/*

 * Copyright (c) 1997, 2007, Oracle and/or its affiliates. All rights reserved.

 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.

 *

 * This code is free software; you can redistribute it and/or modify it

 * under the terms of the GNU General Public License version 2 only, as

 * published by the Free Software Foundation.  Oracle designates this

 * particular file as subject to the "Classpath" exception as provided

 * by Oracle in the LICENSE file that accompanied this code.

 *

 * This code is distributed in the hope that it will be useful, but WITHOUT

 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or

 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License

 * version 2 for more details (a copy is included in the LICENSE file that

 * accompanied this code).

 *

 * You should have received a copy of the GNU General Public License version

 * 2 along with this work; if not, write to the Free Software Foundation,

 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.

 *

 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA

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 */

package java.lang;

import java.lang.ref.*;

import java.util.concurrent.atomic.AtomicInteger;

/**

 * This class provides thread-local variables.  These variables differ from

 * their normal counterparts in that each thread that accesses one (via its

 * <tt>get</tt> or <tt>set</tt> method) has its own, independently initialized

 * copy of the variable.  <tt>ThreadLocal</tt> instances are typically private

 * static fields in classes that wish to associate state with a thread (e.g.,

 * a user ID or Transaction ID).

 *

 * <p>For example, the class below generates unique identifiers local to each

 * thread.

 * A thread's id is assigned the first time it invokes <tt>ThreadId.get()</tt>

 * and remains unchanged on subsequent calls.

 * <pre>

 * import java.util.concurrent.atomic.AtomicInteger;

 *

 * public class ThreadId {

 *     // Atomic integer containing the next thread ID to be assigned

 *     private static final AtomicInteger nextId = new AtomicInteger(0);

 *

 *     // Thread local variable containing each thread's ID

 *     private static final ThreadLocal<Integer> threadId =

 *         new ThreadLocal<Integer>() {

 *             @Override protected Integer initialValue() {

 *                 return nextId.getAndIncrement();

 *         }

 *     };

 *

 *     // Returns the current thread's unique ID, assigning it if necessary

 *     public static int get() {

 *         return threadId.get();

 *     }

 * }

 * </pre>

 * <p>Each thread holds an implicit reference to its copy of a thread-local

 * variable as long as the thread is alive and the <tt>ThreadLocal</tt>

 * instance is accessible; after a thread goes away, all of its copies of

 * thread-local instances are subject to garbage collection (unless other

 * references to these copies exist).

 *

 * @author  Josh Bloch and Doug Lea

 * @since   1.2

 */

public class ThreadLocal<T> {

    /**

     * ThreadLocals rely on per-thread linear-probe hash maps attached

     * to each thread (Thread.threadLocals and

     * inheritableThreadLocals).  The ThreadLocal objects act as keys,

     * searched via threadLocalHashCode.  This is a custom hash code

     * (useful only within ThreadLocalMaps) that eliminates collisions

     * in the common case where consecutively constructed ThreadLocals

     * are used by the same threads, while remaining well-behaved in

     * less common cases.

     */

    private final int threadLocalHashCode = nextHashCode();

    /**

     * The next hash code to be given out. Updated atomically. Starts at

     * zero.

     */

    private static AtomicInteger nextHashCode =

        new AtomicInteger();

    /**

     * The difference between successively generated hash codes - turns

     * implicit sequential thread-local IDs into near-optimally spread

     * multiplicative hash values for power-of-two-sized tables.

     */

    private static final int HASH_INCREMENT = 0x61c88647;

    /**

     * Returns the next hash code.

     */

    private static int nextHashCode() {

        return nextHashCode.getAndAdd(HASH_INCREMENT);

    }

    /**

     * Returns the current thread's "initial value" for this

     * thread-local variable.  This method will be invoked the first

     * time a thread accesses the variable with the {@link #get}

     * method, unless the thread previously invoked the {@link #set}

     * method, in which case the <tt>initialValue</tt> method will not

     * be invoked for the thread.  Normally, this method is invoked at

     * most once per thread, but it may be invoked again in case of

     * subsequent invocations of {@link #remove} followed by {@link #get}.

     *

     * <p>This implementation simply returns <tt>null</tt>; if the

     * programmer desires thread-local variables to have an initial

     * value other than <tt>null</tt>, <tt>ThreadLocal</tt> must be

     * subclassed, and this method overridden.  Typically, an

     * anonymous inner class will be used.

     *

     * @return the initial value for this thread-local

     */

    protected T initialValue() {

        return null;

    }

    /**

     * Creates a thread local variable.

     */

    public ThreadLocal() {

    }

    /**

     * Returns the value in the current thread's copy of this

     * thread-local variable.  If the variable has no value for the

     * current thread, it is first initialized to the value returned

     * by an invocation of the {@link #initialValue} method.

     *

     * @return the current thread's value of this thread-local

     */

    public T get() {

        Thread t = Thread.currentThread();

        ThreadLocalMap map = getMap(t);

        if (map != null) {

            ThreadLocalMap.Entry e = map.getEntry(this);

            if (e != null)

                return (T)e.value;

        }

        return setInitialValue();

    }

    /**

     * Variant of set() to establish initialValue. Used instead

     * of set() in case user has overridden the set() method.

     *

     * @return the initial value

     */

    private T setInitialValue() {

        T value = initialValue();

        Thread t = Thread.currentThread();

        ThreadLocalMap map = getMap(t);

        if (map != null)

            map.set(this, value);

        else

            createMap(t, value);

        return value;

    }

    /**

     * Sets the current thread's copy of this thread-local variable

     * to the specified value.  Most subclasses will have no need to

     * override this method, relying solely on the {@link #initialValue}

     * method to set the values of thread-locals.

     *

     * @param value the value to be stored in the current thread's copy of

     *        this thread-local.

     */

    public void set(T value) {

        Thread t = Thread.currentThread();

        ThreadLocalMap map = getMap(t);

        if (map != null)

            map.set(this, value);

        else

            createMap(t, value);

    }

    /**

     * Removes the current thread's value for this thread-local

     * variable.  If this thread-local variable is subsequently

     * {@linkplain #get read} by the current thread, its value will be

     * reinitialized by invoking its {@link #initialValue} method,

     * unless its value is {@linkplain #set set} by the current thread

     * in the interim.  This may result in multiple invocations of the

     * <tt>initialValue</tt> method in the current thread.

     *

     * @since 1.5

     */

     public void remove() {

         ThreadLocalMap m = getMap(Thread.currentThread());

         if (m != null)

             m.remove(this);

     }

    /**

     * Get the map associated with a ThreadLocal. Overridden in

     * InheritableThreadLocal.

     *

     * @param  t the current thread

     * @return the map

     */

    ThreadLocalMap getMap(Thread t) {

        return t.threadLocals;

    }

    /**

     * Create the map associated with a ThreadLocal. Overridden in

     * InheritableThreadLocal.

     *

     * @param t the current thread

     * @param firstValue value for the initial entry of the map

     * @param map the map to store.

     */

    void createMap(Thread t, T firstValue) {

        t.threadLocals = new ThreadLocalMap(this, firstValue);

    }

    /**

     * Factory method to create map of inherited thread locals.

     * Designed to be called only from Thread constructor.

     *

     * @param  parentMap the map associated with parent thread

     * @return a map containing the parent's inheritable bindings

     */

    static ThreadLocalMap createInheritedMap(ThreadLocalMap parentMap) {

        return new ThreadLocalMap(parentMap);

    }

    /**

     * Method childValue is visibly defined in subclass

     * InheritableThreadLocal, but is internally defined here for the

     * sake of providing createInheritedMap factory method without

     * needing to subclass the map class in InheritableThreadLocal.

     * This technique is preferable to the alternative of embedding

     * instanceof tests in methods.

     */

    T childValue(T parentValue) {

        throw new UnsupportedOperationException();

    }

    /**

     * ThreadLocalMap is a customized hash map suitable only for

     * maintaining thread local values. No operations are exported

     * outside of the ThreadLocal class. The class is package private to

     * allow declaration of fields in class Thread.  To help deal with

     * very large and long-lived usages, the hash table entries use

     * WeakReferences for keys. However, since reference queues are not

     * used, stale entries are guaranteed to be removed only when

     * the table starts running out of space.

     */

    static class ThreadLocalMap {

        /**

         * The entries in this hash map extend WeakReference, using

         * its main ref field as the key (which is always a

         * ThreadLocal object).  Note that null keys (i.e. entry.get()

         * == null) mean that the key is no longer referenced, so the

         * entry can be expunged from table.  Such entries are referred to

         * as "stale entries" in the code that follows.

         */

        static class Entry extends WeakReference<ThreadLocal> {

            /** The value associated with this ThreadLocal. */

            Object value;

            Entry(ThreadLocal k, Object v) {

                super(k);

                value = v;

            }

        }

        /**

         * The initial capacity -- MUST be a power of two.

         */

        private static final int INITIAL_CAPACITY = 16;

        /**

         * The table, resized as necessary.

         * table.length MUST always be a power of two.

         */

        private Entry[] table;

        /**

         * The number of entries in the table.

         */

        private int size = 0;

        /**

         * The next size value at which to resize.

         */

        private int threshold; // Default to 0

        /**

         * Set the resize threshold to maintain at worst a 2/3 load factor.

         */

        private void setThreshold(int len) {

            threshold = len * 2 / 3;

        }

        /**

         * Increment i modulo len.

         */

        private static int nextIndex(int i, int len) {

            return ((i + 1 < len) ? i + 1 : 0);

        }

        /**

         * Decrement i modulo len.

         */

        private static int prevIndex(int i, int len) {

            return ((i - 1 >= 0) ? i - 1 : len - 1);

        }

        /**

         * Construct a new map initially containing (firstKey, firstValue).

         * ThreadLocalMaps are constructed lazily, so we only create

         * one when we have at least one entry to put in it.

         */

        ThreadLocalMap(ThreadLocal firstKey, Object firstValue) {

            table = new Entry[INITIAL_CAPACITY];

            int i = firstKey.threadLocalHashCode & (INITIAL_CAPACITY - 1);

            table[i] = new Entry(firstKey, firstValue);

            size = 1;

            setThreshold(INITIAL_CAPACITY);

        }

        /**

         * Construct a new map including all Inheritable ThreadLocals

         * from given parent map. Called only by createInheritedMap.

         *

         * @param parentMap the map associated with parent thread.

         */

        private ThreadLocalMap(ThreadLocalMap parentMap) {

            Entry[] parentTable = parentMap.table;

            int len = parentTable.length;

            setThreshold(len);

            table = new Entry[len];

            for (int j = 0; j < len; j++) {

                Entry e = parentTable[j];

                if (e != null) {

                    ThreadLocal key = e.get();

                    if (key != null) {

                        Object value = key.childValue(e.value);

                        Entry c = new Entry(key, value);

                        int h = key.threadLocalHashCode & (len - 1);

                        while (table[h] != null)

                            h = nextIndex(h, len);

                        table[h] = c;

                        size++;

                    }

                }

            }

        }

        /**

         * Get the entry associated with key.  This method

         * itself handles only the fast path: a direct hit of existing

         * key. It otherwise relays to getEntryAfterMiss.  This is

         * designed to maximize performance for direct hits, in part

         * by making this method readily inlinable.

         *

         * @param  key the thread local object

         * @return the entry associated with key, or null if no such

         */

        private Entry getEntry(ThreadLocal key) {

            int i = key.threadLocalHashCode & (table.length - 1);

            Entry e = table[i];

            if (e != null && e.get() == key)

                return e;

            else

                return getEntryAfterMiss(key, i, e);

        }

        /**

         * Version of getEntry method for use when key is not found in

         * its direct hash slot.

         *

         * @param  key the thread local object

         * @param  i the table index for key's hash code

         * @param  e the entry at table[i]

         * @return the entry associated with key, or null if no such

         */

        private Entry getEntryAfterMiss(ThreadLocal key, int i, Entry e) {

            Entry[] tab = table;

            int len = tab.length;

            while (e != null) {

                ThreadLocal k = e.get();

                if (k == key)

                    return e;

                if (k == null)

                    expungeStaleEntry(i);

                else

                    i = nextIndex(i, len);

                e = tab[i];

            }

            return null;

        }

        /**

         * Set the value associated with key.

         *

         * @param key the thread local object

         * @param value the value to be set

         */

        private void set(ThreadLocal key, Object value) {

            // We don't use a fast path as with get() because it is at

            // least as common to use set() to create new entries as

            // it is to replace existing ones, in which case, a fast

            // path would fail more often than not.

            Entry[] tab = table;

            int len = tab.length;

            int i = key.threadLocalHashCode & (len-1);

            for (Entry e = tab[i];

                 e != null;

                 e = tab[i = nextIndex(i, len)]) {

                ThreadLocal k = e.get();

                if (k == key) {

                    e.value = value;

                    return;

                }

                if (k == null) {

                    replaceStaleEntry(key, value, i);

                    return;

                }

            }

            tab[i] = new Entry(key, value);

            int sz = ++size;

            if (!cleanSomeSlots(i, sz) && sz >= threshold)

                rehash();

        }

        /**

         * Remove the entry for key.

         */

        private void remove(ThreadLocal key) {

            Entry[] tab = table;

            int len = tab.length;

            int i = key.threadLocalHashCode & (len-1);

            for (Entry e = tab[i];

                 e != null;

                 e = tab[i = nextIndex(i, len)]) {

                if (e.get() == key) {

                    e.clear();

                    expungeStaleEntry(i);

                    return;

                }

            }

        }

        /**

         * Replace a stale entry encountered during a set operation

         * with an entry for the specified key.  The value passed in

         * the value parameter is stored in the entry, whether or not

         * an entry already exists for the specified key.

         *

         * As a side effect, this method expunges all stale entries in the

         * "run" containing the stale entry.  (A run is a sequence of entries

         * between two null slots.)

         *

         * @param  key the key

         * @param  value the value to be associated with key

         * @param  staleSlot index of the first stale entry encountered while

         *         searching for key.

         */

        private void replaceStaleEntry(ThreadLocal key, Object value,

                                       int staleSlot) {

            Entry[] tab = table;

            int len = tab.length;

            Entry e;

            // Back up to check for prior stale entry in current run.

            // We clean out whole runs at a time to avoid continual

            // incremental rehashing due to garbage collector freeing

            // up refs in bunches (i.e., whenever the collector runs).

            int slotToExpunge = staleSlot;

            for (int i = prevIndex(staleSlot, len);

                 (e = tab[i]) != null;

                 i = prevIndex(i, len))

                if (e.get() == null)

                    slotToExpunge = i;

            // Find either the key or trailing null slot of run, whichever

            // occurs first

            for (int i = nextIndex(staleSlot, len);

                 (e = tab[i]) != null;

                 i = nextIndex(i, len)) {

                ThreadLocal k = e.get();

                // If we find key, then we need to swap it

                // with the stale entry to maintain hash table order.

                // The newly stale slot, or any other stale slot

                // encountered above it, can then be sent to expungeStaleEntry

                // to remove or rehash all of the other entries in run.

                if (k == key) {

                    e.value = value;

                    tab[i] = tab[staleSlot];

                    tab[staleSlot] = e;

                    // Start expunge at preceding stale entry if it exists

                    if (slotToExpunge == staleSlot)

                        slotToExpunge = i;

                    cleanSomeSlots(expungeStaleEntry(slotToExpunge), len);

                    return;

                }

                // If we didn't find stale entry on backward scan, the

                // first stale entry seen while scanning for key is the

                // first still present in the run.

                if (k == null && slotToExpunge == staleSlot)

                    slotToExpunge = i;

            }

            // If key not found, put new entry in stale slot

            tab[staleSlot].value = null;

            tab[staleSlot] = new Entry(key, value);

            // If there are any other stale entries in run, expunge them

            if (slotToExpunge != staleSlot)

                cleanSomeSlots(expungeStaleEntry(slotToExpunge), len);

        }

        /**

         * Expunge a stale entry by rehashing any possibly colliding entries

         * lying between staleSlot and the next null slot.  This also expunges

         * any other stale entries encountered before the trailing null.  See

         * Knuth, Section 6.4

         *

         * @param staleSlot index of slot known to have null key

         * @return the index of the next null slot after staleSlot

         * (all between staleSlot and this slot will have been checked

         * for expunging).

         */

        private int expungeStaleEntry(int staleSlot) {

            Entry[] tab = table;

            int len = tab.length;

            // expunge entry at staleSlot

            tab[staleSlot].value = null;

            tab[staleSlot] = null;

            size--;

            // Rehash until we encounter null

            Entry e;

            int i;

            for (i = nextIndex(staleSlot, len);

                 (e = tab[i]) != null;

                 i = nextIndex(i, len)) {

                ThreadLocal k = e.get();

                if (k == null) {

                    e.value = null;

                    tab[i] = null;

                    size--;

                } else {

                    int h = k.threadLocalHashCode & (len - 1);

                    if (h != i) {

                        tab[i] = null;

                        // Unlike Knuth 6.4 Algorithm R, we must scan until

                        // null because multiple entries could have been stale.

                        while (tab[h] != null)

                            h = nextIndex(h, len);

                        tab[h] = e;

                    }

                }

            }

            return i;

        }

        /**

         * Heuristically scan some cells looking for stale entries.

         * This is invoked when either a new element is added, or

         * another stale one has been expunged. It performs a

         * logarithmic number of scans, as a balance between no

         * scanning (fast but retains garbage) and a number of scans

         * proportional to number of elements, that would find all

         * garbage but would cause some insertions to take O(n) time.

         *

         * @param i a position known NOT to hold a stale entry. The

         * scan starts at the element after i.

         *

         * @param n scan control: <tt>log2(n)</tt> cells are scanned,

         * unless a stale entry is found, in which case

         * <tt>log2(table.length)-1</tt> additional cells are scanned.

         * When called from insertions, this parameter is the number

         * of elements, but when from replaceStaleEntry, it is the

         * table length. (Note: all this could be changed to be either

         * more or less aggressive by weighting n instead of just

         * using straight log n. But this version is simple, fast, and

         * seems to work well.)

         *

         * @return true if any stale entries have been removed.

         */

        private boolean cleanSomeSlots(int i, int n) {

            boolean removed = false;

            Entry[] tab = table;

            int len = tab.length;

            do {

                i = nextIndex(i, len);

                Entry e = tab[i];

                if (e != null && e.get() == null) {

                    n = len;

                    removed = true;

                    i = expungeStaleEntry(i);

                }

            } while ( (n >>>= 1) != 0);

            return removed;

        }

        /**

         * Re-pack and/or re-size the table. First scan the entire

         * table removing stale entries. If this doesn't sufficiently

         * shrink the size of the table, double the table size.

         */

        private void rehash() {

            expungeStaleEntries();

            // Use lower threshold for doubling to avoid hysteresis

            if (size >= threshold - threshold / 4)

                resize();

        }

        /**

         * Double the capacity of the table.

         */

        private void resize() {

            Entry[] oldTab = table;

            int oldLen = oldTab.length;

            int newLen = oldLen * 2;

            Entry[] newTab = new Entry[newLen];

            int count = 0;

            for (int j = 0; j < oldLen; ++j) {

                Entry e = oldTab[j];

                if (e != null) {

                    ThreadLocal k = e.get();

                    if (k == null) {

                        e.value = null; // Help the GC

                    } else {

                        int h = k.threadLocalHashCode & (newLen - 1);

                        while (newTab[h] != null)

                            h = nextIndex(h, newLen);

                        newTab[h] = e;

                        count++;

                    }

                }

            }

            setThreshold(newLen);

            size = count;

            table = newTab;

        }

        /**

         * Expunge all stale entries in the table.

         */

        private void expungeStaleEntries() {

            Entry[] tab = table;

            int len = tab.length;

            for (int j = 0; j < len; j++) {

                Entry e = tab[j];

                if (e != null && e.get() == null)

                    expungeStaleEntry(j);

            }

        }

    }

}