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 import jdk.internal.misc.JdkThreadLocal;
32
33 /**
34 * This class provides thread-local variables. These variables differ from
35 * their normal counterparts in that each thread that accesses one (via its
36 * {@code get} or {@code set} method) has its own, independently initialized
37 * copy of the variable. {@code ThreadLocal} instances are typically private
38 * static fields in classes that wish to associate state with a thread (e.g.,
39 * a user ID or Transaction ID).
40 *
41 * <p>For example, the class below generates unique identifiers local to each
42 * thread.
43 * A thread's id is assigned the first time it invokes {@code ThreadId.get()}
44 * and remains unchanged on subsequent calls.
45 * <pre>
46 * import java.util.concurrent.atomic.AtomicInteger;
47 *
48 * public class ThreadId {
49 * // Atomic integer containing the next thread ID to be assigned
50 * private static final AtomicInteger nextId = new AtomicInteger(0);
51 *
52 * // Thread local variable containing each thread's ID
53 * private static final ThreadLocal<Integer> threadId =
54 * new ThreadLocal<Integer>() {
55 * @Override protected Integer initialValue() {
56 * return nextId.getAndIncrement();
57 * }
58 * };
59 *
60 * // Returns the current thread's unique ID, assigning it if necessary
61 * public static int get() {
62 * return threadId.get();
63 * }
64 * }
65 * </pre>
66 * <p>Each thread holds an implicit reference to its copy of a thread-local
67 * variable as long as the thread is alive and the {@code ThreadLocal}
68 * instance is accessible; after a thread goes away, all of its copies of
69 * thread-local instances are subject to garbage collection (unless other
70 * references to these copies exist).
71 *
72 * @author Josh Bloch and Doug Lea
73 * @since 1.2
74 */
75 public class ThreadLocal<T> {
76 /**
77 * ThreadLocals rely on per-thread linear-probe hash maps attached
78 * to each thread (Thread.threadLocals and
79 * inheritableThreadLocals). The ThreadLocal objects act as keys,
80 * searched via threadLocalHashCode. This is a custom hash code
81 * (useful only within ThreadLocalMaps) that eliminates collisions
82 * in the common case where consecutively constructed ThreadLocals
83 * are used by the same threads, while remaining well-behaved in
84 * less common cases.
85 */
86 private final int threadLocalHashCode = nextHashCode();
87
88 static void callThreadTerminated(ThreadLocalMap map) {
89 if (map == null) {
90 return;
91 }
92
93 // We expect to have a very small number of hooks and
94 // potentially a lot of entries in the map. So, it should be
95 // faster to iterate over the hooks and do look-ups on the map
96 // instead of iterating over the map.
97 JdkThreadLocal.forEach(tl -> {
98 final ThreadLocalMap.Entry entry = map.getEntry(tl);
99 if (entry != null) {
100 tl.callThreadTerminated(entry.value);
101 }
102 });
103 }
104
105 /**
106 * The next hash code to be given out. Updated atomically. Starts at
107 * zero.
108 */
109 private static AtomicInteger nextHashCode =
110 new AtomicInteger();
111
112 /**
113 * The difference between successively generated hash codes - turns
114 * implicit sequential thread-local IDs into near-optimally spread
115 * multiplicative hash values for power-of-two-sized tables.
116 */
117 private static final int HASH_INCREMENT = 0x61c88647;
118
119 /**
120 * Returns the next hash code.
121 */
122 private static int nextHashCode() {
123 return nextHashCode.getAndAdd(HASH_INCREMENT);
124 }
125
126 /**
127 * Returns the current thread's "initial value" for this
128 * thread-local variable. This method will be invoked the first
129 * time a thread accesses the variable with the {@link #get}
130 * method, unless the thread previously invoked the {@link #set}
131 * method, in which case the {@code initialValue} method will not
132 * be invoked for the thread. Normally, this method is invoked at
133 * most once per thread, but it may be invoked again in case of
134 * subsequent invocations of {@link #remove} followed by {@link #get}.
135 *
136 * <p>This implementation simply returns {@code null}; if the
137 * programmer desires thread-local variables to have an initial
138 * value other than {@code null}, {@code ThreadLocal} must be
139 * subclassed, and this method overridden. Typically, an
140 * anonymous inner class will be used.
141 *
142 * @return the initial value for this thread-local
143 */
144 protected T initialValue() {
145 return null;
146 }
147
148 /**
149 * Creates a thread local variable. The initial value of the variable is
150 * determined by invoking the {@code get} method on the {@code Supplier}.
151 *
152 * @param <S> the type of the thread local's value
153 * @param supplier the supplier to be used to determine the initial value
154 * @return a new thread local variable
155 * @throws NullPointerException if the specified supplier is null
156 * @since 1.8
157 */
158 public static <S> ThreadLocal<S> withInitial(Supplier<? extends S> supplier) {
159 return new SuppliedThreadLocal<>(supplier);
160 }
161
162 /**
163 * Creates a thread local variable.
164 * @see #withInitial(java.util.function.Supplier)
165 */
166 public ThreadLocal() {
167 }
168
169 /**
170 * Returns the value in the current thread's copy of this
171 * thread-local variable. If the variable has no value for the
172 * current thread, it is first initialized to the value returned
173 * by an invocation of the {@link #initialValue} method.
174 *
175 * @return the current thread's value of this thread-local
176 */
177 public T get() {
178 Thread t = Thread.currentThread();
179 ThreadLocalMap map = getMap(t);
180 if (map != null) {
181 ThreadLocalMap.Entry e = map.getEntry(this);
182 if (e != null) {
183 @SuppressWarnings("unchecked")
184 T result = (T)e.value;
185 return result;
186 }
187 }
188 return setInitialValue();
189 }
190
191 /**
192 * Variant of set() to establish initialValue. Used instead
193 * of set() in case user has overridden the set() method.
194 *
195 * @return the initial value
196 */
197 private T setInitialValue() {
198 T value = initialValue();
199 Thread t = Thread.currentThread();
200 ThreadLocalMap map = getMap(t);
201 if (map != null)
202 map.set(this, value);
203 else
204 createMap(t, value);
205 return value;
206 }
207
208 /**
209 * Sets the current thread's copy of this thread-local variable
210 * to the specified value. Most subclasses will have no need to
211 * override this method, relying solely on the {@link #initialValue}
212 * method to set the values of thread-locals.
213 *
214 * @param value the value to be stored in the current thread's copy of
215 * this thread-local.
216 */
217 public void set(T value) {
218 Thread t = Thread.currentThread();
219 ThreadLocalMap map = getMap(t);
220 if (map != null)
221 map.set(this, value);
222 else
223 createMap(t, value);
224 }
225
226 /**
227 * Removes the current thread's value for this thread-local
228 * variable. If this thread-local variable is subsequently
229 * {@linkplain #get read} by the current thread, its value will be
230 * reinitialized by invoking its {@link #initialValue} method,
231 * unless its value is {@linkplain #set set} by the current thread
232 * in the interim. This may result in multiple invocations of the
233 * {@code initialValue} method in the current thread.
234 *
235 * @since 1.5
236 */
237 public void remove() {
238 ThreadLocalMap m = getMap(Thread.currentThread());
239 if (m != null)
240 m.remove(this);
241 }
242
243 /**
244 * Get the map associated with a ThreadLocal. Overridden in
245 * InheritableThreadLocal.
246 *
247 * @param t the current thread
248 * @return the map
249 */
250 ThreadLocalMap getMap(Thread t) {
251 return t.threadLocals;
252 }
253
254 /**
255 * Create the map associated with a ThreadLocal. Overridden in
256 * InheritableThreadLocal.
257 *
258 * @param t the current thread
259 * @param firstValue value for the initial entry of the map
260 */
261 void createMap(Thread t, T firstValue) {
262 t.threadLocals = new ThreadLocalMap(this, firstValue);
263 }
264
265 /**
266 * Factory method to create map of inherited thread locals.
267 * Designed to be called only from Thread constructor.
268 *
269 * @param parentMap the map associated with parent thread
270 * @return a map containing the parent's inheritable bindings
271 */
272 static ThreadLocalMap createInheritedMap(ThreadLocalMap parentMap) {
273 return new ThreadLocalMap(parentMap);
274 }
275
276 /**
277 * Method childValue is visibly defined in subclass
278 * InheritableThreadLocal, but is internally defined here for the
279 * sake of providing createInheritedMap factory method without
280 * needing to subclass the map class in InheritableThreadLocal.
281 * This technique is preferable to the alternative of embedding
282 * instanceof tests in methods.
283 */
284 T childValue(T parentValue) {
285 throw new UnsupportedOperationException();
286 }
287
288 /**
289 * An extension of ThreadLocal that obtains its initial value from
290 * the specified {@code Supplier}.
291 */
292 static final class SuppliedThreadLocal<T> extends ThreadLocal<T> {
293
294 private final Supplier<? extends T> supplier;
295
296 SuppliedThreadLocal(Supplier<? extends T> supplier) {
297 this.supplier = Objects.requireNonNull(supplier);
298 }
299
300 @Override
301 protected T initialValue() {
302 return supplier.get();
303 }
304 }
305
306 /**
307 * ThreadLocalMap is a customized hash map suitable only for
308 * maintaining thread local values. No operations are exported
309 * outside of the ThreadLocal class. The class is package private to
310 * allow declaration of fields in class Thread. To help deal with
311 * very large and long-lived usages, the hash table entries use
312 * WeakReferences for keys. However, since reference queues are not
313 * used, stale entries are guaranteed to be removed only when
314 * the table starts running out of space.
315 */
316 static class ThreadLocalMap {
317
318 /**
319 * The entries in this hash map extend WeakReference, using
320 * its main ref field as the key (which is always a
321 * ThreadLocal object). Note that null keys (i.e. entry.get()
322 * == null) mean that the key is no longer referenced, so the
323 * entry can be expunged from table. Such entries are referred to
324 * as "stale entries" in the code that follows.
325 */
326 static class Entry extends WeakReference<ThreadLocal<?>> {
327 /** The value associated with this ThreadLocal. */
328 Object value;
329
330 Entry(ThreadLocal<?> k, Object v) {
331 super(k);
332 value = v;
333 }
334 }
335
336 /**
337 * The initial capacity -- MUST be a power of two.
338 */
339 private static final int INITIAL_CAPACITY = 16;
340
341 /**
342 * The table, resized as necessary.
343 * table.length MUST always be a power of two.
344 */
345 private Entry[] table;
346
347 /**
348 * The number of entries in the table.
349 */
350 private int size = 0;
351
352 /**
353 * The next size value at which to resize.
354 */
355 private int threshold; // Default to 0
356
357 /**
358 * Set the resize threshold to maintain at worst a 2/3 load factor.
359 */
360 private void setThreshold(int len) {
361 threshold = len * 2 / 3;
362 }
363
364 /**
365 * Increment i modulo len.
366 */
367 private static int nextIndex(int i, int len) {
368 return ((i + 1 < len) ? i + 1 : 0);
369 }
370
371 /**
372 * Decrement i modulo len.
373 */
374 private static int prevIndex(int i, int len) {
375 return ((i - 1 >= 0) ? i - 1 : len - 1);
376 }
377
378 /**
379 * Construct a new map initially containing (firstKey, firstValue).
380 * ThreadLocalMaps are constructed lazily, so we only create
381 * one when we have at least one entry to put in it.
382 */
383 ThreadLocalMap(ThreadLocal<?> firstKey, Object firstValue) {
384 table = new Entry[INITIAL_CAPACITY];
385 int i = firstKey.threadLocalHashCode & (INITIAL_CAPACITY - 1);
386 table[i] = new Entry(firstKey, firstValue);
387 size = 1;
388 setThreshold(INITIAL_CAPACITY);
389 }
390
391 /**
392 * Construct a new map including all Inheritable ThreadLocals
393 * from given parent map. Called only by createInheritedMap.
394 *
395 * @param parentMap the map associated with parent thread.
396 */
397 private ThreadLocalMap(ThreadLocalMap parentMap) {
398 Entry[] parentTable = parentMap.table;
399 int len = parentTable.length;
400 setThreshold(len);
401 table = new Entry[len];
402
403 for (Entry e : parentTable) {
404 if (e != null) {
405 @SuppressWarnings("unchecked")
406 ThreadLocal<Object> key = (ThreadLocal<Object>) e.get();
407 if (key != null) {
408 Object value = key.childValue(e.value);
409 Entry c = new Entry(key, value);
410 int h = key.threadLocalHashCode & (len - 1);
411 while (table[h] != null)
412 h = nextIndex(h, len);
413 table[h] = c;
414 size++;
415 }
416 }
417 }
418 }
419
420 /**
421 * Get the entry associated with key. This method
422 * itself handles only the fast path: a direct hit of existing
423 * key. It otherwise relays to getEntryAfterMiss. This is
424 * designed to maximize performance for direct hits, in part
425 * by making this method readily inlinable.
426 *
427 * @param key the thread local object
428 * @return the entry associated with key, or null if no such
429 */
430 private Entry getEntry(ThreadLocal<?> key) {
431 int i = key.threadLocalHashCode & (table.length - 1);
432 Entry e = table[i];
433 if (e != null && e.get() == key)
434 return e;
435 else
436 return getEntryAfterMiss(key, i, e);
437 }
438
439 /**
440 * Version of getEntry method for use when key is not found in
441 * its direct hash slot.
442 *
443 * @param key the thread local object
444 * @param i the table index for key's hash code
445 * @param e the entry at table[i]
446 * @return the entry associated with key, or null if no such
447 */
448 private Entry getEntryAfterMiss(ThreadLocal<?> key, int i, Entry e) {
449 Entry[] tab = table;
450 int len = tab.length;
451
452 while (e != null) {
453 ThreadLocal<?> k = e.get();
454 if (k == key)
455 return e;
456 if (k == null)
457 expungeStaleEntry(i);
458 else
459 i = nextIndex(i, len);
460 e = tab[i];
461 }
462 return null;
463 }
464
465 /**
466 * Set the value associated with key.
467 *
468 * @param key the thread local object
469 * @param value the value to be set
470 */
471 private void set(ThreadLocal<?> key, Object value) {
472
473 // We don't use a fast path as with get() because it is at
474 // least as common to use set() to create new entries as
475 // it is to replace existing ones, in which case, a fast
476 // path would fail more often than not.
477
478 Entry[] tab = table;
479 int len = tab.length;
480 int i = key.threadLocalHashCode & (len-1);
481
482 for (Entry e = tab[i];
483 e != null;
484 e = tab[i = nextIndex(i, len)]) {
485 ThreadLocal<?> k = e.get();
486
487 if (k == key) {
488 e.value = value;
489 return;
490 }
491
492 if (k == null) {
493 replaceStaleEntry(key, value, i);
494 return;
495 }
496 }
497
498 tab[i] = new Entry(key, value);
499 int sz = ++size;
500 if (!cleanSomeSlots(i, sz) && sz >= threshold)
501 rehash();
502 }
503
504 /**
505 * Remove the entry for key.
506 */
507 private void remove(ThreadLocal<?> key) {
508 Entry[] tab = table;
509 int len = tab.length;
510 int i = key.threadLocalHashCode & (len-1);
511 for (Entry e = tab[i];
512 e != null;
513 e = tab[i = nextIndex(i, len)]) {
514 if (e.get() == key) {
515 e.clear();
516 expungeStaleEntry(i);
517 return;
518 }
519 }
520 }
521
522 /**
523 * Replace a stale entry encountered during a set operation
524 * with an entry for the specified key. The value passed in
525 * the value parameter is stored in the entry, whether or not
526 * an entry already exists for the specified key.
527 *
528 * As a side effect, this method expunges all stale entries in the
529 * "run" containing the stale entry. (A run is a sequence of entries
530 * between two null slots.)
531 *
532 * @param key the key
533 * @param value the value to be associated with key
534 * @param staleSlot index of the first stale entry encountered while
535 * searching for key.
536 */
537 private void replaceStaleEntry(ThreadLocal<?> key, Object value,
538 int staleSlot) {
539 Entry[] tab = table;
540 int len = tab.length;
541 Entry e;
542
543 // Back up to check for prior stale entry in current run.
544 // We clean out whole runs at a time to avoid continual
545 // incremental rehashing due to garbage collector freeing
546 // up refs in bunches (i.e., whenever the collector runs).
547 int slotToExpunge = staleSlot;
548 for (int i = prevIndex(staleSlot, len);
549 (e = tab[i]) != null;
550 i = prevIndex(i, len))
551 if (e.get() == null)
552 slotToExpunge = i;
553
554 // Find either the key or trailing null slot of run, whichever
555 // occurs first
556 for (int i = nextIndex(staleSlot, len);
557 (e = tab[i]) != null;
558 i = nextIndex(i, len)) {
559 ThreadLocal<?> k = e.get();
560
561 // If we find key, then we need to swap it
562 // with the stale entry to maintain hash table order.
563 // The newly stale slot, or any other stale slot
564 // encountered above it, can then be sent to expungeStaleEntry
565 // to remove or rehash all of the other entries in run.
566 if (k == key) {
567 e.value = value;
568
569 tab[i] = tab[staleSlot];
570 tab[staleSlot] = e;
571
572 // Start expunge at preceding stale entry if it exists
573 if (slotToExpunge == staleSlot)
574 slotToExpunge = i;
575 cleanSomeSlots(expungeStaleEntry(slotToExpunge), len);
576 return;
577 }
578
579 // If we didn't find stale entry on backward scan, the
580 // first stale entry seen while scanning for key is the
581 // first still present in the run.
582 if (k == null && slotToExpunge == staleSlot)
583 slotToExpunge = i;
584 }
585
586 // If key not found, put new entry in stale slot
587 tab[staleSlot].value = null;
588 tab[staleSlot] = new Entry(key, value);
589
590 // If there are any other stale entries in run, expunge them
591 if (slotToExpunge != staleSlot)
592 cleanSomeSlots(expungeStaleEntry(slotToExpunge), len);
593 }
594
595 /**
596 * Expunge a stale entry by rehashing any possibly colliding entries
597 * lying between staleSlot and the next null slot. This also expunges
598 * any other stale entries encountered before the trailing null. See
599 * Knuth, Section 6.4
600 *
601 * @param staleSlot index of slot known to have null key
602 * @return the index of the next null slot after staleSlot
603 * (all between staleSlot and this slot will have been checked
604 * for expunging).
605 */
606 private int expungeStaleEntry(int staleSlot) {
607 Entry[] tab = table;
608 int len = tab.length;
609
610 // expunge entry at staleSlot
611 tab[staleSlot].value = null;
612 tab[staleSlot] = null;
613 size--;
614
615 // Rehash until we encounter null
616 Entry e;
617 int i;
618 for (i = nextIndex(staleSlot, len);
619 (e = tab[i]) != null;
620 i = nextIndex(i, len)) {
621 ThreadLocal<?> k = e.get();
622 if (k == null) {
623 e.value = null;
624 tab[i] = null;
625 size--;
626 } else {
627 int h = k.threadLocalHashCode & (len - 1);
628 if (h != i) {
629 tab[i] = null;
630
631 // Unlike Knuth 6.4 Algorithm R, we must scan until
632 // null because multiple entries could have been stale.
633 while (tab[h] != null)
634 h = nextIndex(h, len);
635 tab[h] = e;
636 }
637 }
638 }
639 return i;
640 }
641
642 /**
643 * Heuristically scan some cells looking for stale entries.
644 * This is invoked when either a new element is added, or
645 * another stale one has been expunged. It performs a
646 * logarithmic number of scans, as a balance between no
647 * scanning (fast but retains garbage) and a number of scans
648 * proportional to number of elements, that would find all
649 * garbage but would cause some insertions to take O(n) time.
650 *
651 * @param i a position known NOT to hold a stale entry. The
652 * scan starts at the element after i.
653 *
654 * @param n scan control: {@code log2(n)} cells are scanned,
655 * unless a stale entry is found, in which case
656 * {@code log2(table.length)-1} additional cells are scanned.
657 * When called from insertions, this parameter is the number
658 * of elements, but when from replaceStaleEntry, it is the
659 * table length. (Note: all this could be changed to be either
660 * more or less aggressive by weighting n instead of just
661 * using straight log n. But this version is simple, fast, and
662 * seems to work well.)
663 *
664 * @return true if any stale entries have been removed.
665 */
666 private boolean cleanSomeSlots(int i, int n) {
667 boolean removed = false;
668 Entry[] tab = table;
669 int len = tab.length;
670 do {
671 i = nextIndex(i, len);
672 Entry e = tab[i];
673 if (e != null && e.get() == null) {
674 n = len;
675 removed = true;
676 i = expungeStaleEntry(i);
677 }
678 } while ( (n >>>= 1) != 0);
679 return removed;
680 }
681
682 /**
683 * Re-pack and/or re-size the table. First scan the entire
684 * table removing stale entries. If this doesn't sufficiently
685 * shrink the size of the table, double the table size.
686 */
687 private void rehash() {
688 expungeStaleEntries();
689
690 // Use lower threshold for doubling to avoid hysteresis
691 if (size >= threshold - threshold / 4)
692 resize();
693 }
694
695 /**
696 * Double the capacity of the table.
697 */
698 private void resize() {
699 Entry[] oldTab = table;
700 int oldLen = oldTab.length;
701 int newLen = oldLen * 2;
702 Entry[] newTab = new Entry[newLen];
703 int count = 0;
704
705 for (Entry e : oldTab) {
706 if (e != null) {
707 ThreadLocal<?> k = e.get();
708 if (k == null) {
709 e.value = null; // Help the GC
710 } else {
711 int h = k.threadLocalHashCode & (newLen - 1);
712 while (newTab[h] != null)
713 h = nextIndex(h, newLen);
714 newTab[h] = e;
715 count++;
716 }
717 }
718 }
719
720 setThreshold(newLen);
721 size = count;
722 table = newTab;
723 }
724
725 /**
726 * Expunge all stale entries in the table.
727 */
728 private void expungeStaleEntries() {
729 Entry[] tab = table;
730 int len = tab.length;
731 for (int j = 0; j < len; j++) {
732 Entry e = tab[j];
733 if (e != null && e.get() == null)
734 expungeStaleEntry(j);
735 }
736 }
737 }
738 }