ThreadLocalT

ThreadLocal<T>

java.lang

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Java 1.2

This class provides a convenient way to create thread-local variables . When you declare a static field in a class, there is only one value for that field, shared by all objects of the class. When you declare a nonstatic instance field in a class, every object of the class has its own separate copy of that variable. ThreadLocal provides an option between these two extremes. If you declare a static field to hold a THReadLocal object, that ThreadLocal holds a different value for each thread. Objects running in the same thread see the same value when they call the get( ) method of the ThreadLocal object. Objects running in different threads obtain different values from get( ) , however.

In Java 5.0, this class has been made generic and the type variable T represents the type of the object referenced by this ThreadLocal .

The set( ) method sets the value held by the THReadLocal object for the currently running thread. get( ) returns the value held for the currently running thread. Note that there is no way to obtain the value of the ThreadLocal object for any thread other than the one that calls get( ) . To understand the ThreadLocal class, you may find it helpful to think of a ThreadLocal object as a hashtable or java.util.Map that maps from Thread objects to arbitrary values. Calling set( ) creates an association between the current Thread ( Thread.currentThread( ) ) and the specified value. Calling get( ) first looks up the current thread, then uses the hashtable to look up the value associated with that current thread.

If a thread calls get( ) for the first time without having first called set( ) to establish a thread-local value, get( ) calls the protected initialValue( ) method to obtain the initial value to return. The default implementation of initialValue( ) simply returns null , but subclasses can override this if they desire .

See also InheritableThreadLocal , which allows thread-local values to be inherited from parent threads by child threads.

 public class  ThreadLocal<T>  {  // Public Constructors  public  ThreadLocal  ( );  // Public Instance Methods  public T  get  ( );  5.0  public void  remove  ( );        public void  set  (T  value  );  // Protected Instance Methods  protected T  initialValue  ( );  constant  } 

Subclasses

InheritableThreadLocal