ThreadLocal是Java语言提供的用于支持线程局部变量的类。所谓的线程局部变量，就是仅仅只能被本线程访问，不能在线程之间进行共享访问的变量(每个线程一个拷贝)。在各个Java web的各种框架中ThreadLocal几乎已经被用烂了，spring中有使用，mybatis中也有使用，hibernate中也有使用，甚至我们写个分页也用ThreadLocal来传递参数......这也从侧面说明了ThreadLocal十分的给力。
从使用者的角度而言，一般我们可以将ThreadLocal看做是一个：ConcurrentHashMap<Thread, Object>，以Thread引用为key, 来保存本线程的局部变量。但是从实现的角度而言，ThreadLocal的实现根本就不是这样的。下面从源码分析ThreadLocal的实现。
1. 既然是线程局部变量，那么理所当然就应该存储在自己的线程对象中，我们可以从 Thread 的源码中找到线程局部变量存储的地方：
public class Thread implements Runnable { /* Make sure registerNatives is the first thing <clinit> does. */ private static native void registerNatives(); static { registerNatives(); } // ... ... /* ThreadLocal values pertaining to this thread. This map is maintained * by the ThreadLocal class. */ ThreadLocal.ThreadLocalMap threadLocals = null; /* * InheritableThreadLocal values pertaining to this thread. This map is * maintained by the InheritableThreadLocal class. */ ThreadLocal.ThreadLocalMap inheritableThreadLocals = null;
我们可以看到线程局部变量是存储在Thread对象的 threadLocals 属性中，而 threadLocals 属性是一个 ThreadLocal.ThreadLocalMap 对象。
2. 我们接着看 ThreadLocal.ThreadLocalMap 是何方神圣
/** * 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 { /** * 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; } } /** * The initial capacity -- MUST be a power of two. */ private static final int INITIAL_CAPACITY = 16; /** * The table, resized as necessary. * table.length MUST always be a power of two. */ private Entry[] table; // ... ... /** * 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) { table = new Entry[INITIAL_CAPACITY]; int i = firstKey.threadLocalHashCode & (INITIAL_CAPACITY - 1); table[i] = new Entry(firstKey, firstValue); size = 1; setThreshold(INITIAL_CAPACITY); }
可以看到ThreadLocal.ThreadLocalMap 是 ThreadLocal 的一个静态内部类。ThreadLocalMap从字面上就可以看出这是一个保存ThreadLocal对象的map(其实是以它为Key)，没错，不过是经过了两层包装的ThreadLocal对象。第一层包装是使用 WeakReference<ThreadLocal<?>> 将ThreadLocal对象变成一个弱引用的对象；第二层包装是 定义了一个专门的类 Entry 来扩展 WeakReference<ThreadLocal<?>>：
static class Entry extends WeakReference<ThreadLocal<?>> { /** The value associated with this ThreadLocal. */ Object value; Entry(ThreadLocal<?> k, Object v) { super(k); value = v; } }
类 Entry 很显然是一个保存map键值对的实体，ThreadLocal<?>为key, 要保存的线程局部变量的值为value。super(k)调用的WeakReference的构造函数，表示将ThreadLocal<?>对象转换成弱引用对象，用做key。
从 ThreadLocalMap 的构造函数：
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); }
可以看出，ThreadLocalMap这个map的实现是使用一个数组 private Entry[] table 来保存键值对的实体，初始大小为16，ThreadLocalMap自己实现了如何从 key 到 value 的映射： firstKey.threadLocalHashCode & (INITIAL_CAPACITY - 1)
/** * ThreadLocals rely on per-thread linear-probe hash maps attached * to each thread (Thread.threadLocals and * inheritableThreadLocals). The ThreadLocal objects act as keys, * searched via threadLocalHashCode. 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. */ private final int threadLocalHashCode = nextHashCode(); /** * The next hash code to be given out. Updated atomically. Starts at * zero. */ 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. */ private static int nextHashCode() { return nextHashCode.getAndAdd(HASH_INCREMENT); }
使用一个 static 的原子属性 AtomicInteger nextHashCode，通过每次增加 HASH_INCREMENT = 0x61c88647 ，然后 & (INITIAL_CAPACITY - 1) 取得在数组 private Entry[] table 中的索引。
3. 我们先看一下 Thread 对象中的 ThreadLocal.ThreadLocalMap threadLocals = null; 如何初始化：
/** * 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. * * @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); } /** * 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); }
ThreadLocal在调用set方法时，如果 getMap(注意是以Thread引用为key) 返回的 t.threadLocals 为null，那么表示该线程的 ThreadLocalMap 还没有初始化，所以调用createMap进行初始化：t.threadLocals = new ThreadLocalMap(this, firstValue);
注意这里使用到了延迟初始化的技术：
/** * 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) { table = new Entry[INITIAL_CAPACITY]; int i = firstKey.threadLocalHashCode & (INITIAL_CAPACITY - 1); table[i] = new Entry(firstKey, firstValue); size = 1; setThreshold(INITIAL_CAPACITY); }
这里仅仅是初始化了16个元素的引用数组，并没有初始化16个 Entry 对象。而是一个线程中有多少个线程局部对象要保存，那么就初始化多少个 Entry 对象来保存它们。
到了这里，我们可以思考一下，为什么要这样实现了。为什么要用 ThreadLocalMap 来保存线程局部对象呢？原因是一个线程拥有的的局部对象可能有很多，这样实现的话，那么不管你一个线程拥有多少个局部变量，都是使用同一个 ThreadLocalMap 来保存的，ThreadLocalMap 中 private Entry[] table 的初始大小是16。超过容量的2/3时，会扩容。
4. 我们在看一下 ThreadLocal.set 方法：
/** * 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. * * @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); }
我们看到是以当前 thread 的引用为 key, 获得 ThreadLocalMap ，然后调用 map.set(this, value); 保存进 private Entry[] table :
/** * 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. 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(); }
5. ThreadLocal 涉及到的两个层面的内存自动回收
1）在 ThreadLocal 层面的内存回收：
/* * 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).
当线程死亡时，那么所有的保存在的线程局部变量就会被回收，其实这里是指线程Thread对象中的 ThreadLocal.ThreadLocalMap threadLocals 会被回收，这是显然的。
2）ThreadLocalMap 层面的内存回收：
/**
* 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.
*/
如果线程可以活很长的时间，并且该线程保存的线程局部变量有很多(也就是 Entry 对象很多)，那么就涉及到在线程的生命期内如何回收 ThreadLocalMap 的内存了，不然的话，Entry对象越多，那么ThreadLocalMap 就会越来越大，占用的内存就会越来越多，所以对于已经不需要了的线程局部变量，就应该清理掉其对应的Entry对象。使用的方式是，Entry对象的key是WeakReference 的包装，当ThreadLocalMap 的 private Entry[] table，已经被占用达到了三分之二时 threshold = 2/3(也就是线程拥有的局部变量超过了10个) ，就会尝试回收 Entry 对象，我们可以看到 ThreadLocalMap.set方法中有下面的代码：
if (!cleanSomeSlots(i, sz) && sz >= threshold)
rehash();
cleanSomeSlots 就是进行回收内存：
/** * 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. * * @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; }
e.get() == null 调用的是 Entry 的父类 WeakReference<ThreadLocal<?>> 的方法：
/** * Returns this reference object's referent. If this reference object has * been cleared, either by the program or by the garbage collector, then * this method returns <code>null</code>. * * @return The object to which this reference refers, or * <code>null</code> if this reference object has been cleared */ public T get() { return this.referent; }
返回 null ，表示 Entry 的 key 已经被回收了，所以可以回收该 Entry 对象了：expungeStaleEntry(i)
/** * 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; // expunge entry at staleSlot tab[staleSlot].value = null; tab[staleSlot] = null; size--;
6. ThreadLocal常用的接口：
1）需要制定初始值时，可以覆盖protected T initialValue()方法；
2）public T get()；
3）public void set(T value)；
4）public void remove()；
7. 总结
1）一个线程中的所有的局部变量其实存储在该线程自己的同一个map属性中；
2）线程死亡时，线程局部变量会自动回收内存；
3）线程局部变量时通过一个 Entry 保存在map中，该Entry 的key是一个 WeakReference包装的ThreadLocal, value为线程局部变量;
key 到 value 的映射是通过：ThreadLocal.threadLocalHashCode & (INITIAL_CAPACITY - 1) 来完成的；
4）当线程拥有的局部变量超过了容量的2/3(没有扩大容量时是10个)，会涉及到ThreadLocalMap中Entry的回收；
