Java Software Engineer Interview Question:

Please explain what is the ThreadLocal class? How and why would you use it?

Submitted by: Muhammad

A single ThreadLocal instance can store different values for each thread independently. Each thread that accesses the get() or set() method of a ThreadLocal instance is accessing 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). The example below, from the ThreadLocal Javadoc, generates unique identifiers local to each thread. A thread’s id is assigned the first time it invokes ThreadId.get() and remains unchanged on subsequent calls.

public class ThreadId {
// Next thread ID to be assigned
private static final AtomicInteger nextId = new AtomicInteger(0);

// Thread local variable containing each thread's ID
private static final ThreadLocal<Integer> threadId =
new ThreadLocal<Integer>() {
@Override protected Integer initialValue() {
return nextId.getAndIncrement();
}
};

// Returns the current thread's unique ID, assigning it if necessary
public static int get() {
return threadId.get();
}
}
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).
Submitted by: Muhammad