1、概述
ThreadLocal，可以理解为线程的局部变量，作用就是为每一个使用该变量的线程都提供一个变量值的副本，每一个线程都可以独立地改变自己的副本，而不会和其它线程的副本冲突。
ThreadLocal是如何做到为每一个线程维护变量的副本的呢？
每个线程中都有一个ThreadLocalMap（Thread.threadLocals），用于存储每一个线程的变量的副本。
ThreadLocalMap使用数组Entry[] table保存ThreadLocal-->Object键值对象，数组保存位置：int i = key.nextHashCode() & (table.length - 1);。
ThreadLocal和Synchonized区别：
都用于解决多线程并发访问。
Synchronized用于线程间的数据共享（使变量或代码块在某一时该只能被一个线程访问），是一种以延长访问时间来换取线程安全性的策略；
而ThreadLocal则用于线程间的数据隔离（为每一个线程都提供了变量的副本），是一种以空间来换取线程安全性的策略。
2、ThreadLocalMap
用于存储每一个线程的变量的副本
/** * 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 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); } /** * 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); } /** * 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. * * @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; } } } ...... }
3、ThreadLocal
public class ThreadLocal<T> { /** * 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); } /** * 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}. * * <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. * * @return the initial value for this thread-local */ //返回此线程局部变量的当前线程的初始值 protected T initialValue() { return null; } /** * 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. * * @return the current thread's value of this thread-local */ //返回此线程局部变量的当前线程副本中的值 public T get() { Thread t = Thread.currentThread(); ThreadLocalMap map = t.threadLocals; if (map != null) { ThreadLocalMap.Entry e = map.getEntry(this); if (e != null) { @SuppressWarnings("unchecked") T result = (T)e.value; return result; } } return setInitialValue(); } /** * 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. */ //将此线程局部变量的当前线程副本中的值设置为指定值 public void set(T value) { Thread t = Thread.currentThread(); ThreadLocalMap map = t.threadLocals; 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. * * @since 1.5 */ //移除此线程局部变量的值 public void remove() { ThreadLocalMap m = Thread.currentThread().threadLocals; if (m != null) m.remove(this); } /** * 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); } ...... }
为什么取HASH_INCREMENT = 0x61c88647?
（可以阅读Why 0x61c88647?）
由来：
This number represents the golden ratio (sqrt(5)-1) times two to the power of 31. The result is then a golden number, either 2654435769 or -1640531527. You can see the calculation here:
long l1 = (long) ((1L << 31) * (Math.sqrt(5) - 1)); //Math.sqrt(5) - 1 = 1.2360679774997898 System.out.println("as 32 bit unsigned: " + l1); //2654435769 int i1 = (int) l1; System.out.println("as 32 bit signed: " + i1); //-1640531527 = -0x61c88647
与fibonacci hashing(斐波那契散列法)以及黄金分割有关，特殊的哈希码0x61c88647大大降低碰撞的几率，能让哈希码能均匀的分布在2的N次方的数组里。
key.threadLocalHashCode & (len-1)，ThreadLocalMap 中 Entry[] table 的大小必须是2的N次方呀(len = 2^N)，那 len-1 的二进制表示就是低位连续的N个1， 那 key.threadLocalHashCode & (len-1) 的值就是 threadLocalHashCode 的低N位。
测试：
private static AtomicInteger nextHashCode = new AtomicInteger(); private static final int HASH_INCREMENT = 0x61c88647; private static int nextHashCode() { return nextHashCode.getAndAdd(HASH_INCREMENT); } public static void main(String[] args) { for (int j = 0; j < 5; j++) { int size = 2 << j; // hash = 0; int[] indexArray = new int[size]; for (int i = 0; i < size; i++) { indexArray[i] = nextHashCode() & (size - 1); } System.out.println("indexs = "+ Arrays.toString(indexArray)); } }
结果：
indexs = [0, 1]
indexs = [2, 1, 0, 3]
indexs = [2, 1, 0, 7, 6, 5, 4, 3]
indexs = [2, 9, 0, 7, 14, 5, 12, 3, 10, 1, 8, 15, 6, 13, 4, 11]
indexs = [18, 25, 0, 7, 14, 21, 28, 3, 10, 17, 24, 31, 6, 13, 20, 27, 2, 9, 16, 23, 30, 5, 12, 19, 26, 1, 8, 15, 22, 29, 4, 11]
没有看到重复的索引值，要哈希表的大小是2的N次方,那么基本上可以保证每次计算出的index值都不会重复。
为什么HashCode不直接用自增的方式(HASH_INCREMENT=1)?
我的理解是，随着不用的 ThreadLocal 变量被回收掉，这种自增的方式的性能会越来越差，因为临近的 slot 为空的可能性很小。而 ThreadLocal 实际所采用的方式，其下标是在跳跃分布，这样即使出现冲突，在临近找到空 slot 的可能性更大一些，性能也会更好。
4、例子
Android Looper的实现：
public final class Looper {
......
// sThreadLocal.get() will return null unless you've called prepare().
static final ThreadLocal<Looper> sThreadLocal = new ThreadLocal<Looper>();
private static void prepare(boolean quitAllowed) {
if (sThreadLocal.get() != null) {
throw new RuntimeException("Only one Looper may be created per thread");
}
sThreadLocal.set(new Looper(quitAllowed));
}
/**
* Return the Looper object associated with the current thread. Returns
* null if the calling thread is not associated with a Looper.
*/
public static Looper myLooper() {
return sThreadLocal.get();
}
......
}
ThreadId:
维护每个线程的id
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(); } }