【Java】深入理解ThreadLocal
2024-08-24 10:13:46
一、前言
要理解ThreadLocal，首先必须理解线程安全。线程可以看做是一个具有一定独立功能的处理过程，它是比进程更细度的单位。当程序以单线程运行的时候，我们不需要考虑线程安全。然而当一个进程中包含多个线程的时候，就需要考虑线程安全问题，因为此时线程可能会同时操作同一个资源，当两个或者两个以上线程同时操作一个资源的时候，就会造成冲突、不一致等问题，即线程不安全。
解决线程安全问题，本质上就是解决资源共享问题，一般有以下手段：
1）可重入（不依赖环境）；2）互斥（同一时间段只允许一个线程使用）；3）原子操作；4）Thread-Local
二、Thread-Local
Thread-Local是一个很简单的思想：如果一个资源会引起线程竞争，那就为每一个线程配备一个资源。这就是ThreadLocal需要做的事情。
三、ThreadLocal的用法
在理解ThreadLocal之前，首先看一下它的用法：
public class ThreadLocalTest {
public static ThreadLocal<Integer> intLocal = new ThreadLocal<Integer>() {
@Override
protected Integer initialValue() {
// TODO Auto-generated method stub
return 0;
}
@Override
public Integer get() {
// TODO Auto-generated method stub
set(super.get() + 1);
return super.get();
}
@Override
public void set(Integer value) {
// TODO Auto-generated method stub
super.set(value);
}
@Override
public void remove() {
// TODO Auto-generated method stub
super.remove();
}
};
public static void main(String[] args) {
// TODO Auto-generated method stub
for(int index = 0; index < 3; index ++)
new MyThread(index).start();
}
}
class MyThread extends Thread{
int id;
public MyThread(int id){
this.id = id;
}
@Override
public void run() {
// TODO Auto-generated method stub
for(int index = 0; index < 3; index ++){
System.out.println("Thread-" + id + " : " + ThreadLocalTest.intLocal.get());
try {
Thread.sleep((int)(100 * Math.random()));
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
}
}
其打印结果如下：
Thread-1 : 1
Thread-0 : 1
Thread-2 : 1
Thread-0 : 2
Thread-2 : 2
Thread-1 : 2
Thread-2 : 3
Thread-0 : 3
Thread-1 : 3
这是一个很典型的问题，在学习多线程以及同步的时候，几乎所有的书本都会使用类似的一个例子：银行存钱取钱问题。我们知道当多线程并发操作一个int值的加减操作的时候，最后的数值会产生很大的不确定性，得不到最终正确的结果。
而从例子中我们可以看到：每一个线程对int值的操作都是独立的，我们使用的只是同一个静态的intLocal类型！通过使用TreadLocal，我们可以为每一个线程提供独立的资源副本，从而完成对资源的“共享”操作。
ThreadLocal类中可重载的方法只有四个：
1）set()：设置值，也就是说，我们选择将某个值设置为ThreadLocal类型的；
2）get()：将设置进去的值取出来；
3）remove()：我们不想将某个值设置为ThreadLocal了，移除掉；
4）initialValue()：如果get的时候还没有设置值，就使用这个方法进行初始化；
使用过程简单明了，一般重载initialValue()提供一个初始值就可以了，其余方法不需要重载。
四、ThreadLocal的实现
看源码是最直接也是最有效的学习方式，不但可以掌握其原理，也可以学习Java源码精巧的实现方式。
ThreadLocal的实现代码略长，我们选取重要的方法作为切入点：
public T get() {
Thread t = Thread.currentThread();
ThreadLocalMap map = getMap(t);
if (map != null) {
ThreadLocalMap.Entry e = map.getEntry(this);
if (e != null)
return (T)e.value;
}
return setInitialValue();
}
第一个方法是get()方法。这边出现了Thread和Map，联想到Thread的作用，大致应该可以猜到ThreadLocal的实现：维护一个Map，以Thread作为键，以变量作为值，每一次要使用变量的时候，就以当前的Thread作为键去取值，如果没有，就初始化一个值返回，如果有，就直接返回。
事实上，ThreadLocal的实现思路的确大致如此。但是我们要做的事情其实更多：
A.如果要设置更多的值怎么办？也就是说，我们有多种资源需要共享怎么办？
B.为每一个线程共享一个资源，如何回收？
我们言归正传，从源码中获取答案。
get()方法的第3行中出现了一个ThreadLocalMap实例，它是从getMap()方法获取的，其方法如下：
ThreadLocalMap getMap(Thread t) {
return t.threadLocals;
}
t表示当前的线程，从Thread的源码中可以看到，的确是有一个ThreadLocalMap实例，其声明和注解如下：
/* ThreadLocal values pertaining to this thread. This map is maintained
* by the ThreadLocal class. */
ThreadLocal.ThreadLocalMap threadLocals = null;
这个属性是专门为ThreadLocal类而存在的，而它的实现也存在于ThreadLocal中，是ThreadLocal的一个静态内部类。其类注释如下：
/**
* 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.
*/
它是一个定制的HashMap（自然具有HashMap的相关特性，比如自动扩增容量等）。
前面提到A,B两个问题，从源码来看，A问题的解决方案就是，为每一个Thread维护一个HashMap，在这里就是维护一个ThreadLocalMap属性，这个属性的键是ThreadLocal，值就是资源副本，详细描述如下：
每一个Thread都有一个ThreadLocalMap属性，这个属性是类似于HashMap的，它以ThreadLocal为键，以属于该线程的资源副本为值。我们可以这样看待ThreadLocal：ThreadLocal是为一组线程维护资源副本的对象，通过它，可以为每一个线程创建资源副本，也可以正确获得属于某一线程的资源副本。
每一个ThreadLocal只能维护一个共享资源，一旦声明ThreadLocal实例，线程在调用的其get()方法获取资源副本的时候，就可以自动设置绑定到该线程本身。
好，现在转了一小圈回到get方法()。get()方法的第2、3行很明显是获取属于当前线程的ThreadLocalMap，如果这个map不为空，我们就以当前的ThreadLocal为键，去获取相应的Entry，Entry是ThreadLocalMap的静态内部类，其定义如下：
static class Entry extends WeakReference<ThreadLocal> {
/** The value associated with this ThreadLocal. */
Object value;
Entry(ThreadLocal k, Object v) {
super(k);
value = v;
}
}
它继承与弱引用，所以在get()方法里面如第7行一样调用e.value方法就可以获取实际的资源副本值。但是如果有一个为空，说明属于该线程的资源副本还不存在，则需要去创建资源副本，从代码中可以看到是调用setInitialValue()方法，其定义如下：
/**
* Variant of set() to establish initialValue. Used instead
* of set() in case user has overridden the set() method.
*
* @return the initial value
*/
private T setInitialValue() {
T value = initialValue();
Thread t = Thread.currentThread();
ThreadLocalMap map = getMap(t);
if (map != null)
map.set(this, value);
else
createMap(t, value);
return value;
}
第8行调用initialValue()方法初始化一个值，还记得在一开始的例子中，我们重载这个方法产生一个初始化的值么？
接下来是判断线程的ThreadLocalMap是否为空，不为空就直接这是值（键为this，值为value），为空则创建一个Map，调用方法为createMap()，其定义如下：
void createMap(Thread t, T firstValue) {
t.threadLocals = new ThreadLocalMap(this, firstValue);
}
简单明了，而ThreadLocalMap的这个构造方法的实现如下：
/**
* 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);
}
实例化table数组用于存储键值对，然后通过映射将键值对存储进入相应的位置。
至于set方法，看完get()后应该很简单了，自己都可以实现：
/**
* 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);
}
现在还剩一个问题B，会造成内存泄露吗？ThreadLocal的类注释里面有一段话：
/* 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).
*/
每一个线程对资源副本都有一个隐式引用：只要线程还在运行，只要ThreadLocal还是可以获取的。当一个线程运行结束销毁时，所有的资源副本都是可以被垃圾回收的。这段注释表明，ThreadLocal的使用是不会造成内训泄露的。
但是我们来仔细分析一下，我想画一张Thread、ThreadLocal和ThreadLocalMap的依赖关系图，但是在绘制过程中，我发现：根本就没有依赖！
非常惊讶，我在这里把ThreadLocal完整的源码贴一遍，读者可以自行审视。
package java.lang;
import java.lang.ref.*;
import java.util.concurrent.atomic.AtomicInteger; /**
* 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>ThreadId.get()</tt>
* and remains unchanged on subsequent calls.
* <pre>
* import java.util.concurrent.atomic.AtomicInteger;
*
* 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();
* }
* }
* </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
* @since 1.2
*/
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 <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;
} /**
* Creates a thread local variable.
*/
public ThreadLocal() {
} /**
* 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
*/
public T get() {
Thread t = Thread.currentThread();
ThreadLocalMap map = getMap(t);
if (map != null) {
ThreadLocalMap.Entry e = map.getEntry(this);
if (e != null)
return (T)e.value;
}
return setInitialValue();
} /**
* Variant of set() to establish initialValue. Used instead
* of set() in case user has overridden the set() method.
*
* @return the initial value
*/
private T setInitialValue() {
T value = initialValue();
Thread t = Thread.currentThread();
ThreadLocalMap map = getMap(t);
if (map != null)
map.set(this, value);
else
createMap(t, value);
return value;
} /**
* 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);
} /**
* 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
* <tt>initialValue</tt> method in the current thread.
*
* @since 1.5
*/
public void remove() {
ThreadLocalMap m = getMap(Thread.currentThread());
if (m != null)
m.remove(this);
} /**
* 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
* @param map the map to store.
*/
void createMap(Thread t, T firstValue) {
t.threadLocals = new ThreadLocalMap(this, firstValue);
} /**
* Factory method to create map of inherited thread locals.
* Designed to be called only from Thread constructor.
*
* @param parentMap the map associated with parent thread
* @return a map containing the parent's inheritable bindings
*/
static ThreadLocalMap createInheritedMap(ThreadLocalMap parentMap) {
return new ThreadLocalMap(parentMap);
} /**
* Method childValue is visibly defined in subclass
* InheritableThreadLocal, but is internally defined here for the
* sake of providing createInheritedMap factory method without
* needing to subclass the map class in InheritableThreadLocal.
* This technique is preferable to the alternative of embedding
* instanceof tests in methods.
*/
T childValue(T parentValue) {
throw new UnsupportedOperationException();
} /**
* 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; /**
* The number of entries in the table.
*/
private int size = 0; /**
* The next size value at which to resize.
*/
private int threshold; // Default to 0 /**
* Set the resize threshold to maintain at worst a 2/3 load factor.
*/
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);
} /**
* 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);
} /**
* 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) {
Entry[] parentTable = parentMap.table;
int len = parentTable.length;
setThreshold(len);
table = new Entry[len]; for (int j = 0; j < len; j++) {
Entry e = parentTable[j];
if (e != null) {
ThreadLocal key = e.get();
if (key != null) {
Object value = key.childValue(e.value);
Entry c = new Entry(key, value);
int h = key.threadLocalHashCode & (len - 1);
while (table[h] != null)
h = nextIndex(h, len);
table[h] = c;
size++;
}
}
}
} /**
* 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;
}
}
} /**
* 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);
} /**
* 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--; // Rehash until we encounter null
Entry e;
int i;
for (i = nextIndex(staleSlot, len);
(e = tab[i]) != null;
i = nextIndex(i, len)) {
ThreadLocal k = e.get();
if (k == null) {
e.value = null;
tab[i] = null;
size--;
} else {
int h = k.threadLocalHashCode & (len - 1);
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;
} /**
* 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: <tt>log2(n)</tt> cells are scanned,
* unless a stale entry is found, in which case
* <tt>log2(table.length)-1</tt> 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;
} /**
* Re-pack and/or re-size the table. First scan the entire
* table removing stale entries. If this doesn't sufficiently
* shrink the size of the table, double the table size.
*/
private void rehash() {
expungeStaleEntries(); // Use lower threshold for doubling to avoid hysteresis
if (size >= threshold - threshold / 4)
resize();
} /**
* Double the capacity of the table.
*/
private void resize() {
Entry[] oldTab = table;
int oldLen = oldTab.length;
int newLen = oldLen * 2;
Entry[] newTab = new Entry[newLen];
int count = 0; for (int j = 0; j < oldLen; ++j) {
Entry e = oldTab[j];
if (e != null) {
ThreadLocal k = e.get();
if (k == null) {
e.value = null; // Help the GC
} else {
int h = k.threadLocalHashCode & (newLen - 1);
while (newTab[h] != null)
h = nextIndex(h, newLen);
newTab[h] = e;
count++;
}
}
} setThreshold(newLen);
size = count;
table = newTab;
} /**
* Expunge all stale entries in the table.
*/
private void expungeStaleEntries() {
Entry[] tab = table;
int len = tab.length;
for (int j = 0; j < len; j++) {
Entry e = tab[j];
if (e != null && e.get() == null)
expungeStaleEntry(j);
}
}
}
}
ThreadLocal对Thread的引用全部通过局部变量完成，而没有一个全局变量。而实际的资源副本则存储在Thread的自身的属性ThreadLocalMap中，这说明，其实ThreadLocal只是关联一个Thread和其资源副本的桥梁，并且实际上Thread和资源副本的生命周期是紧密相连的，的的确确如ThreadLocal所说，在线程被回收的时候，其资源副本也会被回收，虽然ThreadLocal是静态的，但是它既不引用Thread，也不引用ThreadLocalMap。真是精巧的设计！
五、ThreadLocal作用
其实ThreadLocal所实现的功能前面已经描述的很清楚了，但是从网上的讨论来看，大家对ThreadLocal所要达成的目的意见却不是很统一，比如博客《彻底理解ThreadLocal》，具体可以看一下类的注释：
/**
* 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).
*/
从这个注解来看，和TLS（可以参见《Thread Local Storage》）大致是一样的，其实不用去深究其具体目的。个人觉得，ThreadLocal的功能就是为每一个线程提供一个资源副本，当你有这样的需求的时候，就可以使用ThreadLocal去解决问题。
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