ThreadLocal

最新推荐文章于 2024-10-20 08:30:00 发布

转载最新推荐文章于 2024-10-20 08:30:00 发布 · 149 阅读

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原文链接：https://www.jianshu.com/p/3c5d7f09dfbd

本文深入探讨了ThreadLocal的实现原理，包括其在不同线程间存储数据的独特方式，以及ThreadLocalMap如何管理线程局部变量。通过代码示例，详细解释了set和get方法的工作流程，展示了ThreadLocal在多线程环境中提供线程隔离特性的机制。

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莫忘记，人类情感上最大的需要是感恩。

在阅读Handler源码时发现了这么一个东西，本想直混在其他博客中一笔带过，但仔细想了下这个东西还是蛮重要的，于是开了这篇博客。

ThreadLocal

threadlocal使用方法很简单

static final ThreadLocal<T> sThreadLocal = new ThreadLocal<T>();
sThreadLocal.set()
sThreadLocal.get()

threadlocal而是一个线程内部的存储类，可以在指定线程内存储数据，数据存储以后，只有指定线程可以得到存储数据，官方解释如下。

/**
 * 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).
 */

大致意思就是ThreadLocal提供了线程内存储变量的能力，这些变量不同之处在于每一个线程读取的变量是对应的互相独立的。通过get和set方法就可以得到当前线程对应的值。

做个不恰当的比喻，从表面上看ThreadLocal相当于维护了一个map，key就是当前的线程，value就是需要存储的对象。

这里的这个比喻是不恰当的，实际上是ThreadLocal的静态内部类ThreadLocalMap为每个Thread都维护了一个数组table，ThreadLocal确定了一个数组下标，而这个下标就是value存储的对应位置。。

作为一个存储数据的类，关键点就在get和set方法。

//set 方法
public void set(T value) {
      //获取当前线程
      Thread t = Thread.currentThread();
      //实际存储的数据结构类型
      ThreadLocalMap map = getMap(t);
      //如果存在map就直接set，没有则创建map并set
      if (map != null)
          map.set(this, value);
      else
          createMap(t, value);
  }

//getMap方法
ThreadLocalMap getMap(Thread t) {
//thred中维护了一个ThreadLocalMap
return t.threadLocals;
}

//createMap
void createMap(Thread t, T firstValue) {
//实例化一个新的ThreadLocalMap，并赋值给线程的成员变量threadLocals
t.threadLocals = new ThreadLocalMap(this, firstValue);
}

从上面代码可以看出每个线程持有一个ThreadLocalMap对象。每一个新的线程Thread都会实例化一个ThreadLocalMap并赋值给成员变量threadLocals，使用时若已经存在threadLocals则直接使用已经存在的对象。

Thread

/* ThreadLocal values pertaining to this thread. This map is maintained
     * by the ThreadLocal class. */
    ThreadLocal.ThreadLocalMap threadLocals = null;

Thread中关于ThreadLocalMap部分的相关声明，接下来看一下createMap方法中的实例化过程。

ThreadLocalMap

set方法

//Entry为ThreadLocalMap静态内部类，对ThreadLocal的若引用
//同时让ThreadLocal和储值形成key-value的关系
static class Entry extends WeakReference<ThreadLocal<?>> {
    /** The value associated with this ThreadLocal. */
    Object value;
<span class="token class-name">Entry</span><span class="token punctuation">(</span><span class="token class-name">ThreadLocal</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token operator">?</span><span class="token punctuation">&gt;</span></span> k<span class="token punctuation">,</span> <span class="token class-name">Object</span> v<span class="token punctuation">)</span> <span class="token punctuation">{</span>
       <span class="token keyword">super</span><span class="token punctuation">(</span>k<span class="token punctuation">)</span><span class="token punctuation">;</span>
        value <span class="token operator">=</span> v<span class="token punctuation">;</span>
<span class="token punctuation">}</span>

}

//ThreadLocalMap构造方法
ThreadLocalMap(ThreadLocal<?> firstKey, Object firstValue) {
//内部成员数组，INITIAL_CAPACITY值为16的常量
table = new Entry[INITIAL_CAPACITY];
//位运算，结果与取模相同，计算出需要存放的位置
//threadLocalHashCode比较有趣
int i = firstKey.threadLocalHashCode & (INITIAL_CAPACITY - 1);
table[i] = new Entry(firstKey, firstValue);
size = 1;
setThreshold(INITIAL_CAPACITY);
}

通过上面的代码不难看出在实例化ThreadLocalMap时创建了一个长度为16的Entry数组。通过hashCode与length位运算确定出一个索引值i，这个i就是被存储在table数组中的位置。

前面讲过每个线程Thread持有一个ThreadLocalMap类型的实例threadLocals，结合此处的构造方法可以理解成每个线程Thread都持有一个Entry型的数组table，而一切的读取过程都是通过操作这个数组table完成的。

显然table是set和get的焦点，在看具体的set和get方法前，先看下面这段代码。

//在某一线程声明了ABC三种类型的ThreadLocal
ThreadLocal<A> sThreadLocalA = new ThreadLocal<A>();
ThreadLocal<B> sThreadLocalB = new ThreadLocal<B>();
ThreadLocal<C> sThreadLocalC = new ThreadLocal<C>();

由前面我们知道对于一个Thread来说只有持有一个ThreadLocalMap，所以ABC对应同一个ThreadLocalMap对象。为了管理ABC，于是将他们存储在一个数组的不同位置，而这个数组就是上面提到的Entry型的数组table。

那么问题来了，ABC在table中的位置是如何确定的？为了能正常够正常的访问对应的值，肯定存在一种方法计算出确定的索引值i，show me code。

  //ThreadLocalMap中set方法。
  private void set(ThreadLocal<?> key, Object value) {
        <span class="token comment">// We don't use a fast path as with get() because it is at</span>
        <span class="token comment">// least as common to use set() to create new entries as</span>
        <span class="token comment">// it is to replace existing ones, in which case, a fast</span>
        <span class="token comment">// path would fail more often than not.</span>

        <span class="token class-name">Entry</span><span class="token punctuation">[</span><span class="token punctuation">]</span> tab <span class="token operator">=</span> table<span class="token punctuation">;</span>
        <span class="token keyword">int</span> len <span class="token operator">=</span> tab<span class="token punctuation">.</span>length<span class="token punctuation">;</span>
        <span class="token comment">//获取索引值，这个地方是比较特别的地方</span>
        <span class="token keyword">int</span> i <span class="token operator">=</span> key<span class="token punctuation">.</span>threadLocalHashCode <span class="token operator">&amp;</span> <span class="token punctuation">(</span>len<span class="token operator">-</span><span class="token number">1</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

        <span class="token comment">//遍历tab如果已经存在则更新值</span>
        <span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token class-name">Entry</span> e <span class="token operator">=</span> tab<span class="token punctuation">[</span>i<span class="token punctuation">]</span><span class="token punctuation">;</span>
             e <span class="token operator">!=</span> <span class="token keyword">null</span><span class="token punctuation">;</span>
             e <span class="token operator">=</span> tab<span class="token punctuation">[</span>i <span class="token operator">=</span> <span class="token function">nextIndex</span><span class="token punctuation">(</span>i<span class="token punctuation">,</span> len<span class="token punctuation">)</span><span class="token punctuation">]</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
            <span class="token class-name">ThreadLocal</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token operator">?</span><span class="token punctuation">&gt;</span></span> k <span class="token operator">=</span> e<span class="token punctuation">.</span><span class="token function">get</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

            <span class="token keyword">if</span> <span class="token punctuation">(</span>k <span class="token operator">==</span> key<span class="token punctuation">)</span> <span class="token punctuation">{</span>
                e<span class="token punctuation">.</span>value <span class="token operator">=</span> value<span class="token punctuation">;</span>
                <span class="token keyword">return</span><span class="token punctuation">;</span>
            <span class="token punctuation">}</span>

            <span class="token keyword">if</span> <span class="token punctuation">(</span>k <span class="token operator">==</span> <span class="token keyword">null</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
                <span class="token function">replaceStaleEntry</span><span class="token punctuation">(</span>key<span class="token punctuation">,</span> value<span class="token punctuation">,</span> i<span class="token punctuation">)</span><span class="token punctuation">;</span>
                <span class="token keyword">return</span><span class="token punctuation">;</span>
            <span class="token punctuation">}</span>
        <span class="token punctuation">}</span>
        
        <span class="token comment">//如果上面没有遍历成功则创建新值</span>
        tab<span class="token punctuation">[</span>i<span class="token punctuation">]</span> <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Entry</span><span class="token punctuation">(</span>key<span class="token punctuation">,</span> value<span class="token punctuation">)</span><span class="token punctuation">;</span>
        <span class="token keyword">int</span> sz <span class="token operator">=</span> <span class="token operator">++</span>size<span class="token punctuation">;</span>
        <span class="token comment">//满足条件数组扩容x2</span>
        <span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token operator">!</span><span class="token function">cleanSomeSlots</span><span class="token punctuation">(</span>i<span class="token punctuation">,</span> sz<span class="token punctuation">)</span> <span class="token operator">&amp;&amp;</span> sz <span class="token operator">&gt;=</span> threshold<span class="token punctuation">)</span>
            <span class="token function">rehash</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span>

在ThreadLocalMap中的set方法与构造方法能看到以下代码片段。

int i = key.threadLocalHashCode & (len-1)
int i = firstKey.threadLocalHashCode & (INITIAL_CAPACITY - 1)
简而言之就是将threadLocalHashCode进行一个位运算（取模）得到索引i，threadLocalHashCode代码如下。

    //ThreadLocal中threadLocalHashCode相关代码.
<span class="token keyword">private</span> <span class="token keyword">final</span> <span class="token keyword">int</span> threadLocalHashCode <span class="token operator">=</span> <span class="token function">nextHashCode</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

<span class="token comment">/**
 * The next hash code to be given out. Updated atomically. Starts at
 * zero.
 */</span>
<span class="token keyword">private</span> <span class="token keyword">static</span> <span class="token class-name">AtomicInteger</span> nextHashCode <span class="token operator">=</span>
    <span class="token keyword">new</span> <span class="token class-name">AtomicInteger</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

<span class="token comment">/**
 * 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.
 */</span>
<span class="token keyword">private</span> <span class="token keyword">static</span> <span class="token keyword">final</span> <span class="token keyword">int</span> HASH_INCREMENT <span class="token operator">=</span> <span class="token number">0x61c88647</span><span class="token punctuation">;</span>

<span class="token comment">/**
 * Returns the next hash code.
 */</span>
<span class="token keyword">private</span> <span class="token keyword">static</span> <span class="token keyword">int</span> <span class="token function">nextHashCode</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token comment">//自增</span>
    <span class="token keyword">return</span> nextHashCode<span class="token punctuation">.</span><span class="token function">getAndAdd</span><span class="token punctuation">(</span>HASH_INCREMENT<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>

因为static的原因，在每次new ThreadLocal时因为threadLocalHashCode的初始化，会使threadLocalHashCode值自增一次，增量为0x61c88647。

0x61c88647是斐波那契散列乘数,它的优点是通过它散列(hash)出来的结果分布会比较均匀，可以很大程度上避免hash冲突，已初始容量16为例，hash并与15位运算计算数组下标结果如下：

hashCode	数组下标
0x61c88647	7
0xc3910c8e	14
0x255992d5	5
0x8722191c	12
0xe8ea9f63	3
0x4ab325aa	10
0xac7babf1	1
0xe443238	8
0x700cb87f	15

总结如下：

对于某一ThreadLocal来讲，他的索引值i是确定的，在不同线程之间访问时访问的是不同的table数组的同一位置即都为table[i]，只不过这个不同线程之间的table是独立的。
对于同一线程的不同ThreadLocal来讲，这些ThreadLocal实例共享一个table数组，然后每个ThreadLocal实例在table中的索引i是不同的。

get()方法

//ThreadLocal中get方法
public T get() {
    Thread t = Thread.currentThread();
    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();
}

//ThreadLocalMap中getEntry方法
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);
}