ThreadLocal

AI-generated Key Takeaways

• ThreadLocal provides thread-local variables, giving each thread its own independent copy of a variable.

• These variables are typically used for associating thread-specific state, like user IDs or transaction IDs.

• initialValue() can be overridden to provide a non-null initial value for the thread-local variable.

• When a thread terminates, its thread-local variables are eligible for garbage collection.

• get(), set(), and remove() are the primary methods for interacting with a ThreadLocal variable within a thread.

public class ThreadLocal extends Object

Known Direct Subclasses InheritableThreadLocal<T> InheritableThreadLocal<T> This class extends ThreadLocal to provide inheritance of values from parent thread to child thread: when a child thread is created, the child receives initial values for all inheritable thread-local variables for which the parent has values.

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, the class below 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.

 import java.util.concurrent.atomic.AtomicInteger;

 public class ThreadId {
     // Atomic integer containing the 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).

Public Constructor Summary

ThreadLocal() Creates a thread local variable.

Public Method Summary

T	get() Returns the value in the current thread's copy of this thread-local variable.
void	remove() Removes the current thread's value for this thread-local variable.
void	set(T value) Sets the current thread's copy of this thread-local variable to the specified value.
static <S> ThreadLocal<S>	withInitial(Supplier<? extends S> supplier) Creates a thread local variable.

Protected Method Summary

T	initialValue() Returns the current thread's "initial value" for this thread-local variable.

Inherited Method Summary

From class java.lang.Object

Object	clone() Creates and returns a copy of this Object.
boolean	equals(Object obj) Compares this instance with the specified object and indicates if they are equal.
void	finalize() Invoked when the garbage collector has detected that this instance is no longer reachable.
final Class<?>	getClass() Returns the unique instance of Class that represents this object's class.
int	hashCode() Returns an integer hash code for this object.
final void	notify() Causes a thread which is waiting on this object's monitor (by means of calling one of the wait() methods) to be woken up.
final void	notifyAll() Causes all threads which are waiting on this object's monitor (by means of calling one of the wait() methods) to be woken up.
String	toString() Returns a string containing a concise, human-readable description of this object.
final void	wait(long timeout, int nanos) Causes the calling thread to wait until another thread calls the notify() or notifyAll() method of this object or until the specified timeout expires.
final void	wait(long timeout) Causes the calling thread to wait until another thread calls the notify() or notifyAll() method of this object or until the specified timeout expires.
final void	wait() Causes the calling thread to wait until another thread calls the notify() or notifyAll() method of this object.

Public Constructors

public ThreadLocal ()

Creates a thread local variable.

See Also

• withInitial(java.util.function.Supplier)

Public Methods

public T get ()

Returns the value in the current thread's copy of this thread-local variable. If the variable has no value for the current thread, it is first initialized to the value returned by an invocation of the initialValue() method.

Returns

• the current thread's value of this thread-local

public void remove ()

Removes the current thread's value for this thread-local variable. If this thread-local variable is subsequently {@linkplain #get read} by the current thread, its value will be reinitialized by invoking its initialValue() method, unless its value is {@linkplain #set set} by the current thread in the interim. This may result in multiple invocations of the initialValue method in the current thread.

public void set (T value)

Sets the current thread's copy of this thread-local variable to the specified value. Most subclasses will have no need to override this method, relying solely on the initialValue() method to set the values of thread-locals.

Parameters

value	the value to be stored in the current thread's copy of this thread-local.

public static ThreadLocal<S> withInitial (Supplier<? extends S> supplier)

Creates a thread local variable. The initial value of the variable is determined by invoking the get method on the Supplier.

Parameters

supplier	the supplier to be used to determine the initial value

Returns

• a new thread local variable

Throws

NullPointerException	if the specified supplier is null

Protected Methods

protected T initialValue ()

Returns the current thread's "initial value" for this thread-local variable. This method will be invoked the first time a thread accesses the variable with the get() method, unless the thread previously invoked the set(T) method, in which case the initialValue method will not be invoked for the thread. Normally, this method is invoked at most once per thread, but it may be invoked again in case of subsequent invocations of remove() followed by get().

This implementation simply returns null; if the programmer desires thread-local variables to have an initial value other than null, ThreadLocal must be subclassed, and this method overridden. Typically, an anonymous inner class will be used.

Returns

• the initial value for this thread-local