the queue are not flushed to disk).

No, I didn't profile it yet, these are rather conceptual thoughts.

Need not be.

FileOutputStream.write(byte[]) is a native method. Common sense would suggest that write() may just write to the internal buffers, and a later call to flush() would actually commit it.

You can use the log4j org.apache.log4j.AsyncAppender and logging calls will not block. The actual logging is done in another thread so you won't need to worry about calls to log4j not returning in a timely manner.

By default immediateFlush is enabled which means that logging is slower but ensures that each append request is actually written out. You can set this to false if you don't care whether or not the last lines are written out if your application crashes.

log4j.appender.R.ImmediateFlush=false

Also, take a look at this post on Log4j: Performance Tips, in which the author has got some test stats on using immediateFlush, bufferedIO and asyncAppender. He concludes, that for local logging "set immediateFlush=false, and leave bufferedIO at the default of don't buffer" and that "asycAppender actually takes longer than normal non-asyc".

It's likely going to depend on the OS, drivers and underlying file system. If write caching is enabled for example it'll probably return right away. I've seen gigabytes/day of logs written synchronously without affecting performance too much, as long as IO isn't bottlenecked. It's still probably worth writing them asynchronously if you're concerned about response times. And it eliminates potential future issues, e.g. if you changed to writing to network drive and the network has issues.