Best practices for Java logging from multiple threads?

Question

I want to have a diagnostic log that is produced by several tasks managing data. These tasks may be in multiple threads. Each task needs to write an element (possibly with subelements) to the log; get in and get out quickly. If this were a single-task situation I'd use XMLStreamWriter as it seems like the best match for simplicity/functionality without having to hold a ballooning XML document in memory.

But it's not a single-task situation, and I'm not sure how to best make sure this is "threadsafe", where "threadsafe" in this application means that each log element should be written to the log correctly and serially (one after the other and not interleaved in any way).

Any suggestions? I have a vague intuition that the way to go is to use a queue of log elements (with each one able to be produced quickly: my application is busy doing real work that's performance-sensitive), and have a separate thread which handles the log elements and sends them to a file so the logging doesn't interrupt the producers.

The logging doesn't necessarily have to be XML, but I do want it to be structured and machine-readable.

edit: I put "threadsafe" in quotes. Log4j seems to be the obvious choice (new to me but old to the community), why reinvent the wheel...

Answer 1

Use a logging framework, such as Log4j.

Answer 2

I think you are on the wrong path. You say “threadsafe” but you actually mean “serialized”. Threadsafe means that one thread will not interfere with data from other thread. Most of the time, threading issues are resolved beforehand and you should not worry about it just for logging sake. For example, if your write:

myVariableSum = 0 + myVariable; //here comes other thread - Not very likely! logger.info("Log some INFO; myVariable has value" + myVariable.toString()); 

You have to make sure that myVariable has not been changed by some other thread from the moment calculation (first line) was performed but before logging method was called. If this happens, you will log dirty value that was not used to perform the operation but value that was assigned by some other thread. This is generally taken care of; for example local (method level) variable can not be changed by other thread. Anyway, if you have to worry about this when logging, than 99% that your program has serious threading issues already.
All major logging frameworks are by themselves “threadsafe” meaning they can be deployed in multithreaded environments and will not display problems similar to one described above internally.
Getting traces to appear in log in order they happen is actually usually called “serialization” of calls. Serializing log writes will be a major performance bottleneck on any multithreaded app. If you use logging framework, like log4j, traces from all threads will appear in single place more or less in order they happen. However, one column is generally Thread name, so you can easily filter your log data by thread; each thread will log its data in chronological order. Check out this link: http://logging.apache.org/log4j/1.2/faq.html#1.7
Finally, if serializing log writes is what you really need, then you could use some kind of structure, like java.util.concurrent.BlockingQueue to route your messages.