概述
ThreadLocal是一个本地线程副本变量工具类，很多地方称作线程本地变量，也有些地方称作线程本地存储。其原理就是为每个线程都提供一个副本变量，使得这些变量是线程级别的、私有的变量。所以对于不同的线程，每次获取副本值时，别的线程并不能获取到当前线程的副本值，形成了对副本的隔离，使得各个线程之间互不影响，从而在高并发场景下实现无状态的调用，特别适用于各个线程依赖不同的变量值完成操作的场景。如为每个线程创建一个独立的数据库连接。
我们来看一个并发问题：当一个可变对象被多个线程访问时，可能会得到非预期的结果。例如，在《避免创建不必要的对象》一文中，DateUtils 的两个方法 format(Date date)和parse(String strDate)都是非线程安全的，它们在格式化日期的时候，共享从父类 DateFormat 继承而来的 Calendar 对象。为了解决这个并发问题，文中给出了一种基于ThreadLocal的解决方案，本文在此基础上，从源码的角度分析ThreadLocal是怎么实现线程安全的，所用java.version为1.8.0_05。
ThreadLocal get 源码解读
在使用 ThreadLocal 时，当前线程通过 get() 方法访问 ThreadLocal 中包含的变量。
/**
* 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(); // ① 获取当前线程
ThreadLocalMap map = getMap(t); // 获取当前线程的ThreadLocalMap
if (map != null) { // 判断 ThreadLocalMap 是否存在
ThreadLocalMap.Entry e = map.getEntry(this); // 调用 ThreadLocalMap 的 getEntry 方法
if (e != null) {
@SuppressWarnings("unchecked")
T result = (T)e.value;
return result;
}
}
return setInitialValue();
}
首先，在①处取得当前线程t；然后，通过getMap(t)方法获取一个变量map，map的类型为ThreadLocalMap；其次，获取<key,value>键值对e，注意，这里获取键值对时，传进去的请求参数是 this，而非当前线程t。最后，判断map是否为null。如果map为null，则调用setInitialValue方法返回value；否则，返回value值。现在，对get方法的每一句来仔细分析。首先看一下getMap方法中做了什么：
/**
* 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;
}
getMap返回当前线程t中的一个成员变量threadLocals，即
public class Thread implements Runnable {
/* ThreadLocal values pertaining to this thread. This map is maintained
* by the ThreadLocal class.
*/
ThreadLocal.ThreadLocalMap threadLocals = null;
}
从注释可以看到，threadLocals就是ThreadLocal的内部类ThreadLocalMap。所以每个 Thread 都会拥有一个 ThreadLocalMap 变量，用于存放属于该 Thread 私有的 ThreadLocal 变量。因此，ThreadLocal就相当于一个调度中心，每次调用 get 方法的时候，都会先找到当前线程的 ThreadLocalMap，然后再在这个 ThreadLocalMap 中找到对应的线程本地变量。
注意：ThreadLocal中之所以可以直接使用t.threadLocals，是因为Thread与ThreadLocal在同一个包下，同样Thread可以直接访问ThreadLocal.ThreadLocalMap threadLocals = null;来进行声明属性。ThreadLocalMap实现如下：
/**
* 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.
*/
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;
ThreadLocalMap 是ThreadLocal 内部的一个Map实现，然而它并没有实现任何集合的接口规范，因为它仅供内部使用，数据结构采用 数组 + 开方地址法，Entry 继承 WeakReference，是基于 ThreadLocal 这种特殊场景实现的 Map，以ThreadLocal作为key。
然后再继续看setInitialValue方法的具体实现：
/**
* 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;
}
/**
* 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;
}
显而易见，如果变量 map 不为空，就设置键值对；否则，调用createMap方法创建一个Map对象：
/**
* 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);
}
下面综述ThreadLocal是如何为每个线程创建变量副本的：
首先，在每个线程Thread内部有一个ThreadLocal.ThreadLocalMap类型的成员变量threadLocals，这个threadLocals就是用来存储实际的变量副本的，key为当前ThreadLocal变量，value为变量副本（即T类型的变量）。
初始化时，在Thread里面，threadLocals为空，当通过ThreadLocal变量调用get()方法或者set()方法，就会对Thread类中的threadLocals进行初始化，并且以当前ThreadLocal变量为键值，以ThreadLocal要保存的副本变量为value，存到threadLocals。
然后在当前线程里面，如果要使用副本变量，就可以通过get方法在threadLocals里面查找。
ThreadLocal set源码解读
ThreadLocal 还提供了修改和删除当前包含对象的方法，修改的方法为 set，删除的方法为 remove：
/**
* 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); // 调用 ThreadLocalMap 的 set 方法
else
createMap(t, value);
}
很好理解，如果当前 ThredLocal 还没有包含值，那么就调用 createMap 来初始化当前线程的 ThreadLocalMap 对象；否则，直接在 map 中修改当前 ThreadLocal（this）包含的值。
ThreadLocal remove源码解读
/**
* 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);
}
remove 方法就是获得当前线程的 ThreadLocalMap 对象，然后调用这个 map 的remove(ThreadLocal) 方法。查看 ThreadLocalMap 的 remove(ThreadLocal) 方法的实现：
/**
* 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;
}
}
}
关于remove(ThreadLocal<?> key)，在下一节中再展开介绍。
ThreadLocal和synchronized
ThreadLocal和Synchonized都用于解决多线程并发访问。synchronized是利用锁的机制，使变量或代码块在某一时刻仅仅能被一个线程访问，依此实现数据共享。它以“时间换空间”，访问串行化，对象共享化。而ThreadLocal为每个线程都提供了变量的副本，使得每个线程在某一时刻访问到的并非同一个对象，这样就隔离了多个线程对数据的共享，以“空间换时间”，访问并行化，对象独享化，缺点是增加了线程间的竞争，降低了效率。
Synchronized用于线程间的数据共享，而ThreadLocal则用于线程间的数据隔离。
Reference
- https://blog.csdn.net/sonny543/article/details/51336457
- https://www.jianshu.com/p/56f64e3c1b6c
- https://segmentfault.com/a/1190000010251063?utm_medium=referral&utm_source=tuicool