Class ThreadLocal<T>

java.lang.Object
java.lang.ThreadLocal<T>
Type Parameters:
T - the type of the thread local's value
Direct Known Subclasses:
InheritableThreadLocal
public classThreadLocal<T>extendsObject
This class provides thread-local variables. These variables differ fromtheir normal counterparts in that each thread that accesses one (via itsget orset method) has its own, independently initializedcopy of the variable.ThreadLocal instances are typically privatestatic 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 eachthread.A thread's id is assigned the first time it invokesThreadId.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-localvariable as long as the thread is alive and theThreadLocalinstance is accessible; after a thread goes away, all of its copies ofthread-local instances are subject to garbage collection (unless otherreferences to these copies exist).

Since:
1.2

  • Constructor Summary

    Constructors
    Constructor
    Description
    Creates a thread local variable.

  • Method Summary

    Modifier and Type
    Method
    Description
    get()
    Returns the value in the current thread's copy of thisthread-local variable.
    protectedT
    Returns the current thread's "initial value" for thisthread-local variable.
    void
    Removes the current thread's value for this thread-localvariable.
    void
    set(T value)
    Sets the current thread's copy of this thread-local variableto the specified value.
    static <S> ThreadLocal<S>
    withInitial(Supplier<? extends S> supplier)
    Creates a thread local variable.

  • Constructor Details

  • Method Details

    • initialValue

      protected T initialValue()
      Returns the current thread's "initial value" for thisthread-local variable. This method will be invoked the firsttime a thread accesses the variable with theget()method, unless the thread previously invoked theset(T)method, in which case theinitialValue method will notbe invoked for the thread. Normally, this method is invoked atmost once per thread, but it may be invoked again in case ofsubsequent invocations ofremove() followed byget().
      Implementation Requirements:
      This implementation simply returnsnull; if theprogrammer desires thread-local variables to have an initialvalue other thannull, then eitherThreadLocalcan be subclassed and this method overridden or the methodwithInitial(Supplier) can be used toconstruct aThreadLocal.
      Returns:
      the initial value for this thread-local
      See Also:

    • withInitial

      public static <S> ThreadLocal<S> withInitial(Supplier<? extends S> supplier)
      Creates a thread local variable. The initial value of the variable isdetermined by invoking theget method on theSupplier.
      Type Parameters:
      S - the type of the thread local's value
      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
      Since:
      1.8

    • get

      public T get()
      Returns the value in the current thread's copy of thisthread-local variable. If the variable has no value for thecurrent thread, it is first initialized to the value returnedby an invocation of theinitialValue() method.
      Returns:
      the current thread's value of this thread-local

    • set

      public void set(T value)
      Sets the current thread's copy of this thread-local variableto the specified value. Most subclasses will have no need tooverride this method, relying solely on theinitialValue()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.

    • remove

      public void remove()
      Removes the current thread's value for this thread-localvariable. If this thread-local variable is subsequentlyread by the current thread, its value will bereinitialized by invoking itsinitialValue() method,unless its value isset by the current threadin the interim. This may result in multiple invocations of theinitialValue method in the current thread.
      Since:
      1.5