1 /*
2 * Copyright (c) 1997, 2007, 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.concurrent.atomic.AtomicInteger;
29
30 /**
31 * This class provides thread-local variables. These variables differ from
32 * their normal counterparts in that each thread that accesses one (via its
33 * <tt>get</tt> or <tt>set</tt> method) has its own, independently initialized
34 * copy of the variable. <tt>ThreadLocal</tt> instances are typically private
35 * static fields in classes that wish to associate state with a thread (e.g.,
36 * a user ID or Transaction ID).
37 *
38 * <p>For example, the class below generates unique identifiers local to each
39 * thread.
40 * A thread's id is assigned the first time it invokes <tt>ThreadId.get()</tt>
41 * and remains unchanged on subsequent calls.
42 * <pre>
43 * import java.util.concurrent.atomic.AtomicInteger;
44 *
45 * public class ThreadId {
46 * // Atomic integer containing the next thread ID to be assigned
47 * private static final AtomicInteger nextId = new AtomicInteger(0);
48 *
49 * // Thread local variable containing each thread's ID
50 * private static final ThreadLocal<Integer> threadId =
51 * new ThreadLocal<Integer>() {
52 * @Override protected Integer initialValue() {
53 * return nextId.getAndIncrement();
54 * }
55 * };
56 *
57 * // Returns the current thread's unique ID, assigning it if necessary
58 * public static int get() {
59 * return threadId.get();
60 * }
61 * }
62 * </pre>
63 * <p>Each thread holds an implicit reference to its copy of a thread-local
64 * variable as long as the thread is alive and the <tt>ThreadLocal</tt>
65 * instance is accessible; after a thread goes away, all of its copies of
66 * thread-local instances are subject to garbage collection (unless other
67 * references to these copies exist).
68 *
69 * @author Josh Bloch and Doug Lea
70 * @since 1.2
71 */
72 public class ThreadLocal<T> {
73 /**
74 * ThreadLocals rely on per-thread linear-probe hash maps attached
75 * to each thread (Thread.threadLocals and
76 * inheritableThreadLocals). The ThreadLocal objects act as keys,
77 * searched via threadLocalHashCode. This is a custom hash code
78 * (useful only within ThreadLocalMaps) that eliminates collisions
79 * in the common case where consecutively constructed ThreadLocals
80 * are used by the same threads, while remaining well-behaved in
81 * less common cases.
82 */
83 private final int threadLocalHashCode = nextHashCode();
84
85 /**
86 * The next hash code to be given out. Updated atomically. Starts at
87 * zero.
88 */
89 private static AtomicInteger nextHashCode =
90 new AtomicInteger();
91
92 /**
93 * The difference between successively generated hash codes - turns
94 * implicit sequential thread-local IDs into near-optimally spread
95 * multiplicative hash values for power-of-two-sized tables.
96 */
97 private static final int HASH_INCREMENT = 0x61c88647;
98
99 /**
100 * Returns the next hash code.
101 */
102 private static int nextHashCode() {
103 return nextHashCode.getAndAdd(HASH_INCREMENT);
104 }
105
106 /**
107 * Returns the current thread's "initial value" for this
108 * thread-local variable. This method will be invoked the first
109 * time a thread accesses the variable with the {@link #get}
110 * method, unless the thread previously invoked the {@link #set}
111 * method, in which case the <tt>initialValue</tt> method will not
112 * be invoked for the thread. Normally, this method is invoked at
113 * most once per thread, but it may be invoked again in case of
114 * subsequent invocations of {@link #remove} followed by {@link #get}.
115 *
116 * <p>This implementation simply returns <tt>null</tt>; if the
117 * programmer desires thread-local variables to have an initial
118 * value other than <tt>null</tt>, <tt>ThreadLocal</tt> must be
119 * subclassed, and this method overridden. Typically, an
120 * anonymous inner class will be used.
121 *
122 * @return the initial value for this thread-local
123 */
124 protected T initialValue() {
125 return null;
126 }
127
128 /**
129 * Creates a thread local variable.
130 */
131 public ThreadLocal() {
132 }
133
134 /**
135 * Returns the value in the current thread's copy of this
136 * thread-local variable. If the variable has no value for the
137 * current thread, it is first initialized to the value returned
138 * by an invocation of the {@link #initialValue} method.
139 *
140 * @return the current thread's value of this thread-local
141 */
142 public T get() {
143 Thread t = Thread.currentThread();
144 ThreadLocalMap map = getMap(t);
145 if (map != null) {
146 ThreadLocalMap.Entry e = map.getEntry(this);
147 if (e != null) {
148 @SuppressWarnings("unchecked")
149 T result = (T)e.value;
150 return result;
151 }
152 }
153 return setInitialValue();
154 }
155
156 /**
157 * Variant of set() to establish initialValue. Used instead
158 * of set() in case user has overridden the set() method.
159 *
160 * @return the initial value
161 */
162 private T setInitialValue() {
163 T value = initialValue();
164 Thread t = Thread.currentThread();
165 ThreadLocalMap map = getMap(t);
166 if (map != null)
167 map.set(this, value);
168 else
169 createMap(t, value);
170 return value;
171 }
172
173 /**
174 * Sets the current thread's copy of this thread-local variable
175 * to the specified value. Most subclasses will have no need to
176 * override this method, relying solely on the {@link #initialValue}
177 * method to set the values of thread-locals.
178 *
179 * @param value the value to be stored in the current thread's copy of
180 * this thread-local.
181 */
182 public void set(T value) {
183 Thread t = Thread.currentThread();
184 ThreadLocalMap map = getMap(t);
185 if (map != null)
186 map.set(this, value);
187 else
188 createMap(t, value);
189 }
190
191 /**
192 * Removes the current thread's value for this thread-local
193 * variable. If this thread-local variable is subsequently
194 * {@linkplain #get read} by the current thread, its value will be
195 * reinitialized by invoking its {@link #initialValue} method,
196 * unless its value is {@linkplain #set set} by the current thread
197 * in the interim. This may result in multiple invocations of the
198 * <tt>initialValue</tt> method in the current thread.
199 *
200 * @since 1.5
201 */
202 public void remove() {
203 ThreadLocalMap m = getMap(Thread.currentThread());
204 if (m != null)
205 m.remove(this);
206 }
207
208 /**
209 * Get the map associated with a ThreadLocal. Overridden in
210 * InheritableThreadLocal.
211 *
212 * @param t the current thread
213 * @return the map
214 */
215 ThreadLocalMap getMap(Thread t) {
216 return t.threadLocals;
217 }
218
219 /**
220 * Create the map associated with a ThreadLocal. Overridden in
221 * InheritableThreadLocal.
222 *
223 * @param t the current thread
224 * @param firstValue value for the initial entry of the map
225 */
226 void createMap(Thread t, T firstValue) {
227 t.threadLocals = new ThreadLocalMap(this, firstValue);
228 }
229
230 /**
231 * Factory method to create map of inherited thread locals.
232 * Designed to be called only from Thread constructor.
233 *
234 * @param parentMap the map associated with parent thread
235 * @return a map containing the parent's inheritable bindings
236 */
237 static ThreadLocalMap createInheritedMap(ThreadLocalMap parentMap) {
238 return new ThreadLocalMap(parentMap);
239 }
240
241 /**
242 * Method childValue is visibly defined in subclass
243 * InheritableThreadLocal, but is internally defined here for the
244 * sake of providing createInheritedMap factory method without
245 * needing to subclass the map class in InheritableThreadLocal.
246 * This technique is preferable to the alternative of embedding
247 * instanceof tests in methods.
248 */
249 T childValue(T parentValue) {
250 throw new UnsupportedOperationException();
251 }
252
253 /**
254 * ThreadLocalMap is a customized hash map suitable only for
255 * maintaining thread local values. No operations are exported
256 * outside of the ThreadLocal class. The class is package private to
257 * allow declaration of fields in class Thread. To help deal with
258 * very large and long-lived usages, the hash table entries use
259 * WeakReferences for keys. However, since reference queues are not
260 * used, stale entries are guaranteed to be removed only when
261 * the table starts running out of space.
262 */
263 static class ThreadLocalMap {
264
265 /**
266 * The entries in this hash map extend WeakReference, using
267 * its main ref field as the key (which is always a
268 * ThreadLocal object). Note that null keys (i.e. entry.get()
269 * == null) mean that the key is no longer referenced, so the
270 * entry can be expunged from table. Such entries are referred to
271 * as "stale entries" in the code that follows.
272 */
273 static class Entry extends WeakReference<ThreadLocal<?>> {
274 /** The value associated with this ThreadLocal. */
275 Object value;
276
277 Entry(ThreadLocal<?> k, Object v) {
278 super(k);
279 value = v;
280 }
281 }
282
283 /**
284 * The initial capacity -- MUST be a power of two.
285 */
286 private static final int INITIAL_CAPACITY = 16;
287
288 /**
289 * The table, resized as necessary.
290 * table.length MUST always be a power of two.
291 */
292 private Entry[] table;
293
294 /**
295 * The number of entries in the table.
296 */
297 private int size = 0;
298
299 /**
300 * The next size value at which to resize.
301 */
302 private int threshold; // Default to 0
303
304 /**
305 * Set the resize threshold to maintain at worst a 2/3 load factor.
306 */
307 private void setThreshold(int len) {
308 threshold = len * 2 / 3;
309 }
310
311 /**
312 * Increment i modulo len.
313 */
314 private static int nextIndex(int i, int len) {
315 return ((i + 1 < len) ? i + 1 : 0);
316 }
317
318 /**
319 * Decrement i modulo len.
320 */
321 private static int prevIndex(int i, int len) {
322 return ((i - 1 >= 0) ? i - 1 : len - 1);
323 }
324
325 /**
326 * Construct a new map initially containing (firstKey, firstValue).
327 * ThreadLocalMaps are constructed lazily, so we only create
328 * one when we have at least one entry to put in it.
329 */
330 ThreadLocalMap(ThreadLocal<?> firstKey, Object firstValue) {
331 table = new Entry[INITIAL_CAPACITY];
332 int i = firstKey.threadLocalHashCode & (INITIAL_CAPACITY - 1);
333 table[i] = new Entry(firstKey, firstValue);
334 size = 1;
335 setThreshold(INITIAL_CAPACITY);
336 }
337
338 /**
339 * Construct a new map including all Inheritable ThreadLocals
340 * from given parent map. Called only by createInheritedMap.
341 *
342 * @param parentMap the map associated with parent thread.
343 */
344 private ThreadLocalMap(ThreadLocalMap parentMap) {
345 Entry[] parentTable = parentMap.table;
346 int len = parentTable.length;
347 setThreshold(len);
348 table = new Entry[len];
349
350 for (int j = 0; j < len; j++) {
351 Entry e = parentTable[j];
352 if (e != null) {
353 @SuppressWarnings("unchecked")
354 ThreadLocal<Object> key = (ThreadLocal<Object>) e.get();
355 if (key != null) {
356 Object value = key.childValue(e.value);
357 Entry c = new Entry(key, value);
358 int h = key.threadLocalHashCode & (len - 1);
359 while (table[h] != null)
360 h = nextIndex(h, len);
361 table[h] = c;
362 size++;
363 }
364 }
365 }
366 }
367
368 /**
369 * Get the entry associated with key. This method
370 * itself handles only the fast path: a direct hit of existing
371 * key. It otherwise relays to getEntryAfterMiss. This is
372 * designed to maximize performance for direct hits, in part
373 * by making this method readily inlinable.
374 *
375 * @param key the thread local object
376 * @return the entry associated with key, or null if no such
377 */
378 private Entry getEntry(ThreadLocal<?> key) {
379 int i = key.threadLocalHashCode & (table.length - 1);
380 Entry e = table[i];
381 if (e != null && e.get() == key)
382 return e;
383 else
384 return getEntryAfterMiss(key, i, e);
385 }
386
387 /**
388 * Version of getEntry method for use when key is not found in
389 * its direct hash slot.
390 *
391 * @param key the thread local object
392 * @param i the table index for key's hash code
393 * @param e the entry at table[i]
394 * @return the entry associated with key, or null if no such
395 */
396 private Entry getEntryAfterMiss(ThreadLocal<?> key, int i, Entry e) {
397 Entry[] tab = table;
398 int len = tab.length;
399
400 while (e != null) {
401 ThreadLocal<?> k = e.get();
402 if (k == key)
403 return e;
404 if (k == null)
405 expungeStaleEntry(i);
406 else
407 i = nextIndex(i, len);
408 e = tab[i];
409 }
410 return null;
411 }
412
413 /**
414 * Set the value associated with key.
415 *
416 * @param key the thread local object
417 * @param value the value to be set
418 */
419 private void set(ThreadLocal<?> key, Object value) {
420
421 // We don't use a fast path as with get() because it is at
422 // least as common to use set() to create new entries as
423 // it is to replace existing ones, in which case, a fast
424 // path would fail more often than not.
425
426 Entry[] tab = table;
427 int len = tab.length;
428 int i = key.threadLocalHashCode & (len-1);
429
430 for (Entry e = tab[i];
431 e != null;
432 e = tab[i = nextIndex(i, len)]) {
433 ThreadLocal<?> k = e.get();
434
435 if (k == key) {
436 e.value = value;
437 return;
438 }
439
440 if (k == null) {
441 replaceStaleEntry(key, value, i);
442 return;
443 }
444 }
445
446 tab[i] = new Entry(key, value);
447 int sz = ++size;
448 if (!cleanSomeSlots(i, sz) && sz >= threshold)
449 rehash();
450 }
451
452 /**
453 * Remove the entry for key.
454 */
455 private void remove(ThreadLocal<?> key) {
456 Entry[] tab = table;
457 int len = tab.length;
458 int i = key.threadLocalHashCode & (len-1);
459 for (Entry e = tab[i];
460 e != null;
461 e = tab[i = nextIndex(i, len)]) {
462 if (e.get() == key) {
463 e.clear();
464 expungeStaleEntry(i);
465 return;
466 }
467 }
468 }
469
470 /**
471 * Replace a stale entry encountered during a set operation
472 * with an entry for the specified key. The value passed in
473 * the value parameter is stored in the entry, whether or not
474 * an entry already exists for the specified key.
475 *
476 * As a side effect, this method expunges all stale entries in the
477 * "run" containing the stale entry. (A run is a sequence of entries
478 * between two null slots.)
479 *
480 * @param key the key
481 * @param value the value to be associated with key
482 * @param staleSlot index of the first stale entry encountered while
483 * searching for key.
484 */
485 private void replaceStaleEntry(ThreadLocal<?> key, Object value,
486 int staleSlot) {
487 Entry[] tab = table;
488 int len = tab.length;
489 Entry e;
490
491 // Back up to check for prior stale entry in current run.
492 // We clean out whole runs at a time to avoid continual
493 // incremental rehashing due to garbage collector freeing
494 // up refs in bunches (i.e., whenever the collector runs).
495 int slotToExpunge = staleSlot;
496 for (int i = prevIndex(staleSlot, len);
497 (e = tab[i]) != null;
498 i = prevIndex(i, len))
499 if (e.get() == null)
500 slotToExpunge = i;
501
502 // Find either the key or trailing null slot of run, whichever
503 // occurs first
504 for (int i = nextIndex(staleSlot, len);
505 (e = tab[i]) != null;
506 i = nextIndex(i, len)) {
507 ThreadLocal<?> k = e.get();
508
509 // If we find key, then we need to swap it
510 // with the stale entry to maintain hash table order.
511 // The newly stale slot, or any other stale slot
512 // encountered above it, can then be sent to expungeStaleEntry
513 // to remove or rehash all of the other entries in run.
514 if (k == key) {
515 e.value = value;
516
517 tab[i] = tab[staleSlot];
518 tab[staleSlot] = e;
519
520 // Start expunge at preceding stale entry if it exists
521 if (slotToExpunge == staleSlot)
522 slotToExpunge = i;
523 cleanSomeSlots(expungeStaleEntry(slotToExpunge), len);
524 return;
525 }
526
527 // If we didn't find stale entry on backward scan, the
528 // first stale entry seen while scanning for key is the
529 // first still present in the run.
530 if (k == null && slotToExpunge == staleSlot)
531 slotToExpunge = i;
532 }
533
534 // If key not found, put new entry in stale slot
535 tab[staleSlot].value = null;
536 tab[staleSlot] = new Entry(key, value);
537
538 // If there are any other stale entries in run, expunge them
539 if (slotToExpunge != staleSlot)
540 cleanSomeSlots(expungeStaleEntry(slotToExpunge), len);
541 }
542
543 /**
544 * Expunge a stale entry by rehashing any possibly colliding entries
545 * lying between staleSlot and the next null slot. This also expunges
546 * any other stale entries encountered before the trailing null. See
547 * Knuth, Section 6.4
548 *
549 * @param staleSlot index of slot known to have null key
550 * @return the index of the next null slot after staleSlot
551 * (all between staleSlot and this slot will have been checked
552 * for expunging).
553 */
554 private int expungeStaleEntry(int staleSlot) {
555 Entry[] tab = table;
556 int len = tab.length;
557
558 // expunge entry at staleSlot
559 tab[staleSlot].value = null;
560 tab[staleSlot] = null;
561 size--;
562
563 // Rehash until we encounter null
564 Entry e;
565 int i;
566 for (i = nextIndex(staleSlot, len);
567 (e = tab[i]) != null;
568 i = nextIndex(i, len)) {
569 ThreadLocal<?> k = e.get();
570 if (k == null) {
571 e.value = null;
572 tab[i] = null;
573 size--;
574 } else {
575 int h = k.threadLocalHashCode & (len - 1);
576 if (h != i) {
577 tab[i] = null;
578
579 // Unlike Knuth 6.4 Algorithm R, we must scan until
580 // null because multiple entries could have been stale.
581 while (tab[h] != null)
582 h = nextIndex(h, len);
583 tab[h] = e;
584 }
585 }
586 }
587 return i;
588 }
589
590 /**
591 * Heuristically scan some cells looking for stale entries.
592 * This is invoked when either a new element is added, or
593 * another stale one has been expunged. It performs a
594 * logarithmic number of scans, as a balance between no
595 * scanning (fast but retains garbage) and a number of scans
596 * proportional to number of elements, that would find all
597 * garbage but would cause some insertions to take O(n) time.
598 *
599 * @param i a position known NOT to hold a stale entry. The
600 * scan starts at the element after i.
601 *
602 * @param n scan control: <tt>log2(n)</tt> cells are scanned,
603 * unless a stale entry is found, in which case
604 * <tt>log2(table.length)-1</tt> additional cells are scanned.
605 * When called from insertions, this parameter is the number
606 * of elements, but when from replaceStaleEntry, it is the
607 * table length. (Note: all this could be changed to be either
608 * more or less aggressive by weighting n instead of just
609 * using straight log n. But this version is simple, fast, and
610 * seems to work well.)
611 *
612 * @return true if any stale entries have been removed.
613 */
614 private boolean cleanSomeSlots(int i, int n) {
615 boolean removed = false;
616 Entry[] tab = table;
617 int len = tab.length;
618 do {
619 i = nextIndex(i, len);
620 Entry e = tab[i];
621 if (e != null && e.get() == null) {
622 n = len;
623 removed = true;
624 i = expungeStaleEntry(i);
625 }
626 } while ( (n >>>= 1) != 0);
627 return removed;
628 }
629
630 /**
631 * Re-pack and/or re-size the table. First scan the entire
632 * table removing stale entries. If this doesn't sufficiently
633 * shrink the size of the table, double the table size.
634 */
635 private void rehash() {
636 expungeStaleEntries();
637
638 // Use lower threshold for doubling to avoid hysteresis
639 if (size >= threshold - threshold / 4)
640 resize();
641 }
642
643 /**
644 * Double the capacity of the table.
645 */
646 private void resize() {
647 Entry[] oldTab = table;
648 int oldLen = oldTab.length;
649 int newLen = oldLen * 2;
650 Entry[] newTab = new Entry[newLen];
651 int count = 0;
652
653 for (int j = 0; j < oldLen; ++j) {
654 Entry e = oldTab[j];
655 if (e != null) {
656 ThreadLocal<?> k = e.get();
657 if (k == null) {
658 e.value = null; // Help the GC
659 } else {
660 int h = k.threadLocalHashCode & (newLen - 1);
661 while (newTab[h] != null)
662 h = nextIndex(h, newLen);
663 newTab[h] = e;
664 count++;
665 }
666 }
667 }
668
669 setThreshold(newLen);
670 size = count;
671 table = newTab;
672 }
673
674 /**
675 * Expunge all stale entries in the table.
676 */
677 private void expungeStaleEntries() {
678 Entry[] tab = table;
679 int len = tab.length;
680 for (int j = 0; j < len; j++) {
681 Entry e = tab[j];
682 if (e != null && e.get() == null)
683 expungeStaleEntry(j);
684 }
685 }
686 }
687 }