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ThreadLocal

public class ThreadLocal
extends Object

java.lang.Object
   ↳ java.lang.ThreadLocal<T>
Known Direct Subclasses


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.

 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).

Summary

Public constructors

ThreadLocal()

Creates a thread local variable.

Public methods

T get()

Returns the value in the current thread's copy of 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.

Protected methods

T initialValue()

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

Inherited methods

From class java.lang.Object

Public constructors

ThreadLocal

Added in API level 1
ThreadLocal ()

Creates a thread local variable.

Public methods

get

Added in API level 1
T 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
T the current thread's value of this thread-local

remove

Added in API level 1
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.

set

Added in API level 1
void set (T value)

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 T: the value to be stored in the current thread's copy of this thread-local.

Protected methods

initialValue

Added in API level 1
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().

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.

Returns
T the initial value for this thread-local
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