总结
在线程创建时，把父线程的ThreadLocal 的值 放到了inheritableThreadLocals中
InheritableThreadLocal 的源码
public class InheritableThreadLocal<T> extends ThreadLocal<T> {
// ①
protected T childValue(T parentValue) {
return parentValue;
}
// ②
ThreadLocalMap getMap(Thread t) {
return t.inheritableThreadLocals;
}
// ③
void createMap(Thread t, T firstValue) {
t.inheritableThreadLocals = new ThreadLocalMap(this, firstValue);
}
}
public class Thread implements Runnable {
/*
* InheritableThreadLocal values pertaining to this thread. This map is
* maintained by the InheritableThreadLocal class.
*/
ThreadLocal.ThreadLocalMap inheritableThreadLocals = null;
可以看到，InheritableThreadLocal 继承了ThreadLocal，并且重写了三个方法，看来实现的门道就在这三个方法里面。
先看代码③，InheritableThreadLocal 重写了createMap方法，那么现在当第一次调用set方法时，创建的是当前线程的inheritableThreadLocals 变量的实例而不再是threadLocals。由代码②可知，当调用get方法获取当前线程内部的map变量时，获取的是inheritableThreadLocals而不再是threadLocals。
可以这么说，在InheritableThreadLocal的世界里，变量inheritableThreadLocals替代了threadLocals。
代码②③都讲了，再来看看代码①，以及如何让子线程可以访问父线程的本地变量。
这要从创建Thread的代码说起，打开Thread类的默认构造函数，代码如下。
public Thread(Runnable target) {
init(null, target, "Thread-" + nextThreadNum(), 0);
}
private void init(ThreadGroup g, Runnable target, String name,
long stackSize, AccessControlContext acc,
boolean inheritThreadLocals) {
// ... 省略无关部分
// 获取父线程 - 当前线程
Thread parent = currentThread();
// ... 省略无关部分
// 如果父线程的inheritThreadLocals不为null 且 inheritThreadLocals=true
if (inheritThreadLocals && parent.inheritableThreadLocals != null)
// 设置子线程中的inheritableThreadLocals变量
this.inheritableThreadLocals =
ThreadLocal.createInheritedMap(parent.inheritableThreadLocals);
// ... 省略无关部分
}
static ThreadLocalMap createInheritedMap(ThreadLocalMap parentMap) {
return new ThreadLocalMap(parentMap);
}
再来看看里面是如何执行createInheritedMap 的。
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) {
// 这里调用了重写的代码① childValue
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++;
}
}
}
}
在该构造函数内部把父线程的inheritableThreadLocals成员变量的值复制到新的ThreadLocalMap 对象中。
参考：ThreadLoacal
