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();
 }