ThreadLocal

public class ThreadLocal
extends Object

java.lang.Object
↳	java.lang.ThreadLocal<T>

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).

Summary

Public constructors
`ThreadLocal()` Creates a thread local variable.

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

Inherited methods

From class


        
          java.lang.Object

`Object`	`clone()` Creates and returns a copy of this object.
`boolean`	`equals(Object obj)` Indicates whether some other object is "equal to" this one.
`void`	`finalize()` Called by the garbage collector on an object when garbage collection determines that there are no more references to the object.
`final Class<?>`	`getClass()` Returns the runtime class of this `Object`.
`int`	`hashCode()` Returns a hash code value for the object.
`final void`	`notify()` Wakes up a single thread that is waiting on this object's monitor.
`final void`	`notifyAll()` Wakes up all threads that are waiting on this object's monitor.
`String`	`toString()` Returns a string representation of the object.
`final void`	`wait(long timeoutMillis, int nanos)` Causes the current thread to wait until it is awakened, typically by being notified or interrupted, or until a certain amount of real time has elapsed.
`final void`	`wait(long timeoutMillis)` Causes the current thread to wait until it is awakened, typically by being notified or interrupted, or until a certain amount of real time has elapsed.
`final void`	`wait()` Causes the current thread to wait until it is awakened, typically by being notified or interrupted.

Public constructors

ThreadLocal

Added in API level 1

public ThreadLocal ()

Creates a thread local variable.

See also:

withInitial(java.util.function.Supplier)

Public methods

get

Added in API level 1

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
`T`	the current thread's value of this thread-local

remove

Added in API level 1

public void remove ()

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

set

Added in API level 1

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`	`T`: the value to be stored in the current thread's copy of this thread-local.

withInitial

Added in API level 26

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`	`Supplier`: the supplier to be used to determine the initial value

Returns
`ThreadLocal<S>`	a new thread local variable

Throws
`NullPointerException`	if the specified supplier is null

Protected methods

initialValue

Added in API level 1

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
`T`	the initial value for this thread-local