public class ThreadLocal<T> extends Object
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<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).
| Constructor and Description | 
|---|
| ThreadLocal()Creates a thread local variable. | 
| Modifier and Type | Method and Description | 
|---|---|
| T | get()Returns the value in the current thread's copy of this
 thread-local variable. | 
| protected T | initialValue()Returns the current thread's "initial value" for this
 thread-local variable. | 
| void | remove()Removes the current thread's value for this thread-local
 variable. | 
| void | set(T value)Sets 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. | 
public ThreadLocal()
withInitial(java.util.function.Supplier)protected T initialValue()
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().
 This implementation simply returns null; if the
 programmer desires thread-local variables to have an initial
 value other than null, ThreadLocal must be
 subclassed, and this method overridden.  Typically, an
 anonymous inner class will be used.
public static <S> ThreadLocal<S> withInitial(Supplier<? extends S> supplier)
get method on the Supplier.S - the type of the thread local's valuesupplier - the supplier to be used to determine the initial valueNullPointerException - if the specified supplier is nullpublic T get()
initialValue() method.public void set(T value)
initialValue()
 method to set the values of thread-locals.value - the value to be stored in the current thread's copy of
        this thread-local.public void remove()
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. Submit a bug or feature 
For further API reference and developer documentation, see Java SE Documentation. That documentation contains more detailed, developer-targeted descriptions, with conceptual overviews, definitions of terms, workarounds, and working code examples.
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