* It is possible to make the value associated with the child Thread a function
* of the value that is associated with the parent Thread by overriding the
* childValue() method.
*
* @since 1.2
* @author Mark Wielaard (mark@klomp.org)
*/
public class InheritableThreadLocal extends ThreadLocal {
/**
* Maps Threads to a Set of InheritableThreadLocals
* (the heritage of that Thread).
* Uses a WeakHashMap so if the Thread is garbage collected the reference
* to that Set disappears.
* Both AddToHeritage access and modify it so they have to
* synchronize on the threadMap when they do.
*/
private static Map threadMap = new WeakHashMap();
/**
* Creates a new InheritableThreadLocal that has no values associated
* with it yet.
*/
public InheritableThreadLocal() {
super();
}
/**
* Determines the value associated with a newly created child Thread
* as a function of the value associated with the currently executing
* (parent) Thread.
*
* The default implementation just returns the parentValue.
*/
protected Object childValue(Object parentValue) {
return parentValue;
}
/**
* Adds this InheritableThreadLocal to the heritage of the
* current Thread and returns the value of the ThreadLocal
* for the Thread. The value will be either the last value that the
* current Thread has set, or the childValue of the last value that the
* parent Thread set before the current Thread was created, or the
* initialValue of the ThreadLocal.
*
* @see ThreadLocal#get()
*/
public Object get() {
addToHeritage();
return super.get();
}
/**
* Adds this InheritableThreadLocal to the heritage of the
* current Thread and sets the value of the ThreadLocal
* for the Thread.
*
* @see ThreadLocal#set(Object)
*/
public void set(Object value) {
addToHeritage();
super.set(value);
}
/**
* Adds this InheritableThreadLocal to the heritage
* of the current Thread.
*/
private void addToHeritage() {
Thread currentThread = Thread.currentThread();
Set heritage;
synchronized(threadMap) {
heritage = (Set)threadMap.get(currentThread);
}
// Note that we don't have to synchronize on the heritage Set
// since only this Thread (or the parent Thread when creating
// the heritage) ever modifies it.
if (heritage == null) {
heritage = new HashSet();
synchronized(threadMap) {
threadMap.put(currentThread, heritage);
}
}
if (!heritage.contains(this)) {
heritage.add(this);
}
}
/**
* Generates the childValues of all InheritableThreadLocals
* that are in the heritage of the current Thread for the newly created
* childThread.
* Should be called from the contructor of java.lang.Thread.
*/
static void newChildThread(Thread childThread) {
// The currentThread is the parent of the new thread
Thread parentThread = Thread.currentThread();
// Inherit all the InheritableThreadLocals of the parent thread
Set heritage;
synchronized(threadMap) {
heritage = (Set)threadMap.get(parentThread);
}
// Note that we don't have to synchronize on the heritage Set
// since only this Thread (or the parent Thread when creating
// the heritage) ever modifies it.
if (heritage != null) {
synchronized(threadMap) {
threadMap.put(childThread, new HashSet(heritage));
}
// And constructs all the new child values
// (has to be done now that we are executing in the parentThread)
Iterator it = heritage.iterator();
while (it.hasNext()) {
InheritableThreadLocal local =
(InheritableThreadLocal) it.next();
// Note that the parentValue cannot be null
// If it was it would not be in the heritage
Object parentValue = local.get(parentThread).getValue();
Object childValue = local.childValue(parentValue);
ThreadLocal.Value v = new ThreadLocal.Value(childValue);
local.set(childThread, v);
}
}
}
}