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

Author:

Josh Bloch and Doug Lea

Nested Class Summary

(package private) static class

ThreadLocal.ThreadLocalMap ThreadLocalMap is a customized hash map suitable only for maintaining thread local values.

Field Summary

private static int	HASH_INCREMENT The difference between successively generated hash codes - turns implicit sequential thread-local IDs into near-optimally spread multiplicative hash values for power-of-two-sized tables.
private static int	nextHashCode The next hash code to be given out.
private int	threadLocalHashCode ThreadLocals rely on per-thread hash maps attached to each thread (Thread.threadLocals and inheritableThreadLocals).

Constructor Summary

ThreadLocal()

Method Summary

(package private) Object	childValue(Object parentValue) Method childValue is visibly defined in subclass InheritableThreadLocal, but is internally defined here for the sake of providing createInheritedMap factory method without needing to subclass the map class in InheritableThreadLocal.
(package private) static ThreadLocal.ThreadLocalMap	createInheritedMap(ThreadLocal.ThreadLocalMap parentMap) Factory method to create map of inherited thread locals.
(package private) void	createMap(Thread t, Object firstValue) Create the map associated with a ThreadLocal.
Object	get() Returns the value in the current thread's copy of this thread-local variable.
(package private) ThreadLocal.ThreadLocalMap	getMap(Thread t) Get the map associated with a ThreadLocal.
protected Object	initialValue() Returns the current thread's initial value for this thread-local variable.
private static int	nextHashCode() Compute the next hash code.
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

Field Detail

threadLocalHashCode

private final int threadLocalHashCode

ThreadLocals rely on per-thread hash maps attached to each thread (Thread.threadLocals and inheritableThreadLocals). The ThreadLocal objects act as keys, searched via threadLocalHashCode. This is a custom hash code (useful only within ThreadLocalMaps) that eliminates collisions in the common case where consecutively constructed ThreadLocals are used by the same threads, while remaining well-behaved in less common cases.

nextHashCode

private static int nextHashCode

The next hash code to be given out. Accessed only by like-named method.

HASH_INCREMENT

private static final int HASH_INCREMENT

The difference between successively generated hash codes - turns implicit sequential thread-local IDs into near-optimally spread multiplicative hash values for power-of-two-sized tables.

See Also:

Constant Field Values

Constructor Detail

ThreadLocal

public ThreadLocal()

Method Detail

nextHashCode

private static int nextHashCode()

Compute the next hash code. The static synchronization used here should not be a performance bottleneck. When ThreadLocals are generated in different threads at a fast enough rate to regularly contend on this lock, memory contention is by far a more serious problem than lock contention.

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.

getMap

ThreadLocal.ThreadLocalMap getMap(Thread t)

Get the map associated with a ThreadLocal. Overridden in InheritableThreadLocal.

Parameters:

t - the current thread

Returns:

the map

createMap

void createMap(Thread t,
               Object firstValue)

Create the map associated with a ThreadLocal. Overridden in InheritableThreadLocal.

Parameters:

t - the current thread

firstValue - value for the initial entry of the map

createInheritedMap

static ThreadLocal.ThreadLocalMap createInheritedMap(ThreadLocal.ThreadLocalMap parentMap)

Factory method to create map of inherited thread locals. Designed to be called only from Thread constructor.

Parameters:

parentMap - the map associated with parent thread

Returns:

a map containing the parent's inheritable bindings

childValue

Object childValue(Object parentValue)

Method childValue is visibly defined in subclass InheritableThreadLocal, but is internally defined here for the sake of providing createInheritedMap factory method without needing to subclass the map class in InheritableThreadLocal. This technique is preferable to the alternative of embedding instanceof tests in methods.