Sharing Objects - Java Concurrency in Practice - page 49

get and set accessormethods that maintain a separate copy of the value for each thread that
uses it, so a get returns the most recent value passed to set from the currently executing
thread .
Thread-local variables are often used to prevent sharing in designs based on mutable
Singletons or global variables. For example, a single-threaded application might maintain a
global database connection that is initialized at startup to avoid having to pass a Connec-
tion to every method. Since JDBC connections may not be thread-safe, a multithreaded
application that uses a global connection without additional coordination is not thread-
safe either. By using a ThreadLocal to store the JDBC connection, as in Connec-
tionHolder in Listing 3.10 , each thread will have its own connection.
Listing 3.10. Using ThreadLocal to Ensure thread Confinement.
This technique can also be used when a frequently used operation requires a temporary object
such as a buffer and wants to avoid reallocating the temporary object on each invocation.
For example, before Java 5.0, Integer.toString used a ThreadLocal to store the
12-byte buffer used for formatting its result, rather than using a shared static buffer (which
would require locking) or allocating a new buffer for each invocation. [11]
When a thread calls ThreadLocal.get for the first time, initialValue is consulted
to provide the initial value for that thread. Conceptually, you can think of a
ThreadLocal<T> as holding a Map<Thread,T> that stores the thread-specific values,
though this is not how it is actually implemented. The thread-specific values are stored in the