File	Doc	Category	Size	Date	Package
ThreadLocal.java	API Doc	Java SE 6 API	23872	Tue Jun 10 00:25:38 BST 2008	java.lang

ThreadLocal

• java.lang.Object

import java.util.concurrent.atomic.AtomicInteger;

public class UniqueThreadIdGenerator {

private static final AtomicInteger uniqueId = new AtomicInteger(0);

private static final ThreadLocal < Integer > uniqueNum =
new ThreadLocal < Integer > () {
@Override protected Integer initialValue() {
return uniqueId.getAndIncrement();
}
};

public static int getCurrentThreadId() {
return uniqueId.get();
}
} // UniqueThreadIdGenerator

Fields Summary
private final int	threadLocalHashCode ThreadLocals rely on per-thread linear-probe 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 AtomicInteger	nextHashCode The next hash code to be given out. Updated atomically. Starts at zero.
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. If the variable has no value for the current thread, it is first initialized to the value returned by an invocation of the {@link #initialValue} method. return the current thread's value of this thread-local Thread t = Thread.currentThread(); ThreadLocalMap map = getMap(t); if (map != null) { ThreadLocalMap.Entry e = map.getEntry(this); if (e != null) return (T)e.value; } return setInitialValue();
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 the first time a thread accesses the variable with the {@link #get} method, unless the thread previously invoked the {@link #set} method, in which case the initialValue method will not be invoked for the thread. Normally, this method is invoked at most once per thread, but it may be invoked again in case of subsequent invocations of {@link #remove} followed by {@link #get}. This implementation simply returns null; if the programmer desires thread-local variables to have an initial value other than null, ThreadLocal must be subclassed, and this method overridden. Typically, an anonymous inner class will be used. return the initial value for this thread-local return null;
private static int	nextHashCode() Returns the next hash code. return nextHashCode.getAndAdd(HASH_INCREMENT);
public void	remove() Removes the current thread's value for this thread-local variable. If this thread-local variable is subsequently {@linkplain #get read} by the current thread, its value will be reinitialized by invoking its {@link #initialValue} method, unless its value is {@linkplain #set set} by the current thread in the interim. This may result in multiple invocations of the initialValue method in the current thread. 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. Most subclasses will have no need to override this method, relying solely on the {@link #initialValue} method to set the values of thread-locals. param value the value to be stored in the current thread's copy of this thread-local. Thread t = Thread.currentThread(); ThreadLocalMap map = getMap(t); if (map != null) map.set(this, value); else createMap(t, value);
private T	setInitialValue() Variant of set() to establish initialValue. Used instead of set() in case user has overridden the set() method. return the initial value T value = initialValue(); Thread t = Thread.currentThread(); ThreadLocalMap map = getMap(t); if (map != null) map.set(this, value); else createMap(t, value); return value;