ThreadLocal: history of performance improvment

From "Threading lightly, Part 3: Sometimes it's best not to share. Exploiting ThreadLocal to enhance scalability"

ThreadLocal performance
While the concept of a thread-local variable has been around for a long time and is supported by many threading frameworks including the Posix pthreads specification, thread-local support was omitted from the initial Java Threads design and only added in version 1.2 of the Java platform. In many ways, ThreadLocal is still a work in progress; it was rewritten for version 1.3 and again for version 1.4, both times to address performance problems.

In JDK 1.2, ThreadLocal was implemented in a manner very similar to Listing 2, except that a synchronized WeakHashMap was used to store the values instead of a HashMap. (Using WeakHashMap solves the problem of Thread objects not getting garbage collected, at some additional performance cost.) Needless to say, the performance of ThreadLocal was quite poor.

Concurrency lessons from eBay engineers

On JavaOne Technical Sessions 2009 3 engineers from eBay speek about Java concurrency troubles/lessons that they learned.
- Lazy Initialization patern
- Holder pattern
- correct Double-Checked Locking pattern
- Interesting example: ConcurrentMap of AtomicInteger
- DateFormat, MessageDigest, CharsetEncoder/CharsetDecoder - not thread safe
- ThreadLocal pattern
- potential lock contention on Class.forName(...) when missing class
- SAXParserFactory.newInstance().newSAXParser() - potentially significant lock contention on META-INF
P.S. You MUST be resistered at Sun. Its easy.
P.P.S. And, Yes, eBay, Amazon and part of Google writed in Java:)