ThreadLocal
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2022-05-14 18:00:44
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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 < Integer > uniqueNum =
* new ThreadLocal < Integer > () {
* @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