Luke Butters created LOG4J2-1434:
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Summary: StringBuffer in thread local causes memory leaks.
Key: LOG4J2-1434
URL: https://issues.apache.org/jira/browse/LOG4J2-1434
Project: Log4j 2
Issue Type: Bug
Affects Versions: 2.6.1
Reporter: Luke Butters
In an effort to speed up logging ThreadLocals have been introduced see
LOG4J2-1125 however this does causes memory issues reported
http://stackoverflow.com/questions/32867327/log4j2-threadlocal-memory-leak-in-servletcontextlistener
and subsequently ignored in LOG4J2-1142
The problem of the ThreadLocal occurs when threads are re-used which is an
absolutely valid way of using java. For example an executor service can re-use
threads as well as Jetty.
Below I demonstrate a contrived example of the memory leak:
{code}
int stringSize = 1024*1024*10; //~10MB maybe 20MB for UTF-16
StringBuilder sb = new StringBuilder(stringSize);
for(int i = 0; i < stringSize; i++) {
sb.append('a' + i % 5);
}
String largeString = sb.toString();
sb = null; //Let it be GC'ed
ExecutorService es = Executors.newFixedThreadPool(100);
final CountDownLatch countDownLatch = new CountDownLatch(100);
for(int i = 0; i < 100; i++) {
es.execute(()-> {
//Log the big string to demonstrate the issue.
log.fatal(largeString);
//Ensure we use all 100 of our threads by not releasing this
thread yet.
countDownLatch.countDown();
});
//We sleep for 2s so we more easily watch memory growth
Thread.sleep(2000);
}
{code}
I recommend that log4j2 immediately remove the ThreadLocal as a small gain in
performance does not outweigh the problems associated with memory leaks.
Finally other options for caching the StringBuilder with a ThreadLocal could be
considered for example we might re-use StringBuilders that are no larger than
3k while removing the ones which are larger than 3k.
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