1 /*
2 * Copyright (c) 1997, 2012, 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 * The initial value of the variable is set by (1) calling the
72 * {@code initialValue method}, (2) obtaining it from a {@code Supplier<T>}
73 * which has been provided via the
74 * constructor, or (3) by calling the {@code set} method.
75 *
76 * If (1) is used and an initial value other than the default of null is required,
77 * the {@code initialValue} method is typically overridden with an
78 * anonymous-inner class.
79 *
80 * @author Josh Bloch and Doug Lea
81 * @since 1.2
82 */
83 public class ThreadLocal<T> {
84 /**
85 * ThreadLocals rely on per-thread linear-probe hash maps attached
86 * to each thread (Thread.threadLocals and
87 * inheritableThreadLocals). The ThreadLocal objects act as keys,
88 * searched via threadLocalHashCode. This is a custom hash code
89 * (useful only within ThreadLocalMaps) that eliminates collisions
90 * in the common case where consecutively constructed ThreadLocals
91 * are used by the same threads, while remaining well-behaved in
92 * less common cases.
93 */
94 private final int threadLocalHashCode = nextHashCode();
95
96 /**
97 * The next hash code to be given out. Updated atomically. Starts at
98 * zero.
99 */
100 private static AtomicInteger nextHashCode =
101 new AtomicInteger();
102
103 /**
104 * The difference between successively generated hash codes - turns
105 * implicit sequential thread-local IDs into near-optimally spread
106 * multiplicative hash values for power-of-two-sized tables.
107 */
108 private static final int HASH_INCREMENT = 0x61c88647;
109
110 /**
111 * Supplied by the invoker of the constructor to set the initial value
112 */
113 private final Supplier<T> supplier;
114
115 /**
116 * Returns the next hash code.
117 */
118 private static int nextHashCode() {
119 return nextHashCode.getAndAdd(HASH_INCREMENT);
120 }
121
122 /**
123 * Returns the current thread's "initial value" for this
124 * thread-local variable unless a {@code Supplier<T>} has been passed
125 * to the constructor, in which case the Supplier is consulted in
126 * preference to this method. This method will be invoked the first
127 * time a thread accesses the variable with the {@link #get}
128 * method, unless the thread previously invoked the {@link #set}
129 * method, in which case the {@code initialValue} method will not
130 * be invoked for the thread. Normally, this method is invoked at
131 * most once per thread, but it may be invoked again in case of
132 * subsequent invocations of {@link #remove} followed by {@link #get}.
133 *
134 * <p>This implementation simply returns {@code null}
135 *
136 * @return the initial value for this thread-local
137 */
138 protected T initialValue() {
139 return null;
140 }
141
142 /**
143 * Creates a thread local variable. The initial value of the variable is
144 * provided by calling the {@code intialValue} method.
145 */
146 public ThreadLocal() {
147 supplier = null;
148 }
149
150 /**
151 * Creates a thread local variable. The initial value of the variable is
152 * determined by invoking the {@code get} method on the supplier.
153 *
154 * @param supplier the supplier to be used to determine the initial value
155 */
156 public ThreadLocal(Supplier<T> supplier) {
157 this.supplier = Objects.requireNonNull(supplier);
158 }
159
160 /**
161 * Returns the value in the current thread's copy of this
162 * thread-local variable. If the variable has no value for the
163 * current thread, it is first initialized to the value returned
164 * by an invocation of the {@link #initialValue} method.
165 *
166 * @return the current thread's value of this thread-local
167 */
168 public T get() {
169 Thread t = Thread.currentThread();
170 ThreadLocalMap map = getMap(t);
171 if (map != null) {
172 ThreadLocalMap.Entry e = map.getEntry(this);
173 if (e != null) {
174 @SuppressWarnings("unchecked")
175 T result = (T)e.value;
176 return result;
177 }
178 }
179 return setInitialValue();
180 }
181
182 /**
183 * Variant of set() to establish initialValue. Used instead
184 * of set() in case user has overridden the set() method.
185 *
186 * @return the initial value
187 */
188 private T setInitialValue() {
189 T value = (supplier != null ? supplier.get() : initialValue());
190 Thread t = Thread.currentThread();
191 ThreadLocalMap map = getMap(t);
192 if (map != null)
193 map.set(this, value);
194 else
195 createMap(t, value);
196 return value;
197 }
198
199 /**
200 * Sets the current thread's copy of this thread-local variable
201 * to the specified value. Most subclasses will have no need to
202 * override this method, relying solely on the {@link #initialValue}
203 * method to set the values of thread-locals.
204 *
205 * @param value the value to be stored in the current thread's copy of
206 * this thread-local.
207 */
208 public void set(T value) {
209 Thread t = Thread.currentThread();
210 ThreadLocalMap map = getMap(t);
211 if (map != null)
212 map.set(this, value);
213 else
214 createMap(t, value);
215 }
216
217 /**
218 * Removes the current thread's value for this thread-local
219 * variable. If this thread-local variable is subsequently
220 * {@linkplain #get read} by the current thread, its value will be
221 * reinitialized by invoking its {@link #initialValue} method,
222 * unless its value is {@linkplain #set set} by the current thread
223 * in the interim. This may result in multiple invocations of the
224 * {@code initialValue} method in the current thread.
225 *
226 * @since 1.5
227 */
228 public void remove() {
229 ThreadLocalMap m = getMap(Thread.currentThread());
230 if (m != null)
231 m.remove(this);
232 }
233
234 /**
235 * Get the map associated with a ThreadLocal. Overridden in
236 * InheritableThreadLocal.
237 *
238 * @param t the current thread
239 * @return the map
240 */
241 ThreadLocalMap getMap(Thread t) {
242 return t.threadLocals;
243 }
244
245 /**
246 * Create the map associated with a ThreadLocal. Overridden in
247 * InheritableThreadLocal.
248 *
249 * @param t the current thread
250 * @param firstValue value for the initial entry of the map
251 */
252 void createMap(Thread t, T firstValue) {
253 t.threadLocals = new ThreadLocalMap(this, firstValue);
254 }
255
256 /**
257 * Factory method to create map of inherited thread locals.
258 * Designed to be called only from Thread constructor.
259 *
260 * @param parentMap the map associated with parent thread
261 * @return a map containing the parent's inheritable bindings
262 */
263 static ThreadLocalMap createInheritedMap(ThreadLocalMap parentMap) {
264 return new ThreadLocalMap(parentMap);
265 }
266
267 /**
268 * Method childValue is visibly defined in subclass
269 * InheritableThreadLocal, but is internally defined here for the
270 * sake of providing createInheritedMap factory method without
271 * needing to subclass the map class in InheritableThreadLocal.
272 * This technique is preferable to the alternative of embedding
273 * instanceof tests in methods.
274 */
275 T childValue(T parentValue) {
276 throw new UnsupportedOperationException();
277 }
278
279 /**
280 * ThreadLocalMap is a customized hash map suitable only for
281 * maintaining thread local values. No operations are exported
282 * outside of the ThreadLocal class. The class is package private to
283 * allow declaration of fields in class Thread. To help deal with
284 * very large and long-lived usages, the hash table entries use
285 * WeakReferences for keys. However, since reference queues are not
286 * used, stale entries are guaranteed to be removed only when
287 * the table starts running out of space.
288 */
289 static class ThreadLocalMap {
290
291 /**
292 * The entries in this hash map extend WeakReference, using
293 * its main ref field as the key (which is always a
294 * ThreadLocal object). Note that null keys (i.e. entry.get()
295 * == null) mean that the key is no longer referenced, so the
296 * entry can be expunged from table. Such entries are referred to
297 * as "stale entries" in the code that follows.
298 */
299 static class Entry extends WeakReference<ThreadLocal<?>> {
300 /** The value associated with this ThreadLocal. */
301 Object value;
302
303 Entry(ThreadLocal<?> k, Object v) {
304 super(k);
305 value = v;
306 }
307 }
308
309 /**
310 * The initial capacity -- MUST be a power of two.
311 */
312 private static final int INITIAL_CAPACITY = 16;
313
314 /**
315 * The table, resized as necessary.
316 * table.length MUST always be a power of two.
317 */
318 private Entry[] table;
319
320 /**
321 * The number of entries in the table.
322 */
323 private int size = 0;
324
325 /**
326 * The next size value at which to resize.
327 */
328 private int threshold; // Default to 0
329
330 /**
331 * Set the resize threshold to maintain at worst a 2/3 load factor.
332 */
333 private void setThreshold(int len) {
334 threshold = len * 2 / 3;
335 }
336
337 /**
338 * Increment i modulo len.
339 */
340 private static int nextIndex(int i, int len) {
341 return ((i + 1 < len) ? i + 1 : 0);
342 }
343
344 /**
345 * Decrement i modulo len.
346 */
347 private static int prevIndex(int i, int len) {
348 return ((i - 1 >= 0) ? i - 1 : len - 1);
349 }
350
351 /**
352 * Construct a new map initially containing (firstKey, firstValue).
353 * ThreadLocalMaps are constructed lazily, so we only create
354 * one when we have at least one entry to put in it.
355 */
356 ThreadLocalMap(ThreadLocal<?> firstKey, Object firstValue) {
357 table = new Entry[INITIAL_CAPACITY];
358 int i = firstKey.threadLocalHashCode & (INITIAL_CAPACITY - 1);
359 table[i] = new Entry(firstKey, firstValue);
360 size = 1;
361 setThreshold(INITIAL_CAPACITY);
362 }
363
364 /**
365 * Construct a new map including all Inheritable ThreadLocals
366 * from given parent map. Called only by createInheritedMap.
367 *
368 * @param parentMap the map associated with parent thread.
369 */
370 private ThreadLocalMap(ThreadLocalMap parentMap) {
371 Entry[] parentTable = parentMap.table;
372 int len = parentTable.length;
373 setThreshold(len);
374 table = new Entry[len];
375
376 for (int j = 0; j < len; j++) {
377 Entry e = parentTable[j];
378 if (e != null) {
379 @SuppressWarnings("unchecked")
380 ThreadLocal<Object> key = (ThreadLocal<Object>) e.get();
381 if (key != null) {
382 Object value = key.childValue(e.value);
383 Entry c = new Entry(key, value);
384 int h = key.threadLocalHashCode & (len - 1);
385 while (table[h] != null)
386 h = nextIndex(h, len);
387 table[h] = c;
388 size++;
389 }
390 }
391 }
392 }
393
394 /**
395 * Get the entry associated with key. This method
396 * itself handles only the fast path: a direct hit of existing
397 * key. It otherwise relays to getEntryAfterMiss. This is
398 * designed to maximize performance for direct hits, in part
399 * by making this method readily inlinable.
400 *
401 * @param key the thread local object
402 * @return the entry associated with key, or null if no such
403 */
404 private Entry getEntry(ThreadLocal<?> key) {
405 int i = key.threadLocalHashCode & (table.length - 1);
406 Entry e = table[i];
407 if (e != null && e.get() == key)
408 return e;
409 else
410 return getEntryAfterMiss(key, i, e);
411 }
412
413 /**
414 * Version of getEntry method for use when key is not found in
415 * its direct hash slot.
416 *
417 * @param key the thread local object
418 * @param i the table index for key's hash code
419 * @param e the entry at table[i]
420 * @return the entry associated with key, or null if no such
421 */
422 private Entry getEntryAfterMiss(ThreadLocal<?> key, int i, Entry e) {
423 Entry[] tab = table;
424 int len = tab.length;
425
426 while (e != null) {
427 ThreadLocal<?> k = e.get();
428 if (k == key)
429 return e;
430 if (k == null)
431 expungeStaleEntry(i);
432 else
433 i = nextIndex(i, len);
434 e = tab[i];
435 }
436 return null;
437 }
438
439 /**
440 * Set the value associated with key.
441 *
442 * @param key the thread local object
443 * @param value the value to be set
444 */
445 private void set(ThreadLocal<?> key, Object value) {
446
447 // We don't use a fast path as with get() because it is at
448 // least as common to use set() to create new entries as
449 // it is to replace existing ones, in which case, a fast
450 // path would fail more often than not.
451
452 Entry[] tab = table;
453 int len = tab.length;
454 int i = key.threadLocalHashCode & (len-1);
455
456 for (Entry e = tab[i];
457 e != null;
458 e = tab[i = nextIndex(i, len)]) {
459 ThreadLocal<?> k = e.get();
460
461 if (k == key) {
462 e.value = value;
463 return;
464 }
465
466 if (k == null) {
467 replaceStaleEntry(key, value, i);
468 return;
469 }
470 }
471
472 tab[i] = new Entry(key, value);
473 int sz = ++size;
474 if (!cleanSomeSlots(i, sz) && sz >= threshold)
475 rehash();
476 }
477
478 /**
479 * Remove the entry for key.
480 */
481 private void remove(ThreadLocal<?> key) {
482 Entry[] tab = table;
483 int len = tab.length;
484 int i = key.threadLocalHashCode & (len-1);
485 for (Entry e = tab[i];
486 e != null;
487 e = tab[i = nextIndex(i, len)]) {
488 if (e.get() == key) {
489 e.clear();
490 expungeStaleEntry(i);
491 return;
492 }
493 }
494 }
495
496 /**
497 * Replace a stale entry encountered during a set operation
498 * with an entry for the specified key. The value passed in
499 * the value parameter is stored in the entry, whether or not
500 * an entry already exists for the specified key.
501 *
502 * As a side effect, this method expunges all stale entries in the
503 * "run" containing the stale entry. (A run is a sequence of entries
504 * between two null slots.)
505 *
506 * @param key the key
507 * @param value the value to be associated with key
508 * @param staleSlot index of the first stale entry encountered while
509 * searching for key.
510 */
511 private void replaceStaleEntry(ThreadLocal<?> key, Object value,
512 int staleSlot) {
513 Entry[] tab = table;
514 int len = tab.length;
515 Entry e;
516
517 // Back up to check for prior stale entry in current run.
518 // We clean out whole runs at a time to avoid continual
519 // incremental rehashing due to garbage collector freeing
520 // up refs in bunches (i.e., whenever the collector runs).
521 int slotToExpunge = staleSlot;
522 for (int i = prevIndex(staleSlot, len);
523 (e = tab[i]) != null;
524 i = prevIndex(i, len))
525 if (e.get() == null)
526 slotToExpunge = i;
527
528 // Find either the key or trailing null slot of run, whichever
529 // occurs first
530 for (int i = nextIndex(staleSlot, len);
531 (e = tab[i]) != null;
532 i = nextIndex(i, len)) {
533 ThreadLocal<?> k = e.get();
534
535 // If we find key, then we need to swap it
536 // with the stale entry to maintain hash table order.
537 // The newly stale slot, or any other stale slot
538 // encountered above it, can then be sent to expungeStaleEntry
539 // to remove or rehash all of the other entries in run.
540 if (k == key) {
541 e.value = value;
542
543 tab[i] = tab[staleSlot];
544 tab[staleSlot] = e;
545
546 // Start expunge at preceding stale entry if it exists
547 if (slotToExpunge == staleSlot)
548 slotToExpunge = i;
549 cleanSomeSlots(expungeStaleEntry(slotToExpunge), len);
550 return;
551 }
552
553 // If we didn't find stale entry on backward scan, the
554 // first stale entry seen while scanning for key is the
555 // first still present in the run.
556 if (k == null && slotToExpunge == staleSlot)
557 slotToExpunge = i;
558 }
559
560 // If key not found, put new entry in stale slot
561 tab[staleSlot].value = null;
562 tab[staleSlot] = new Entry(key, value);
563
564 // If there are any other stale entries in run, expunge them
565 if (slotToExpunge != staleSlot)
566 cleanSomeSlots(expungeStaleEntry(slotToExpunge), len);
567 }
568
569 /**
570 * Expunge a stale entry by rehashing any possibly colliding entries
571 * lying between staleSlot and the next null slot. This also expunges
572 * any other stale entries encountered before the trailing null. See
573 * Knuth, Section 6.4
574 *
575 * @param staleSlot index of slot known to have null key
576 * @return the index of the next null slot after staleSlot
577 * (all between staleSlot and this slot will have been checked
578 * for expunging).
579 */
580 private int expungeStaleEntry(int staleSlot) {
581 Entry[] tab = table;
582 int len = tab.length;
583
584 // expunge entry at staleSlot
585 tab[staleSlot].value = null;
586 tab[staleSlot] = null;
587 size--;
588
589 // Rehash until we encounter null
590 Entry e;
591 int i;
592 for (i = nextIndex(staleSlot, len);
593 (e = tab[i]) != null;
594 i = nextIndex(i, len)) {
595 ThreadLocal<?> k = e.get();
596 if (k == null) {
597 e.value = null;
598 tab[i] = null;
599 size--;
600 } else {
601 int h = k.threadLocalHashCode & (len - 1);
602 if (h != i) {
603 tab[i] = null;
604
605 // Unlike Knuth 6.4 Algorithm R, we must scan until
606 // null because multiple entries could have been stale.
607 while (tab[h] != null)
608 h = nextIndex(h, len);
609 tab[h] = e;
610 }
611 }
612 }
613 return i;
614 }
615
616 /**
617 * Heuristically scan some cells looking for stale entries.
618 * This is invoked when either a new element is added, or
619 * another stale one has been expunged. It performs a
620 * logarithmic number of scans, as a balance between no
621 * scanning (fast but retains garbage) and a number of scans
622 * proportional to number of elements, that would find all
623 * garbage but would cause some insertions to take O(n) time.
624 *
625 * @param i a position known NOT to hold a stale entry. The
626 * scan starts at the element after i.
627 *
628 * @param n scan control: {@code log2(n)} cells are scanned,
629 * unless a stale entry is found, in which case
630 * {@code log2(table.length)-1} additional cells are scanned.
631 * When called from insertions, this parameter is the number
632 * of elements, but when from replaceStaleEntry, it is the
633 * table length. (Note: all this could be changed to be either
634 * more or less aggressive by weighting n instead of just
635 * using straight log n. But this version is simple, fast, and
636 * seems to work well.)
637 *
638 * @return true if any stale entries have been removed.
639 */
640 private boolean cleanSomeSlots(int i, int n) {
641 boolean removed = false;
642 Entry[] tab = table;
643 int len = tab.length;
644 do {
645 i = nextIndex(i, len);
646 Entry e = tab[i];
647 if (e != null && e.get() == null) {
648 n = len;
649 removed = true;
650 i = expungeStaleEntry(i);
651 }
652 } while ( (n >>>= 1) != 0);
653 return removed;
654 }
655
656 /**
657 * Re-pack and/or re-size the table. First scan the entire
658 * table removing stale entries. If this doesn't sufficiently
659 * shrink the size of the table, double the table size.
660 */
661 private void rehash() {
662 expungeStaleEntries();
663
664 // Use lower threshold for doubling to avoid hysteresis
665 if (size >= threshold - threshold / 4)
666 resize();
667 }
668
669 /**
670 * Double the capacity of the table.
671 */
672 private void resize() {
673 Entry[] oldTab = table;
674 int oldLen = oldTab.length;
675 int newLen = oldLen * 2;
676 Entry[] newTab = new Entry[newLen];
677 int count = 0;
678
679 for (int j = 0; j < oldLen; ++j) {
680 Entry e = oldTab[j];
681 if (e != null) {
682 ThreadLocal<?> k = e.get();
683 if (k == null) {
684 e.value = null; // Help the GC
685 } else {
686 int h = k.threadLocalHashCode & (newLen - 1);
687 while (newTab[h] != null)
688 h = nextIndex(h, newLen);
689 newTab[h] = e;
690 count++;
691 }
692 }
693 }
694
695 setThreshold(newLen);
696 size = count;
697 table = newTab;
698 }
699
700 /**
701 * Expunge all stale entries in the table.
702 */
703 private void expungeStaleEntries() {
704 Entry[] tab = table;
705 int len = tab.length;
706 for (int j = 0; j < len; j++) {
707 Entry e = tab[j];
708 if (e != null && e.get() == null)
709 expungeStaleEntry(j);
710 }
711 }
712 }
713 }