package java.lang;
import java.lang.ref.Reference;
import java.lang.ref.WeakReference;
import java.util.concurrent.atomic.AtomicInteger;
/**
* Implements a thread-local storage, that is, a variable for which each thread
* has its own value. All threads share the same {@code ThreadLocal} object,
* but each sees a different value when accessing it, and changes made by one
* thread do not affect the other threads. The implementation supports
* {@code null} values.
*
* @see java.lang.Thread
* @author Bob Lee
*/
public class ThreadLocal<T> {
/* Thanks to Josh Bloch and Doug Lea for code reviews and impl advice. */
/**
* Creates a new thread-local variable.
*/
public ThreadLocal() {}
/**
* Returns the value of this variable for the current thread. If an entry
* doesn't yet exist for this variable on this thread, this method will
* create an entry, populating the value with the result of
* {@link #initialValue()}.
*
* @return the current value of the variable for the calling thread.
*/
@SuppressWarnings("unchecked")
public T get() {
// Optimized for the fast path.
Thread currentThread = Thread.currentThread();
Values values = values(currentThread);
if (values != null) {
Object[] table = values.table;
int index = hash & values.mask;
if (this.reference == table[index]) {
return (T) table[index + 1];
}
} else {
values = initializeValues(currentThread);
}
return (T) values.getAfterMiss(this);
}
/**
* Provides the initial value of this variable for the current thread.
* The default implementation returns {@code null}.
*
* @return the initial value of the variable.
*/
protected T initialValue() {
return null;
}
/**
* Sets the value of this variable for the current thread. If set to
* {@code null}, the value will be set to null and the underlying entry will
* still be present.
*
* @param value the new value of the variable for the caller thread.
*/
public void set(T value) {
Thread currentThread = Thread.currentThread();
Values values = values(currentThread);
if (values == null) {
values = initializeValues(currentThread);
}
values.put(this, value);
}
/**
* Removes the entry for this variable in the current thread. If this call
* is followed by a {@link #get()} before a {@link #set},
* {@code #get()} will call {@link #initialValue()} and create a new
* entry with the resulting value.
*
* @since 1.5
*/
public void remove() {
Thread currentThread = Thread.currentThread();
Values values = values(currentThread);
if (values != null) {
values.remove(this);
}
}
/**
* Creates Values instance for this thread and variable type.
*/
Values initializeValues(Thread current) {
return current.localValues = new Values();
}
/**
* Gets Values instance for this thread and variable type.
*/
Values values(Thread current) {
return current.localValues;
}
/** Weak reference to this thread local instance. */
private final Reference<ThreadLocal<T>> reference
= new WeakReference<ThreadLocal<T>>(this);
/** Hash counter. */
private static AtomicInteger hashCounter = new AtomicInteger(0);
/**
* Internal hash. We deliberately don't bother with #hashCode().
* Hashes must be even. This ensures that the result of
* (hash & (table.length - 1)) points to a key and not a value.
*
* We increment by Doug Lea's Magic Number(TM) (*2 since keys are in
* every other bucket) to help prevent clustering.
*/
private final int hash = hashCounter.getAndAdd(0x61c88647 * 2);
/**
* Per-thread map of ThreadLocal instances to values.
*/
static class Values {
/**
* Size must always be a power of 2.
*/
private static final int INITIAL_SIZE = 16;
/**
* Placeholder for deleted entries.
*/
private static final Object TOMBSTONE = new Object();
/**
* Map entries. Contains alternating keys (ThreadLocal) and values.
* The length is always a power of 2.
*/
private Object[] table;
/** Used to turn hashes into indices. */
private int mask;
/** Number of live entries. */
private int size;
/** Number of tombstones. */
private int tombstones;
/** Maximum number of live entries and tombstones. */
private int maximumLoad;
/** Points to the next cell to clean up. */
private int clean;
/**
* Constructs a new, empty instance.
*/
Values() {
initializeTable(INITIAL_SIZE);
this.size = 0;
this.tombstones = 0;
}
/**
* Used for InheritableThreadLocals.
*/
Values(Values fromParent) {
this.table = fromParent.table.clone();
this.mask = fromParent.mask;
this.size = fromParent.size;
this.tombstones = fromParent.tombstones;
this.maximumLoad = fromParent.maximumLoad;
this.clean = fromParent.clean;
inheritValues(fromParent);
}
/**
* Inherits values from a parent thread.
*/
@SuppressWarnings({"unchecked"})
private void inheritValues(Values fromParent) {
// Transfer values from parent to child thread.
Object[] table = this.table;
for (int i = table.length - 2; i >= 0; i -= 2) {
Object k = table[i];
if (k == null || k == TOMBSTONE) {
// Skip this entry.
continue;
}
// The table can only contain null, tombstones and references.
Reference<InheritableThreadLocal<?>> reference
= (Reference<InheritableThreadLocal<?>>) k;
// Raw type enables us to pass in an Object below.
InheritableThreadLocal key = reference.get();
if (key != null) {
// Replace value with filtered value.
// We should just let exceptions bubble out and tank
// the thread creation
table[i + 1] = key.childValue(fromParent.table[i + 1]);
} else {
// The key was reclaimed.
table[i] = TOMBSTONE;
table[i + 1] = null;
fromParent.table[i] = TOMBSTONE;
fromParent.table[i + 1] = null;
tombstones++;
fromParent.tombstones++;
size--;
fromParent.size--;
}
}
}
/**
* Creates a new, empty table with the given capacity.
*/
private void initializeTable(int capacity) {
this.table = new Object[capacity * 2];
this.mask = table.length - 1;
this.clean = 0;
this.maximumLoad = capacity * 2 / 3; // 2/3
}
/**
* Cleans up after garbage-collected thread locals.
*/
private void cleanUp() {
if (rehash()) {
// If we rehashed, we needn't clean up (clean up happens as
// a side effect).
return;
}
if (size == 0) {
// No live entries == nothing to clean.
return;
}
// Clean log(table.length) entries picking up where we left off
// last time.
int index = clean;
Object[] table = this.table;
for (int counter = table.length; counter > 0; counter >>= 1,
index = next(index)) {
Object k = table[index];
if (k == TOMBSTONE || k == null) {
continue; // on to next entry
}
// The table can only contain null, tombstones and references.
@SuppressWarnings("unchecked")
Reference<ThreadLocal<?>> reference
= (Reference<ThreadLocal<?>>) k;
if (reference.get() == null) {
// This thread local was reclaimed by the garbage collector.
table[index] = TOMBSTONE;
table[index + 1] = null;
tombstones++;
size--;
}
}
// Point cursor to next index.
clean = index;
}
/**
* Rehashes the table, expanding or contracting it as necessary.
* Gets rid of tombstones. Returns true if a rehash occurred.
* We must rehash every time we fill a null slot; we depend on the
* presence of null slots to end searches (otherwise, we'll infinitely
* loop).
*/
private boolean rehash() {
if (tombstones + size < maximumLoad) {
return false;
}
int capacity = table.length >> 1;
// Default to the same capacity. This will create a table of the
// same size and move over the live entries, analogous to a
// garbage collection. This should only happen if you churn a
// bunch of thread local garbage (removing and reinserting
// the same thread locals over and over will overwrite tombstones
// and not fill up the table).
int newCapacity = capacity;
if (size > (capacity >> 1)) {
// More than 1/2 filled w/ live entries.
// Double size.
newCapacity = capacity * 2;
}
Object[] oldTable = this.table;
// Allocate new table.
initializeTable(newCapacity);
// We won't have any tombstones after this.
this.tombstones = 0;
// If we have no live entries, we can quit here.
if (size == 0) {
return true;
}
// Move over entries.
for (int i = oldTable.length - 2; i >= 0; i -= 2) {
Object k = oldTable[i];
if (k == null || k == TOMBSTONE) {
// Skip this entry.
continue;
}
// The table can only contain null, tombstones and references.
@SuppressWarnings("unchecked")
Reference<ThreadLocal<?>> reference
= (Reference<ThreadLocal<?>>) k;
ThreadLocal<?> key = reference.get();
if (key != null) {
// Entry is still live. Move it over.
add(key, oldTable[i + 1]);
} else {
// The key was reclaimed.
size--;
}
}
return true;
}
/**
* Adds an entry during rehashing. Compared to put(), this method
* doesn't have to clean up, check for existing entries, account for
* tombstones, etc.
*/
void add(ThreadLocal<?> key, Object value) {
for (int index = key.hash & mask;; index = next(index)) {
Object k = table[index];
if (k == null) {
table[index] = key.reference;
table[index + 1] = value;
return;
}
}
}
/**
* Sets entry for given ThreadLocal to given value, creating an
* entry if necessary.
*/
void put(ThreadLocal<?> key, Object value) {
cleanUp();
// Keep track of first tombstone. That's where we want to go back
// and add an entry if necessary.
int firstTombstone = -1;
for (int index = key.hash & mask;; index = next(index)) {
Object k = table[index];
if (k == key.reference) {
// Replace existing entry.
table[index + 1] = value;
return;
}
if (k == null) {
if (firstTombstone == -1) {
// Fill in null slot.
table[index] = key.reference;
table[index + 1] = value;
size++;
return;
}
// Go back and replace first tombstone.
table[firstTombstone] = key.reference;
table[firstTombstone + 1] = value;
tombstones--;
size++;
return;
}
// Remember first tombstone.
if (firstTombstone == -1 && k == TOMBSTONE) {
firstTombstone = index;
}
}
}
/**
* Gets value for given ThreadLocal after not finding it in the first
* slot.
*/
Object getAfterMiss(ThreadLocal<?> key) {
Object[] table = this.table;
int index = key.hash & mask;
// If the first slot is empty, the search is over.
if (table[index] == null) {
Object value = key.initialValue();
// If the table is still the same and the slot is still empty...
if (this.table == table && table[index] == null) {
table[index] = key.reference;
table[index + 1] = value;
size++;
cleanUp();
return value;
}
// The table changed during initialValue().
put(key, value);
return value;
}
// Keep track of first tombstone. That's where we want to go back
// and add an entry if necessary.
int firstTombstone = -1;
// Continue search.
for (index = next(index);; index = next(index)) {
Object reference = table[index];
if (reference == key.reference) {
return table[index + 1];
}
// If no entry was found...
if (reference == null) {
Object value = key.initialValue();
// If the table is still the same...
if (this.table == table) {
// If we passed a tombstone and that slot still
// contains a tombstone...
if (firstTombstone > -1
&& table[firstTombstone] == TOMBSTONE) {
table[firstTombstone] = key.reference;
table[firstTombstone + 1] = value;
tombstones--;
size++;
// No need to clean up here. We aren't filling
// in a null slot.
return value;
}
// If this slot is still empty...
if (table[index] == null) {
table[index] = key.reference;
table[index + 1] = value;
size++;
cleanUp();
return value;
}
}
// The table changed during initialValue().
put(key, value);
return value;
}
if (firstTombstone == -1 && reference == TOMBSTONE) {
// Keep track of this tombstone so we can overwrite it.
firstTombstone = index;
}
}
}
/**
* Removes entry for the given ThreadLocal.
*/
void remove(ThreadLocal<?> key) {
cleanUp();
for (int index = key.hash & mask;; index = next(index)) {
Object reference = table[index];
if (reference == key.reference) {
// Success!
table[index] = TOMBSTONE;
table[index + 1] = null;
tombstones++;
size--;
return;
}
if (reference == null) {
// No entry found.
return;
}
}
}
/**
* Gets the next index. If we're at the end of the table, we wrap back
* around to 0.
*/
private int next(int index) {
return (index + 2) & mask;
}
}
}
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