1 /*
2 * Copyright (c) 1997, 2013, Oracle and/or its affiliates. All rights reserved.
3 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
4 *
5 * This code is free software; you can redistribute it and/or modify it
6 * under the terms of the GNU General Public License version 2 only, as
7 * published by the Free Software Foundation. Oracle designates this
8 * particular file as subject to the "Classpath" exception as provided
9 * by Oracle in the LICENSE file that accompanied this code.
10 *
11 * This code is distributed in the hope that it will be useful, but WITHOUT
12 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
13 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
14 * version 2 for more details (a copy is included in the LICENSE file that
15 * accompanied this code).
16 *
17 * You should have received a copy of the GNU General Public License version
18 * 2 along with this work; if not, write to the Free Software Foundation,
19 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
20 *
21 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
22 * or visit www.oracle.com if you need additional information or have any
23 * questions.
24 */
25
26 package java.lang;
27 import java.lang.ref.*;
28 import java.util.Objects;
29 import java.util.concurrent.atomic.AtomicInteger;
30 import java.util.function.Supplier;
31
32 /**
33 * This class provides thread-local variables. These variables differ from
34 * their normal counterparts in that each thread that accesses one (via its
35 * {@code get} or {@code set} method) has its own, independently initialized
36 * copy of the variable. {@code ThreadLocal} instances are typically private
37 * static fields in classes that wish to associate state with a thread (e.g.,
38 * a user ID or Transaction ID).
39 *
40 * <p>For example, the class below generates unique identifiers local to each
41 * thread.
42 * A thread's id is assigned the first time it invokes {@code ThreadId.get()}
43 * and remains unchanged on subsequent calls.
44 * <pre>
45 * import java.util.concurrent.atomic.AtomicInteger;
46 *
47 * public class ThreadId {
48 * // Atomic integer containing the next thread ID to be assigned
49 * private static final AtomicInteger nextId = new AtomicInteger(0);
50 *
51 * // Thread local variable containing each thread's ID
52 * private static final ThreadLocal<Integer> threadId =
53 * new ThreadLocal<Integer>() {
54 * @Override protected Integer initialValue() {
55 * return nextId.getAndIncrement();
56 * }
57 * };
58 *
59 * // Returns the current thread's unique ID, assigning it if necessary
60 * public static int get() {
61 * return threadId.get();
62 * }
63 * }
64 * </pre>
65 * <p>Each thread holds an implicit reference to its copy of a thread-local
66 * variable as long as the thread is alive and the {@code ThreadLocal}
67 * instance is accessible; after a thread goes away, all of its copies of
68 * thread-local instances are subject to garbage collection (unless other
69 * references to these copies exist).
70 *
71 * @author Josh Bloch and Doug Lea
72 * @since 1.2
73 */
74 public class ThreadLocal<T> {
75 /**
76 * ThreadLocals rely on per-thread linear-probe hash maps attached
77 * to each thread (Thread.threadLocals and
78 * inheritableThreadLocals). The ThreadLocal objects act as keys,
79 * searched via threadLocalHashCode. This is a custom hash code
80 * (useful only within ThreadLocalMaps) that eliminates collisions
81 * in the common case where consecutively constructed ThreadLocals
82 * are used by the same threads, while remaining well-behaved in
83 * less common cases.
84 */
85 private final int threadLocalHashCode = nextHashCode();
86
87 /**
88 * The next hash code to be given out. Updated atomically. Starts at
89 * zero.
90 */
91 private static AtomicInteger nextHashCode =
92 new AtomicInteger();
93
94 /**
95 * The difference between successively generated hash codes - turns
96 * implicit sequential thread-local IDs into near-optimally spread
97 * multiplicative hash values for power-of-two-sized tables.
98 */
99 private static final int HASH_INCREMENT = 0x61c88647;
100
101 /**
102 * Returns the next hash code.
103 */
104 private static int nextHashCode() {
105 return nextHashCode.getAndAdd(HASH_INCREMENT);
106 }
107
108 /**
109 * Returns the current thread's "initial value" for this
110 * thread-local variable. This method will be invoked the first
111 * time a thread accesses the variable with the {@link #get}
112 * method, unless the thread previously invoked the {@link #set}
113 * method, in which case the {@code initialValue} method will not
114 * be invoked for the thread. Normally, this method is invoked at
115 * most once per thread, but it may be invoked again in case of
116 * subsequent invocations of {@link #remove} followed by {@link #get}.
117 *
118 * <p>This implementation simply returns {@code null}; if the
119 * programmer desires thread-local variables to have an initial
120 * value other than {@code null}, {@code ThreadLocal} must be
121 * subclassed, and this method overridden. Typically, an
122 * anonymous inner class will be used.
123 *
124 * @return the initial value for this thread-local
125 */
126 protected T initialValue() {
127 return null;
128 }
129
130 /**
131 * Creates a thread local variable. The initial value of the variable is
132 * determined by invoking the {@code get} method on the {@code Supplier}.
133 *
134 * @param <S> the type of the thread local's value
135 * @param supplier the supplier to be used to determine the initial value
136 * @return a new thread local variable
137 * @throws NullPointerException if the specified supplier is null
138 * @since 1.8
139 */
140 public static <S> ThreadLocal<S> withInitial(Supplier<? extends S> supplier) {
141 return new SuppliedThreadLocal<>(supplier);
142 }
143
144 /**
145 * Creates a thread local variable.
146 * @see #withInitial(java.util.function.Supplier)
147 */
148 public ThreadLocal() {
149 }
150
151 /**
152 * Returns the value in the current thread's copy of this
153 * thread-local variable. If the variable has no value for the
154 * current thread, it is first initialized to the value returned
155 * by an invocation of the {@link #initialValue} method.
156 *
157 * @return the current thread's value of this thread-local
158 */
159 public T get() {
160 Thread t = Thread.currentThread();
161 ThreadLocalMap map = getMap(t);
162 if (map != null) {
163 ThreadLocalMap.Entry e = map.getEntry(this);
164 if (e != null) {
165 @SuppressWarnings("unchecked")
166 T result = (T)e.value;
167 return result;
168 }
169 }
170 return setInitialValue();
171 }
172
173 /**
174 * Variant of set() to establish initialValue. Used instead
175 * of set() in case user has overridden the set() method.
176 *
177 * @return the initial value
178 */
179 private T setInitialValue() {
180 T value = initialValue();
181 Thread t = Thread.currentThread();
182 ThreadLocalMap map = getMap(t);
183 if (map != null)
184 map.set(this, value);
185 else
186 createMap(t, value);
187 return value;
188 }
189
190 /**
191 * Sets the current thread's copy of this thread-local variable
192 * to the specified value. Most subclasses will have no need to
193 * override this method, relying solely on the {@link #initialValue}
194 * method to set the values of thread-locals.
195 *
196 * @param value the value to be stored in the current thread's copy of
197 * this thread-local.
198 */
199 public void set(T value) {
200 Thread t = Thread.currentThread();
201 ThreadLocalMap map = getMap(t);
202 if (map != null)
203 map.set(this, value);
204 else
205 createMap(t, value);
206 }
207
208 /**
209 * Removes the current thread's value for this thread-local
210 * variable. If this thread-local variable is subsequently
211 * {@linkplain #get read} by the current thread, its value will be
212 * reinitialized by invoking its {@link #initialValue} method,
213 * unless its value is {@linkplain #set set} by the current thread
214 * in the interim. This may result in multiple invocations of the
215 * {@code initialValue} method in the current thread.
216 *
217 * @since 1.5
218 */
219 public void remove() {
220 ThreadLocalMap m = getMap(Thread.currentThread());
221 if (m != null)
222 m.remove(this);
223 }
224
225 /**
226 * Get the map associated with a ThreadLocal. Overridden in
227 * InheritableThreadLocal.
228 *
229 * @param t the current thread
230 * @return the map
231 */
232 ThreadLocalMap getMap(Thread t) {
233 return t.threadLocals;
234 }
235
236 /**
237 * Create the map associated with a ThreadLocal. Overridden in
238 * InheritableThreadLocal.
239 *
240 * @param t the current thread
241 * @param firstValue value for the initial entry of the map
242 */
243 void createMap(Thread t, T firstValue) {
244 t.threadLocals = new ThreadLocalMap(this, firstValue);
245 }
246
247 /**
248 * Factory method to create map of inherited thread locals.
249 * Designed to be called only from Thread constructor.
250 *
251 * @param parentMap the map associated with parent thread
252 * @return a map containing the parent's inheritable bindings
253 */
254 static ThreadLocalMap createInheritedMap(ThreadLocalMap parentMap) {
255 return new ThreadLocalMap(parentMap);
256 }
257
258 /**
259 * Method childValue is visibly defined in subclass
260 * InheritableThreadLocal, but is internally defined here for the
261 * sake of providing createInheritedMap factory method without
262 * needing to subclass the map class in InheritableThreadLocal.
263 * This technique is preferable to the alternative of embedding
264 * instanceof tests in methods.
265 */
266 T childValue(T parentValue) {
267 throw new UnsupportedOperationException();
268 }
269
270 /**
271 * An extension of ThreadLocal that obtains its initial value from
272 * the specified {@code Supplier}.
273 */
274 static final class SuppliedThreadLocal<T> extends ThreadLocal<T> {
275
276 private final Supplier<? extends T> supplier;
277
278 SuppliedThreadLocal(Supplier<? extends T> supplier) {
279 this.supplier = Objects.requireNonNull(supplier);
280 }
281
282 @Override
283 protected T initialValue() {
284 return supplier.get();
285 }
286 }
287
288 /**
289 * ThreadLocalMap is a customized hash map suitable only for
290 * maintaining thread local values. No operations are exported
291 * outside of the ThreadLocal class. The class is package private to
292 * allow declaration of fields in class Thread. To help deal with
293 * very large and long-lived usages, the hash table entries use
294 * WeakReferences for keys. However, since reference queues are not
295 * used, stale entries are guaranteed to be removed only when
296 * the table starts running out of space.
297 */
298 static class ThreadLocalMap {
299
300 /**
301 * The entries in this hash map extend WeakReference, using
302 * its main ref field as the key (which is always a
303 * ThreadLocal object). Note that null keys (i.e. entry.get()
304 * == null) mean that the key is no longer referenced, so the
305 * entry can be expunged from table. Such entries are referred to
306 * as "stale entries" in the code that follows.
307 */
308 static class Entry extends WeakReference<ThreadLocal<?>> {
309 /** The value associated with this ThreadLocal. */
310 Object value;
311
312 Entry(ThreadLocal<?> k, Object v) {
313 super(k);
314 value = v;
315 }
316 }
317
318 /**
319 * The initial capacity -- MUST be a power of two.
320 */
321 private static final int INITIAL_CAPACITY = 16;
322
323 /**
324 * The table, resized as necessary.
325 * table.length MUST always be a power of two.
326 */
327 private Entry[] table;
328
329 /**
330 * The number of entries in the table.
331 */
332 private int size = 0;
333
334 /**
335 * The next size value at which to resize.
336 */
337 private int threshold; // Default to 0
338
339 /**
340 * Set the resize threshold to maintain at worst a 2/3 load factor.
341 */
342 private void setThreshold(int len) {
343 threshold = len * 2 / 3;
344 }
345
346 /**
347 * Increment i modulo len.
348 */
349 private static int nextIndex(int i, int len) {
350 return ((i + 1 < len) ? i + 1 : 0);
351 }
352
353 /**
354 * Decrement i modulo len.
355 */
356 private static int prevIndex(int i, int len) {
357 return ((i - 1 >= 0) ? i - 1 : len - 1);
358 }
359
360 /**
361 * Construct a new map initially containing (firstKey, firstValue).
362 * ThreadLocalMaps are constructed lazily, so we only create
363 * one when we have at least one entry to put in it.
364 */
365 ThreadLocalMap(ThreadLocal<?> firstKey, Object firstValue) {
366 table = new Entry[INITIAL_CAPACITY];
367 int i = firstKey.threadLocalHashCode & (INITIAL_CAPACITY - 1);
368 table[i] = new Entry(firstKey, firstValue);
369 size = 1;
370 setThreshold(INITIAL_CAPACITY);
371 }
372
373 /**
374 * Construct a new map including all Inheritable ThreadLocals
375 * from given parent map. Called only by createInheritedMap.
376 *
377 * @param parentMap the map associated with parent thread.
378 */
379 private ThreadLocalMap(ThreadLocalMap parentMap) {
380 Entry[] parentTable = parentMap.table;
381 int len = parentTable.length;
382 setThreshold(len);
383 table = new Entry[len];
384
385 for (Entry e : parentTable) {
386 if (e != null) {
387 @SuppressWarnings("unchecked")
388 ThreadLocal<Object> key = (ThreadLocal<Object>) e.get();
389 if (key != null) {
390 Object value = key.childValue(e.value);
391 Entry c = new Entry(key, value);
392 int h = key.threadLocalHashCode & (len - 1);
393 while (table[h] != null)
394 h = nextIndex(h, len);
395 table[h] = c;
396 size++;
397 }
398 }
399 }
400 }
401
402 /**
403 * Get the entry associated with key. This method
404 * itself handles only the fast path: a direct hit of existing
405 * key. It otherwise relays to getEntryAfterMiss. This is
406 * designed to maximize performance for direct hits, in part
407 * by making this method readily inlinable.
408 *
409 * @param key the thread local object
410 * @return the entry associated with key, or null if no such
411 */
412 private Entry getEntry(ThreadLocal<?> key) {
413 int i = key.threadLocalHashCode & (table.length - 1);
414 Entry e = table[i];
415 if (e != null && e.get() == key)
416 return e;
417 else
418 return getEntryAfterMiss(key, i, e);
419 }
420
421 /**
422 * Version of getEntry method for use when key is not found in
423 * its direct hash slot.
424 *
425 * @param key the thread local object
426 * @param i the table index for key's hash code
427 * @param e the entry at table[i]
428 * @return the entry associated with key, or null if no such
429 */
430 private Entry getEntryAfterMiss(ThreadLocal<?> key, int i, Entry e) {
431 Entry[] tab = table;
432 int len = tab.length;
433
434 while (e != null) {
435 ThreadLocal<?> k = e.get();
436 if (k == key)
437 return e;
438 if (k == null)
439 expungeStaleEntry(i);
440 else
441 i = nextIndex(i, len);
442 e = tab[i];
443 }
444 return null;
445 }
446
447 /**
448 * Set the value associated with key.
449 *
450 * @param key the thread local object
451 * @param value the value to be set
452 */
453 private void set(ThreadLocal<?> key, Object value) {
454
455 // We don't use a fast path as with get() because it is at
456 // least as common to use set() to create new entries as
457 // it is to replace existing ones, in which case, a fast
458 // path would fail more often than not.
459
460 Entry[] tab = table;
461 int len = tab.length;
462 int i = key.threadLocalHashCode & (len-1);
463
464 for (Entry e = tab[i];
465 e != null;
466 e = tab[i = nextIndex(i, len)]) {
467 ThreadLocal<?> k = e.get();
468
469 if (k == key) {
470 e.value = value;
471 return;
472 }
473
474 if (k == null) {
475 replaceStaleEntry(key, value, i);
476 return;
477 }
478 }
479
480 tab[i] = new Entry(key, value);
481 int sz = ++size;
482 if (!cleanSomeSlots(i, sz) && sz >= threshold)
483 rehash();
484 }
485
486 /**
487 * Remove the entry for key.
488 */
489 private void remove(ThreadLocal<?> key) {
490 Entry[] tab = table;
491 int len = tab.length;
492 int i = key.threadLocalHashCode & (len-1);
493 for (Entry e = tab[i];
494 e != null;
495 e = tab[i = nextIndex(i, len)]) {
496 if (e.get() == key) {
497 e.clear();
498 expungeStaleEntry(i);
499 return;
500 }
501 }
502 }
503
504 /**
505 * Replace a stale entry encountered during a set operation
506 * with an entry for the specified key. The value passed in
507 * the value parameter is stored in the entry, whether or not
508 * an entry already exists for the specified key.
509 *
510 * As a side effect, this method expunges all stale entries in the
511 * "run" containing the stale entry. (A run is a sequence of entries
512 * between two null slots.)
513 *
514 * @param key the key
515 * @param value the value to be associated with key
516 * @param staleSlot index of the first stale entry encountered while
517 * searching for key.
518 */
519 private void replaceStaleEntry(ThreadLocal<?> key, Object value,
520 int staleSlot) {
521 Entry[] tab = table;
522 int len = tab.length;
523 Entry e;
524
525 // Back up to check for prior stale entry in current run.
526 // We clean out whole runs at a time to avoid continual
527 // incremental rehashing due to garbage collector freeing
528 // up refs in bunches (i.e., whenever the collector runs).
529 int slotToExpunge = staleSlot;
530 for (int i = prevIndex(staleSlot, len);
531 (e = tab[i]) != null;
532 i = prevIndex(i, len))
533 if (e.get() == null)
534 slotToExpunge = i;
535
536 // Find either the key or trailing null slot of run, whichever
537 // occurs first
538 for (int i = nextIndex(staleSlot, len);
539 (e = tab[i]) != null;
540 i = nextIndex(i, len)) {
541 ThreadLocal<?> k = e.get();
542
543 // If we find key, then we need to swap it
544 // with the stale entry to maintain hash table order.
545 // The newly stale slot, or any other stale slot
546 // encountered above it, can then be sent to expungeStaleEntry
547 // to remove or rehash all of the other entries in run.
548 if (k == key) {
549 e.value = value;
550
551 tab[i] = tab[staleSlot];
552 tab[staleSlot] = e;
553
554 // Start expunge at preceding stale entry if it exists
555 if (slotToExpunge == staleSlot)
556 slotToExpunge = i;
557 cleanSomeSlots(expungeStaleEntry(slotToExpunge), len);
558 return;
559 }
560
561 // If we didn't find stale entry on backward scan, the
562 // first stale entry seen while scanning for key is the
563 // first still present in the run.
564 if (k == null && slotToExpunge == staleSlot)
565 slotToExpunge = i;
566 }
567
568 // If key not found, put new entry in stale slot
569 tab[staleSlot].value = null;
570 tab[staleSlot] = new Entry(key, value);
571
572 // If there are any other stale entries in run, expunge them
573 if (slotToExpunge != staleSlot)
574 cleanSomeSlots(expungeStaleEntry(slotToExpunge), len);
575 }
576
577 /**
578 * Expunge a stale entry by rehashing any possibly colliding entries
579 * lying between staleSlot and the next null slot. This also expunges
580 * any other stale entries encountered before the trailing null. See
581 * Knuth, Section 6.4
582 *
583 * @param staleSlot index of slot known to have null key
584 * @return the index of the next null slot after staleSlot
585 * (all between staleSlot and this slot will have been checked
586 * for expunging).
587 */
588 private int expungeStaleEntry(int staleSlot) {
589 Entry[] tab = table;
590 int len = tab.length;
591
592 // expunge entry at staleSlot
593 tab[staleSlot].value = null;
594 tab[staleSlot] = null;
595 size--;
596
597 // Rehash until we encounter null
598 Entry e;
599 int i;
600 for (i = nextIndex(staleSlot, len);
601 (e = tab[i]) != null;
602 i = nextIndex(i, len)) {
603 ThreadLocal<?> k = e.get();
604 if (k == null) {
605 e.value = null;
606 tab[i] = null;
607 size--;
608 } else {
609 int h = k.threadLocalHashCode & (len - 1);
610 if (h != i) {
611 tab[i] = null;
612
613 // Unlike Knuth 6.4 Algorithm R, we must scan until
614 // null because multiple entries could have been stale.
615 while (tab[h] != null)
616 h = nextIndex(h, len);
617 tab[h] = e;
618 }
619 }
620 }
621 return i;
622 }
623
624 /**
625 * Heuristically scan some cells looking for stale entries.
626 * This is invoked when either a new element is added, or
627 * another stale one has been expunged. It performs a
628 * logarithmic number of scans, as a balance between no
629 * scanning (fast but retains garbage) and a number of scans
630 * proportional to number of elements, that would find all
631 * garbage but would cause some insertions to take O(n) time.
632 *
633 * @param i a position known NOT to hold a stale entry. The
634 * scan starts at the element after i.
635 *
636 * @param n scan control: {@code log2(n)} cells are scanned,
637 * unless a stale entry is found, in which case
638 * {@code log2(table.length)-1} additional cells are scanned.
639 * When called from insertions, this parameter is the number
640 * of elements, but when from replaceStaleEntry, it is the
641 * table length. (Note: all this could be changed to be either
642 * more or less aggressive by weighting n instead of just
643 * using straight log n. But this version is simple, fast, and
644 * seems to work well.)
645 *
646 * @return true if any stale entries have been removed.
647 */
648 private boolean cleanSomeSlots(int i, int n) {
649 boolean removed = false;
650 Entry[] tab = table;
651 int len = tab.length;
652 do {
653 i = nextIndex(i, len);
654 Entry e = tab[i];
655 if (e != null && e.get() == null) {
656 n = len;
657 removed = true;
658 i = expungeStaleEntry(i);
659 }
660 } while ( (n >>>= 1) != 0);
661 return removed;
662 }
663
664 /**
665 * Re-pack and/or re-size the table. First scan the entire
666 * table removing stale entries. If this doesn't sufficiently
667 * shrink the size of the table, double the table size.
668 */
669 private void rehash() {
670 expungeStaleEntries();
671
672 // Use lower threshold for doubling to avoid hysteresis
673 if (size >= threshold - threshold / 4)
674 resize();
675 }
676
677 /**
678 * Double the capacity of the table.
679 */
680 private void resize() {
681 Entry[] oldTab = table;
682 int oldLen = oldTab.length;
683 int newLen = oldLen * 2;
684 Entry[] newTab = new Entry[newLen];
685 int count = 0;
686
687 for (Entry e : oldTab) {
688 if (e != null) {
689 ThreadLocal<?> k = e.get();
690 if (k == null) {
691 e.value = null; // Help the GC
692 } else {
693 int h = k.threadLocalHashCode & (newLen - 1);
694 while (newTab[h] != null)
695 h = nextIndex(h, newLen);
696 newTab[h] = e;
697 count++;
698 }
699 }
700 }
701
702 setThreshold(newLen);
703 size = count;
704 table = newTab;
705 }
706
707 /**
708 * Expunge all stale entries in the table.
709 */
710 private void expungeStaleEntries() {
711 Entry[] tab = table;
712 int len = tab.length;
713 for (int j = 0; j < len; j++) {
714 Entry e = tab[j];
715 if (e != null && e.get() == null)
716 expungeStaleEntry(j);
717 }
718 }
719 }
720 }