ThreadLocal源码阅读

package java.lang;

import java.lang.ref.WeakReference;
import java.util.Objects;
import java.util.concurrent.atomic.AtomicInteger;
import java.util.function.Supplier;

/**
 * This class provides thread-local variables.  These variables differ from
 * their normal counterparts in that each thread that accesses one (via its
 * {@code get} or {@code set} method) has its own, independently initialized
 * copy of the variable.  {@code ThreadLocal} instances are typically private
 * static fields in classes that wish to associate state with a thread (e.g.,
 * a user ID or Transaction ID).
 * 本类提供线程局部变量。这些变量和普通变量不同，每一个线程访问（通过get和set方法）
 * 的变量都是属于线程的独立初始化的一个线程局部变量的副本。线程局部变量通常是类中希望
 * 将状态与线程关联的私有静态域（如用户ID和事务ID）。
 *
 * <p>For example, the class below generates unique identifiers local to each
 * thread.
 * 例如，下面的类生成每个线程本地的唯一标识符。
 * A thread's id is assigned the first time it invokes {@code ThreadId.get()}
 * and remains unchanged on subsequent calls.
 * 一个线程的Id在该线程首次调用get方法时生成，且在后续通话中保持不变。
 * <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 {@code ThreadLocal}
 * 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).
 * 只要线程还活着，且ThreadLocal实例能够被访问，每个线程均保留其线程局部变量
 * 副本的隐式引用；线程死后它的所有线程局部变量副本被垃圾回收器回收（除非存在
 * 对这些线程局部变量副本的其他应用）
 *
 * @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).
     * ThreadLocal的实例依赖于每个线程的线性探针哈希图附加到每个线程。
     * The ThreadLocal objects act as keys,searched via threadLocalHashCode.
     * ThreadLocal对象充当键，通过threadLocalHashCode搜索。
     * 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.
     * 这是一个自定义哈希码（仅在ThreadLocalMaps中有用），在相同的线程使用连续构造
     * 的ThreadLocals的常见情况下，它消除了冲突，而在不太常见的情况下，它们表现良好。
     */
    private final int threadLocalHashCode = nextHashCode();

    /**
     * The next hash code to be given out. Updated atomically. Starts at
     * zero.
     * 下一次给出的哈希码，该值的自动更新不能被中断（原子性），值从0开始累加。
     */
    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.
     * 连续生成的哈希码间的差值，
     */
    private static final int HASH_INCREMENT = 0x61c88647;

    /**
     * Returns the next hash code.
     * 返回下一个哈希码
     */
    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 {@code initialValue} 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}.
     * 翻译：
     * 返回当前线程持有的线程局部变量副本的初始值。线程在第一次通过get方法访问局部
     * 变量前如果没有调用过set方法为局部变量设置值，本函数就会被调用。通常每个线程
     * 最多会调用一次本方法，但是如果在之后依次调用了remove和get方法，它可能会再次被调用。
     *
     * <p>This implementation simply returns {@code null}; if the
     * programmer desires thread-local variables to have an initial
     * value other than {@code null}, {@code ThreadLocal} must be
     * subclassed, and this method overridden.  Typically, an
     * anonymous inner class will be used.
     * 本实现仅返回null，如果开发者希望线程的局部变量具体其他初始值，只能通过声明子类并
     * 重写本方法。通常，将使用匿名内部类。（下面就有一个内部类）
     *
     * @return the initial value for this thread-local
     */
    protected T initialValue() {
        return null;
    }

    /**
     * Creates a thread local variable. The initial value of the variable is
     * determined by invoking the {@code get} method on the {@code Supplier}.
     * 创建一个线程局部变量的副本，变量的初始值是通过调用Supplier上的get方法确定的。
     * （Supplier是一个函数式接口）
     *
     * @param <S>      the type of the thread local's value
     * @param supplier the supplier to be used to determine the initial value
     * @return a new thread local variable
     * @throws NullPointerException if the specified supplier is null
     * @since 1.8
     */
    public static <S> ThreadLocal<S> withInitial(Supplier<? extends S> supplier) {
        return new SuppliedThreadLocal<>(supplier);
    }

    /**
     * 构造函数
     * Creates a thread local variable.
     *
     * @see #withInitial(java.util.function.Supplier)
     */
    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.
     * 译：
     * 返回当前线程持有的线程局部变量副本的值。如果该变量还没赋值，
     * 则先使用initialValue方法为其赋予初始值。
     *
     * @return the current thread's value of this thread-local
     */
    public T get() {
        //1、确定当前线程
        Thread t = Thread.currentThread();
        //2、
        ThreadLocalMap map = getMap(t);
        if (map != null) {
            ThreadLocalMap.Entry e = map.getEntry(this);
            if (e != null) {
                @SuppressWarnings("unchecked")
                T result = (T) e.value;
                return result;
            }
        }
        return setInitialValue();
    }

    /**
     * Variant of set() to establish initialValue. Used instead
     * of set() in case user has overridden the set() method.
     * 译：
     * set方法的变体，用于设置初始值。如果用户已覆盖set（）方法，
     * 请使用它代替set（）。
     *
     * @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.
     * 译：
     * 设置当前线程持有的线程局部变量副本的值为指定值。大部分子类不需要重写本
     * 方法，仅依靠initialValue方法来设置值。
     *
     * @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
     * {@code initialValue} method in the current thread.
     * 译：
     * 移除单前线程持有的线程本地变量的值。如果随后马上又通过get方法试图获取
     * 这个线程本地变量的值，这个变量的值会再次通过调用initialValue方法确定，
     * 除非在调用get方法前通过set方法设置了值就不会。这可能会导致initialValue
     * 方法被当前线程多次调用。
     *
     * @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
     */
    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();
    }

    /**
     * An extension of ThreadLocal that obtains its initial value from
     * the specified {@code Supplier}.
     * SuppliedThreadLocal类拓展了ThreadLocal类，该类重写了initialValue函数，
     * 用于从指定的Supplier获取初始值。
     */
    static final class SuppliedThreadLocal<T> extends ThreadLocal<T> {

        /**
         * Supplier<T>是一个函数式接口
         */
        private final Supplier<? extends T> supplier;

        /**
         * 构造函数
         *
         * @param supplier - 一个函数式接口，用来返回线程本地变量的初始值
         */
        SuppliedThreadLocal(Supplier<? extends T> supplier) {
            /**
             *     public static <T> T requireNonNull(T obj) {
             *         if (obj == null)
             *             throw new NullPointerException();
             *         return obj;
             *     }
             */
            this.supplier = Objects.requireNonNull(supplier);
        }

        /**
         * 这里很关键，需要去看看Supplier接口的源码。
         */
        @Override
        protected T initialValue() {
            return supplier.get();
        }
    }

    /**
     * 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.
     * 译：
     * ThreadLocalMap是自定义的HashMap，仅适用于维护线程局部变量的值。
     * 没有操作导出到ThreadLocal类之外。这个类是包私有的，用于Thread类
     * 域声明。为了处理存储空间消耗大，使用时间长的使用情况，这个HashTable
     * 使用WeakReferences（弱引用）作为键。但是，由于未使用参考队列，因此
     * 仅在表开始空间不足时，才保证删除过时的条目。
     */
    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.
         * 译：
         * 该哈希表中的条目是WeakReference子类的对象，以其主域作为键（大多数情
         * 况下是一个ThreadLocal对象）。需要注意，当以null作为键时，意味着这个
         * 键已经不再被引用，因此这个键所在的条目能被表移除。这样的条目我们在下文
         * 中称为“过时的条目”
         */
        static class Entry extends WeakReference<ThreadLocal<?>> {
            /**
             * The value associated with this ThreadLocal.
             * 与ThreadLocal相关联的值
             */
            Object value;

            /**
             * @param k - 以线程作为键
             * @param v - 线程持有的threal-local变量副本的值
             */
            Entry(ThreadLocal<?> k, Object v) {
                super(k);
                value = v;
            }
        }

        /**
         * 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.
         * 译：
         * 表在必要的时候会扩容，表的大小必须是2的幂
         */
        private Entry[] table;

        /**
         * The number of entries in the table.
         * 表中条目（键值对）的数量
         */
        private int size = 0;

        /**
         * the next size value at which to resize.
         * 下一次调整表的大小时要增加的值
         */
        private int threshold; // Default to 0

        /**
         * Set the resize threshold to maintain at worst a 2/3 load factor.
         * 从这里可以看出，每次扩容，容量增加2/3。
         */
        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.
         * 译：
         * 构造一个最初包含以下内容的新Map。ThreadLocalMaps是延迟创建的，所以
         * 只有当至少要添加一条条目时才创建一个。
         */
        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) {
                    @SuppressWarnings("unchecked")
                    ThreadLocal<Object> key = (ThreadLocal<Object>) 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.
         * 译：
         * 获取键关联的条目。
         * 理解：
         * 散列表中通过哈希函数和键计算哈希码，使用哈希码来决定数据存储的位置，
         * 不同的键通过哈希函数可能计算出相同的哈希码，这被称为“冲突”。遇到冲突
         * 时需要使用某种“冲突解决策略”重新确定一个哈希码，本函数假设没有冲突，
         * 尝试通过计算出来的哈希码直接取值。事实上好的哈希算法计算出来的哈希码
         * 分布的比较均匀，即很少发生冲突。
         *
         * @param key the thread local object
         * @return the entry associated with key, or null if no such
         */
        private Entry getEntry(ThreadLocal<?> key) {
            //1、尝试直接取值
            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);//2、冲突解决方案
        }

        /**
         * 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.
         * 设置与键关联的值
         *
         * @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.
         * <p>
         * 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
         * 译：
         * 通过重新散列位于staleSlot和下一个null插槽之间的任何可能冲突的条目来清除
         * 陈旧的条目。这还将删除尾随null之前遇到的所有其他过时的条目。
         *
         * @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.
         * 译：
         * 启发式扫描某些单元以查找陈旧条目。当添加了新元素或已删除另一旧
         * 元素时，将调用此方法。它执行对数扫描，作为无扫描（快速但保留垃
         * 圾）和与元素数量成正比的扫描数量之间的平衡，这会发现所有垃圾，
         * 但会导致某些插入花费O（n）时间。
         *
         * @param i a position known NOT to hold a stale entry. The
         *          scan starts at the element after i.
         * @param n scan control: {@code log2(n)} cells are scanned,
         *          unless a stale entry is found, in which case
         *          {@code log2(table.length)-1} 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.
         * 译：
         * 重新包装和/或调整表大小。首先扫描整个表，删除陈旧的条目。
         * 如果这还不足以缩小表格的大小，请将表格大小加倍。
         */
        private void rehash() {
            expungeStaleEntries();

            // Use lower threshold for doubling to avoid hysteresis
            if (size >= threshold - threshold / 4)
                resize();
        }

        /**
         * Double the capacity of the table.
         * 使表的容积几倍。
         */
        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;
        }

        /**
         * Expunge all stale entries in the table.
         * 清除表中所有过时条目
         */
        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);
            }
        }
    }
}

/*
 * Copyright (c) 2012, 2013, Oracle and/or its affiliates. All rights reserved.
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 */
package java.util.function;

/**
 * 该类函数式接口应用于支持Lambda表达式的API中，
 * Supplier接口用于提供一个T类对象。
 */

/**
 * Represents a supplier of results.
 * 代表结果的提供者
 *
 * <p>There is no requirement that a new or distinct result be returned each
 * time the supplier is invoked.
 * 并不要求每次调用都返回一个新的或者不同的结果。
 *
 * <p>This is a <a href="package-summary.html">functional interface</a>
 * whose functional method is {@link #get()}.
 *
 * @param <T> the type of results supplied by this supplier
 * @since 1.8
 */
@FunctionalInterface
public interface Supplier<T> {

    /**
     * Gets a result.
     *
     * @return a result
     */
    T get();
}