/*
* Copyright (c) 1997, 2013, Oracle and/or its affiliates. All rights reserved.
* ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
*
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*/
package java.lang;
import java.lang.ref.*;
import java.util.
Objects;
import java.util.concurrent.atomic.
AtomicInteger;
import java.util.function.
Supplier;
/**
* This class provides thread-local variables. These variables differ from
* their normal counterparts in that each thread that accesses one (via its
* {@code get} or {@code set} method) has its own, independently initialized
* copy of the variable. {@code ThreadLocal} instances are typically private
* static fields in classes that wish to associate state with a thread (e.g.,
* a user ID or Transaction ID).
*
* <p>For example, the class below generates unique identifiers local to each
* thread.
* A thread's id is assigned the first time it invokes {@code ThreadId.get()}
* and remains unchanged on subsequent calls.
* <pre>
* import java.util.concurrent.atomic.AtomicInteger;
*
* public class ThreadId {
* // Atomic integer containing the next thread ID to be assigned
* private static final AtomicInteger nextId = new AtomicInteger(0);
*
* // Thread local variable containing each thread's ID
* private static final ThreadLocal<Integer> threadId =
* new ThreadLocal<Integer>() {
* @Override protected Integer initialValue() {
* return nextId.getAndIncrement();
* }
* };
*
* // Returns the current thread's unique ID, assigning it if necessary
* public static int get() {
* return threadId.get();
* }
* }
* </pre>
* <p>Each thread holds an implicit reference to its copy of a thread-local
* variable as long as the thread is alive and the {@code ThreadLocal}
* instance is accessible; after a thread goes away, all of its copies of
* thread-local instances are subject to garbage collection (unless other
* references to these copies exist).
*
* @author Josh Bloch and Doug Lea
* @since 1.2
*/
public class
ThreadLocal<T> {
/**
* 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 final int
threadLocalHashCode =
nextHashCode();
/**
* The next hash code to be given out. Updated atomically. Starts at
* zero.
*/
private static
AtomicInteger nextHashCode =
new
AtomicInteger();
/**
* 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.
*/
private static final int
HASH_INCREMENT = 0x61c88647;
/**
* Returns the next hash code.
*/
private static int
nextHashCode() {
return
nextHashCode.
getAndAdd(
HASH_INCREMENT);
}
/**
* 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 {@code 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}.
*
* <p>This implementation simply returns {@code null}; if the
* programmer desires thread-local variables to have an initial
* value other than {@code null}, {@code ThreadLocal} must be
* subclassed, and this method overridden. Typically, an
* anonymous inner class will be used.
*
* @return the initial value for this thread-local
*/
protected T
initialValue() {
return null;
}
/**
* Creates a thread local variable. The initial value of the variable is
* determined by invoking the {@code get} method on the {@code Supplier}.
*
* @param <S> the type of the thread local's value
* @param supplier the supplier to be used to determine the initial value
* @return a new thread local variable
* @throws NullPointerException if the specified supplier is null
* @since 1.8
*/
public static <S>
ThreadLocal<S>
withInitial(
Supplier<? extends S>
supplier) {
return new
SuppliedThreadLocal<>(
supplier);
}
/**
* Creates a thread local variable.
* @see #withInitial(java.util.function.Supplier)
*/
public
ThreadLocal() {
}
/**
* 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
*/
public T
get() {
Thread t =
Thread.
currentThread();
ThreadLocalMap map =
getMap(
t);
if (
map != null) {
ThreadLocalMap.
Entry e =
map.
getEntry(this);
if (
e != null) {
@
SuppressWarnings("unchecked")
T
result = (T)
e.
value;
return
result;
}
}
return
setInitialValue();
}
/**
* Variant of set() to establish initialValue. Used instead
* of set() in case user has overridden the set() method.
*
* @return the initial value
*/
private T
setInitialValue() {
T
value =
initialValue();
Thread t =
Thread.
currentThread();
ThreadLocalMap map =
getMap(
t);
if (
map != null)
map.
set(this,
value);
else
createMap(
t,
value);
return
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.
*/
public void
set(T
value) {
Thread t =
Thread.
currentThread();
ThreadLocalMap map =
getMap(
t);
if (
map != null)
map.
set(this,
value);
else
createMap(
t,
value);
}
/**
* 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
* {@code initialValue} method in the current thread.
*
* @since 1.5
*/
public void
remove() {
ThreadLocalMap m =
getMap(
Thread.
currentThread());
if (
m != null)
m.
remove(this);
}
/**
* Get the map associated with a ThreadLocal. Overridden in
* InheritableThreadLocal.
*
* @param t the current thread
* @return the map
*/
ThreadLocalMap getMap(
Thread t) {
return
t.
threadLocals;
}
/**
* 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
*/
void
createMap(
Thread t, T
firstValue) {
t.
threadLocals = new
ThreadLocalMap(this,
firstValue);
}
/**
* 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
*/
static
ThreadLocalMap createInheritedMap(
ThreadLocalMap parentMap) {
return new
ThreadLocalMap(
parentMap);
}
/**
* 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.
*/
T
childValue(T
parentValue) {
throw new
UnsupportedOperationException();
}
/**
* An extension of ThreadLocal that obtains its initial value from
* the specified {@code Supplier}.
*/
static final class
SuppliedThreadLocal<T> extends
ThreadLocal<T> {
private final
Supplier<? extends T>
supplier;
SuppliedThreadLocal(
Supplier<? extends T>
supplier) {
this.
supplier =
Objects.
requireNonNull(
supplier);
}
@
Override
protected T
initialValue() {
return
supplier.
get();
}
}
/**
* ThreadLocalMap is a customized hash map suitable only for
* maintaining thread local values. No operations are exported
* outside of the ThreadLocal class. The class is package private to
* allow declaration of fields in class Thread. To help deal with
* very large and long-lived usages, the hash table entries use
* WeakReferences for keys. However, since reference queues are not
* used, stale entries are guaranteed to be removed only when
* the table starts running out of space.
*/
static class
ThreadLocalMap {
/**
* The entries in this hash map extend WeakReference, using
* its main ref field as the key (which is always a
* ThreadLocal object). Note that null keys (i.e. entry.get()
* == null) mean that the key is no longer referenced, so the
* entry can be expunged from table. Such entries are referred to
* as "stale entries" in the code that follows.
*/
static class
Entry extends
WeakReference<
ThreadLocal<?>> {
/** The value associated with this ThreadLocal. */
Object value;
Entry(
ThreadLocal<?>
k,
Object v) {
super(
k);
value =
v;
}
}
/**
* The initial capacity -- MUST be a power of two.
*/
private static final int
INITIAL_CAPACITY = 16;
/**
* The table, resized as necessary.
* table.length MUST always be a power of two.
*/
private
Entry[]
table;
/**
* The number of entries in the table.
*/
private int
size = 0;
/**
* The next size value at which to resize.
*/
private int
threshold; // Default to 0
/**
* Set the resize threshold to maintain at worst a 2/3 load factor.
*/
private void
setThreshold(int
len) {
threshold =
len * 2 / 3;
}
/**
* Increment i modulo len.
*/
private static int
nextIndex(int
i, int
len) {
return ((
i + 1 <
len) ?
i + 1 : 0);
}
/**
* Decrement i modulo len.
*/
private static int
prevIndex(int
i, int
len) {
return ((
i - 1 >= 0) ?
i - 1 :
len - 1);
}
/**
* Construct a new map initially containing (firstKey, firstValue).
* ThreadLocalMaps are constructed lazily, so we only create
* one when we have at least one entry to put in it.
*/
ThreadLocalMap(
ThreadLocal<?>
firstKey,
Object firstValue) {
table = new
Entry[
INITIAL_CAPACITY];
int
i =
firstKey.
threadLocalHashCode & (
INITIAL_CAPACITY - 1);
table[
i] = new
Entry(
firstKey,
firstValue);
size = 1;
setThreshold(
INITIAL_CAPACITY);
}
/**
* Construct a new map including all Inheritable ThreadLocals
* from given parent map. Called only by createInheritedMap.
*
* @param parentMap the map associated with parent thread.
*/
private
ThreadLocalMap(
ThreadLocalMap parentMap) {
Entry[]
parentTable =
parentMap.
table;
int
len =
parentTable.length;
setThreshold(
len);
table = new
Entry[
len];
for (int
j = 0;
j <
len;
j++) {
Entry e =
parentTable[
j];
if (
e != null) {
@
SuppressWarnings("unchecked")
ThreadLocal<
Object>
key = (
ThreadLocal<
Object>)
e.
get();
if (
key != null) {
Object value =
key.
childValue(
e.
value);
Entry c = new
Entry(
key,
value);
int
h =
key.
threadLocalHashCode & (
len - 1);
while (
table[
h] != null)
h =
nextIndex(
h,
len);
table[
h] =
c;
size++;
}
}
}
}
/**
* Get the entry associated with key. This method
* itself handles only the fast path: a direct hit of existing
* key. It otherwise relays to getEntryAfterMiss. This is
* designed to maximize performance for direct hits, in part
* by making this method readily inlinable.
*
* @param key the thread local object
* @return the entry associated with key, or null if no such
*/
private
Entry getEntry(
ThreadLocal<?>
key) {
int
i =
key.
threadLocalHashCode & (
table.length - 1);
Entry e =
table[
i];
if (
e != null &&
e.
get() ==
key)
return
e;
else
return
getEntryAfterMiss(
key,
i,
e);
}
/**
* Version of getEntry method for use when key is not found in
* its direct hash slot.
*
* @param key the thread local object
* @param i the table index for key's hash code
* @param e the entry at table[i]
* @return the entry associated with key, or null if no such
*/
private
Entry getEntryAfterMiss(
ThreadLocal<?>
key, int
i,
Entry e) {
Entry[]
tab =
table;
int
len =
tab.length;
while (
e != null) {
ThreadLocal<?>
k =
e.
get();
if (
k ==
key)
return
e;
if (
k == null)
expungeStaleEntry(
i);
else
i =
nextIndex(
i,
len);
e =
tab[
i];
}
return null;
}
/**
* Set the value associated with key.
*
* @param key the thread local object
* @param value the value to be set
*/
private void
set(
ThreadLocal<?>
key,
Object value) {
// We don't use a fast path as with get() because it is at
// least as common to use set() to create new entries as
// it is to replace existing ones, in which case, a fast
// path would fail more often than not.
Entry[]
tab =
table;
int
len =
tab.length;
int
i =
key.
threadLocalHashCode & (
len-1);
for (
Entry e =
tab[
i];
e != null;
e =
tab[
i =
nextIndex(
i,
len)]) {
ThreadLocal<?>
k =
e.
get();
if (
k ==
key) {
e.
value =
value;
return;
}
if (
k == null) {
replaceStaleEntry(
key,
value,
i);
return;
}
}
tab[
i] = new
Entry(
key,
value);
int
sz = ++
size;
if (!
cleanSomeSlots(
i,
sz) &&
sz >=
threshold)
rehash();
}
/**
* Remove the entry for key.
*/
private void
remove(
ThreadLocal<?>
key) {
Entry[]
tab =
table;
int
len =
tab.length;
int
i =
key.
threadLocalHashCode & (
len-1);
for (
Entry e =
tab[
i];
e != null;
e =
tab[
i =
nextIndex(
i,
len)]) {
if (
e.
get() ==
key) {
e.
clear();
expungeStaleEntry(
i);
return;
}
}
}
/**
* Replace a stale entry encountered during a set operation
* with an entry for the specified key. The value passed in
* the value parameter is stored in the entry, whether or not
* an entry already exists for the specified key.
*
* As a side effect, this method expunges all stale entries in the
* "run" containing the stale entry. (A run is a sequence of entries
* between two null slots.)
*
* @param key the key
* @param value the value to be associated with key
* @param staleSlot index of the first stale entry encountered while
* searching for key.
*/
private void
replaceStaleEntry(
ThreadLocal<?>
key,
Object value,
int
staleSlot) {
Entry[]
tab =
table;
int
len =
tab.length;
Entry e;
// Back up to check for prior stale entry in current run.
// We clean out whole runs at a time to avoid continual
// incremental rehashing due to garbage collector freeing
// up refs in bunches (i.e., whenever the collector runs).
int
slotToExpunge =
staleSlot;
for (int
i =
prevIndex(
staleSlot,
len);
(
e =
tab[
i]) != null;
i =
prevIndex(
i,
len))
if (
e.
get() == null)
slotToExpunge =
i;
// Find either the key or trailing null slot of run, whichever
// occurs first
for (int
i =
nextIndex(
staleSlot,
len);
(
e =
tab[
i]) != null;
i =
nextIndex(
i,
len)) {
ThreadLocal<?>
k =
e.
get();
// If we find key, then we need to swap it
// with the stale entry to maintain hash table order.
// The newly stale slot, or any other stale slot
// encountered above it, can then be sent to expungeStaleEntry
// to remove or rehash all of the other entries in run.
if (
k ==
key) {
e.
value =
value;
tab[
i] =
tab[
staleSlot];
tab[
staleSlot] =
e;
// Start expunge at preceding stale entry if it exists
if (
slotToExpunge ==
staleSlot)
slotToExpunge =
i;
cleanSomeSlots(
expungeStaleEntry(
slotToExpunge),
len);
return;
}
// If we didn't find stale entry on backward scan, the
// first stale entry seen while scanning for key is the
// first still present in the run.
if (
k == null &&
slotToExpunge ==
staleSlot)
slotToExpunge =
i;
}
// If key not found, put new entry in stale slot
tab[
staleSlot].
value = null;
tab[
staleSlot] = new
Entry(
key,
value);
// If there are any other stale entries in run, expunge them
if (
slotToExpunge !=
staleSlot)
cleanSomeSlots(
expungeStaleEntry(
slotToExpunge),
len);
}
/**
* Expunge a stale entry by rehashing any possibly colliding entries
* lying between staleSlot and the next null slot. This also expunges
* any other stale entries encountered before the trailing null. See
* Knuth, Section 6.4
*
* @param staleSlot index of slot known to have null key
* @return the index of the next null slot after staleSlot
* (all between staleSlot and this slot will have been checked
* for expunging).
*/
private int
expungeStaleEntry(int
staleSlot) {
Entry[]
tab =
table;
int
len =
tab.length;
// expunge entry at staleSlot
tab[
staleSlot].
value = null;
tab[
staleSlot] = null;
size--;
// Rehash until we encounter null
Entry e;
int
i;
for (
i =
nextIndex(
staleSlot,
len);
(
e =
tab[
i]) != null;
i =
nextIndex(
i,
len)) {
ThreadLocal<?>
k =
e.
get();
if (
k == null) {
e.
value = null;
tab[
i] = null;
size--;
} else {
int
h =
k.
threadLocalHashCode & (
len - 1);
if (
h !=
i) {
tab[
i] = null;
// Unlike Knuth 6.4 Algorithm R, we must scan until
// null because multiple entries could have been stale.
while (
tab[
h] != null)
h =
nextIndex(
h,
len);
tab[
h] =
e;
}
}
}
return
i;
}
/**
* Heuristically scan some cells looking for stale entries.
* This is invoked when either a new element is added, or
* another stale one has been expunged. It performs a
* logarithmic number of scans, as a balance between no
* scanning (fast but retains garbage) and a number of scans
* proportional to number of elements, that would find all
* garbage but would cause some insertions to take O(n) time.
*
* @param i a position known NOT to hold a stale entry. The
* scan starts at the element after i.
*
* @param n scan control: {@code log2(n)} cells are scanned,
* unless a stale entry is found, in which case
* {@code log2(table.length)-1} additional cells are scanned.
* When called from insertions, this parameter is the number
* of elements, but when from replaceStaleEntry, it is the
* table length. (Note: all this could be changed to be either
* more or less aggressive by weighting n instead of just
* using straight log n. But this version is simple, fast, and
* seems to work well.)
*
* @return true if any stale entries have been removed.
*/
private boolean
cleanSomeSlots(int
i, int
n) {
boolean
removed = false;
Entry[]
tab =
table;
int
len =
tab.length;
do {
i =
nextIndex(
i,
len);
Entry e =
tab[
i];
if (
e != null &&
e.
get() == null) {
n =
len;
removed = true;
i =
expungeStaleEntry(
i);
}
} while ( (
n >>>= 1) != 0);
return
removed;
}
/**
* Re-pack and/or re-size the table. First scan the entire
* table removing stale entries. If this doesn't sufficiently
* shrink the size of the table, double the table size.
*/
private void
rehash() {
expungeStaleEntries();
// Use lower threshold for doubling to avoid hysteresis
if (
size >=
threshold -
threshold / 4)
resize();
}
/**
* Double the capacity of the table.
*/
private void
resize() {
Entry[]
oldTab =
table;
int
oldLen =
oldTab.length;
int
newLen =
oldLen * 2;
Entry[]
newTab = new
Entry[
newLen];
int
count = 0;
for (int
j = 0;
j <
oldLen; ++
j) {
Entry e =
oldTab[
j];
if (
e != null) {
ThreadLocal<?>
k =
e.
get();
if (
k == null) {
e.
value = null; // Help the GC
} else {
int
h =
k.
threadLocalHashCode & (
newLen - 1);
while (
newTab[
h] != null)
h =
nextIndex(
h,
newLen);
newTab[
h] =
e;
count++;
}
}
}
setThreshold(
newLen);
size =
count;
table =
newTab;
}
/**
* Expunge all stale entries in the table.
*/
private void
expungeStaleEntries() {
Entry[]
tab =
table;
int
len =
tab.length;
for (int
j = 0;
j <
len;
j++) {
Entry e =
tab[
j];
if (
e != null &&
e.
get() == null)
expungeStaleEntry(
j);
}
}
}
}