1 /*
2 * Copyright 1997-2007 Sun Microsystems, Inc. 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. Sun designates this
8 * particular file as subject to the "Classpath" exception as provided
9 * by Sun 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 Sun Microsystems, Inc., 4150 Network Circle, Santa Clara,
22 * CA 95054 USA or visit www.sun.com if you need additional information or
23 * have any 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 return (T)e.value;
149 }
150 return setInitialValue();
151 }
152
153 /**
154 * Variant of set() to establish initialValue. Used instead
155 * of set() in case user has overridden the set() method.
156 *
157 * @return the initial value
158 */
159 private T setInitialValue() {
160 T value = initialValue();
161 Thread t = Thread.currentThread();
162 ThreadLocalMap map = getMap(t);
163 if (map != null)
164 map.set(this, value);
165 else
166 createMap(t, value);
167 return value;
168 }
169
170 /**
171 * Sets the current thread's copy of this thread-local variable
172 * to the specified value. Most subclasses will have no need to
173 * override this method, relying solely on the {@link #initialValue}
174 * method to set the values of thread-locals.
175 *
176 * @param value the value to be stored in the current thread's copy of
177 * this thread-local.
178 */
179 public void set(T value) {
180 Thread t = Thread.currentThread();
181 ThreadLocalMap map = getMap(t);
182 if (map != null)
183 map.set(this, value);
184 else
185 createMap(t, value);
186 }
187
188 /**
189 * Removes the current thread's value for this thread-local
190 * variable. If this thread-local variable is subsequently
191 * {@linkplain #get read} by the current thread, its value will be
192 * reinitialized by invoking its {@link #initialValue} method,
193 * unless its value is {@linkplain #set set} by the current thread
194 * in the interim. This may result in multiple invocations of the
195 * <tt>initialValue</tt> method in the current thread.
196 *
197 * @since 1.5
198 */
199 public void remove() {
200 ThreadLocalMap m = getMap(Thread.currentThread());
201 if (m != null)
202 m.remove(this);
203 }
204
205 /**
206 * Get the map associated with a ThreadLocal. Overridden in
207 * InheritableThreadLocal.
208 *
209 * @param t the current thread
210 * @return the map
211 */
212 ThreadLocalMap getMap(Thread t) {
213 return t.threadLocals;
214 }
215
216 /**
217 * Create the map associated with a ThreadLocal. Overridden in
218 * InheritableThreadLocal.
219 *
220 * @param t the current thread
221 * @param firstValue value for the initial entry of the map
222 * @param map the map to store.
223 */
224 void createMap(Thread t, T firstValue) {
225 t.threadLocals = new ThreadLocalMap(this, firstValue);
226 }
227
228 /**
229 * Factory method to create map of inherited thread locals.
230 * Designed to be called only from Thread constructor.
231 *
232 * @param parentMap the map associated with parent thread
233 * @return a map containing the parent's inheritable bindings
234 */
235 static ThreadLocalMap createInheritedMap(ThreadLocalMap parentMap) {
236 return new ThreadLocalMap(parentMap);
237 }
238
239 /**
240 * Method childValue is visibly defined in subclass
241 * InheritableThreadLocal, but is internally defined here for the
242 * sake of providing createInheritedMap factory method without
243 * needing to subclass the map class in InheritableThreadLocal.
244 * This technique is preferable to the alternative of embedding
245 * instanceof tests in methods.
246 */
247 T childValue(T parentValue) {
248 throw new UnsupportedOperationException();
249 }
250
251 /**
252 * ThreadLocalMap is a customized hash map suitable only for
253 * maintaining thread local values. No operations are exported
254 * outside of the ThreadLocal class. The class is package private to
255 * allow declaration of fields in class Thread. To help deal with
256 * very large and long-lived usages, the hash table entries use
257 * WeakReferences for keys. However, since reference queues are not
258 * used, stale entries are guaranteed to be removed only when
259 * the table starts running out of space.
260 */
261 static class ThreadLocalMap {
262
263 /**
264 * The entries in this hash map extend WeakReference, using
265 * its main ref field as the key (which is always a
266 * ThreadLocal object). Note that null keys (i.e. entry.get()
267 * == null) mean that the key is no longer referenced, so the
268 * entry can be expunged from table. Such entries are referred to
269 * as "stale entries" in the code that follows.
270 */
271 static class Entry extends WeakReference<ThreadLocal> {
272 /** The value associated with this ThreadLocal. */
273 Object value;
274
275 Entry(ThreadLocal k, Object v) {
276 super(k);
277 value = v;
278 }
279 }
280
281 /**
282 * The initial capacity -- MUST be a power of two.
283 */
284 private static final int INITIAL_CAPACITY = 16;
285
286 /**
287 * The table, resized as necessary.
288 * table.length MUST always be a power of two.
289 */
290 private Entry[] table;
291
292 /**
293 * The number of entries in the table.
294 */
295 private int size = 0;
296
297 /**
298 * The next size value at which to resize.
299 */
300 private int threshold; // Default to 0
301
302 /**
303 * Set the resize threshold to maintain at worst a 2/3 load factor.
304 */
305 private void setThreshold(int len) {
306 threshold = len * 2 / 3;
307 }
308
309 /**
310 * Increment i modulo len.
311 */
312 private static int nextIndex(int i, int len) {
313 return ((i + 1 < len) ? i + 1 : 0);
314 }
315
316 /**
317 * Decrement i modulo len.
318 */
319 private static int prevIndex(int i, int len) {
320 return ((i - 1 >= 0) ? i - 1 : len - 1);
321 }
322
323 /**
324 * Construct a new map initially containing (firstKey, firstValue).
325 * ThreadLocalMaps are constructed lazily, so we only create
326 * one when we have at least one entry to put in it.
327 */
328 ThreadLocalMap(ThreadLocal firstKey, Object firstValue) {
329 table = new Entry[INITIAL_CAPACITY];
330 int i = firstKey.threadLocalHashCode & (INITIAL_CAPACITY - 1);
331 table[i] = new Entry(firstKey, firstValue);
332 size = 1;
333 setThreshold(INITIAL_CAPACITY);
334 }
335
336 /**
337 * Construct a new map including all Inheritable ThreadLocals
338 * from given parent map. Called only by createInheritedMap.
339 *
340 * @param parentMap the map associated with parent thread.
341 */
342 private ThreadLocalMap(ThreadLocalMap parentMap) {
343 Entry[] parentTable = parentMap.table;
344 int len = parentTable.length;
345 setThreshold(len);
346 table = new Entry[len];
347
348 for (int j = 0; j < len; j++) {
349 Entry e = parentTable[j];
350 if (e != null) {
351 ThreadLocal key = e.get();
352 if (key != null) {
353 Object value = key.childValue(e.value);
354 Entry c = new Entry(key, value);
355 int h = key.threadLocalHashCode & (len - 1);
356 while (table[h] != null)
357 h = nextIndex(h, len);
358 table[h] = c;
359 size++;
360 }
361 }
362 }
363 }
364
365 /**
366 * Get the entry associated with key. This method
367 * itself handles only the fast path: a direct hit of existing
368 * key. It otherwise relays to getEntryAfterMiss. This is
369 * designed to maximize performance for direct hits, in part
370 * by making this method readily inlinable.
371 *
372 * @param key the thread local object
373 * @return the entry associated with key, or null if no such
374 */
375 private Entry getEntry(ThreadLocal key) {
376 int i = key.threadLocalHashCode & (table.length - 1);
377 Entry e = table[i];
378 if (e != null && e.get() == key)
379 return e;
380 else
381 return getEntryAfterMiss(key, i, e);
382 }
383
384 /**
385 * Version of getEntry method for use when key is not found in
386 * its direct hash slot.
387 *
388 * @param key the thread local object
389 * @param i the table index for key's hash code
390 * @param e the entry at table[i]
391 * @return the entry associated with key, or null if no such
392 */
393 private Entry getEntryAfterMiss(ThreadLocal key, int i, Entry e) {
394 Entry[] tab = table;
395 int len = tab.length;
396
397 while (e != null) {
398 ThreadLocal k = e.get();
399 if (k == key)
400 return e;
401 if (k == null)
402 expungeStaleEntry(i);
403 else
404 i = nextIndex(i, len);
405 e = tab[i];
406 }
407 return null;
408 }
409
410 /**
411 * Set the value associated with key.
412 *
413 * @param key the thread local object
414 * @param value the value to be set
415 */
416 private void set(ThreadLocal key, Object value) {
417
418 // We don't use a fast path as with get() because it is at
419 // least as common to use set() to create new entries as
420 // it is to replace existing ones, in which case, a fast
421 // path would fail more often than not.
422
423 Entry[] tab = table;
424 int len = tab.length;
425 int i = key.threadLocalHashCode & (len-1);
426
427 for (Entry e = tab[i];
428 e != null;
429 e = tab[i = nextIndex(i, len)]) {
430 ThreadLocal k = e.get();
431
432 if (k == key) {
433 e.value = value;
434 return;
435 }
436
437 if (k == null) {
438 replaceStaleEntry(key, value, i);
439 return;
440 }
441 }
442
443 tab[i] = new Entry(key, value);
444 int sz = ++size;
445 if (!cleanSomeSlots(i, sz) && sz >= threshold)
446 rehash();
447 }
448
449 /**
450 * Remove the entry for key.
451 */
452 private void remove(ThreadLocal key) {
453 Entry[] tab = table;
454 int len = tab.length;
455 int i = key.threadLocalHashCode & (len-1);
456 for (Entry e = tab[i];
457 e != null;
458 e = tab[i = nextIndex(i, len)]) {
459 if (e.get() == key) {
460 e.clear();
461 expungeStaleEntry(i);
462 return;
463 }
464 }
465 }
466
467 /**
468 * Replace a stale entry encountered during a set operation
469 * with an entry for the specified key. The value passed in
470 * the value parameter is stored in the entry, whether or not
471 * an entry already exists for the specified key.
472 *
473 * As a side effect, this method expunges all stale entries in the
474 * "run" containing the stale entry. (A run is a sequence of entries
475 * between two null slots.)
476 *
477 * @param key the key
478 * @param value the value to be associated with key
479 * @param staleSlot index of the first stale entry encountered while
480 * searching for key.
481 */
482 private void replaceStaleEntry(ThreadLocal key, Object value,
483 int staleSlot) {
484 Entry[] tab = table;
485 int len = tab.length;
486 Entry e;
487
488 // Back up to check for prior stale entry in current run.
489 // We clean out whole runs at a time to avoid continual
490 // incremental rehashing due to garbage collector freeing
491 // up refs in bunches (i.e., whenever the collector runs).
492 int slotToExpunge = staleSlot;
493 for (int i = prevIndex(staleSlot, len);
494 (e = tab[i]) != null;
495 i = prevIndex(i, len))
496 if (e.get() == null)
497 slotToExpunge = i;
498
499 // Find either the key or trailing null slot of run, whichever
500 // occurs first
501 for (int i = nextIndex(staleSlot, len);
502 (e = tab[i]) != null;
503 i = nextIndex(i, len)) {
504 ThreadLocal k = e.get();
505
506 // If we find key, then we need to swap it
507 // with the stale entry to maintain hash table order.
508 // The newly stale slot, or any other stale slot
509 // encountered above it, can then be sent to expungeStaleEntry
510 // to remove or rehash all of the other entries in run.
511 if (k == key) {
512 e.value = value;
513
514 tab[i] = tab[staleSlot];
515 tab[staleSlot] = e;
516
517 // Start expunge at preceding stale entry if it exists
518 if (slotToExpunge == staleSlot)
519 slotToExpunge = i;
520 cleanSomeSlots(expungeStaleEntry(slotToExpunge), len);
521 return;
522 }
523
524 // If we didn't find stale entry on backward scan, the
525 // first stale entry seen while scanning for key is the
526 // first still present in the run.
527 if (k == null && slotToExpunge == staleSlot)
528 slotToExpunge = i;
529 }
530
531 // If key not found, put new entry in stale slot
532 tab[staleSlot].value = null;
533 tab[staleSlot] = new Entry(key, value);
534
535 // If there are any other stale entries in run, expunge them
536 if (slotToExpunge != staleSlot)
537 cleanSomeSlots(expungeStaleEntry(slotToExpunge), len);
538 }
539
540 /**
541 * Expunge a stale entry by rehashing any possibly colliding entries
542 * lying between staleSlot and the next null slot. This also expunges
543 * any other stale entries encountered before the trailing null. See
544 * Knuth, Section 6.4
545 *
546 * @param staleSlot index of slot known to have null key
547 * @return the index of the next null slot after staleSlot
548 * (all between staleSlot and this slot will have been checked
549 * for expunging).
550 */
551 private int expungeStaleEntry(int staleSlot) {
552 Entry[] tab = table;
553 int len = tab.length;
554
555 // expunge entry at staleSlot
556 tab[staleSlot].value = null;
557 tab[staleSlot] = null;
558 size--;
559
560 // Rehash until we encounter null
561 Entry e;
562 int i;
563 for (i = nextIndex(staleSlot, len);
564 (e = tab[i]) != null;
565 i = nextIndex(i, len)) {
566 ThreadLocal k = e.get();
567 if (k == null) {
568 e.value = null;
569 tab[i] = null;
570 size--;
571 } else {
572 int h = k.threadLocalHashCode & (len - 1);
573 if (h != i) {
574 tab[i] = null;
575
576 // Unlike Knuth 6.4 Algorithm R, we must scan until
577 // null because multiple entries could have been stale.
578 while (tab[h] != null)
579 h = nextIndex(h, len);
580 tab[h] = e;
581 }
582 }
583 }
584 return i;
585 }
586
587 /**
588 * Heuristically scan some cells looking for stale entries.
589 * This is invoked when either a new element is added, or
590 * another stale one has been expunged. It performs a
591 * logarithmic number of scans, as a balance between no
592 * scanning (fast but retains garbage) and a number of scans
593 * proportional to number of elements, that would find all
594 * garbage but would cause some insertions to take O(n) time.
595 *
596 * @param i a position known NOT to hold a stale entry. The
597 * scan starts at the element after i.
598 *
599 * @param n scan control: <tt>log2(n)</tt> cells are scanned,
600 * unless a stale entry is found, in which case
601 * <tt>log2(table.length)-1</tt> additional cells are scanned.
602 * When called from insertions, this parameter is the number
603 * of elements, but when from replaceStaleEntry, it is the
604 * table length. (Note: all this could be changed to be either
605 * more or less aggressive by weighting n instead of just
606 * using straight log n. But this version is simple, fast, and
607 * seems to work well.)
608 *
609 * @return true if any stale entries have been removed.
610 */
611 private boolean cleanSomeSlots(int i, int n) {
612 boolean removed = false;
613 Entry[] tab = table;
614 int len = tab.length;
615 do {
616 i = nextIndex(i, len);
617 Entry e = tab[i];
618 if (e != null && e.get() == null) {
619 n = len;
620 removed = true;
621 i = expungeStaleEntry(i);
622 }
623 } while ( (n >>>= 1) != 0);
624 return removed;
625 }
626
627 /**
628 * Re-pack and/or re-size the table. First scan the entire
629 * table removing stale entries. If this doesn't sufficiently
630 * shrink the size of the table, double the table size.
631 */
632 private void rehash() {
633 expungeStaleEntries();
634
635 // Use lower threshold for doubling to avoid hysteresis
636 if (size >= threshold - threshold / 4)
637 resize();
638 }
639
640 /**
641 * Double the capacity of the table.
642 */
643 private void resize() {
644 Entry[] oldTab = table;
645 int oldLen = oldTab.length;
646 int newLen = oldLen * 2;
647 Entry[] newTab = new Entry[newLen];
648 int count = 0;
649
650 for (int j = 0; j < oldLen; ++j) {
651 Entry e = oldTab[j];
652 if (e != null) {
653 ThreadLocal k = e.get();
654 if (k == null) {
655 e.value = null; // Help the GC
656 } else {
657 int h = k.threadLocalHashCode & (newLen - 1);
658 while (newTab[h] != null)
659 h = nextIndex(h, newLen);
660 newTab[h] = e;
661 count++;
662 }
663 }
664 }
665
666 setThreshold(newLen);
667 size = count;
668 table = newTab;
669 }
670
671 /**
672 * Expunge all stale entries in the table.
673 */
674 private void expungeStaleEntries() {
675 Entry[] tab = table;
676 int len = tab.length;
677 for (int j = 0; j < len; j++) {
678 Entry e = tab[j];
679 if (e != null && e.get() == null)
680 expungeStaleEntry(j);
681 }
682 }
683 }
684 }
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