File	Doc	Category	Size	Date	Package
ThreadLocal.java	API Doc	Java SE 5 API	24721	Fri Aug 26 14:57:04 BST 2005	java.lang

ThreadLocal

• java.lang.Object

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();
}
}

Fields Summary
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.
private static int	nextHashCode The next hash code to be given out. Accessed only by like-named method.
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.

Constructors Summary
public ThreadLocal() Creates a thread local variable.

Methods Summary
T	childValue(T 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. throw new UnsupportedOperationException();
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. param parentMap the map associated with parent thread return a map containing the parent's inheritable bindings return new ThreadLocalMap(parentMap);
void	createMap(java.lang.Thread t, T firstValue) Create the map associated with a ThreadLocal. Overridden in InheritableThreadLocal. param t the current thread param firstValue value for the initial entry of the map param map the map to store. t.threadLocals = new ThreadLocalMap(this, firstValue);
public T	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. return the current thread's value of this thread-local Thread t = Thread.currentThread(); ThreadLocalMap map = getMap(t); if (map != null) return (T)map.get(this); // Maps are constructed lazily. if the map for this thread // doesn't exist, create it, with this ThreadLocal and its // initial value as its only entry. T value = initialValue(); createMap(t, value); return value;
java.lang.ThreadLocal$ThreadLocalMap	getMap(java.lang.Thread t) Get the map associated with a ThreadLocal. Overridden in InheritableThreadLocal. param t the current thread return the map return t.threadLocals;
protected T	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 {@link #get} method. The initialValue method will not be invoked in a thread if the thread invokes the {@link #set} 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. return the initial value for this thread-local return null;
private static synchronized 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. int h = nextHashCode; nextHashCode = h + HASH_INCREMENT; return h;
public void	remove() Removes the value for this ThreadLocal. This may help reduce the storage requirements of ThreadLocals. If this ThreadLocal is accessed again, it will by default have its initialValue. since 1.5 ThreadLocalMap m = getMap(Thread.currentThread()); if (m != null) m.remove(this);
public void	set(T 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 {@link #initialValue} method to set the values of thread-locals. param value the value to be stored in the current threads' copy of this thread-local. Thread t = Thread.currentThread(); ThreadLocalMap map = getMap(t); if (map != null) map.set(this, value); else createMap(t, value);