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 * @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 supplier the supplier to be used to determine the initial value
135 * @return a new thread local variable
136 * @since 1.8
137 */
138 public static <T> ThreadLocal<T> withInitial(Supplier<? extends T> supplier) {
139 return new SuppliedThreadLocal<>(supplier);
140 }
141
142 /**
143 * Creates a thread local variable.
144 * @see #withInitial(java.util.function.Supplier)
145 */
146 public ThreadLocal() {
147 }
148
149 /**
150 * Returns the value in the current thread's copy of this
151 * thread-local variable. If the variable has no value for the
152 * current thread, it is first initialized to the value returned
153 * by an invocation of the {@link #initialValue} method.
154 *
155 * @return the current thread's value of this thread-local
156 */
157 public T get() {
158 Thread t = Thread.currentThread();
159 ThreadLocalMap map = getMap(t);
160 if (map != null) {
161 ThreadLocalMap.Entry e = map.getEntry(this);
162 if (e != null) {
163 @SuppressWarnings("unchecked")
164 T result = (T)e.value;
165 return result;
166 }
167 }
168 return setInitialValue();
169 }
170
171 /**
172 * Variant of set() to establish initialValue. Used instead
173 * of set() in case user has overridden the set() method.
174 *
175 * @return the initial value
176 */
177 private T setInitialValue() {
178 T value = initialValue();
179 Thread t = Thread.currentThread();
180 ThreadLocalMap map = getMap(t);
181 if (map != null)
182 map.set(this, value);
183 else
184 createMap(t, value);
185 return value;
186 }
187
188 /**
189 * Sets the current thread's copy of this thread-local variable
190 * to the specified value. Most subclasses will have no need to
191 * override this method, relying solely on the {@link #initialValue}
192 * method to set the values of thread-locals.
193 *
194 * @param value the value to be stored in the current thread's copy of
195 * this thread-local.
196 */
197 public void set(T value) {
198 Thread t = Thread.currentThread();
199 ThreadLocalMap map = getMap(t);
200 if (map != null)
201 map.set(this, value);
202 else
203 createMap(t, value);
204 }
205
206 /**
207 * Removes the current thread's value for this thread-local
208 * variable. If this thread-local variable is subsequently
209 * {@linkplain #get read} by the current thread, its value will be
210 * reinitialized by invoking its {@link #initialValue} method,
211 * unless its value is {@linkplain #set set} by the current thread
212 * in the interim. This may result in multiple invocations of the
213 * {@code initialValue} method in the current thread.
214 *
215 * @since 1.5
216 */
217 public void remove() {
218 ThreadLocalMap m = getMap(Thread.currentThread());
219 if (m != null)
220 m.remove(this);
221 }
222
223 /**
224 * Get the map associated with a ThreadLocal. Overridden in
225 * InheritableThreadLocal.
226 *
227 * @param t the current thread
228 * @return the map
229 */
230 ThreadLocalMap getMap(Thread t) {
231 return t.threadLocals;
232 }
233
234 /**
235 * Create the map associated with a ThreadLocal. Overridden in
236 * InheritableThreadLocal.
237 *
238 * @param t the current thread
239 * @param firstValue value for the initial entry of the map
240 */
241 void createMap(Thread t, T firstValue) {
242 t.threadLocals = new ThreadLocalMap(this, firstValue);
243 }
244
245 /**
246 * Factory method to create map of inherited thread locals.
247 * Designed to be called only from Thread constructor.
248 *
249 * @param parentMap the map associated with parent thread
250 * @return a map containing the parent's inheritable bindings
251 */
252 static ThreadLocalMap createInheritedMap(ThreadLocalMap parentMap) {
253 return new ThreadLocalMap(parentMap);
254 }
255
256 /**
257 * Method childValue is visibly defined in subclass
258 * InheritableThreadLocal, but is internally defined here for the
259 * sake of providing createInheritedMap factory method without
260 * needing to subclass the map class in InheritableThreadLocal.
261 * This technique is preferable to the alternative of embedding
262 * instanceof tests in methods.
263 */
264 T childValue(T parentValue) {
265 throw new UnsupportedOperationException();
266 }
267
268 /**
269 * An extension of ThreadLocal that obtains its initial value from
270 * the specified {@code Supplier}.
271 */
272 static final class SuppliedThreadLocal<T> extends ThreadLocal<T> {
273
274 private final Supplier<? extends T> supplier;
275
276 SuppliedThreadLocal(Supplier<? extends T> supplier) {
277 this.supplier = Objects.requireNonNull(supplier);
278 }
279
280 @Override
281 protected T initialValue() {
282 return supplier.get();
283 }
284 }
285
286 /**
287 * ThreadLocalMap is a customized hash map suitable only for
288 * maintaining thread local values. No operations are exported
289 * outside of the ThreadLocal class. The class is package private to
290 * allow declaration of fields in class Thread. To help deal with
291 * very large and long-lived usages, the hash table entries use
292 * WeakReferences for keys. However, since reference queues are not
293 * used, stale entries are guaranteed to be removed only when
294 * the table starts running out of space.
295 */
296 static class ThreadLocalMap {
297
298 /**
299 * The entries in this hash map extend WeakReference, using
300 * its main ref field as the key (which is always a
301 * ThreadLocal object). Note that null keys (i.e. entry.get()
302 * == null) mean that the key is no longer referenced, so the
303 * entry can be expunged from table. Such entries are referred to
304 * as "stale entries" in the code that follows.
305 */
306 static class Entry extends WeakReference<ThreadLocal<?>> {
307 /** The value associated with this ThreadLocal. */
308 Object value;
309
310 Entry(ThreadLocal<?> k, Object v) {
311 super(k);
312 value = v;
313 }
314 }
315
316 /**
317 * The initial capacity -- MUST be a power of two.
318 */
319 private static final int INITIAL_CAPACITY = 16;
320
321 /**
322 * The table, resized as necessary.
323 * table.length MUST always be a power of two.
324 */
325 private Entry[] table;
326
327 /**
328 * The number of entries in the table.
329 */
330 private int size = 0;
331
332 /**
333 * The next size value at which to resize.
334 */
335 private int threshold; // Default to 0
336
337 /**
338 * Set the resize threshold to maintain at worst a 2/3 load factor.
339 */
340 private void setThreshold(int len) {
341 threshold = len * 2 / 3;
342 }
343
344 /**
345 * Increment i modulo len.
346 */
347 private static int nextIndex(int i, int len) {
348 return ((i + 1 < len) ? i + 1 : 0);
349 }
350
351 /**
352 * Decrement i modulo len.
353 */
354 private static int prevIndex(int i, int len) {
355 return ((i - 1 >= 0) ? i - 1 : len - 1);
356 }
357
358 /**
359 * Construct a new map initially containing (firstKey, firstValue).
360 * ThreadLocalMaps are constructed lazily, so we only create
361 * one when we have at least one entry to put in it.
362 */
363 ThreadLocalMap(ThreadLocal<?> firstKey, Object firstValue) {
364 table = new Entry[INITIAL_CAPACITY];
365 int i = firstKey.threadLocalHashCode & (INITIAL_CAPACITY - 1);
366 table[i] = new Entry(firstKey, firstValue);
367 size = 1;
368 setThreshold(INITIAL_CAPACITY);
369 }
370
371 /**
372 * Construct a new map including all Inheritable ThreadLocals
373 * from given parent map. Called only by createInheritedMap.
374 *
375 * @param parentMap the map associated with parent thread.
376 */
377 private ThreadLocalMap(ThreadLocalMap parentMap) {
378 Entry[] parentTable = parentMap.table;
379 int len = parentTable.length;
380 setThreshold(len);
381 table = new Entry[len];
382
383 for (int j = 0; j < len; j++) {
384 Entry e = parentTable[j];
385 if (e != null) {
386 @SuppressWarnings("unchecked")
387 ThreadLocal<Object> key = (ThreadLocal<Object>) e.get();
388 if (key != null) {
389 Object value = key.childValue(e.value);
390 Entry c = new Entry(key, value);
391 int h = key.threadLocalHashCode & (len - 1);
392 while (table[h] != null)
393 h = nextIndex(h, len);
394 table[h] = c;
395 size++;
396 }
397 }
398 }
399 }
400
401 /**
402 * Get the entry associated with key. This method
403 * itself handles only the fast path: a direct hit of existing
404 * key. It otherwise relays to getEntryAfterMiss. This is
405 * designed to maximize performance for direct hits, in part
406 * by making this method readily inlinable.
407 *
408 * @param key the thread local object
409 * @return the entry associated with key, or null if no such
410 */
411 private Entry getEntry(ThreadLocal<?> key) {
412 int i = key.threadLocalHashCode & (table.length - 1);
413 Entry e = table[i];
414 if (e != null && e.get() == key)
415 return e;
416 else
417 return getEntryAfterMiss(key, i, e);
418 }
419
420 /**
421 * Version of getEntry method for use when key is not found in
422 * its direct hash slot.
423 *
424 * @param key the thread local object
425 * @param i the table index for key's hash code
426 * @param e the entry at table[i]
427 * @return the entry associated with key, or null if no such
428 */
429 private Entry getEntryAfterMiss(ThreadLocal<?> key, int i, Entry e) {
430 Entry[] tab = table;
431 int len = tab.length;
432
433 while (e != null) {
434 ThreadLocal<?> k = e.get();
435 if (k == key)
436 return e;
437 if (k == null)
438 expungeStaleEntry(i);
439 else
440 i = nextIndex(i, len);
441 e = tab[i];
442 }
443 return null;
444 }
445
446 /**
447 * Set the value associated with key.
448 *
449 * @param key the thread local object
450 * @param value the value to be set
451 */
452 private void set(ThreadLocal<?> key, Object value) {
453
454 // We don't use a fast path as with get() because it is at
455 // least as common to use set() to create new entries as
456 // it is to replace existing ones, in which case, a fast
457 // path would fail more often than not.
458
459 Entry[] tab = table;
460 int len = tab.length;
461 int i = key.threadLocalHashCode & (len-1);
462
463 for (Entry e = tab[i];
464 e != null;
465 e = tab[i = nextIndex(i, len)]) {
466 ThreadLocal<?> k = e.get();
467
468 if (k == key) {
469 e.value = value;
470 return;
471 }
472
473 if (k == null) {
474 replaceStaleEntry(key, value, i);
475 return;
476 }
477 }
478
479 tab[i] = new Entry(key, value);
480 int sz = ++size;
481 if (!cleanSomeSlots(i, sz) && sz >= threshold)
482 rehash();
483 }
484
485 /**
486 * Remove the entry for key.
487 */
488 private void remove(ThreadLocal<?> key) {
489 Entry[] tab = table;
490 int len = tab.length;
491 int i = key.threadLocalHashCode & (len-1);
492 for (Entry e = tab[i];
493 e != null;
494 e = tab[i = nextIndex(i, len)]) {
495 if (e.get() == key) {
496 e.clear();
497 expungeStaleEntry(i);
498 return;
499 }
500 }
501 }
502
503 /**
504 * Replace a stale entry encountered during a set operation
505 * with an entry for the specified key. The value passed in
506 * the value parameter is stored in the entry, whether or not
507 * an entry already exists for the specified key.
508 *
509 * As a side effect, this method expunges all stale entries in the
510 * "run" containing the stale entry. (A run is a sequence of entries
511 * between two null slots.)
512 *
513 * @param key the key
514 * @param value the value to be associated with key
515 * @param staleSlot index of the first stale entry encountered while
516 * searching for key.
517 */
518 private void replaceStaleEntry(ThreadLocal<?> key, Object value,
519 int staleSlot) {
520 Entry[] tab = table;
521 int len = tab.length;
522 Entry e;
523
524 // Back up to check for prior stale entry in current run.
525 // We clean out whole runs at a time to avoid continual
526 // incremental rehashing due to garbage collector freeing
527 // up refs in bunches (i.e., whenever the collector runs).
528 int slotToExpunge = staleSlot;
529 for (int i = prevIndex(staleSlot, len);
530 (e = tab[i]) != null;
531 i = prevIndex(i, len))
532 if (e.get() == null)
533 slotToExpunge = i;
534
535 // Find either the key or trailing null slot of run, whichever
536 // occurs first
537 for (int i = nextIndex(staleSlot, len);
538 (e = tab[i]) != null;
539 i = nextIndex(i, len)) {
540 ThreadLocal<?> k = e.get();
541
542 // If we find key, then we need to swap it
543 // with the stale entry to maintain hash table order.
544 // The newly stale slot, or any other stale slot
545 // encountered above it, can then be sent to expungeStaleEntry
546 // to remove or rehash all of the other entries in run.
547 if (k == key) {
548 e.value = value;
549
550 tab[i] = tab[staleSlot];
551 tab[staleSlot] = e;
552
553 // Start expunge at preceding stale entry if it exists
554 if (slotToExpunge == staleSlot)
555 slotToExpunge = i;
556 cleanSomeSlots(expungeStaleEntry(slotToExpunge), len);
557 return;
558 }
559
560 // If we didn't find stale entry on backward scan, the
561 // first stale entry seen while scanning for key is the
562 // first still present in the run.
563 if (k == null && slotToExpunge == staleSlot)
564 slotToExpunge = i;
565 }
566
567 // If key not found, put new entry in stale slot
568 tab[staleSlot].value = null;
569 tab[staleSlot] = new Entry(key, value);
570
571 // If there are any other stale entries in run, expunge them
572 if (slotToExpunge != staleSlot)
573 cleanSomeSlots(expungeStaleEntry(slotToExpunge), len);
574 }
575
576 /**
577 * Expunge a stale entry by rehashing any possibly colliding entries
578 * lying between staleSlot and the next null slot. This also expunges
579 * any other stale entries encountered before the trailing null. See
580 * Knuth, Section 6.4
581 *
582 * @param staleSlot index of slot known to have null key
583 * @return the index of the next null slot after staleSlot
584 * (all between staleSlot and this slot will have been checked
585 * for expunging).
586 */
587 private int expungeStaleEntry(int staleSlot) {
588 Entry[] tab = table;
589 int len = tab.length;
590
591 // expunge entry at staleSlot
592 tab[staleSlot].value = null;
593 tab[staleSlot] = null;
594 size--;
595
596 // Rehash until we encounter null
597 Entry e;
598 int i;
599 for (i = nextIndex(staleSlot, len);
600 (e = tab[i]) != null;
601 i = nextIndex(i, len)) {
602 ThreadLocal<?> k = e.get();
603 if (k == null) {
604 e.value = null;
605 tab[i] = null;
606 size--;
607 } else {
608 int h = k.threadLocalHashCode & (len - 1);
609 if (h != i) {
610 tab[i] = null;
611
612 // Unlike Knuth 6.4 Algorithm R, we must scan until
613 // null because multiple entries could have been stale.
614 while (tab[h] != null)
615 h = nextIndex(h, len);
616 tab[h] = e;
617 }
618 }
619 }
620 return i;
621 }
622
623 /**
624 * Heuristically scan some cells looking for stale entries.
625 * This is invoked when either a new element is added, or
626 * another stale one has been expunged. It performs a
627 * logarithmic number of scans, as a balance between no
628 * scanning (fast but retains garbage) and a number of scans
629 * proportional to number of elements, that would find all
630 * garbage but would cause some insertions to take O(n) time.
631 *
632 * @param i a position known NOT to hold a stale entry. The
633 * scan starts at the element after i.
634 *
635 * @param n scan control: {@code log2(n)} cells are scanned,
636 * unless a stale entry is found, in which case
637 * {@code log2(table.length)-1} additional cells are scanned.
638 * When called from insertions, this parameter is the number
639 * of elements, but when from replaceStaleEntry, it is the
640 * table length. (Note: all this could be changed to be either
641 * more or less aggressive by weighting n instead of just
642 * using straight log n. But this version is simple, fast, and
643 * seems to work well.)
644 *
645 * @return true if any stale entries have been removed.
646 */
647 private boolean cleanSomeSlots(int i, int n) {
648 boolean removed = false;
649 Entry[] tab = table;
650 int len = tab.length;
651 do {
652 i = nextIndex(i, len);
653 Entry e = tab[i];
654 if (e != null && e.get() == null) {
655 n = len;
656 removed = true;
657 i = expungeStaleEntry(i);
658 }
659 } while ( (n >>>= 1) != 0);
660 return removed;
661 }
662
663 /**
664 * Re-pack and/or re-size the table. First scan the entire
665 * table removing stale entries. If this doesn't sufficiently
666 * shrink the size of the table, double the table size.
667 */
668 private void rehash() {
669 expungeStaleEntries();
670
671 // Use lower threshold for doubling to avoid hysteresis
672 if (size >= threshold - threshold / 4)
673 resize();
674 }
675
676 /**
677 * Double the capacity of the table.
678 */
679 private void resize() {
680 Entry[] oldTab = table;
681 int oldLen = oldTab.length;
682 int newLen = oldLen * 2;
683 Entry[] newTab = new Entry[newLen];
684 int count = 0;
685
686 for (int j = 0; j < oldLen; ++j) {
687 Entry e = oldTab[j];
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 }