Class ThreadLocal<T>
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).
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Constructor Summary
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Method Summary
All MethodsStatic MethodsInstance MethodsConcrete MethodsModifier and TypeMethodDescriptionget()Returns the value in the current thread's copy of this thread-local variable.protected TReturns the current thread's "initial value" for this thread-local variable.voidremove()Removes the current thread's value for this thread-local variable.voidSets 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.Methods declared in class Object
clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, waitModifier and TypeMethodDescriptionprotected Objectclone()Creates and returns a copy of this object.booleanIndicates whether some other object is "equal to" this one.protected voidfinalize()Deprecated, for removal: This API element is subject to removal in a future version.Finalization is deprecated and subject to removal in a future release.final Class<?>getClass()Returns the runtime class of this Object.inthashCode()Returns a hash code value for this object.final voidnotify()Wakes up a single thread that is waiting on this object's monitor.final voidWakes up all threads that are waiting on this object's monitor.toString()Returns a string representation of the object.final voidwait()Causes the current thread to wait until it is awakened, typically by being notified or interrupted.final voidwait(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 voidwait(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.
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Constructor Details
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ThreadLocal
public ThreadLocal()Creates a thread local variable.See Also:
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Method Details
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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
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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