ThreadLocal

ThreadLocal

一、总结

1.版本 jdk 1.8.0_131 java SE
2.ThreadLocal

• 应用场景：在不同的线程存储不同的上下文信息的场合中
• 实现原理：ThreadLocal只是提供一个thread-local变量，这个变量于当前线程所独有, 每一个线程都有一个隶属与当前线程的thread-local变量

3.ThreadLocal.ThreadLocalMap

• Thread类中持有ThreadLocalMap的对象引用
• ThreadLocalMap 的数据结构是数组 Entry[]
• 数组初始化大小 16，扩展 *2  ，扩展因子：length * 2 / 3
• Entry 继承  WeakReference ，包含 key 与 value 两个属性
• key：不是ThreadLocal本身，而是ThreadLocal的弱引用；value：存入ThreadLocal中的泛型T
• ThreadLocalMap提供了一种为ThreadLocal定制的高效实现，并且自带一种基于弱引用的垃圾清理机制

二、源码及分析

/**
 * 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>UniqueThreadIdGenerator.getCurrentThreadId()</tt> and remains unchanged on subsequent calls.
 * <pre>
 * import java.util.concurrent.atomic.AtomicInteger;
 *
 * public class UniqueThreadIdGenerator {
 *
 *     private static final AtomicInteger uniqueId = new AtomicInteger(0);
 *
 *     private static final ThreadLocal &lt; Integer > uniqueNum = 
 *         new ThreadLocal &lt; Integer > () {
 *             &#64;Override protected Integer initialValue() {
 *                 return uniqueId.getAndIncrement();
 *         }
 *     };
 * 
 *     public static int getCurrentThreadId() {
 *         return uniqueId.get();
 *     }
 * } // UniqueThreadIdGenerator
 * </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
 * @version 1.42, 06/23/06
 * @since   1.2
 */
public class ThreadLocal<T> {

从类的注释中得知：
1.ThreadLocal 类提供了线程本地变量；与普通的变量不同，隶属于每个线程，每个线程可以独立初始化此变量的副本
2.在使用时通常定义为全局的变量 private static ThreadLocal<T> ，通常用来标识每个线程中某属性的一种状态
3.每个线程在首次调用时分配一个本地唯一的标识ID
4.在线程的生命周期内都会拥有一个对本地变量的引用；线程运行结束，所引用的变量会被GC回收

    /**
     * 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; 

1.两个连续生成的 hash codes 的差值。为了让 hash code 能更好地分布在尺寸为 2 的 N 次方的数组里。

2.源码中使用此常量的调用位置

    // 在 ThreadLocal 内部的静态类 ThreadLocalMap 中 
    // 定位当前 hashCode 在数组中的下标时需计算当前线程的 hashCode 
    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);
    }

    // 计算当前线程的 hashCode 
    private final int threadLocalHashCode = nextHashCode();

    /**
     * The next hash code to be given out. Updated atomically. Starts at
     * zero.
     */
    // 原子性操作，线程安全，初始化值为0
    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.
     */
    // 当前的hashCode值加上HASH_INCREMENT 
    private static int nextHashCode() {
	return nextHashCode.getAndAdd(HASH_INCREMENT); 
    }

1.计算数据在数组中的位置，
通常 index % size ，即通过当前数的数值与数组的大小取余
此处 index & (size - 1 )

原因：数组的 INITIAL_CAPACITY初始化大小为 16 ，即 大小为 2 ^ N 次幂，2 ^ N - 1 ，二进制中 N -1 位均为 1 ，即取 index 的低位中的 N-1 位

举例：3 % 16 = 3 <==>
0011 & 1111 = 0011 = 3

位运算比取模效率高很多。
因为对2^n取模，转为二进制，只要不是第n+1位，对结果的贡献显然都是0，会影响结果的只能是第n+1位。
2^1 ==> 0010 ;
2^2 ==> 0100 ;

举例：二进制取模运算的大致步骤
11010110110000(13744)%10011(19)=111(7)
将除数向右移位与被除数位数相同，若被除数大于除数，相减；直到被除数不大于除数；
将除数向右以为与被除数位数相同，若被除数小于除数，则向右少移动一位；重复上述步骤
(1)
11010110110000
10011000000000
--------------
00111110110000
(2)
111110110000
100110000000
------------
011000110000
(3)
11000110000
10011000000
-----------
00101110000
(4)
101110000
100110000
---------
001000000
(5)
1000000
0100110
-------
0011010
(6)
11010
10011
-----
00111
一般的二进制取余运算过程中需要进行移位运算、除数与被除数的大小判断等。

1011100(192)%10000(16)=1100(12)

(1)
1011100
1000000
-------
0011100
(2)
11100
10000
-----
01100
如果 a > 2^N 需要进行至少两次运算

总结：
对于a与2^N取余的运算，等于a&(2^N-1)

2.为何选用 0x61c88647 作为每次生成 hashCode 的间距

        // 目的：查看一串连续的数字存放在大小为16的数组中，是否均匀分布，是否会发生碰撞
	public static void main(String[] args) {
		method();
	}

	private static void method(){
		
		final int HASH_INCREMENT = 0x61c88647 ;
		for(int i = 0 ; i < 16 ; i++){
			int nextHash = i * HASH_INCREMENT + HASH_INCREMENT ;
			System.out.print(nextHash & (16 - 1));
			System.out.print(" ");
		}
	}
// 输出结果如下：
// 7 14 5 12 3 10 1 8 15 6 13 4 11 2 9 0 

1.从输出结果看无重复数值出现：
以此值作为 Map 的KEY 可以减少存入时数据时出现的 HASH 碰撞。

2.
黄金比例： (Math.sqrt(5) - 1))
这个数的选取与斐波那契散列有关，0x61c88647对应的十进制为1640531527。斐波那契散列的乘数可以用(long) ((1L << 31) * (Math.sqrt(5) - 1))可以得到2654435769，如果把这个值给转为带符号的int，则会得到-1640531527。换句话说
(1L << 32) - (long) ((1L << 31) * (Math.sqrt(5) - 1))得到的结果就是1640531527也就是0x61c88647

3.AtomicInteger

    // 构造方法
    /**
     * Creates a thread local variable.
     */
    public ThreadLocal() {
    }

    // 初始化 threadLocal 变量的值
    /**
     * 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;
    }

从方法的注释中得出：
1.返回隶属于当前线程的 thread-local 变量的初始化的值
2.此方法会通过调用get方法来触发；除非在调用get前，调用了set赋值，则不会触发
3.通常每个线程只会执行一次此方法，但是当线程调用remove方法后再次调用get 方法时可能再次执行此方法
4.如果想将变量的初始化赋值为非NULL的值，则需要新增类来继承ThreadLocal，在子类中覆盖此方法来实现赋值；通常，可以通过建立匿名内部类的方式来实现此功能；

    /**
     * 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 {
    }

1.ThreadLocalMap 的作用仅仅是用来保存线程本地变量的值，是一个自适应的 hash map
2.ThreadLocalMap 的所有操作均不会对 ThreadLocal  之外的类提供
3.ThreadLocalMap 可以在Thread类中包装私有的属性

    // 在 Thread 类中含有ThreadLocalMap的声明的对象实例
    /* ThreadLocal values pertaining to this thread. This map is maintained
     * by the ThreadLocal class. */
    ThreadLocal.ThreadLocalMap threadLocals = null;

4.ThreadLocalMap 使用弱引用作为KEY，来解决很大且长时间存活的方法；然而，即使队列的引用已经不被使用了，过期的 entries 仅当table空间不够时才会被移除

        /**
         * 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;
            }
        }

1.ThreadLocalMap 中定义的静态内部类 Entry 继承 WeekReference
弱引用，使用ThreadLocal 的对象的引用作为 key
2.如果key为null说明此时的key不再被引用，此时这个Entry 可以被从表中删除

        /**
         * The initial capacity -- MUST be a power of two.
         */
        // ThreadLocalMap 中的 Entry[] 的初始化容量是16
        private static final int INITIAL_CAPACITY = 16;

        /**
         * The table, resized as necessary.
         * table.length MUST always be a power of two.
         */
        // 数组的长度必须是2的整数次幂
        private Entry[] table;

        /**
         * The number of entries in the table.
         */
        // Entry[] 中 Entry 的数量，而不是数组的长度
        private int size = 0;
 
        /**
         * The next size value at which to resize.
         */
        // 类似 HashMap 中的负载因子，达到此临界值即扩容，而不是达到数组的最大长度
        private int threshold; // Default to 0

        /**
         * Set the resize threshold to maintain at worst a 2/3 load factor.
         */
        // 扩容的临界值 len * 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);
        }

Entry[] 是一个环状的结构
1.求当前 i 的下一个位置
如果 i+1 小于 len ，返回 i+1
如果 i+1 等于 len ，返回 0，即
假设 len = 16 ，数组中的位置依次是0~15，当前i=15，15+1=16=len，即回到了环的起始位置，那15的下一个位置就是0

2.求当前 i 的上一个位置
如果 i-1 大于等于 0 ，返回 i - 1
如果 i-1 小于 0 ，返回 len - 1，即
假设 len = 16 ，数组中的位置依次是0~15，当前i=0，0-11<0<len，即回到了环的结尾位置上，结尾的位置就是len-1,=16-1=15

        /**
         * 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) {
            // 实例化数组，初始化大小16
            table = new Entry[INITIAL_CAPACITY];
            // 获取当前ThreadLocal 的 hashCode 并与 2^N -1 做与操作
            // 目的：计算在数组中的存放位置，等价于 hashcode % length 
            // 但运算效率 & 操作要优于 %
            int i = firstKey.threadLocalHashCode & (INITIAL_CAPACITY - 1);
            // 构造匿名Entry 并放入数组得下标为i的位置上
            table[i] = new Entry(firstKey, firstValue);
            // 初始化数组中entry 的数量为 1
            size = 1;
            // 扩容的临界值值为 16*2/3 = 10 
            setThreshold(INITIAL_CAPACITY);
        }

1.初始化构造一个新的 ThreadLocalMap 包含第一组Entry<key,value>
2.ThreadLocalMap 懒加载构造器，只有当有至少一个entry需要存放的时候才会去构造

        /**
         * 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) {
            // 获取给定的parentMap中的数组
            Entry[] parentTable = parentMap.table;
            // 获取数组的容量
            int len = parentTable.length;
            // 设置扩容的临界值
            setThreshold(len);
            // 创建新的数组
            table = new Entry[len];
            // 遍历原有数组，将数据放入新的数组中
            for (int j = 0; j < len; j++) {
                Entry e = parentTable[j];
                if (e != null) {
                    ThreadLocal key = e.get();
                    if (key != null) {
                        // 此处获取的是子类中定义的value
                        Object value = key.childValue(e.value);
                        Entry c = new Entry(key, value);
                        int h = key.threadLocalHashCode & (len - 1);
                        // 寻址，若当前位置不为空，则继续寻找相邻位置是否有元素
                        while (table[h] != null)
                            h = nextIndex(h, len);
                        table[h] = c;
                        size++;
                    }
                }
            }
        }

1.创建一个包含所有所有从给定的 parentMap 中继承的 TreadLocal 的新的Map
2.仅仅当需要创建继承关系的Map时调用

        /**
         * 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);
        }

1.通过 key 检索绑定的 Entry
2.方法本身值处理快速路径匹配，即直接命中匹配的key
3.否则会关联到 getEntryAfterMiss 方法上，这样设计的目的加大直接命中key 的可能，在某种程度上此方法为一个快速的内联方法
-- 解释：数据尽可能均匀的分布到大小为 2^N 的数组中，但如果出现 hash 碰撞，即 i = hash & (2^N -1) 处已经有值了，那么继续向下寻址，判断当前是否有值，若有，继续向下查询，直到此时i的数据为null，将新增的数据放到 i 的位置上
-- 查询时同样，如果 i 位置就是所需要的KEY，那么直接返回，如果不是，从 i 位置向后检索

        /**
         * 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) {
                // e 非空，获取 Entry 的对应的 KEY
                // 此处再次判断 当前的 k 是否与查询的 k 相等
                // 目的是，当 i 不断后移时，用每次从 i 位置上取到的值与 k 比较
                // 而不是重复判断 
                ThreadLocal k = e.get();
                if (k == key)
                    return e;
                if (k == null)
                    // 如果 k == null 在删除 
                    expungeStaleEntry(i);
                else
                    // i 想后移动一位
                    i = nextIndex(i, len);
                e = tab[i];
            }
            return null;
        }

1.此方法的设计目的是在 getEntry 时，如果未能在 hash 区域内直接命中 key 时的补充
2.从  i 位置开始，向后移动，获取每处 i 对应的值，如果值不为空，那么进行比较，如果为空，那么清除

        /**
         * 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;
            // 删除 entry[] 数组中 staleSlot 位置的元素
            // expunge entry at staleSlot
            tab[staleSlot].value = null;   
            tab[staleSlot] = null;
            size--;

            // Rehash until we encounter null
            Entry e;
            int i;
            // 遍历 i 之后的位置的元素
            for (i = nextIndex(staleSlot, len);
		 (e = tab[i]) != null;
                 i = nextIndex(i, len)) {
                ThreadLocal k = e.get();
                // 如果 k 为 null，则设置数组 i 位置为 null
                if (k == null) {
                    e.value = null;
                    tab[i] = null;
                    size--;
                } else {
                    // 重新计算 当前 key 的 hash值，通过新的 hash 值，计算在数组中的存放位置
                    int h = k.threadLocalHashCode & (len - 1);
                    // 如果位置一样，说明，此时 key 就应该放在 i 处
                    // 如果位置不一样，说明当存放 key 时，发生了 hash 碰撞
                    // 导致  key 不能存放到制定的 h 位置上
                    // 将 i 位置设置为 null 
                    // 判断 h 处是否有数据，无则放入 h 位置
                    // 有则从 h 位置向后寻址
                    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;
        }

1.通过再次hash可能存在碰撞可能的 entry 数组 ，删除位于 staleSlot 与下一个可能为NULL的元素之间的所有
2.删除在遇到 null 元素之前的已经不再使用的引用

        /**
         * 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.

            // 此处和get()方法一样不使用快速路径
            // 因为快速路径至少共同的去调用调用set去创建 entry 同时去替换存在的对象，在这种情况下，更容易出现错误 ？？？？？？？？？
            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();
                // 如果找到了匹配的key，则直接将value替换为最新的
                if (k == key) {
                    e.value = value;
                    return;
                }
                // 如果 key 为 null 
                if (k == null) {
                    replaceStaleEntry(key, value, i);
                    return;
                }
            }

            tab[i] = new Entry(key, value);
            int sz = ++size;
            if (!cleanSomeSlots(i, sz) && sz >= threshold)
                rehash();
        }

1.存入关联的 key 与 value

        /**
         * 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);
        }

1.通过一个特殊的key值在一系列的操作中替换掉一个已经过时的引用
2.无论这个特殊的key值是否在Map中存在，传入的value都会存入到 Entry 中
3.此方法将会删除两个null值之间所有的无效的引用

博文参考：
ThreadLocal 和神奇的 0x61c88647
ThreadLocal源码解读