InheritableThreadLocal 是 Threadlocal的扩展类，核心方法都在Threadlocal中，所以先来看这个类：
1. Threadlocal：
Threadlocal提供了线程内共享的变量。每个访问一个（通过它的get或set方法）的线程都有它自己的、独立初始化的变量空间，即Threadlocal为每个线程单独维护一份副本。 ThreadLocal可以用来在线程内部传递数据（例如，业务中的用户 ID 或事务 ID）。
既然与线程相关，可以看一下Thread的源码，里面有这样一个属性：
/* ThreadLocal values pertaining to this thread. This map is maintained
* by the ThreadLocal class. */
ThreadLocal.ThreadLocalMap threadLocals = null;可以看到每个线程中都维护了一个ThreadlocalMap，map的key是Threadlocal对象，value是该线程中要传递的数据，并且key的哈希算法也是Threadlocal中定制的。
//每次创建一个Threadlocal，就会分配一个对应的哈希码
private final int threadLocalHashCode = nextHashCode();
//注意是static的，所有对象共享，从0开始递增的哈希码
private static AtomicInteger nextHashCode = new AtomicInteger();
//每个哈希码之间的间隔，允许哈希值在map中均匀的分布，防止哈希冲突
private static final int HASH_INCREMENT = 0x61c88647;
private static int nextHashCode() {
return nextHashCode.getAndAdd(HASH_INCREMENT);
}因此想要在线程的map里添加数据，必须要在该线程中创建一个新的Threadlocal对象。只要线程处于活动状态并且ThreadLocal实例可访问，每个线程都持有对其线程局部变量副本的隐式引用（弱引用）；在线程对象销毁后，该线程本地实例副本都将被GC回收（除非存在对这些副本的其他引用）。
//返回此线程局部变量的当前线程的“初始值”。该方法将在线程第一次使用get方法访问变量时调用，除非该线 //程之前调用了set方法，在这种情况下，不会为该线程调用initialValue方法。通常，每个线程最多调用一次
//此方法，但如果后续调用remove后跟get ，则可能会再次调用它。此实现仅返回null ；如果程序员希望线程
//局部变量具有除null以外的初始值，则必须将ThreadLocal子类化，并重写此方法。通常将使用匿名内部类。
protected T initialValue() {
return null;
}
//这个方法使用了一个Supplier<? extends S>对象，其泛型继承了Threadlocal的泛型，也就是可以通过该对象的get方法为Threadlocal变量赋值
public static <S> ThreadLocal<S> withInitial(Supplier<? extends S> supplier) {
return new SuppliedThreadLocal<>(supplier);
}
//Threadlocal的静态内部类
static final class SuppliedThreadLocal<T> extends ThreadLocal<T> {
private final Supplier<? extends T> supplier;
SuppliedThreadLocal(Supplier<? extends T> supplier) {
this.supplier = Objects.requireNonNull(supplier);
}
@Override
protected T initialValue() {
return supplier.get();
}
}接下来要看它核心的get和set方法了：
//这两个方法的逻辑是：先判断当前线程的ThreadlocalMap是否存在
//如果存在直接获取值，如果不存在，创建新的map并设置初始化值
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();
}
//initialValue()需要子类去实现
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;
}
//与上一个方法极其类似
public void set(T value) {
Thread t = Thread.currentThread();
ThreadLocalMap map = getMap(t);
if (map != null)
map.set(this, value);
else
createMap(t, value);
}
//顺便把remove方法也带上吧
public void remove() {
ThreadLocalMap m = getMap(Thread.currentThread());
if (m != null)
m.remove(this);
}
//获取当前线程的ThreadlocalMap
ThreadLocalMap getMap(Thread t) {
return t.threadLocals;
}
这里面用到的ThreadlocalMap，静态内部类，其操作方式跟HashMap类似，只不过哈希值计算的方法不一样，关键点请看注释：
static class ThreadLocalMap {
//这里的map结点继承了弱引用，同时可以看到构造方法里面，使用了super(k)
//表明了这个节点的key是一个弱引用，这样保证了线程结束的时候，key不会出现
//内存泄漏的问题，注意，k是Threadlocal对象，它本身有一个外部的引用指向他
//但是这里仍然没有解决value的泄漏问题，所以当不需要Threadlocal对象的值时，
//在每个线程中需要手动执行Threadlocal.remove()方法，否则会出现内存泄漏
static class Entry extends WeakReference<ThreadLocal<?>> {
Object value;
Entry(ThreadLocal<?> k, Object v) {
super(k);
value = v;
}
}
//下面的属性和方法与HashMap类似
private static final int INITIAL_CAPACITY = 16;
private Entry[] table;
private int size = 0;
private int threshold;
private void setThreshold(int len) {
threshold = len * 2 / 3;
}
private static int nextIndex(int i, int len) {
return ((i + 1 < len) ? i + 1 : 0);
}
private static int prevIndex(int i, int len) {
return ((i - 1 >= 0) ? i - 1 : len - 1);
}
//懒加载方式，只有需要的时候才创建
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);
}
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) {
@SuppressWarnings("unchecked")
ThreadLocal<Object> key = (ThreadLocal<Object>) 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++;
}
}
}
}
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);
}
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;
}
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();
}
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;
}
}
}
private void replaceStaleEntry(ThreadLocal<?> key, Object value,
int staleSlot) {
Entry[] tab = table;
int len = tab.length;
Entry e;
int slotToExpunge = staleSlot;
for (int i = prevIndex(staleSlot, len);
(e = tab[i]) != null;
i = prevIndex(i, len))
if (e.get() == null)
slotToExpunge = i;
for (int i = nextIndex(staleSlot, len);
(e = tab[i]) != null;
i = nextIndex(i, len)) {
ThreadLocal<?> k = e.get();
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 (k == null && slotToExpunge == staleSlot)
slotToExpunge = i;
}
tab[staleSlot].value = null;
tab[staleSlot] = new Entry(key, value);
if (slotToExpunge != staleSlot)
cleanSomeSlots(expungeStaleEntry(slotToExpunge), len);
}
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;
}
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;
}
private void rehash() {
expungeStaleEntries();
// Use lower threshold for doubling to avoid hysteresis
if (size >= threshold - threshold / 4)
resize();
}
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;
}
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);
}
}
}2. InheritableThreadLocal：
有了Threadlocal的基础，这个就很好理解了，它允许被创建的子线程继承创建它的父线程的所有Threadlocal变量，也就是允许Threadlocal在父子线程（注意不是父子类）中传递。
//这里直接传递了父线程的值，没有做任何处理
protected T childValue(T parentValue) {
return parentValue;
}
//可以看到每个线程中还专门维护了一个父子线程共享的ThreadlocalMap
ThreadLocalMap getMap(Thread t) {
return t.inheritableThreadLocals;
}
void createMap(Thread t, T firstValue) {
t.inheritableThreadLocals = new ThreadLocalMap(this, firstValue);
}
