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Showing content from https://docs.oracle.com/en/java/javase/21/docs/api/java.base/java/lang/ThreadLocal.html below:

ThreadLocal (Java SE 21 & JDK 21)

Type Parameters:

T - the type of the thread local's value

Direct Known Subclasses:

InheritableThreadLocal

This class provides thread-local variables. These variables differ from their normal counterparts in that each thread that accesses one (via its

or

method) has its own, independently initialized copy of the variable.

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).

Since:

1.2

• Constructor Summary

  Constructors

  Creates a thread local variable.

• Method Summary

  Returns the value in the current thread's copy of this thread-local variable.

  Returns the current thread's "initial value" for this thread-local variable.

  void

  Removes the current thread's value for this thread-local variable.

  void

  Sets the current thread's copy of this thread-local variable to the specified value.

  Creates a thread local variable.

  Methods declared in class java.lang.Objectclone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

• Constructor Details
  • ThreadLocal

    public ThreadLocal()

    Creates a thread local variable.

    See Also:

    • withInitial(java.util.function.Supplier)

• Method Details
  • initialValue 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()

    .

    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 withInitial(Supplier) can be used to construct a ThreadLocal.

    Returns:

    the initial value for this thread-local

    See Also:

    • withInitial(java.util.function.Supplier)

  • withInitial

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

    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

    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

  • set

    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.

  • remove

    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.

    Since:

    1.5

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