Threads - The Java Programming Language - page 567

14.13. THReadLocal Variables
The THReadLocal class allows you to have a single logical variable that has
independent values in each separate thread. Each THReadLocal object has
a set method that lets you set the value of the variable in the current
thread, and a get method that returns the value for the current thread. It
is as if you had defined a new field in each thread class, but without the
need to actually change any thread class. This is a feature you may well
never need, but if you do need it THReadLocal may simplify your work.
For example, you might want to have a user object associated with all
operations, set on a per-thread basis. You could create a ThreadLocal ob-
ject to hold each thread's current user object:
public class Operations {
private static ThreadLocal<User> users =
new ThreadLocal<User>() {
/** Initially start as the "unknown user". */
protected User initialValue() {
return User.UNKNOWN_USER;
}
};
private static User currentUser() {
return users.get();
}
public static void setUser(User newUser) {
users.set(newUser);
}
public void setValue(int newValue) {
User user = currentUser();
if (!canChange(user))
throw new SecurityException();
// ... modify the value ...
}