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       /**
         * 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.
         * 这个哈希映射中的条目扩展弱引用，使用它的主引用字段作为键（它总是一个ThreadLocal对象）。注意，空键（即entry.get（）== null）
         * 意味着该键不再被引用，所以该条目可以从表中删除。这些条目在下面的代码中被称为"陈旧条目"。
         */
        static class Entry extends WeakReference<ThreadLocal> {
            /** The value associated with this ThreadLocal 与ThreadLocal关联的值. */
            Object value;

            Entry(ThreadLocal k, Object v) {
                super(k);
                value = v;
            }
        }

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    /**
     * 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.
     * ThreadLocalMap是一个自定义哈希映射，仅适用于维护线程本地值。
     * 没有任何操作被导出到ThreadLocal类之外。该类是封装私有的，允许在Thread类中声明字段。为了帮助处理非常大和长寿的用法，
     * 哈希表项使用弱引用来表示键。但是，由于不使用引用队列，所以只有当表开始空间不足时才能删除旧条目。
     */
    static class ThreadLocalMap {
       /**
         * The initial capacity -- MUST be a power of two.
         * 初始容量——必须是2的幂。
         */
        private static final int INITIAL_CAPACITY = 16;

        /**
         * The table, resized as necessary.
         * table.length MUST always be a power of two.
         * 数组保存数据
         */
        private Entry[] table;

        /**
         * The number of entries in the table.
         * 大小
         */
        private int size = 0;

        /**
         * The next size value at which to resize.
         * 调整大小的下一个大小值。
         * 阈值
         */
        private int threshold; // Default to 0

        /**
         * Set the resize threshold to maintain at worst a 2/3 load factor.
         * 设置调整大小阈值以保持最坏的2 / 3负载因素。
         */
        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.
         * 构建一个最初包含（firstKey，firstValue）的新Map。 ThreadLocalMaps是懒惰​​地构造的，所以我们只创建一个，至少有一个条目放在里面。
         */
        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.
         * 构造一个包含给定父映射的所有可继承ThreadLocals的新映射。仅由createInheritedMap调用。
         *
         * @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++;
                    }
                }
            }
        }
     
    }

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        /**
         * 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.
             //我们不像get（）那样使用快速路径，因为它至少和使用set（）来创建新条目一样常见，
             // 因为它将替换现有的条目，在这种情况下，快速路径失败的频率往往比不。
            Entry[] tab = table;
            int len = tab.length;
            int i = key.threadLocalHashCode & (len-1); //定位index

            for (Entry e = tab[i];
                 e != null;
                 e = tab[i = nextIndex(i, len)]) {
                ThreadLocal k = e.get();

                if (k == key) {   //已存在 就替换
                    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)  //扩容
                rehash();
        }
       /**
         * 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.
         *  
         * 替换在设置操作期间遇到的过期条目，并使用指定键的条目。在值参数中传递的值存储在条目中，无论指定的键是否已经存在一个条目。 
         *  
         * 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.)
         * 
         * 作为副作用，该方法在包含过期条目的“运行”中删除所有陈旧的条目。(运行是两个空槽之间的一个序列。)
         *
         * @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).
            //备份以检查当前运行中的以前陈旧的条目。
            // 我们一次清理整个运行，以避免由于垃圾收集器以一束（即每当收集器运行时）释放垃圾收集器而持续增加垃圾回收。
            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.
                //如果我们找到Key，那么我们需要将它与旧的条目交换来维护哈希表的顺序。
                // 然后可以将新陈旧的槽或者其上面遇到的任何其他陈旧的槽发送到expungeStaleEntry以移除或重新运行运行中的所有其他条目。
                if (k == key) {
                    e.value = value;

                    tab[i] = tab[staleSlot];
                    tab[staleSlot] = e;

                    // Start expunge at preceding stale entry if it exists 如果存在之前的陈旧条目，就开始删除
                    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.
                //如果我们没有在反向扫描中找到陈旧的条目，那么在扫描键时看到的第一个陈旧条目是第一个在运行中仍然存在的条目。
                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
            //如果运行中有其他的条目，则删除它们
            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
         *  通过重新处理可能发生在陈旧槽和下一个空槽之间的任何可能发生的冲突来删除陈旧的条目。
         *  这也将删除在尾随null之前遇到的其他的条目。见Knuth，第6.4节
         * @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.
                        //与Knuth 6.4算法不同，我们必须扫描到 null，因为多个条目可能已经过时。
                        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.
         *
         * 启发式地清理slot,  i对应entry是非无效（指向的ThreadLocal没被回收，或者entry本身为空） 
         * n是用于控制控制扫描次数的 * 正常情况下如果log n次扫描没有发现无效slot，函数就结束了 
          * 但是如果发现了无效的slot，将n置为table的长度len，做一次连续段的清理 * 再从下一个空的slot开始继续扫描
         * 
         * @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;
        }        

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 /**
         * 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();
        }
        /**
          * 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);  //清空为空数据
             }
         }       

        /**
         * Double the capacity of the table.
         * 把table的容量翻倍
         */
        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;
        }

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      /**
         * 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.
         * 
         * 获取Entry值。这种方法本身只处理快速路径：直接击中现有的Key。否则，继续getEntryAfterMiss。
         * 这是为了最大限度地提高直接命中的性能，部分是通过使这个方法易于理解。
         *
         * @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.
         * getEntryEntry方法的版本，以便在直接散列槽中找不到key。
         * @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;
        }        

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     /**
         * 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;
                }
            }
        }

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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.
     * ThreadLocals依赖于附加到每个线程（Thread.threadLocals和inheritableThreadLocals）的每线程线性探测哈希映射。
      * ThreadLocal对象充key，通过threadLocalHashCode搜索。这是一个自定义的哈希代码（只在ThreadLocalMaps中有用），
      * 在相同线程使用连续构造的ThreadLocals的常见情况下消除冲突，而在不常见的情况下保持良好行为。
     */
    private final int threadLocalHashCode = nextHashCode();

    /**
     * The next hash code to be given out. Updated atomically. Starts at
     * zero.
     * 下一个哈希码将被给出来。原子更新。从零开始。
     */
    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.
     * 连续生成哈希码的区别 - 将隐式顺序thread-local IDS 转换为近似最佳扩展的2幂大小的表乘散列值。
     */
    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}
     * .
     * 返回此线程局部变量的当前线程的"初始值"。除非线程先前调用{@link #set}方法，在这种情况下，<tt> initialValue </ tt>方法将在第一次线程访问变量时使用此方法将不会被调用的线程。
     * 通常，每个线程最多调用一次此方法，但在后续调用{@link #remove}后跟{@link #get}的情况下，可能会再次调用此方法。
     *
     * <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() {
    }
    
     /**
      * Get the map associated with a ThreadLocal. Overridden in
      * InheritableThreadLocal.
      * 获取与ThreadLocal相关的映射。在可继承线程本地覆盖。
      *
      * @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.
     *  创建与hreadLocal关联的映射
     * @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.
     * 工厂方法创建继承的线程本地Map。设计为仅从线程构造函数调用。
     * @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);
    }     
}

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  long l1 = (long) ((1L << 31) * (Math.sqrt(5) - 1));  //Math.sqrt(5) - 1 = 1.2360679774997898
  System.out.println("as 32 bit unsigned: " + l1);  //2654435769

  int i1 = (int) l1;
  System.out.println("as 32 bit signed:   " + i1);  //-1640531527 = -0x61c88647

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   /**
     * 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);
    }

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    /**
     * 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;
    }

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   /**
     * 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.
     * 返回当前线程的该线程副本中的值 - 局部变量。如果变量对当前线程没有值，则首先将其初始化为调用{@link＃initialValue}方法返回的值。
     * @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();
    }

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    /**
     * 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);
     }