ThreadLocal

http://www.ibm.com/developerworks/java/library/j-threads1/index.html

In the absence of synchronization, it is allowable (according to the JMM) for two threads to see different values in the same memory location. When synchronizing on a monitor (lock), the JMM requires that this cache be invalidated immediately after the lock is acquired, and flushed (writing any modified memory locations back to main memory) before it is released. It's not hard to see why synchronization can have a significant effect on program performance; flushing the cache frequently can be expensive.

A race condition is a situation in which two or more threads or processes are reading or writing some shared data, and the final result depends on the timing of how the threads are scheduled. Race conditions can lead to unpredictable results and subtle program bugs.

A thread-local variable effectively provides a separate copy of its value for each thread that uses it. Each thread can see only the value associated with that thread, and is unaware that other threads may be using or modifying their own copies.
Java compilers offer no special language support for thread-local variables; instead, they are implemented with the ThreadLocal class, which has special support in the core Threadclass.
it's clear that the object stored in the ThreadLocal isnot shared between threads, simplifying the task of determining whether a class is thread-safe or not.