一、什么是ThreadLocal
顾名思义，线程本地变量，用ThreadLocal修饰的变量在线程间相互独立，互不影响。
二、编码体验
创建测试程序，分别启用两个线程，在各个线程中设置并打印当前用户名，观察输出；
public class Test {
private static ThreadLocal<String> usernameLocal = new ThreadLocal<>();
public static void main(String[] args) {
usernameLocal.set("main");
new Thread(() -> {
System.out.println("[a]" + usernameLocal.get());
usernameLocal.set("a");
System.out.println("[a]" + usernameLocal.get());
}).start();
new Thread(() -> {
System.out.println("[b]" + usernameLocal.get());
usernameLocal.set("b");
System.out.println("[b]" + usernameLocal.get());
}).start();
System.out.println("[main]" + usernameLocal.get());
}
}
由于多线程的交错性，结果顺序不一，但最终结果相同，每个线程操作的用户名（usernameLocal）互不影响；
[main]main
[a]null
[b]null
[a]a
[b]b
三、源码剖析
那么ThreadLocal是怎么做到线程变量隔离的？先查看ThreadLocal源码一探究竟。
她的构造方法是一个空的方法，那么我们就从最基础的set开始ThreadLocal的刨根问底之旅；
1. set
/**
* 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);
}
Sets the current thread's copy of this thread-local variable to the specified value，将当前线程局部变量的副本设置为指定值。
值得注意的是ThreadLocalMap这个类，它是ThreadLocal的静态内部类，手动简化下；
2、ThreadLocalMap
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;
// 注：省略一些代码
/**
* 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);
}
/**
* 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;
}
}
}
}
与 map 结构非常类似，根据 get/set 方法可以判断出 key 是 ThreadLocal 类型的，然后 value 就是我们实际需要存放的值。
值得注意的 Entry 是继承自 WeakReference，构造方法中可以观察出弱引用只针对于 key，也就是 ThreadLocal 变量。
了解了ThreadLocalMap的结构，那么回过头继续阅读ThreadLocal的set方法，有个关键的getMap方法：
/**
* 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;
}
原来是获取的是线程ThreadLocalMap类型的私有变量threadLocals，这差不多就能解释上面的为什么问题了。看看Thread类下面的私有变量threadLocals：
/* ThreadLocal values pertaining to this thread. This map is maintained
* by the ThreadLocal class. */
ThreadLocal.ThreadLocalMap threadLocals = null;
初始值为null，然后我们在set中看到当map为null会走createMap方法：
/**
* 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);
}
以当前的ThreadLocal对象实例为key，传入的值为value，初始化t线程的私有变量threadLocals(ThreadLocalMap)，有兴趣可以回到上面看看这个构造方法。
当t线程的threadLocals已经初始化后，再进行set操作，就简单多了，直接类似map的put方法，和上面的初始化设置操作一样。
图示更为清晰：
3、get
/**
* 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) {
@SuppressWarnings("unchecked")
T result = (T)e.value;
return result;
}
}
return setInitialValue();
}
理解了set的操作，get就容易的多了。先根据当前线程获取到threadLocals，然后如果这个threadLocals不为空并且根据当前ThreadLocal为key取到的entry也不为空，才返回entry中保存的值，否则返回setInitialVal()方法的结果；
/**
* 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;
}
先看看第一行的initialValue这个方法：
/**
* 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
*/
protected T initialValue() {
return null;
}
很简单的返回了一个null，结合前面的源码，就是说默认会把当前ThreadLocal实例为key，null为value初始化到threadLocals中取，并且返回null值。
if the programmer desires thread-local variables to have an initial value other than null, ThreadLocal must be subclassed，我们也可以通过重写该方法，改变这个默认值。
4、remove
/**
* 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
*/
public void remove() {
ThreadLocalMap m = getMap(Thread.currentThread());
if (m != null)
m.remove(this);
}
这个方法就是移除掉当前线程threadLocals中这个ThreadLocal实例对应的Entry，那么为什么需要有这个方法呢？
内存泄露，老生常谈的Java话题。当一个对象应该被释放掉但是还持有引用（GCRoot搜索），GC并不会释放掉这块内存，从而造成内存泄露。
5、弱引用
由于ThreadLocalMap.Entry继承自WeakReference的原因，穿插说说弱引用GC回收的情况；
String str = new String("hello");
WeakReference<String> entry = new WeakReference<>(str);
System.out.println("before gc, " + entry.get());
System.gc();
Thread.sleep(1000);
System.out.println("after gc, " + entry.get());
结果是：
before gc, hello
after gc, hello
为什么这个entry引用的String对象没有被回收，那是因为不止entry这个弱引用指向了这个对象，str这个强引用也指向了它，根据GCRoot根搜索原则，它也不会被回收。
那么我们修改测试代码：
WeakReference<String> entry = new WeakReference<>(new String("hello"));
System.out.println("before gc, " + entry.get());
System.gc();
Thread.sleep(1000);
System.out.println("after gc, " + entry.get());
结果是：
before gc, hello
after gc, null
印证了上面的分析。回到本文来说，如果在使用了ThreadLocal后，而没有手动执行remove方法，当这个线程迟迟没有结束，key 是弱引用，但是 value 无法被释放。
最典型的场景就是在线程池中使用 ThreadLocal，由于线程都是可缓存的，线程一直处在存活状况，每个线程 threadLocals 也会一直存在，那么所对应的Entry引用的对象空间除了 gc 也得不到释放。
所以，在每次使用 ThreadLocal 类时，最后记得使用remove方法显得尤为重要。
ThreadLocalMap 每次执行 get/set 都会进行自检操作 expungeStaleEntry，释放 key 等于 null 的 value 对象。
四、结束语
这是刨根问底系列的第一篇，从个人学习钻研源码的角度，一步一步地描述这个过程。相信不管是对于正在阅读这篇文章的你们，还是对于我自己，都是一个很好的学习和总结。
我个人偏向于洁癖，所以每一步粘贴的Java源码都原汁原味，带上官方的注释，有助于大家思考。除非万不得已，才略微插入几行注释加以说明。这也能在未来回顾这些原理时，能加以不同的思考，理解得更透彻。
