This class provides thread-local variables. These variables differ from
their normal counterparts in that each thread that accesses one (via its
get or set method) has its own, independently initialized
copy of the variable. ThreadLocal instances are typically private
static fields in classes that wish to associate state with a thread (e.g.,
a user ID or Transaction ID).
For example, in the class below, the private static ThreadLocal
instance (serialNum) maintains a "serial number" for each thread
that invokes the class's static SerialNum.get() method, which
returns the current thread's serial number. (A thread's serial number is
assigned the first time it invokes SerialNum.get(), and remains
unchanged on subsequent calls.)
public class SerialNum {
// The next serial number to be assigned
private static int nextSerialNum = 0;
private static ThreadLocal serialNum = new ThreadLocal() {
protected synchronized Object initialValue() {
return new Integer(nextSerialNum++);
}
};
public static int get() {
return ((Integer) (serialNum.get())).intValue();
}
}
Each thread holds an implicit reference to its copy of a thread-local
variable as long as the thread is alive and the ThreadLocal
instance is accessible; after a thread goes away, all of its copies of
thread-local instances are subject to garbage collection (unless other
references to these copies exist).
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