java.lang
Class ThreadLocal

java.lang.Object
 |
 +--java.lang.ThreadLocal

Direct Known Subclasses:

InheritableThreadLocal

public class ThreadLocal

extends Object

Untamed:

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.

ThreadLocal

public ThreadLocal()

Enabled:

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 called once per accessing thread for each thread-local, the first time each thread accesses the variable with the get() or set(Object) method. 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 call an appropriate constructor and return the newly constructed object.

Returns:

the initial value for this thread-local

get

public Object get()

Enabled: 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)

Enabled: Sets the current thread's copy of this thread-local variable to the specified value. This is only used to change the value from the one assigned by the initialValue method, and many applications will have no need for this functionality.

Parameters:

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

getMap

java.lang.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 java.lang.ThreadLocal.ThreadLocalMap createInheritedMap(java.lang.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.