# ThreadLocal Added in[API level 1](https://developer.android.com/guide/topics/manifest/uses-sdk-element#ApiLevels) Summary:[Ctors](https://developer.android.com/reference/java/lang/ThreadLocal#pubctors)\|[Methods](https://developer.android.com/reference/java/lang/ThreadLocal#pubmethods)\|[Protected Methods](https://developer.android.com/reference/java/lang/ThreadLocal#promethods)\|[Inherited Methods](https://developer.android.com/reference/java/lang/ThreadLocal#inhmethods) # ThreadLocal ` public class ThreadLocal ` ` extends `[Object](https://developer.android.com/reference/java/lang/Object)` `` ` |---|----------------------------| | [java.lang.Object](https://developer.android.com/reference/java/lang/Object) || | ↳ | java.lang.ThreadLocal\ | |-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------| | Known direct subclasses [InheritableThreadLocal](https://developer.android.com/reference/java/lang/InheritableThreadLocal)\ |---------------------------------------------------------------------------------------------------------|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------| | [InheritableThreadLocal](https://developer.android.com/reference/java/lang/InheritableThreadLocal)\ | 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 threadId = new ThreadLocal() { @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](https://developer.android.com/reference/java/lang/ThreadLocal#ThreadLocal())`() ` Creates a thread local variable. | | ### Public methods || |-------------------------------------------------------------------------------------------------|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------| | ` T` | ` `[get](https://developer.android.com/reference/java/lang/ThreadLocal#get())`() ` Returns the value in the current thread's copy of this thread-local variable. | | ` void` | ` `[remove](https://developer.android.com/reference/java/lang/ThreadLocal#remove())`() ` Removes the current thread's value for this thread-local variable. | | ` void` | ` `[set](https://developer.android.com/reference/java/lang/ThreadLocal#set(T))`(T value) ` Sets the current thread's copy of this thread-local variable to the specified value. | | ` static `[ThreadLocal](https://developer.android.com/reference/java/lang/ThreadLocal)`` | ` `[withInitial](https://developer.android.com/reference/java/lang/ThreadLocal#withInitial(java.util.function.Supplier))`(`[Supplier](https://developer.android.com/reference/java/util/function/Supplier)` supplier) ` Creates a thread local variable. | | ### Protected methods || |------|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------| | ` T` | ` `[initialValue](https://developer.android.com/reference/java/lang/ThreadLocal#initialValue())`() ` Returns the current thread's "initial value" for this thread-local variable. | | ### Inherited methods | |-----------------------|---| | From class` `[java.lang.Object](https://developer.android.com/reference/java/lang/Object)` ` |--------------------------------------------------------------------------------|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------| | ` `[Object](https://developer.android.com/reference/java/lang/Object) | ` `[clone](https://developer.android.com/reference/java/lang/Object#clone())`() ` Creates and returns a copy of this object. | | ` boolean` | ` `[equals](https://developer.android.com/reference/java/lang/Object#equals(java.lang.Object))`(`[Object](https://developer.android.com/reference/java/lang/Object)` obj) ` Indicates whether some other object is "equal to" this one. | | ` void` | ` `[finalize](https://developer.android.com/reference/java/lang/Object#finalize())`() ` Called by the garbage collector on an object when garbage collection determines that there are no more references to the object. | | ` final `[Class](https://developer.android.com/reference/java/lang/Class)`` | ` `[getClass](https://developer.android.com/reference/java/lang/Object#getClass())`() ` Returns the runtime class of this`Object`. | | ` int` | ` `[hashCode](https://developer.android.com/reference/java/lang/Object#hashCode())`() ` Returns a hash code value for the object. | | ` final void` | ` `[notify](https://developer.android.com/reference/java/lang/Object#notify())`() ` Wakes up a single thread that is waiting on this object's monitor. | | ` final void` | ` `[notifyAll](https://developer.android.com/reference/java/lang/Object#notifyAll())`() ` Wakes up all threads that are waiting on this object's monitor. | | ` `[String](https://developer.android.com/reference/java/lang/String) | ` `[toString](https://developer.android.com/reference/java/lang/Object#toString())`() ` Returns a string representation of the object. | | ` final void` | ` `[wait](https://developer.android.com/reference/java/lang/Object#wait(long,%20int))`(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](https://developer.android.com/reference/java/lang/Object#wait(long))`(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](https://developer.android.com/reference/java/lang/Object#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](https://developer.android.com/guide/topics/manifest/uses-sdk-element#ApiLevels) ``` public ThreadLocal () ``` Creates a thread local variable.
**See also:** - [withInitial(java.util.function.Supplier)](https://developer.android.com/reference/java/lang/ThreadLocal#withInitial(java.util.function.Supplier)) ## Public methods ### get Added in[API level 1](https://developer.android.com/guide/topics/manifest/uses-sdk-element#ApiLevels) ``` 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()](https://developer.android.com/reference/java/lang/ThreadLocal#initialValue())method.
| Returns || |-----|--------------------------------------------------------| | `T` | the current thread's value of this thread-local
| ### remove Added in[API level 1](https://developer.android.com/guide/topics/manifest/uses-sdk-element#ApiLevels) ``` public void remove () ``` Removes the current thread's value for this thread-local variable. If this thread-local variable is subsequently[read](https://developer.android.com/reference/java/lang/ThreadLocal#get())by the current thread, its value will be reinitialized by invoking its[initialValue()](https://developer.android.com/reference/java/lang/ThreadLocal#initialValue())method, unless its value is[set](https://developer.android.com/reference/java/lang/ThreadLocal#set(T))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](https://developer.android.com/guide/topics/manifest/uses-sdk-element#ApiLevels) ``` 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()](https://developer.android.com/reference/java/lang/ThreadLocal#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](https://developer.android.com/guide/topics/manifest/uses-sdk-element#ApiLevels) ``` public static ThreadLocal withInitial (Supplier 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](https://developer.android.com/reference/java/lang/ThreadLocal)`` | a new thread local variable
| | Throws || |------------------------------------------------------------------------------------------------|-----------------------------------| | [NullPointerException](https://developer.android.com/reference/java/lang/NullPointerException) | if the specified supplier is null | ## Protected methods ### initialValue Added in[API level 1](https://developer.android.com/guide/topics/manifest/uses-sdk-element#ApiLevels) ``` 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()](https://developer.android.com/reference/java/lang/ThreadLocal#get())method, unless the thread previously invoked the[set(T)](https://developer.android.com/reference/java/lang/ThreadLocal#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()](https://developer.android.com/reference/java/lang/ThreadLocal#remove())followed by[get()](https://developer.android.com/reference/java/lang/ThreadLocal#get()).
##### Implementation Requirements: - This implementation simply returns`null`; if the programmer desires thread-local variables to have an initial value other than`null`, then either`ThreadLocal`can be subclassed and this method overridden or the method[ThreadLocal.withInitial(Supplier)](https://developer.android.com/reference/java/lang/ThreadLocal#withInitial(java.util.function.Supplier))can be used to construct a`ThreadLocal`. | Returns || |-----|------------------------------------------------| | `T` | the initial value for this thread-local
| **See also:** - [withInitial(java.util.function.Supplier)](https://developer.android.com/reference/java/lang/ThreadLocal#withInitial(java.util.function.Supplier))