ThreadLocal理解

/**
* 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);
}

/**
* Get the map associated with a ThreadLocal. Overridden in
* InheritableThreadLocal.
* 获取的是Thread局部变量表的数据，即能保证每个线程私有
* @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
*/
void createMap(Thread t, T firstValue) {
t.threadLocals = new ThreadLocalMap(this, firstValue);
}

/**
* 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.
* 为什么是Map？因为可以new 多个ThreadLocal
* map也是数组加链表，没有什么好说的
*/
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;
//设置扩容值len*2/3
setThreshold(INITIAL_CAPACITY);
}

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();
}

//如果数组中的i位置的第一个entry不是需要找的，遍历链表
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);
}

既然ThreadLocal是线程私有的，那么子线程能获取父线程的ThreadLocal吗?
是可以的

public static void main1(String[] args) {
ThreadLocal<String> threadLocal = new InheritableThreadLocal<>();
threadLocal.set("1231");
threadLocal.get();
Thread thread = new Thread(() -> {
System.out.println("thread:"+threadLocal.get());
});
thread.start();
}

原理是new Thread的时候
if (parent.inheritableThreadLocals != null)
this.inheritableThreadLocals =
ThreadLocal.createInheritedMap(parent.inheritableThreadLocals);
会给Thread中的inheritableThreadLocals初始化，并且InheritableThreadLocal重写了getMap方法。