001: /*
002: * Copyright 1997-2007 Sun Microsystems, Inc. All Rights Reserved.
003: * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
004: *
005: * This code is free software; you can redistribute it and/or modify it
006: * under the terms of the GNU General Public License version 2 only, as
007: * published by the Free Software Foundation. Sun designates this
008: * particular file as subject to the "Classpath" exception as provided
009: * by Sun in the LICENSE file that accompanied this code.
010: *
011: * This code is distributed in the hope that it will be useful, but WITHOUT
012: * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
013: * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
014: * version 2 for more details (a copy is included in the LICENSE file that
015: * accompanied this code).
016: *
017: * You should have received a copy of the GNU General Public License version
018: * 2 along with this work; if not, write to the Free Software Foundation,
019: * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
020: *
021: * Please contact Sun Microsystems, Inc., 4150 Network Circle, Santa Clara,
022: * CA 95054 USA or visit www.sun.com if you need additional information or
023: * have any questions.
024: */
025:
026: package java.lang;
027:
028: import java.lang.ref.*;
029: import java.util.concurrent.atomic.AtomicInteger;
030:
031: /**
032: * This class provides thread-local variables. These variables differ from
033: * their normal counterparts in that each thread that accesses one (via its
034: * <tt>get</tt> or <tt>set</tt> method) has its own, independently initialized
035: * copy of the variable. <tt>ThreadLocal</tt> instances are typically private
036: * static fields in classes that wish to associate state with a thread (e.g.,
037: * a user ID or Transaction ID).
038: *
039: * <p>For example, the class below generates unique identifiers local to each
040: * thread.
041: * A thread's id is assigned the first time it invokes <tt>ThreadId.get()</tt>
042: * and remains unchanged on subsequent calls.
043: * <pre>
044: * import java.util.concurrent.atomic.AtomicInteger;
045: *
046: * public class ThreadId {
047: * // Atomic integer containing the next thread ID to be assigned
048: * private static final AtomicInteger nextId = new AtomicInteger(0);
049: *
050: * // Thread local variable containing each thread's ID
051: * private static final ThreadLocal<Integer> threadId =
052: * new ThreadLocal<Integer>() {
053: * @Override protected Integer initialValue() {
054: * return nextId.getAndIncrement();
055: * }
056: * };
057: *
058: * // Returns the current thread's unique ID, assigning it if necessary
059: * public static int get() {
060: * return threadId.get();
061: * }
062: * }
063: * </pre>
064: * <p>Each thread holds an implicit reference to its copy of a thread-local
065: * variable as long as the thread is alive and the <tt>ThreadLocal</tt>
066: * instance is accessible; after a thread goes away, all of its copies of
067: * thread-local instances are subject to garbage collection (unless other
068: * references to these copies exist).
069: *
070: * @author Josh Bloch and Doug Lea
071: * @version 1.49, 05/05/07
072: * @since 1.2
073: */
074: public class ThreadLocal<T> {
075: /**
076: * ThreadLocals rely on per-thread linear-probe hash maps attached
077: * to each thread (Thread.threadLocals and
078: * inheritableThreadLocals). The ThreadLocal objects act as keys,
079: * searched via threadLocalHashCode. This is a custom hash code
080: * (useful only within ThreadLocalMaps) that eliminates collisions
081: * in the common case where consecutively constructed ThreadLocals
082: * are used by the same threads, while remaining well-behaved in
083: * less common cases.
084: */
085: private final int threadLocalHashCode = nextHashCode();
086:
087: /**
088: * The next hash code to be given out. Updated atomically. Starts at
089: * zero.
090: */
091: private static AtomicInteger nextHashCode = new AtomicInteger();
092:
093: /**
094: * The difference between successively generated hash codes - turns
095: * implicit sequential thread-local IDs into near-optimally spread
096: * multiplicative hash values for power-of-two-sized tables.
097: */
098: private static final int HASH_INCREMENT = 0x61c88647;
099:
100: /**
101: * Returns the next hash code.
102: */
103: private static int nextHashCode() {
104: return nextHashCode.getAndAdd(HASH_INCREMENT);
105: }
106:
107: /**
108: * Returns the current thread's "initial value" for this
109: * thread-local variable. This method will be invoked the first
110: * time a thread accesses the variable with the {@link #get}
111: * method, unless the thread previously invoked the {@link #set}
112: * method, in which case the <tt>initialValue</tt> method will not
113: * be invoked for the thread. Normally, this method is invoked at
114: * most once per thread, but it may be invoked again in case of
115: * subsequent invocations of {@link #remove} followed by {@link #get}.
116: *
117: * <p>This implementation simply returns <tt>null</tt>; if the
118: * programmer desires thread-local variables to have an initial
119: * value other than <tt>null</tt>, <tt>ThreadLocal</tt> must be
120: * subclassed, and this method overridden. Typically, an
121: * anonymous inner class will be used.
122: *
123: * @return the initial value for this thread-local
124: */
125: protected T initialValue() {
126: return null;
127: }
128:
129: /**
130: * Creates a thread local variable.
131: */
132: public ThreadLocal() {
133: }
134:
135: /**
136: * Returns the value in the current thread's copy of this
137: * thread-local variable. If the variable has no value for the
138: * current thread, it is first initialized to the value returned
139: * by an invocation of the {@link #initialValue} method.
140: *
141: * @return the current thread's value of this thread-local
142: */
143: public T get() {
144: Thread t = Thread.currentThread();
145: ThreadLocalMap map = getMap(t);
146: if (map != null) {
147: ThreadLocalMap.Entry e = map.getEntry(this );
148: if (e != null)
149: return (T) e.value;
150: }
151: return setInitialValue();
152: }
153:
154: /**
155: * Variant of set() to establish initialValue. Used instead
156: * of set() in case user has overridden the set() method.
157: *
158: * @return the initial value
159: */
160: private T setInitialValue() {
161: T value = initialValue();
162: Thread t = Thread.currentThread();
163: ThreadLocalMap map = getMap(t);
164: if (map != null)
165: map.set(this , value);
166: else
167: createMap(t, value);
168: return value;
169: }
170:
171: /**
172: * Sets the current thread's copy of this thread-local variable
173: * to the specified value. Most subclasses will have no need to
174: * override this method, relying solely on the {@link #initialValue}
175: * method to set the values of thread-locals.
176: *
177: * @param value the value to be stored in the current thread's copy of
178: * this thread-local.
179: */
180: public void set(T value) {
181: Thread t = Thread.currentThread();
182: ThreadLocalMap map = getMap(t);
183: if (map != null)
184: map.set(this , value);
185: else
186: createMap(t, value);
187: }
188:
189: /**
190: * Removes the current thread's value for this thread-local
191: * variable. If this thread-local variable is subsequently
192: * {@linkplain #get read} by the current thread, its value will be
193: * reinitialized by invoking its {@link #initialValue} method,
194: * unless its value is {@linkplain #set set} by the current thread
195: * in the interim. This may result in multiple invocations of the
196: * <tt>initialValue</tt> method in the current thread.
197: *
198: * @since 1.5
199: */
200: public void remove() {
201: ThreadLocalMap m = getMap(Thread.currentThread());
202: if (m != null)
203: m.remove(this );
204: }
205:
206: /**
207: * Get the map associated with a ThreadLocal. Overridden in
208: * InheritableThreadLocal.
209: *
210: * @param t the current thread
211: * @return the map
212: */
213: ThreadLocalMap getMap(Thread t) {
214: return t.threadLocals;
215: }
216:
217: /**
218: * Create the map associated with a ThreadLocal. Overridden in
219: * InheritableThreadLocal.
220: *
221: * @param t the current thread
222: * @param firstValue value for the initial entry of the map
223: * @param map the map to store.
224: */
225: void createMap(Thread t, T firstValue) {
226: t.threadLocals = new ThreadLocalMap(this , firstValue);
227: }
228:
229: /**
230: * Factory method to create map of inherited thread locals.
231: * Designed to be called only from Thread constructor.
232: *
233: * @param parentMap the map associated with parent thread
234: * @return a map containing the parent's inheritable bindings
235: */
236: static ThreadLocalMap createInheritedMap(ThreadLocalMap parentMap) {
237: return new ThreadLocalMap(parentMap);
238: }
239:
240: /**
241: * Method childValue is visibly defined in subclass
242: * InheritableThreadLocal, but is internally defined here for the
243: * sake of providing createInheritedMap factory method without
244: * needing to subclass the map class in InheritableThreadLocal.
245: * This technique is preferable to the alternative of embedding
246: * instanceof tests in methods.
247: */
248: T childValue(T parentValue) {
249: throw new UnsupportedOperationException();
250: }
251:
252: /**
253: * ThreadLocalMap is a customized hash map suitable only for
254: * maintaining thread local values. No operations are exported
255: * outside of the ThreadLocal class. The class is package private to
256: * allow declaration of fields in class Thread. To help deal with
257: * very large and long-lived usages, the hash table entries use
258: * WeakReferences for keys. However, since reference queues are not
259: * used, stale entries are guaranteed to be removed only when
260: * the table starts running out of space.
261: */
262: static class ThreadLocalMap {
263:
264: /**
265: * The entries in this hash map extend WeakReference, using
266: * its main ref field as the key (which is always a
267: * ThreadLocal object). Note that null keys (i.e. entry.get()
268: * == null) mean that the key is no longer referenced, so the
269: * entry can be expunged from table. Such entries are referred to
270: * as "stale entries" in the code that follows.
271: */
272: static class Entry extends WeakReference<ThreadLocal> {
273: /** The value associated with this ThreadLocal. */
274: Object value;
275:
276: Entry(ThreadLocal k, Object v) {
277: super (k);
278: value = v;
279: }
280: }
281:
282: /**
283: * The initial capacity -- MUST be a power of two.
284: */
285: private static final int INITIAL_CAPACITY = 16;
286:
287: /**
288: * The table, resized as necessary.
289: * table.length MUST always be a power of two.
290: */
291: private Entry[] table;
292:
293: /**
294: * The number of entries in the table.
295: */
296: private int size = 0;
297:
298: /**
299: * The next size value at which to resize.
300: */
301: private int threshold; // Default to 0
302:
303: /**
304: * Set the resize threshold to maintain at worst a 2/3 load factor.
305: */
306: private void setThreshold(int len) {
307: threshold = len * 2 / 3;
308: }
309:
310: /**
311: * Increment i modulo len.
312: */
313: private static int nextIndex(int i, int len) {
314: return ((i + 1 < len) ? i + 1 : 0);
315: }
316:
317: /**
318: * Decrement i modulo len.
319: */
320: private static int prevIndex(int i, int len) {
321: return ((i - 1 >= 0) ? i - 1 : len - 1);
322: }
323:
324: /**
325: * Construct a new map initially containing (firstKey, firstValue).
326: * ThreadLocalMaps are constructed lazily, so we only create
327: * one when we have at least one entry to put in it.
328: */
329: ThreadLocalMap(ThreadLocal firstKey, Object firstValue) {
330: table = new Entry[INITIAL_CAPACITY];
331: int i = firstKey.threadLocalHashCode
332: & (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: ThreadLocal key = e.get();
354: if (key != null) {
355: Object value = key.childValue(e.value);
356: Entry c = new Entry(key, value);
357: int h = key.threadLocalHashCode & (len - 1);
358: while (table[h] != null)
359: h = nextIndex(h, len);
360: table[h] = c;
361: size++;
362: }
363: }
364: }
365: }
366:
367: /**
368: * Get the entry associated with key. This method
369: * itself handles only the fast path: a direct hit of existing
370: * key. It otherwise relays to getEntryAfterMiss. This is
371: * designed to maximize performance for direct hits, in part
372: * by making this method readily inlinable.
373: *
374: * @param key the thread local object
375: * @return the entry associated with key, or null if no such
376: */
377: private Entry getEntry(ThreadLocal key) {
378: int i = key.threadLocalHashCode & (table.length - 1);
379: Entry e = table[i];
380: if (e != null && e.get() == key)
381: return e;
382: else
383: return getEntryAfterMiss(key, i, e);
384: }
385:
386: /**
387: * Version of getEntry method for use when key is not found in
388: * its direct hash slot.
389: *
390: * @param key the thread local object
391: * @param i the table index for key's hash code
392: * @param e the entry at table[i]
393: * @return the entry associated with key, or null if no such
394: */
395: private Entry getEntryAfterMiss(ThreadLocal key, int i, Entry e) {
396: Entry[] tab = table;
397: int len = tab.length;
398:
399: while (e != null) {
400: ThreadLocal k = e.get();
401: if (k == key)
402: return e;
403: if (k == null)
404: expungeStaleEntry(i);
405: else
406: i = nextIndex(i, len);
407: e = tab[i];
408: }
409: return null;
410: }
411:
412: /**
413: * Set the value associated with key.
414: *
415: * @param key the thread local object
416: * @param value the value to be set
417: */
418: private void set(ThreadLocal key, Object value) {
419:
420: // We don't use a fast path as with get() because it is at
421: // least as common to use set() to create new entries as
422: // it is to replace existing ones, in which case, a fast
423: // path would fail more often than not.
424:
425: Entry[] tab = table;
426: int len = tab.length;
427: int i = key.threadLocalHashCode & (len - 1);
428:
429: for (Entry e = tab[i]; e != null; e = tab[i = nextIndex(i,
430: len)]) {
431: ThreadLocal k = e.get();
432:
433: if (k == key) {
434: e.value = value;
435: return;
436: }
437:
438: if (k == null) {
439: replaceStaleEntry(key, value, i);
440: return;
441: }
442: }
443:
444: tab[i] = new Entry(key, value);
445: int sz = ++size;
446: if (!cleanSomeSlots(i, sz) && sz >= threshold)
447: rehash();
448: }
449:
450: /**
451: * Remove the entry for key.
452: */
453: private void remove(ThreadLocal key) {
454: Entry[] tab = table;
455: int len = tab.length;
456: int i = key.threadLocalHashCode & (len - 1);
457: for (Entry e = tab[i]; e != null; e = tab[i = nextIndex(i,
458: 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); (e = tab[i]) != null; i = prevIndex(
494: i, len))
495: if (e.get() == null)
496: slotToExpunge = i;
497:
498: // Find either the key or trailing null slot of run, whichever
499: // occurs first
500: for (int i = nextIndex(staleSlot, len); (e = tab[i]) != null; i = nextIndex(
501: i, len)) {
502: ThreadLocal k = e.get();
503:
504: // If we find key, then we need to swap it
505: // with the stale entry to maintain hash table order.
506: // The newly stale slot, or any other stale slot
507: // encountered above it, can then be sent to expungeStaleEntry
508: // to remove or rehash all of the other entries in run.
509: if (k == key) {
510: e.value = value;
511:
512: tab[i] = tab[staleSlot];
513: tab[staleSlot] = e;
514:
515: // Start expunge at preceding stale entry if it exists
516: if (slotToExpunge == staleSlot)
517: slotToExpunge = i;
518: cleanSomeSlots(expungeStaleEntry(slotToExpunge),
519: len);
520: return;
521: }
522:
523: // If we didn't find stale entry on backward scan, the
524: // first stale entry seen while scanning for key is the
525: // first still present in the run.
526: if (k == null && slotToExpunge == staleSlot)
527: slotToExpunge = i;
528: }
529:
530: // If key not found, put new entry in stale slot
531: tab[staleSlot].value = null;
532: tab[staleSlot] = new Entry(key, value);
533:
534: // If there are any other stale entries in run, expunge them
535: if (slotToExpunge != staleSlot)
536: cleanSomeSlots(expungeStaleEntry(slotToExpunge), len);
537: }
538:
539: /**
540: * Expunge a stale entry by rehashing any possibly colliding entries
541: * lying between staleSlot and the next null slot. This also expunges
542: * any other stale entries encountered before the trailing null. See
543: * Knuth, Section 6.4
544: *
545: * @param staleSlot index of slot known to have null key
546: * @return the index of the next null slot after staleSlot
547: * (all between staleSlot and this slot will have been checked
548: * for expunging).
549: */
550: private int expungeStaleEntry(int staleSlot) {
551: Entry[] tab = table;
552: int len = tab.length;
553:
554: // expunge entry at staleSlot
555: tab[staleSlot].value = null;
556: tab[staleSlot] = null;
557: size--;
558:
559: // Rehash until we encounter null
560: Entry e;
561: int i;
562: for (i = nextIndex(staleSlot, len); (e = tab[i]) != null; i = nextIndex(
563: i, len)) {
564: ThreadLocal k = e.get();
565: if (k == null) {
566: e.value = null;
567: tab[i] = null;
568: size--;
569: } else {
570: int h = k.threadLocalHashCode & (len - 1);
571: if (h != i) {
572: tab[i] = null;
573:
574: // Unlike Knuth 6.4 Algorithm R, we must scan until
575: // null because multiple entries could have been stale.
576: while (tab[h] != null)
577: h = nextIndex(h, len);
578: tab[h] = e;
579: }
580: }
581: }
582: return i;
583: }
584:
585: /**
586: * Heuristically scan some cells looking for stale entries.
587: * This is invoked when either a new element is added, or
588: * another stale one has been expunged. It performs a
589: * logarithmic number of scans, as a balance between no
590: * scanning (fast but retains garbage) and a number of scans
591: * proportional to number of elements, that would find all
592: * garbage but would cause some insertions to take O(n) time.
593: *
594: * @param i a position known NOT to hold a stale entry. The
595: * scan starts at the element after i.
596: *
597: * @param n scan control: <tt>log2(n)</tt> cells are scanned,
598: * unless a stale entry is found, in which case
599: * <tt>log2(table.length)-1</tt> additional cells are scanned.
600: * When called from insertions, this parameter is the number
601: * of elements, but when from replaceStaleEntry, it is the
602: * table length. (Note: all this could be changed to be either
603: * more or less aggressive by weighting n instead of just
604: * using straight log n. But this version is simple, fast, and
605: * seems to work well.)
606: *
607: * @return true if any stale entries have been removed.
608: */
609: private boolean cleanSomeSlots(int i, int n) {
610: boolean removed = false;
611: Entry[] tab = table;
612: int len = tab.length;
613: do {
614: i = nextIndex(i, len);
615: Entry e = tab[i];
616: if (e != null && e.get() == null) {
617: n = len;
618: removed = true;
619: i = expungeStaleEntry(i);
620: }
621: } while ((n >>>= 1) != 0);
622: return removed;
623: }
624:
625: /**
626: * Re-pack and/or re-size the table. First scan the entire
627: * table removing stale entries. If this doesn't sufficiently
628: * shrink the size of the table, double the table size.
629: */
630: private void rehash() {
631: expungeStaleEntries();
632:
633: // Use lower threshold for doubling to avoid hysteresis
634: if (size >= threshold - threshold / 4)
635: resize();
636: }
637:
638: /**
639: * Double the capacity of the table.
640: */
641: private void resize() {
642: Entry[] oldTab = table;
643: int oldLen = oldTab.length;
644: int newLen = oldLen * 2;
645: Entry[] newTab = new Entry[newLen];
646: int count = 0;
647:
648: for (int j = 0; j < oldLen; ++j) {
649: Entry e = oldTab[j];
650: if (e != null) {
651: ThreadLocal k = e.get();
652: if (k == null) {
653: e.value = null; // Help the GC
654: } else {
655: int h = k.threadLocalHashCode & (newLen - 1);
656: while (newTab[h] != null)
657: h = nextIndex(h, newLen);
658: newTab[h] = e;
659: count++;
660: }
661: }
662: }
663:
664: setThreshold(newLen);
665: size = count;
666: table = newTab;
667: }
668:
669: /**
670: * Expunge all stale entries in the table.
671: */
672: private void expungeStaleEntries() {
673: Entry[] tab = table;
674: int len = tab.length;
675: for (int j = 0; j < len; j++) {
676: Entry e = tab[j];
677: if (e != null && e.get() == null)
678: expungeStaleEntry(j);
679: }
680: }
681: }
682: }
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