# ThreadLocal Added in[API level 1](https://developer.android.com/guide/topics/manifest/uses-sdk-element.html#ApiLevels) # ThreadLocal ``` open class ThreadLocal ``` |---|------------------------------------------------------------------------------------------------| | [kotlin.Any](https://kotlinlang.org/api/latest/jvm/stdlib/kotlin/-any/index.html) || | ↳ | [java.lang.ThreadLocal](https://developer.android.com/reference/kotlin/java/lang/ThreadLocal#) | |-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------| | Known Direct Subclasses [InheritableThreadLocal](https://developer.android.com/reference/kotlin/java/lang/InheritableThreadLocal) |-----------------------------------------------------------------------------------------------------------|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------| | [InheritableThreadLocal](https://developer.android.com/reference/kotlin/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. ```kotlin 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/kotlin/java/lang/ThreadLocal#ThreadLocal())`()` Creates a thread local variable. | | Public methods || |-----------------------------------------------------------------------------------------------------|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------| | openT? | [get](https://developer.android.com/reference/kotlin/java/lang/ThreadLocal#get())`()` Returns the value in the current thread's copy of this thread-local variable. | | open[Unit](https://kotlinlang.org/api/latest/jvm/stdlib/kotlin/-unit/index.html) | [remove](https://developer.android.com/reference/kotlin/java/lang/ThreadLocal#remove())`()` Removes the current thread's value for this thread-local variable. | | open[Unit](https://kotlinlang.org/api/latest/jvm/stdlib/kotlin/-unit/index.html) | [set](https://developer.android.com/reference/kotlin/java/lang/ThreadLocal#set(java.lang.ThreadLocal.T))`(`value:` `T`)` Sets the current thread's copy of this thread-local variable to the specified value. | | openstatic[ThreadLocal](https://developer.android.com/reference/kotlin/java/lang/ThreadLocal#)\ | [withInitial](https://developer.android.com/reference/kotlin/java/lang/ThreadLocal#withInitial(java.util.function.Supplier))`(`supplier:` `[Supplier](https://developer.android.com/reference/kotlin/java/util/function/Supplier.html#)`)` Creates a thread local variable. | | Protected methods || |--------|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------| | openT? | [initialValue](https://developer.android.com/reference/kotlin/java/lang/ThreadLocal#initialValue())`()` Returns the current thread's "initial value" for this thread-local variable. | ## Public constructors ### ThreadLocal Added in[API level 1](https://developer.android.com/guide/topics/manifest/uses-sdk-element.html#ApiLevels) ``` ThreadLocal() ``` Creates a thread local variable. **See Also** - ## Public methods ### get Added in[API level 1](https://developer.android.com/guide/topics/manifest/uses-sdk-element.html#ApiLevels) ``` open fun get(): T? ``` 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/kotlin/java/lang/ThreadLocal#initialValue())method. | Return || |----|-------------------------------------------------| | 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.html#ApiLevels) ``` open fun remove(): Unit ``` Removes the current thread's value for this thread-local variable. If this thread-local variable is subsequently[read](https://developer.android.com/reference/kotlin/java/lang/ThreadLocal#get())by the current thread, its value will be reinitialized by invoking its[initialValue](https://developer.android.com/reference/kotlin/java/lang/ThreadLocal#initialValue())method, unless its value is[set](https://developer.android.com/reference/kotlin/java/lang/ThreadLocal#set(java.lang.ThreadLocal.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.html#ApiLevels) ``` open fun set(value: T): Unit ``` 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/kotlin/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.html#ApiLevels) ``` open static fun withInitial(supplier: Supplier): ThreadLocal ``` Creates a thread local variable. The initial value of the variable is determined by invoking the`get`method on the`Supplier`. | Parameters || |------------|-------------------------------------------------------------------------------------------------------------------------------------------------------------| | `` | the type of the thread local's value | | `supplier` | [Supplier](https://developer.android.com/reference/kotlin/java/util/function/Supplier.html#)\:the supplier to be used to determine the initial value | | Return || |-----------------------------------------------------------------------------------------|-----------------------------| | [ThreadLocal](https://developer.android.com/reference/kotlin/java/lang/ThreadLocal#) | a new thread local variable | | Exceptions || |----------------------------------|-----------------------------------| | `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.html#ApiLevels) ``` protected open fun initialValue(): T? ``` 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/kotlin/java/lang/ThreadLocal#get())method, unless the thread previously invoked the[set](https://developer.android.com/reference/kotlin/java/lang/ThreadLocal#set(java.lang.ThreadLocal.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/kotlin/java/lang/ThreadLocal#remove())followed by[get](https://developer.android.com/reference/kotlin/java/lang/ThreadLocal#get()). | Return || |----|-----------------------------------------| | T? | the initial value for this thread-local | **See Also** -