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 jdk.internal.misc.TerminatingThreadLocal;
  28 
  29 import java.lang.ref.*;
  30 import java.util.Objects;
  31 import java.util.concurrent.atomic.AtomicInteger;
  32 import java.util.function.Supplier;
  33 
  34 /**
  35  * This class provides thread-local variables.  These variables differ from
  36  * their normal counterparts in that each thread that accesses one (via its
  37  * {@code get} or {@code set} method) has its own, independently initialized
  38  * copy of the variable.  {@code ThreadLocal} instances are typically private
  39  * static fields in classes that wish to associate state with a thread (e.g.,
  40  * a user ID or Transaction ID).
  41  *
  42  * <p>For example, the class below generates unique identifiers local to each
  43  * thread.
  44  * A thread's id is assigned the first time it invokes {@code ThreadId.get()}
  45  * and remains unchanged on subsequent calls.
  46  * <pre>
  47  * import java.util.concurrent.atomic.AtomicInteger;
  48  *
  49  * public class ThreadId {
  50  *     // Atomic integer containing the next thread ID to be assigned
  51  *     private static final AtomicInteger nextId = new AtomicInteger(0);
  52  *
  53  *     // Thread local variable containing each thread's ID
  54  *     private static final ThreadLocal&lt;Integer&gt; threadId =
  55  *         new ThreadLocal&lt;Integer&gt;() {
  56  *             @Override protected Integer initialValue() {
  57  *                 return nextId.getAndIncrement();
  58  *         }
  59  *     };
  60  *
  61  *     // Returns the current thread's unique ID, assigning it if necessary
  62  *     public static int get() {
  63  *         return threadId.get();
  64  *     }
  65  * }
  66  * </pre>
  67  * <p>Each thread holds an implicit reference to its copy of a thread-local
  68  * variable as long as the thread is alive and the {@code ThreadLocal}
  69  * instance is accessible; after a thread goes away, all of its copies of
  70  * thread-local instances are subject to garbage collection (unless other
  71  * references to these copies exist).
  72  *
  73  * @author  Josh Bloch and Doug Lea
  74  * @since   1.2
  75  */
  76 public class ThreadLocal<T> {
  77     /**
  78      * ThreadLocals rely on per-thread linear-probe hash maps attached
  79      * to each thread (Thread.threadLocals and
  80      * inheritableThreadLocals).  The ThreadLocal objects act as keys,
  81      * searched via threadLocalHashCode.  This is a custom hash code
  82      * (useful only within ThreadLocalMaps) that eliminates collisions
  83      * in the common case where consecutively constructed ThreadLocals
  84      * are used by the same threads, while remaining well-behaved in
  85      * less common cases.
  86      */
  87     private final int threadLocalHashCode = nextHashCode();
  88 
  89     /**
  90      * The next hash code to be given out. Updated atomically. Starts at
  91      * zero.
  92      */
  93     private static AtomicInteger nextHashCode =
  94         new AtomicInteger();
  95 
  96     /**
  97      * The difference between successively generated hash codes - turns
  98      * implicit sequential thread-local IDs into near-optimally spread
  99      * multiplicative hash values for power-of-two-sized tables.
 100      */
 101     private static final int HASH_INCREMENT = 0x61c88647;
 102 
 103     /**
 104      * Returns the next hash code.
 105      */
 106     private static int nextHashCode() {
 107         return nextHashCode.getAndAdd(HASH_INCREMENT);
 108     }
 109 
 110     /**
 111      * Returns the current thread's "initial value" for this
 112      * thread-local variable.  This method will be invoked the first
 113      * time a thread accesses the variable with the {@link #get}
 114      * method, unless the thread previously invoked the {@link #set}
 115      * method, in which case the {@code initialValue} method will not
 116      * be invoked for the thread.  Normally, this method is invoked at
 117      * most once per thread, but it may be invoked again in case of
 118      * subsequent invocations of {@link #remove} followed by {@link #get}.
 119      *
 120      * <p>This implementation simply returns {@code null}; if the
 121      * programmer desires thread-local variables to have an initial
 122      * value other than {@code null}, {@code ThreadLocal} must be
 123      * subclassed, and this method overridden.  Typically, an
 124      * anonymous inner class will be used.
 125      *
 126      * @return the initial value for this thread-local
 127      */
 128     protected T initialValue() {
 129         return null;
 130     }
 131 
 132     /**
 133      * Creates a thread local variable. The initial value of the variable is
 134      * determined by invoking the {@code get} method on the {@code Supplier}.
 135      *
 136      * @param <S> the type of the thread local's value
 137      * @param supplier the supplier to be used to determine the initial value
 138      * @return a new thread local variable
 139      * @throws NullPointerException if the specified supplier is null
 140      * @since 1.8
 141      */
 142     public static <S> ThreadLocal<S> withInitial(Supplier<? extends S> supplier) {
 143         return new SuppliedThreadLocal<>(supplier);
 144     }
 145 
 146     /**
 147      * Creates a thread local variable.
 148      * @see #withInitial(java.util.function.Supplier)
 149      */
 150     public ThreadLocal() {
 151     }
 152 
 153     /**
 154      * Returns the value in the current thread's copy of this
 155      * thread-local variable.  If the variable has no value for the
 156      * current thread, it is first initialized to the value returned
 157      * by an invocation of the {@link #initialValue} method.
 158      *
 159      * @return the current thread's value of this thread-local
 160      */
 161     public T get() {
 162         Thread t = Thread.currentThread();
 163         ThreadLocalMap map = getMap(t);
 164         if (map != null) {
 165             ThreadLocalMap.Entry e = map.getEntry(this);
 166             if (e != null) {
 167                 @SuppressWarnings("unchecked")
 168                 T result = (T)e.value;
 169                 return result;
 170             }
 171         }
 172         return setInitialValue();
 173     }
 174 
 175     /**
 176      * Returns {@code true} if there is a value in the current thread's copy of
 177      * this thread-local variable, even if that values is {@code null}.
 178      *
 179      * @return {@code true} if current thread has associated value in this
 180      *         thread-local variable; {@code false} if not
 181      */
 182     boolean isPresent() {
 183         Thread t = Thread.currentThread();
 184         ThreadLocalMap map = getMap(t);
 185         return map != null && map.getEntry(this) != null;
 186     }
 187 
 188     /**
 189      * Variant of set() to establish initialValue. Used instead
 190      * of set() in case user has overridden the set() method.
 191      *
 192      * @return the initial value
 193      */
 194     private T setInitialValue() {
 195         T value = initialValue();
 196         Thread t = Thread.currentThread();
 197         ThreadLocalMap map = getMap(t);
 198         if (map != null) {
 199             map.set(this, value);
 200         } else {
 201             createMap(t, value);
 202         }
 203         if (this instanceof TerminatingThreadLocal) {
 204             TerminatingThreadLocal.register((TerminatingThreadLocal<?>) this);
 205         }
 206         return value;
 207     }
 208 
 209     /**
 210      * Sets the current thread's copy of this thread-local variable
 211      * to the specified value.  Most subclasses will have no need to
 212      * override this method, relying solely on the {@link #initialValue}
 213      * method to set the values of thread-locals.
 214      *
 215      * @param value the value to be stored in the current thread's copy of
 216      *        this thread-local.
 217      */
 218     public void set(T value) {
 219         Thread t = Thread.currentThread();
 220         ThreadLocalMap map = getMap(t);
 221         if (map != null) {
 222             map.set(this, value);
 223         } else {
 224             createMap(t, value);
 225         }
 226     }
 227 
 228     /**
 229      * Removes the current thread's value for this thread-local
 230      * variable.  If this thread-local variable is subsequently
 231      * {@linkplain #get read} by the current thread, its value will be
 232      * reinitialized by invoking its {@link #initialValue} method,
 233      * unless its value is {@linkplain #set set} by the current thread
 234      * in the interim.  This may result in multiple invocations of the
 235      * {@code initialValue} method in the current thread.
 236      *
 237      * @since 1.5
 238      */
 239      public void remove() {
 240          ThreadLocalMap m = getMap(Thread.currentThread());
 241          if (m != null) {
 242              m.remove(this);
 243          }
 244      }
 245 
 246     /**
 247      * Get the map associated with a ThreadLocal. Overridden in
 248      * InheritableThreadLocal.
 249      *
 250      * @param  t the current thread
 251      * @return the map
 252      */
 253     ThreadLocalMap getMap(Thread t) {
 254         return t.threadLocals;
 255     }
 256 
 257     /**
 258      * Create the map associated with a ThreadLocal. Overridden in
 259      * InheritableThreadLocal.
 260      *
 261      * @param t the current thread
 262      * @param firstValue value for the initial entry of the map
 263      */
 264     void createMap(Thread t, T firstValue) {
 265         t.threadLocals = new ThreadLocalMap(this, firstValue);
 266     }
 267 
 268     /**
 269      * Factory method to create map of inherited thread locals.
 270      * Designed to be called only from Thread constructor.
 271      *
 272      * @param  parentMap the map associated with parent thread
 273      * @return a map containing the parent's inheritable bindings
 274      */
 275     static ThreadLocalMap createInheritedMap(ThreadLocalMap parentMap) {
 276         return new ThreadLocalMap(parentMap);
 277     }
 278 
 279     /**
 280      * Method childValue is visibly defined in subclass
 281      * InheritableThreadLocal, but is internally defined here for the
 282      * sake of providing createInheritedMap factory method without
 283      * needing to subclass the map class in InheritableThreadLocal.
 284      * This technique is preferable to the alternative of embedding
 285      * instanceof tests in methods.
 286      */
 287     T childValue(T parentValue) {
 288         throw new UnsupportedOperationException();
 289     }
 290 
 291     /**
 292      * An extension of ThreadLocal that obtains its initial value from
 293      * the specified {@code Supplier}.
 294      */
 295     static final class SuppliedThreadLocal<T> extends ThreadLocal<T> {
 296 
 297         private final Supplier<? extends T> supplier;
 298 
 299         SuppliedThreadLocal(Supplier<? extends T> supplier) {
 300             this.supplier = Objects.requireNonNull(supplier);
 301         }
 302 
 303         @Override
 304         protected T initialValue() {
 305             return supplier.get();
 306         }
 307     }
 308 
 309     /**
 310      * ThreadLocalMap is a customized hash map suitable only for
 311      * maintaining thread local values. No operations are exported
 312      * outside of the ThreadLocal class. The class is package private to
 313      * allow declaration of fields in class Thread.  To help deal with
 314      * very large and long-lived usages, the hash table entries use
 315      * WeakReferences for keys. However, since reference queues are not
 316      * used, stale entries are guaranteed to be removed only when
 317      * the table starts running out of space.
 318      */
 319     static class ThreadLocalMap {
 320 
 321         /**
 322          * The entries in this hash map extend WeakReference, using
 323          * its main ref field as the key (which is always a
 324          * ThreadLocal object).  Note that null keys (i.e. entry.get()
 325          * == null) mean that the key is no longer referenced, so the
 326          * entry can be expunged from table.  Such entries are referred to
 327          * as "stale entries" in the code that follows.
 328          */
 329         static class Entry extends WeakReference<ThreadLocal<?>> {
 330             /** The value associated with this ThreadLocal. */
 331             Object value;
 332 
 333             Entry(ThreadLocal<?> k, Object v) {
 334                 super(k);
 335                 value = v;
 336             }
 337         }
 338 
 339         /**
 340          * The initial capacity -- MUST be a power of two.
 341          */
 342         private static final int INITIAL_CAPACITY = 16;
 343 
 344         /**
 345          * The table, resized as necessary.
 346          * table.length MUST always be a power of two.
 347          */
 348         private Entry[] table;
 349 
 350         /**
 351          * The number of entries in the table.
 352          */
 353         private int size = 0;
 354 
 355         /**
 356          * The next size value at which to resize.
 357          */
 358         private int threshold; // Default to 0
 359 
 360         /**
 361          * Set the resize threshold to maintain at worst a 2/3 load factor.
 362          */
 363         private void setThreshold(int len) {
 364             threshold = len * 2 / 3;
 365         }
 366 
 367         /**
 368          * Increment i modulo len.
 369          */
 370         private static int nextIndex(int i, int len) {
 371             return ((i + 1 < len) ? i + 1 : 0);
 372         }
 373 
 374         /**
 375          * Decrement i modulo len.
 376          */
 377         private static int prevIndex(int i, int len) {
 378             return ((i - 1 >= 0) ? i - 1 : len - 1);
 379         }
 380 
 381         /**
 382          * Construct a new map initially containing (firstKey, firstValue).
 383          * ThreadLocalMaps are constructed lazily, so we only create
 384          * one when we have at least one entry to put in it.
 385          */
 386         ThreadLocalMap(ThreadLocal<?> firstKey, Object firstValue) {
 387             table = new Entry[INITIAL_CAPACITY];
 388             int i = firstKey.threadLocalHashCode & (INITIAL_CAPACITY - 1);
 389             table[i] = new Entry(firstKey, firstValue);
 390             size = 1;
 391             setThreshold(INITIAL_CAPACITY);
 392         }
 393 
 394         /**
 395          * Construct a new map including all Inheritable ThreadLocals
 396          * from given parent map. Called only by createInheritedMap.
 397          *
 398          * @param parentMap the map associated with parent thread.
 399          */
 400         private ThreadLocalMap(ThreadLocalMap parentMap) {
 401             Entry[] parentTable = parentMap.table;
 402             int len = parentTable.length;
 403             setThreshold(len);
 404             table = new Entry[len];
 405 
 406             for (Entry e : parentTable) {
 407                 if (e != null) {
 408                     @SuppressWarnings("unchecked")
 409                     ThreadLocal<Object> key = (ThreadLocal<Object>) e.get();
 410                     if (key != null) {
 411                         Object value = key.childValue(e.value);
 412                         Entry c = new Entry(key, value);
 413                         int h = key.threadLocalHashCode & (len - 1);
 414                         while (table[h] != null)
 415                             h = nextIndex(h, len);
 416                         table[h] = c;
 417                         size++;
 418                     }
 419                 }
 420             }
 421         }
 422 
 423         /**
 424          * Get the entry associated with key.  This method
 425          * itself handles only the fast path: a direct hit of existing
 426          * key. It otherwise relays to getEntryAfterMiss.  This is
 427          * designed to maximize performance for direct hits, in part
 428          * by making this method readily inlinable.
 429          *
 430          * @param  key the thread local object
 431          * @return the entry associated with key, or null if no such
 432          */
 433         private Entry getEntry(ThreadLocal<?> key) {
 434             int i = key.threadLocalHashCode & (table.length - 1);
 435             Entry e = table[i];
 436             if (e != null && e.get() == key)
 437                 return e;
 438             else
 439                 return getEntryAfterMiss(key, i, e);
 440         }
 441 
 442         /**
 443          * Version of getEntry method for use when key is not found in
 444          * its direct hash slot.
 445          *
 446          * @param  key the thread local object
 447          * @param  i the table index for key's hash code
 448          * @param  e the entry at table[i]
 449          * @return the entry associated with key, or null if no such
 450          */
 451         private Entry getEntryAfterMiss(ThreadLocal<?> key, int i, Entry e) {
 452             Entry[] tab = table;
 453             int len = tab.length;
 454 
 455             while (e != null) {
 456                 ThreadLocal<?> k = e.get();
 457                 if (k == key)
 458                     return e;
 459                 if (k == null)
 460                     expungeStaleEntry(i);
 461                 else
 462                     i = nextIndex(i, len);
 463                 e = tab[i];
 464             }
 465             return null;
 466         }
 467 
 468         /**
 469          * Set the value associated with key.
 470          *
 471          * @param key the thread local object
 472          * @param value the value to be set
 473          */
 474         private void set(ThreadLocal<?> key, Object value) {
 475 
 476             // We don't use a fast path as with get() because it is at
 477             // least as common to use set() to create new entries as
 478             // it is to replace existing ones, in which case, a fast
 479             // path would fail more often than not.
 480 
 481             Entry[] tab = table;
 482             int len = tab.length;
 483             int i = key.threadLocalHashCode & (len-1);
 484 
 485             for (Entry e = tab[i];
 486                  e != null;
 487                  e = tab[i = nextIndex(i, len)]) {
 488                 ThreadLocal<?> k = e.get();
 489 
 490                 if (k == key) {
 491                     e.value = value;
 492                     return;
 493                 }
 494 
 495                 if (k == null) {
 496                     replaceStaleEntry(key, value, i);
 497                     return;
 498                 }
 499             }
 500 
 501             tab[i] = new Entry(key, value);
 502             int sz = ++size;
 503             if (!cleanSomeSlots(i, sz) && sz >= threshold)
 504                 rehash();
 505         }
 506 
 507         /**
 508          * Remove the entry for key.
 509          */
 510         private void remove(ThreadLocal<?> key) {
 511             Entry[] tab = table;
 512             int len = tab.length;
 513             int i = key.threadLocalHashCode & (len-1);
 514             for (Entry e = tab[i];
 515                  e != null;
 516                  e = tab[i = nextIndex(i, len)]) {
 517                 if (e.get() == key) {
 518                     e.clear();
 519                     expungeStaleEntry(i);
 520                     return;
 521                 }
 522             }
 523         }
 524 
 525         /**
 526          * Replace a stale entry encountered during a set operation
 527          * with an entry for the specified key.  The value passed in
 528          * the value parameter is stored in the entry, whether or not
 529          * an entry already exists for the specified key.
 530          *
 531          * As a side effect, this method expunges all stale entries in the
 532          * "run" containing the stale entry.  (A run is a sequence of entries
 533          * between two null slots.)
 534          *
 535          * @param  key the key
 536          * @param  value the value to be associated with key
 537          * @param  staleSlot index of the first stale entry encountered while
 538          *         searching for key.
 539          */
 540         private void replaceStaleEntry(ThreadLocal<?> key, Object value,
 541                                        int staleSlot) {
 542             Entry[] tab = table;
 543             int len = tab.length;
 544             Entry e;
 545 
 546             // Back up to check for prior stale entry in current run.
 547             // We clean out whole runs at a time to avoid continual
 548             // incremental rehashing due to garbage collector freeing
 549             // up refs in bunches (i.e., whenever the collector runs).
 550             int slotToExpunge = staleSlot;
 551             for (int i = prevIndex(staleSlot, len);
 552                  (e = tab[i]) != null;
 553                  i = prevIndex(i, len))
 554                 if (e.get() == null)
 555                     slotToExpunge = i;
 556 
 557             // Find either the key or trailing null slot of run, whichever
 558             // occurs first
 559             for (int i = nextIndex(staleSlot, len);
 560                  (e = tab[i]) != null;
 561                  i = nextIndex(i, len)) {
 562                 ThreadLocal<?> k = e.get();
 563 
 564                 // If we find key, then we need to swap it
 565                 // with the stale entry to maintain hash table order.
 566                 // The newly stale slot, or any other stale slot
 567                 // encountered above it, can then be sent to expungeStaleEntry
 568                 // to remove or rehash all of the other entries in run.
 569                 if (k == key) {
 570                     e.value = value;
 571 
 572                     tab[i] = tab[staleSlot];
 573                     tab[staleSlot] = e;
 574 
 575                     // Start expunge at preceding stale entry if it exists
 576                     if (slotToExpunge == staleSlot)
 577                         slotToExpunge = i;
 578                     cleanSomeSlots(expungeStaleEntry(slotToExpunge), len);
 579                     return;
 580                 }
 581 
 582                 // If we didn't find stale entry on backward scan, the
 583                 // first stale entry seen while scanning for key is the
 584                 // first still present in the run.
 585                 if (k == null && slotToExpunge == staleSlot)
 586                     slotToExpunge = i;
 587             }
 588 
 589             // If key not found, put new entry in stale slot
 590             tab[staleSlot].value = null;
 591             tab[staleSlot] = new Entry(key, value);
 592 
 593             // If there are any other stale entries in run, expunge them
 594             if (slotToExpunge != staleSlot)
 595                 cleanSomeSlots(expungeStaleEntry(slotToExpunge), len);
 596         }
 597 
 598         /**
 599          * Expunge a stale entry by rehashing any possibly colliding entries
 600          * lying between staleSlot and the next null slot.  This also expunges
 601          * any other stale entries encountered before the trailing null.  See
 602          * Knuth, Section 6.4
 603          *
 604          * @param staleSlot index of slot known to have null key
 605          * @return the index of the next null slot after staleSlot
 606          * (all between staleSlot and this slot will have been checked
 607          * for expunging).
 608          */
 609         private int expungeStaleEntry(int staleSlot) {
 610             Entry[] tab = table;
 611             int len = tab.length;
 612 
 613             // expunge entry at staleSlot
 614             tab[staleSlot].value = null;
 615             tab[staleSlot] = null;
 616             size--;
 617 
 618             // Rehash until we encounter null
 619             Entry e;
 620             int i;
 621             for (i = nextIndex(staleSlot, len);
 622                  (e = tab[i]) != null;
 623                  i = nextIndex(i, len)) {
 624                 ThreadLocal<?> k = e.get();
 625                 if (k == null) {
 626                     e.value = null;
 627                     tab[i] = null;
 628                     size--;
 629                 } else {
 630                     int h = k.threadLocalHashCode & (len - 1);
 631                     if (h != i) {
 632                         tab[i] = null;
 633 
 634                         // Unlike Knuth 6.4 Algorithm R, we must scan until
 635                         // null because multiple entries could have been stale.
 636                         while (tab[h] != null)
 637                             h = nextIndex(h, len);
 638                         tab[h] = e;
 639                     }
 640                 }
 641             }
 642             return i;
 643         }
 644 
 645         /**
 646          * Heuristically scan some cells looking for stale entries.
 647          * This is invoked when either a new element is added, or
 648          * another stale one has been expunged. It performs a
 649          * logarithmic number of scans, as a balance between no
 650          * scanning (fast but retains garbage) and a number of scans
 651          * proportional to number of elements, that would find all
 652          * garbage but would cause some insertions to take O(n) time.
 653          *
 654          * @param i a position known NOT to hold a stale entry. The
 655          * scan starts at the element after i.
 656          *
 657          * @param n scan control: {@code log2(n)} cells are scanned,
 658          * unless a stale entry is found, in which case
 659          * {@code log2(table.length)-1} additional cells are scanned.
 660          * When called from insertions, this parameter is the number
 661          * of elements, but when from replaceStaleEntry, it is the
 662          * table length. (Note: all this could be changed to be either
 663          * more or less aggressive by weighting n instead of just
 664          * using straight log n. But this version is simple, fast, and
 665          * seems to work well.)
 666          *
 667          * @return true if any stale entries have been removed.
 668          */
 669         private boolean cleanSomeSlots(int i, int n) {
 670             boolean removed = false;
 671             Entry[] tab = table;
 672             int len = tab.length;
 673             do {
 674                 i = nextIndex(i, len);
 675                 Entry e = tab[i];
 676                 if (e != null && e.get() == null) {
 677                     n = len;
 678                     removed = true;
 679                     i = expungeStaleEntry(i);
 680                 }
 681             } while ( (n >>>= 1) != 0);
 682             return removed;
 683         }
 684 
 685         /**
 686          * Re-pack and/or re-size the table. First scan the entire
 687          * table removing stale entries. If this doesn't sufficiently
 688          * shrink the size of the table, double the table size.
 689          */
 690         private void rehash() {
 691             expungeStaleEntries();
 692 
 693             // Use lower threshold for doubling to avoid hysteresis
 694             if (size >= threshold - threshold / 4)
 695                 resize();
 696         }
 697 
 698         /**
 699          * Double the capacity of the table.
 700          */
 701         private void resize() {
 702             Entry[] oldTab = table;
 703             int oldLen = oldTab.length;
 704             int newLen = oldLen * 2;
 705             Entry[] newTab = new Entry[newLen];
 706             int count = 0;
 707 
 708             for (Entry e : oldTab) {
 709                 if (e != null) {
 710                     ThreadLocal<?> k = e.get();
 711                     if (k == null) {
 712                         e.value = null; // Help the GC
 713                     } else {
 714                         int h = k.threadLocalHashCode & (newLen - 1);
 715                         while (newTab[h] != null)
 716                             h = nextIndex(h, newLen);
 717                         newTab[h] = e;
 718                         count++;
 719                     }
 720                 }
 721             }
 722 
 723             setThreshold(newLen);
 724             size = count;
 725             table = newTab;
 726         }
 727 
 728         /**
 729          * Expunge all stale entries in the table.
 730          */
 731         private void expungeStaleEntries() {
 732             Entry[] tab = table;
 733             int len = tab.length;
 734             for (int j = 0; j < len; j++) {
 735                 Entry e = tab[j];
 736                 if (e != null && e.get() == null)
 737                     expungeStaleEntry(j);
 738             }
 739         }
 740     }
 741 }