jtulach@1280: /*
jtulach@1280:  * Copyright (c) 1997, 2007, Oracle and/or its affiliates. All rights reserved.
jtulach@1280:  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
jtulach@1280:  *
jtulach@1280:  * This code is free software; you can redistribute it and/or modify it
jtulach@1280:  * under the terms of the GNU General Public License version 2 only, as
jtulach@1280:  * published by the Free Software Foundation.  Oracle designates this
jtulach@1280:  * particular file as subject to the "Classpath" exception as provided
jtulach@1280:  * by Oracle in the LICENSE file that accompanied this code.
jtulach@1280:  *
jtulach@1280:  * This code is distributed in the hope that it will be useful, but WITHOUT
jtulach@1280:  * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
jtulach@1280:  * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
jtulach@1280:  * version 2 for more details (a copy is included in the LICENSE file that
jtulach@1280:  * accompanied this code).
jtulach@1280:  *
jtulach@1280:  * You should have received a copy of the GNU General Public License version
jtulach@1280:  * 2 along with this work; if not, write to the Free Software Foundation,
jtulach@1280:  * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
jtulach@1280:  *
jtulach@1280:  * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
jtulach@1280:  * or visit www.oracle.com if you need additional information or have any
jtulach@1280:  * questions.
jtulach@1280:  */
jtulach@1280: 
jtulach@1280: package java.lang;
jtulach@1280: import java.lang.ref.*;
jtulach@1280: import java.util.concurrent.atomic.AtomicInteger;
jtulach@1280: 
jtulach@1280: /**
jtulach@1280:  * This class provides thread-local variables.  These variables differ from
jtulach@1280:  * their normal counterparts in that each thread that accesses one (via its
jtulach@1280:  * <tt>get</tt> or <tt>set</tt> method) has its own, independently initialized
jtulach@1280:  * copy of the variable.  <tt>ThreadLocal</tt> instances are typically private
jtulach@1280:  * static fields in classes that wish to associate state with a thread (e.g.,
jtulach@1280:  * a user ID or Transaction ID).
jtulach@1280:  *
jtulach@1280:  * <p>For example, the class below generates unique identifiers local to each
jtulach@1280:  * thread.
jtulach@1280:  * A thread's id is assigned the first time it invokes <tt>ThreadId.get()</tt>
jtulach@1280:  * and remains unchanged on subsequent calls.
jtulach@1280:  * <pre>
jtulach@1280:  * import java.util.concurrent.atomic.AtomicInteger;
jtulach@1280:  *
jtulach@1280:  * public class ThreadId {
jtulach@1280:  *     // Atomic integer containing the next thread ID to be assigned
jtulach@1280:  *     private static final AtomicInteger nextId = new AtomicInteger(0);
jtulach@1280:  *
jtulach@1280:  *     // Thread local variable containing each thread's ID
jtulach@1280:  *     private static final ThreadLocal&lt;Integer> threadId =
jtulach@1280:  *         new ThreadLocal&lt;Integer>() {
jtulach@1280:  *             &#64;Override protected Integer initialValue() {
jtulach@1280:  *                 return nextId.getAndIncrement();
jtulach@1280:  *         }
jtulach@1280:  *     };
jtulach@1280:  *
jtulach@1280:  *     // Returns the current thread's unique ID, assigning it if necessary
jtulach@1280:  *     public static int get() {
jtulach@1280:  *         return threadId.get();
jtulach@1280:  *     }
jtulach@1280:  * }
jtulach@1280:  * </pre>
jtulach@1280:  * <p>Each thread holds an implicit reference to its copy of a thread-local
jtulach@1280:  * variable as long as the thread is alive and the <tt>ThreadLocal</tt>
jtulach@1280:  * instance is accessible; after a thread goes away, all of its copies of
jtulach@1280:  * thread-local instances are subject to garbage collection (unless other
jtulach@1280:  * references to these copies exist).
jtulach@1280:  *
jtulach@1280:  * @author  Josh Bloch and Doug Lea
jtulach@1280:  * @since   1.2
jtulach@1280:  */
jtulach@1280: public class ThreadLocal<T> {
jtulach@1280:     /**
jtulach@1280:      * ThreadLocals rely on per-thread linear-probe hash maps attached
jtulach@1280:      * to each thread (Thread.threadLocals and
jtulach@1280:      * inheritableThreadLocals).  The ThreadLocal objects act as keys,
jtulach@1280:      * searched via threadLocalHashCode.  This is a custom hash code
jtulach@1280:      * (useful only within ThreadLocalMaps) that eliminates collisions
jtulach@1280:      * in the common case where consecutively constructed ThreadLocals
jtulach@1280:      * are used by the same threads, while remaining well-behaved in
jtulach@1280:      * less common cases.
jtulach@1280:      */
jtulach@1280:     private final int threadLocalHashCode = nextHashCode();
jtulach@1280: 
jtulach@1280:     /**
jtulach@1280:      * The next hash code to be given out. Updated atomically. Starts at
jtulach@1280:      * zero.
jtulach@1280:      */
jtulach@1280:     private static AtomicInteger nextHashCode =
jtulach@1280:         new AtomicInteger();
jtulach@1280: 
jtulach@1280:     /**
jtulach@1280:      * The difference between successively generated hash codes - turns
jtulach@1280:      * implicit sequential thread-local IDs into near-optimally spread
jtulach@1280:      * multiplicative hash values for power-of-two-sized tables.
jtulach@1280:      */
jtulach@1280:     private static final int HASH_INCREMENT = 0x61c88647;
jtulach@1280: 
jtulach@1280:     /**
jtulach@1280:      * Returns the next hash code.
jtulach@1280:      */
jtulach@1280:     private static int nextHashCode() {
jtulach@1280:         return nextHashCode.getAndAdd(HASH_INCREMENT);
jtulach@1280:     }
jtulach@1280: 
jtulach@1280:     /**
jtulach@1280:      * Returns the current thread's "initial value" for this
jtulach@1280:      * thread-local variable.  This method will be invoked the first
jtulach@1280:      * time a thread accesses the variable with the {@link #get}
jtulach@1280:      * method, unless the thread previously invoked the {@link #set}
jtulach@1280:      * method, in which case the <tt>initialValue</tt> method will not
jtulach@1280:      * be invoked for the thread.  Normally, this method is invoked at
jtulach@1280:      * most once per thread, but it may be invoked again in case of
jtulach@1280:      * subsequent invocations of {@link #remove} followed by {@link #get}.
jtulach@1280:      *
jtulach@1280:      * <p>This implementation simply returns <tt>null</tt>; if the
jtulach@1280:      * programmer desires thread-local variables to have an initial
jtulach@1280:      * value other than <tt>null</tt>, <tt>ThreadLocal</tt> must be
jtulach@1280:      * subclassed, and this method overridden.  Typically, an
jtulach@1280:      * anonymous inner class will be used.
jtulach@1280:      *
jtulach@1280:      * @return the initial value for this thread-local
jtulach@1280:      */
jtulach@1280:     protected T initialValue() {
jtulach@1280:         return null;
jtulach@1280:     }
jtulach@1280: 
jtulach@1280:     /**
jtulach@1280:      * Creates a thread local variable.
jtulach@1280:      */
jtulach@1280:     public ThreadLocal() {
jtulach@1280:     }
jtulach@1280: 
jtulach@1280:     /**
jtulach@1280:      * Returns the value in the current thread's copy of this
jtulach@1280:      * thread-local variable.  If the variable has no value for the
jtulach@1280:      * current thread, it is first initialized to the value returned
jtulach@1280:      * by an invocation of the {@link #initialValue} method.
jtulach@1280:      *
jtulach@1280:      * @return the current thread's value of this thread-local
jtulach@1280:      */
jtulach@1280:     public T get() {
jtulach@1280:         Thread t = Thread.currentThread();
jtulach@1280:         ThreadLocalMap map = getMap(t);
jtulach@1280:         if (map != null) {
jtulach@1280:             ThreadLocalMap.Entry e = map.getEntry(this);
jtulach@1280:             if (e != null)
jtulach@1280:                 return (T)e.value;
jtulach@1280:         }
jtulach@1280:         return setInitialValue();
jtulach@1280:     }
jtulach@1280: 
jtulach@1280:     /**
jtulach@1280:      * Variant of set() to establish initialValue. Used instead
jtulach@1280:      * of set() in case user has overridden the set() method.
jtulach@1280:      *
jtulach@1280:      * @return the initial value
jtulach@1280:      */
jtulach@1280:     private T setInitialValue() {
jtulach@1280:         T value = initialValue();
jtulach@1280:         Thread t = Thread.currentThread();
jtulach@1280:         ThreadLocalMap map = getMap(t);
jtulach@1280:         if (map != null)
jtulach@1280:             map.set(this, value);
jtulach@1280:         else
jtulach@1280:             createMap(t, value);
jtulach@1280:         return value;
jtulach@1280:     }
jtulach@1280: 
jtulach@1280:     /**
jtulach@1280:      * Sets the current thread's copy of this thread-local variable
jtulach@1280:      * to the specified value.  Most subclasses will have no need to
jtulach@1280:      * override this method, relying solely on the {@link #initialValue}
jtulach@1280:      * method to set the values of thread-locals.
jtulach@1280:      *
jtulach@1280:      * @param value the value to be stored in the current thread's copy of
jtulach@1280:      *        this thread-local.
jtulach@1280:      */
jtulach@1280:     public void set(T value) {
jtulach@1280:         Thread t = Thread.currentThread();
jtulach@1280:         ThreadLocalMap map = getMap(t);
jtulach@1280:         if (map != null)
jtulach@1280:             map.set(this, value);
jtulach@1280:         else
jtulach@1280:             createMap(t, value);
jtulach@1280:     }
jtulach@1280: 
jtulach@1280:     /**
jtulach@1280:      * Removes the current thread's value for this thread-local
jtulach@1280:      * variable.  If this thread-local variable is subsequently
jtulach@1280:      * {@linkplain #get read} by the current thread, its value will be
jtulach@1280:      * reinitialized by invoking its {@link #initialValue} method,
jtulach@1280:      * unless its value is {@linkplain #set set} by the current thread
jtulach@1280:      * in the interim.  This may result in multiple invocations of the
jtulach@1280:      * <tt>initialValue</tt> method in the current thread.
jtulach@1280:      *
jtulach@1280:      * @since 1.5
jtulach@1280:      */
jtulach@1280:      public void remove() {
jtulach@1280:          ThreadLocalMap m = getMap(Thread.currentThread());
jtulach@1280:          if (m != null)
jtulach@1280:              m.remove(this);
jtulach@1280:      }
jtulach@1280: 
jtulach@1280:     /**
jtulach@1280:      * Get the map associated with a ThreadLocal. Overridden in
jtulach@1280:      * InheritableThreadLocal.
jtulach@1280:      *
jtulach@1280:      * @param  t the current thread
jtulach@1280:      * @return the map
jtulach@1280:      */
jtulach@1280:     ThreadLocalMap getMap(Thread t) {
jtulach@1280:         return t.threadLocals;
jtulach@1280:     }
jtulach@1280: 
jtulach@1280:     /**
jtulach@1280:      * Create the map associated with a ThreadLocal. Overridden in
jtulach@1280:      * InheritableThreadLocal.
jtulach@1280:      *
jtulach@1280:      * @param t the current thread
jtulach@1280:      * @param firstValue value for the initial entry of the map
jtulach@1280:      * @param map the map to store.
jtulach@1280:      */
jtulach@1280:     void createMap(Thread t, T firstValue) {
jtulach@1280:         t.threadLocals = new ThreadLocalMap(this, firstValue);
jtulach@1280:     }
jtulach@1280: 
jtulach@1280:     /**
jtulach@1280:      * Factory method to create map of inherited thread locals.
jtulach@1280:      * Designed to be called only from Thread constructor.
jtulach@1280:      *
jtulach@1280:      * @param  parentMap the map associated with parent thread
jtulach@1280:      * @return a map containing the parent's inheritable bindings
jtulach@1280:      */
jtulach@1280:     static ThreadLocalMap createInheritedMap(ThreadLocalMap parentMap) {
jtulach@1280:         return new ThreadLocalMap(parentMap);
jtulach@1280:     }
jtulach@1280: 
jtulach@1280:     /**
jtulach@1280:      * Method childValue is visibly defined in subclass
jtulach@1280:      * InheritableThreadLocal, but is internally defined here for the
jtulach@1280:      * sake of providing createInheritedMap factory method without
jtulach@1280:      * needing to subclass the map class in InheritableThreadLocal.
jtulach@1280:      * This technique is preferable to the alternative of embedding
jtulach@1280:      * instanceof tests in methods.
jtulach@1280:      */
jtulach@1280:     T childValue(T parentValue) {
jtulach@1280:         throw new UnsupportedOperationException();
jtulach@1280:     }
jtulach@1280: 
jtulach@1280:     /**
jtulach@1280:      * ThreadLocalMap is a customized hash map suitable only for
jtulach@1280:      * maintaining thread local values. No operations are exported
jtulach@1280:      * outside of the ThreadLocal class. The class is package private to
jtulach@1280:      * allow declaration of fields in class Thread.  To help deal with
jtulach@1280:      * very large and long-lived usages, the hash table entries use
jtulach@1280:      * WeakReferences for keys. However, since reference queues are not
jtulach@1280:      * used, stale entries are guaranteed to be removed only when
jtulach@1280:      * the table starts running out of space.
jtulach@1280:      */
jtulach@1280:     static class ThreadLocalMap {
jtulach@1280: 
jtulach@1280:         /**
jtulach@1280:          * The entries in this hash map extend WeakReference, using
jtulach@1280:          * its main ref field as the key (which is always a
jtulach@1280:          * ThreadLocal object).  Note that null keys (i.e. entry.get()
jtulach@1280:          * == null) mean that the key is no longer referenced, so the
jtulach@1280:          * entry can be expunged from table.  Such entries are referred to
jtulach@1280:          * as "stale entries" in the code that follows.
jtulach@1280:          */
jtulach@1280:         static class Entry extends WeakReference<ThreadLocal> {
jtulach@1280:             /** The value associated with this ThreadLocal. */
jtulach@1280:             Object value;
jtulach@1280: 
jtulach@1280:             Entry(ThreadLocal k, Object v) {
jtulach@1280:                 super(k);
jtulach@1280:                 value = v;
jtulach@1280:             }
jtulach@1280:         }
jtulach@1280: 
jtulach@1280:         /**
jtulach@1280:          * The initial capacity -- MUST be a power of two.
jtulach@1280:          */
jtulach@1280:         private static final int INITIAL_CAPACITY = 16;
jtulach@1280: 
jtulach@1280:         /**
jtulach@1280:          * The table, resized as necessary.
jtulach@1280:          * table.length MUST always be a power of two.
jtulach@1280:          */
jtulach@1280:         private Entry[] table;
jtulach@1280: 
jtulach@1280:         /**
jtulach@1280:          * The number of entries in the table.
jtulach@1280:          */
jtulach@1280:         private int size = 0;
jtulach@1280: 
jtulach@1280:         /**
jtulach@1280:          * The next size value at which to resize.
jtulach@1280:          */
jtulach@1280:         private int threshold; // Default to 0
jtulach@1280: 
jtulach@1280:         /**
jtulach@1280:          * Set the resize threshold to maintain at worst a 2/3 load factor.
jtulach@1280:          */
jtulach@1280:         private void setThreshold(int len) {
jtulach@1280:             threshold = len * 2 / 3;
jtulach@1280:         }
jtulach@1280: 
jtulach@1280:         /**
jtulach@1280:          * Increment i modulo len.
jtulach@1280:          */
jtulach@1280:         private static int nextIndex(int i, int len) {
jtulach@1280:             return ((i + 1 < len) ? i + 1 : 0);
jtulach@1280:         }
jtulach@1280: 
jtulach@1280:         /**
jtulach@1280:          * Decrement i modulo len.
jtulach@1280:          */
jtulach@1280:         private static int prevIndex(int i, int len) {
jtulach@1280:             return ((i - 1 >= 0) ? i - 1 : len - 1);
jtulach@1280:         }
jtulach@1280: 
jtulach@1280:         /**
jtulach@1280:          * Construct a new map initially containing (firstKey, firstValue).
jtulach@1280:          * ThreadLocalMaps are constructed lazily, so we only create
jtulach@1280:          * one when we have at least one entry to put in it.
jtulach@1280:          */
jtulach@1280:         ThreadLocalMap(ThreadLocal firstKey, Object firstValue) {
jtulach@1280:             table = new Entry[INITIAL_CAPACITY];
jtulach@1280:             int i = firstKey.threadLocalHashCode & (INITIAL_CAPACITY - 1);
jtulach@1280:             table[i] = new Entry(firstKey, firstValue);
jtulach@1280:             size = 1;
jtulach@1280:             setThreshold(INITIAL_CAPACITY);
jtulach@1280:         }
jtulach@1280: 
jtulach@1280:         /**
jtulach@1280:          * Construct a new map including all Inheritable ThreadLocals
jtulach@1280:          * from given parent map. Called only by createInheritedMap.
jtulach@1280:          *
jtulach@1280:          * @param parentMap the map associated with parent thread.
jtulach@1280:          */
jtulach@1280:         private ThreadLocalMap(ThreadLocalMap parentMap) {
jtulach@1280:             Entry[] parentTable = parentMap.table;
jtulach@1280:             int len = parentTable.length;
jtulach@1280:             setThreshold(len);
jtulach@1280:             table = new Entry[len];
jtulach@1280: 
jtulach@1280:             for (int j = 0; j < len; j++) {
jtulach@1280:                 Entry e = parentTable[j];
jtulach@1280:                 if (e != null) {
jtulach@1280:                     ThreadLocal key = e.get();
jtulach@1280:                     if (key != null) {
jtulach@1280:                         Object value = key.childValue(e.value);
jtulach@1280:                         Entry c = new Entry(key, value);
jtulach@1280:                         int h = key.threadLocalHashCode & (len - 1);
jtulach@1280:                         while (table[h] != null)
jtulach@1280:                             h = nextIndex(h, len);
jtulach@1280:                         table[h] = c;
jtulach@1280:                         size++;
jtulach@1280:                     }
jtulach@1280:                 }
jtulach@1280:             }
jtulach@1280:         }
jtulach@1280: 
jtulach@1280:         /**
jtulach@1280:          * Get the entry associated with key.  This method
jtulach@1280:          * itself handles only the fast path: a direct hit of existing
jtulach@1280:          * key. It otherwise relays to getEntryAfterMiss.  This is
jtulach@1280:          * designed to maximize performance for direct hits, in part
jtulach@1280:          * by making this method readily inlinable.
jtulach@1280:          *
jtulach@1280:          * @param  key the thread local object
jtulach@1280:          * @return the entry associated with key, or null if no such
jtulach@1280:          */
jtulach@1280:         private Entry getEntry(ThreadLocal key) {
jtulach@1280:             int i = key.threadLocalHashCode & (table.length - 1);
jtulach@1280:             Entry e = table[i];
jtulach@1280:             if (e != null && e.get() == key)
jtulach@1280:                 return e;
jtulach@1280:             else
jtulach@1280:                 return getEntryAfterMiss(key, i, e);
jtulach@1280:         }
jtulach@1280: 
jtulach@1280:         /**
jtulach@1280:          * Version of getEntry method for use when key is not found in
jtulach@1280:          * its direct hash slot.
jtulach@1280:          *
jtulach@1280:          * @param  key the thread local object
jtulach@1280:          * @param  i the table index for key's hash code
jtulach@1280:          * @param  e the entry at table[i]
jtulach@1280:          * @return the entry associated with key, or null if no such
jtulach@1280:          */
jtulach@1280:         private Entry getEntryAfterMiss(ThreadLocal key, int i, Entry e) {
jtulach@1280:             Entry[] tab = table;
jtulach@1280:             int len = tab.length;
jtulach@1280: 
jtulach@1280:             while (e != null) {
jtulach@1280:                 ThreadLocal k = e.get();
jtulach@1280:                 if (k == key)
jtulach@1280:                     return e;
jtulach@1280:                 if (k == null)
jtulach@1280:                     expungeStaleEntry(i);
jtulach@1280:                 else
jtulach@1280:                     i = nextIndex(i, len);
jtulach@1280:                 e = tab[i];
jtulach@1280:             }
jtulach@1280:             return null;
jtulach@1280:         }
jtulach@1280: 
jtulach@1280:         /**
jtulach@1280:          * Set the value associated with key.
jtulach@1280:          *
jtulach@1280:          * @param key the thread local object
jtulach@1280:          * @param value the value to be set
jtulach@1280:          */
jtulach@1280:         private void set(ThreadLocal key, Object value) {
jtulach@1280: 
jtulach@1280:             // We don't use a fast path as with get() because it is at
jtulach@1280:             // least as common to use set() to create new entries as
jtulach@1280:             // it is to replace existing ones, in which case, a fast
jtulach@1280:             // path would fail more often than not.
jtulach@1280: 
jtulach@1280:             Entry[] tab = table;
jtulach@1280:             int len = tab.length;
jtulach@1280:             int i = key.threadLocalHashCode & (len-1);
jtulach@1280: 
jtulach@1280:             for (Entry e = tab[i];
jtulach@1280:                  e != null;
jtulach@1280:                  e = tab[i = nextIndex(i, len)]) {
jtulach@1280:                 ThreadLocal k = e.get();
jtulach@1280: 
jtulach@1280:                 if (k == key) {
jtulach@1280:                     e.value = value;
jtulach@1280:                     return;
jtulach@1280:                 }
jtulach@1280: 
jtulach@1280:                 if (k == null) {
jtulach@1280:                     replaceStaleEntry(key, value, i);
jtulach@1280:                     return;
jtulach@1280:                 }
jtulach@1280:             }
jtulach@1280: 
jtulach@1280:             tab[i] = new Entry(key, value);
jtulach@1280:             int sz = ++size;
jtulach@1280:             if (!cleanSomeSlots(i, sz) && sz >= threshold)
jtulach@1280:                 rehash();
jtulach@1280:         }
jtulach@1280: 
jtulach@1280:         /**
jtulach@1280:          * Remove the entry for key.
jtulach@1280:          */
jtulach@1280:         private void remove(ThreadLocal key) {
jtulach@1280:             Entry[] tab = table;
jtulach@1280:             int len = tab.length;
jtulach@1280:             int i = key.threadLocalHashCode & (len-1);
jtulach@1280:             for (Entry e = tab[i];
jtulach@1280:                  e != null;
jtulach@1280:                  e = tab[i = nextIndex(i, len)]) {
jtulach@1280:                 if (e.get() == key) {
jtulach@1280:                     e.clear();
jtulach@1280:                     expungeStaleEntry(i);
jtulach@1280:                     return;
jtulach@1280:                 }
jtulach@1280:             }
jtulach@1280:         }
jtulach@1280: 
jtulach@1280:         /**
jtulach@1280:          * Replace a stale entry encountered during a set operation
jtulach@1280:          * with an entry for the specified key.  The value passed in
jtulach@1280:          * the value parameter is stored in the entry, whether or not
jtulach@1280:          * an entry already exists for the specified key.
jtulach@1280:          *
jtulach@1280:          * As a side effect, this method expunges all stale entries in the
jtulach@1280:          * "run" containing the stale entry.  (A run is a sequence of entries
jtulach@1280:          * between two null slots.)
jtulach@1280:          *
jtulach@1280:          * @param  key the key
jtulach@1280:          * @param  value the value to be associated with key
jtulach@1280:          * @param  staleSlot index of the first stale entry encountered while
jtulach@1280:          *         searching for key.
jtulach@1280:          */
jtulach@1280:         private void replaceStaleEntry(ThreadLocal key, Object value,
jtulach@1280:                                        int staleSlot) {
jtulach@1280:             Entry[] tab = table;
jtulach@1280:             int len = tab.length;
jtulach@1280:             Entry e;
jtulach@1280: 
jtulach@1280:             // Back up to check for prior stale entry in current run.
jtulach@1280:             // We clean out whole runs at a time to avoid continual
jtulach@1280:             // incremental rehashing due to garbage collector freeing
jtulach@1280:             // up refs in bunches (i.e., whenever the collector runs).
jtulach@1280:             int slotToExpunge = staleSlot;
jtulach@1280:             for (int i = prevIndex(staleSlot, len);
jtulach@1280:                  (e = tab[i]) != null;
jtulach@1280:                  i = prevIndex(i, len))
jtulach@1280:                 if (e.get() == null)
jtulach@1280:                     slotToExpunge = i;
jtulach@1280: 
jtulach@1280:             // Find either the key or trailing null slot of run, whichever
jtulach@1280:             // occurs first
jtulach@1280:             for (int i = nextIndex(staleSlot, len);
jtulach@1280:                  (e = tab[i]) != null;
jtulach@1280:                  i = nextIndex(i, len)) {
jtulach@1280:                 ThreadLocal k = e.get();
jtulach@1280: 
jtulach@1280:                 // If we find key, then we need to swap it
jtulach@1280:                 // with the stale entry to maintain hash table order.
jtulach@1280:                 // The newly stale slot, or any other stale slot
jtulach@1280:                 // encountered above it, can then be sent to expungeStaleEntry
jtulach@1280:                 // to remove or rehash all of the other entries in run.
jtulach@1280:                 if (k == key) {
jtulach@1280:                     e.value = value;
jtulach@1280: 
jtulach@1280:                     tab[i] = tab[staleSlot];
jtulach@1280:                     tab[staleSlot] = e;
jtulach@1280: 
jtulach@1280:                     // Start expunge at preceding stale entry if it exists
jtulach@1280:                     if (slotToExpunge == staleSlot)
jtulach@1280:                         slotToExpunge = i;
jtulach@1280:                     cleanSomeSlots(expungeStaleEntry(slotToExpunge), len);
jtulach@1280:                     return;
jtulach@1280:                 }
jtulach@1280: 
jtulach@1280:                 // If we didn't find stale entry on backward scan, the
jtulach@1280:                 // first stale entry seen while scanning for key is the
jtulach@1280:                 // first still present in the run.
jtulach@1280:                 if (k == null && slotToExpunge == staleSlot)
jtulach@1280:                     slotToExpunge = i;
jtulach@1280:             }
jtulach@1280: 
jtulach@1280:             // If key not found, put new entry in stale slot
jtulach@1280:             tab[staleSlot].value = null;
jtulach@1280:             tab[staleSlot] = new Entry(key, value);
jtulach@1280: 
jtulach@1280:             // If there are any other stale entries in run, expunge them
jtulach@1280:             if (slotToExpunge != staleSlot)
jtulach@1280:                 cleanSomeSlots(expungeStaleEntry(slotToExpunge), len);
jtulach@1280:         }
jtulach@1280: 
jtulach@1280:         /**
jtulach@1280:          * Expunge a stale entry by rehashing any possibly colliding entries
jtulach@1280:          * lying between staleSlot and the next null slot.  This also expunges
jtulach@1280:          * any other stale entries encountered before the trailing null.  See
jtulach@1280:          * Knuth, Section 6.4
jtulach@1280:          *
jtulach@1280:          * @param staleSlot index of slot known to have null key
jtulach@1280:          * @return the index of the next null slot after staleSlot
jtulach@1280:          * (all between staleSlot and this slot will have been checked
jtulach@1280:          * for expunging).
jtulach@1280:          */
jtulach@1280:         private int expungeStaleEntry(int staleSlot) {
jtulach@1280:             Entry[] tab = table;
jtulach@1280:             int len = tab.length;
jtulach@1280: 
jtulach@1280:             // expunge entry at staleSlot
jtulach@1280:             tab[staleSlot].value = null;
jtulach@1280:             tab[staleSlot] = null;
jtulach@1280:             size--;
jtulach@1280: 
jtulach@1280:             // Rehash until we encounter null
jtulach@1280:             Entry e;
jtulach@1280:             int i;
jtulach@1280:             for (i = nextIndex(staleSlot, len);
jtulach@1280:                  (e = tab[i]) != null;
jtulach@1280:                  i = nextIndex(i, len)) {
jtulach@1280:                 ThreadLocal k = e.get();
jtulach@1280:                 if (k == null) {
jtulach@1280:                     e.value = null;
jtulach@1280:                     tab[i] = null;
jtulach@1280:                     size--;
jtulach@1280:                 } else {
jtulach@1280:                     int h = k.threadLocalHashCode & (len - 1);
jtulach@1280:                     if (h != i) {
jtulach@1280:                         tab[i] = null;
jtulach@1280: 
jtulach@1280:                         // Unlike Knuth 6.4 Algorithm R, we must scan until
jtulach@1280:                         // null because multiple entries could have been stale.
jtulach@1280:                         while (tab[h] != null)
jtulach@1280:                             h = nextIndex(h, len);
jtulach@1280:                         tab[h] = e;
jtulach@1280:                     }
jtulach@1280:                 }
jtulach@1280:             }
jtulach@1280:             return i;
jtulach@1280:         }
jtulach@1280: 
jtulach@1280:         /**
jtulach@1280:          * Heuristically scan some cells looking for stale entries.
jtulach@1280:          * This is invoked when either a new element is added, or
jtulach@1280:          * another stale one has been expunged. It performs a
jtulach@1280:          * logarithmic number of scans, as a balance between no
jtulach@1280:          * scanning (fast but retains garbage) and a number of scans
jtulach@1280:          * proportional to number of elements, that would find all
jtulach@1280:          * garbage but would cause some insertions to take O(n) time.
jtulach@1280:          *
jtulach@1280:          * @param i a position known NOT to hold a stale entry. The
jtulach@1280:          * scan starts at the element after i.
jtulach@1280:          *
jtulach@1280:          * @param n scan control: <tt>log2(n)</tt> cells are scanned,
jtulach@1280:          * unless a stale entry is found, in which case
jtulach@1280:          * <tt>log2(table.length)-1</tt> additional cells are scanned.
jtulach@1280:          * When called from insertions, this parameter is the number
jtulach@1280:          * of elements, but when from replaceStaleEntry, it is the
jtulach@1280:          * table length. (Note: all this could be changed to be either
jtulach@1280:          * more or less aggressive by weighting n instead of just
jtulach@1280:          * using straight log n. But this version is simple, fast, and
jtulach@1280:          * seems to work well.)
jtulach@1280:          *
jtulach@1280:          * @return true if any stale entries have been removed.
jtulach@1280:          */
jtulach@1280:         private boolean cleanSomeSlots(int i, int n) {
jtulach@1280:             boolean removed = false;
jtulach@1280:             Entry[] tab = table;
jtulach@1280:             int len = tab.length;
jtulach@1280:             do {
jtulach@1280:                 i = nextIndex(i, len);
jtulach@1280:                 Entry e = tab[i];
jtulach@1280:                 if (e != null && e.get() == null) {
jtulach@1280:                     n = len;
jtulach@1280:                     removed = true;
jtulach@1280:                     i = expungeStaleEntry(i);
jtulach@1280:                 }
jtulach@1280:             } while ( (n >>>= 1) != 0);
jtulach@1280:             return removed;
jtulach@1280:         }
jtulach@1280: 
jtulach@1280:         /**
jtulach@1280:          * Re-pack and/or re-size the table. First scan the entire
jtulach@1280:          * table removing stale entries. If this doesn't sufficiently
jtulach@1280:          * shrink the size of the table, double the table size.
jtulach@1280:          */
jtulach@1280:         private void rehash() {
jtulach@1280:             expungeStaleEntries();
jtulach@1280: 
jtulach@1280:             // Use lower threshold for doubling to avoid hysteresis
jtulach@1280:             if (size >= threshold - threshold / 4)
jtulach@1280:                 resize();
jtulach@1280:         }
jtulach@1280: 
jtulach@1280:         /**
jtulach@1280:          * Double the capacity of the table.
jtulach@1280:          */
jtulach@1280:         private void resize() {
jtulach@1280:             Entry[] oldTab = table;
jtulach@1280:             int oldLen = oldTab.length;
jtulach@1280:             int newLen = oldLen * 2;
jtulach@1280:             Entry[] newTab = new Entry[newLen];
jtulach@1280:             int count = 0;
jtulach@1280: 
jtulach@1280:             for (int j = 0; j < oldLen; ++j) {
jtulach@1280:                 Entry e = oldTab[j];
jtulach@1280:                 if (e != null) {
jtulach@1280:                     ThreadLocal k = e.get();
jtulach@1280:                     if (k == null) {
jtulach@1280:                         e.value = null; // Help the GC
jtulach@1280:                     } else {
jtulach@1280:                         int h = k.threadLocalHashCode & (newLen - 1);
jtulach@1280:                         while (newTab[h] != null)
jtulach@1280:                             h = nextIndex(h, newLen);
jtulach@1280:                         newTab[h] = e;
jtulach@1280:                         count++;
jtulach@1280:                     }
jtulach@1280:                 }
jtulach@1280:             }
jtulach@1280: 
jtulach@1280:             setThreshold(newLen);
jtulach@1280:             size = count;
jtulach@1280:             table = newTab;
jtulach@1280:         }
jtulach@1280: 
jtulach@1280:         /**
jtulach@1280:          * Expunge all stale entries in the table.
jtulach@1280:          */
jtulach@1280:         private void expungeStaleEntries() {
jtulach@1280:             Entry[] tab = table;
jtulach@1280:             int len = tab.length;
jtulach@1280:             for (int j = 0; j < len; j++) {
jtulach@1280:                 Entry e = tab[j];
jtulach@1280:                 if (e != null && e.get() == null)
jtulach@1280:                     expungeStaleEntry(j);
jtulach@1280:             }
jtulach@1280:         }
jtulach@1280:     }
jtulach@1280: }