java.lang

Class ThreadLocal

java.lang.Object

java.lang.ThreadLocal

Direct Known Subclasses:

InheritableThreadLocal

public class ThreadLocal
extends Object

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, in the class below, the private static ThreadLocal instance (serialNum) maintains a "serial number" for each thread that invokes the class's static SerialNum.get() method, which returns the current thread's serial number. (A thread's serial number is assigned the first time it invokes SerialNum.get(), and remains unchanged on subsequent calls.)

 public class SerialNum {
     // The next serial number to be assigned
     private static int nextSerialNum = 0;
 
     private static ThreadLocal serialNum = new ThreadLocal() {
         protected synchronized Object initialValue() {
             return new Integer(nextSerialNum++);
         }
     };
 
     public static int get() {
         return ((Integer) (serialNum.get())).intValue();
     }
 }
 

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

Version:

1.21, 03/12/05

Author:

Josh Bloch and Doug Lea

Constructor Summary

Constructors

Constructor and Description
ThreadLocal()

Method Summary

Methods

Modifier and Type	Method and Description
Object	get() Returns the value in the current thread's copy of this thread-local variable.
protected Object	initialValue() Returns the current thread's initial value for this thread-local variable.
void	set(Object value) Sets the current thread's copy of this thread-local variable to the specified value.

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Constructor Detail

ThreadLocal

public ThreadLocal()

Method Detail

initialValue

protected Object initialValue()

Returns the current thread's initial value for this thread-local variable. This method will be invoked at most once per accessing thread for each thread-local, the first time the thread accesses the variable with the get() method. The initialValue method will not be invoked in a thread if the thread invokes the set(Object) method prior to the get method.

This implementation simply returns null; if the programmer desires thread-local variables to be initialized to some value other than null, ThreadLocal must be subclassed, and this method overridden. Typically, an anonymous inner class will be used. Typical implementations of initialValue will invoke an appropriate constructor and return the newly constructed object.

Returns:

the initial value for this thread-local

get

public Object get()

Returns the value in the current thread's copy of this thread-local variable. Creates and initializes the copy if this is the first time the thread has called this method.

Returns:

the current thread's value of this thread-local

set

public void set(Object value)

Sets the current thread's copy of this thread-local variable to the specified value. Many applications will have no need for this functionality, relying solely on the initialValue() method to set the values of thread-locals.

Parameters:

value - the value to be stored in the current threads' copy of this thread-local.