带你读《2022技术人的百宝黑皮书》——合理使用线程池以及线程变量（10）

public T get() {
// 1. 获取当前线程
Thread t = Thread.currentThread();
// 2. 获取当前线程内部的ThreadLocalMap变量t.threadLocals;
ThreadLocalMap map = getMap(t);
// 3. 判断map是否为null
if (map != null) {
// 4. 使用当前threadLocal变量获取entry
ThreadLocalMap.Entry e = map.getEntry(this);
// 5. 判断entry是否为null
if (e != null) {
// 6.返回Entry.value
@SuppressWarnings("unchecked")
T result = (T)e.value;
return result;
  }
  }
// 7. 如果map/entry为null设置初始值
return setInitialValue(); 
}
  /**
* 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() {
// 1. 初始化value，如果重写就用重写后的value，默认null
T value = initialValue();
// 2. 获取当前线程
Thread t = Thread.currentThread();
// 3. 获取当前线程内部的ThreadLocalMap变量
ThreadLocalMap map = getMap(t);
if (map != null)
// 4. 不为null就set, key: threadLocal, value: value
map.set(this, value);
else
// 5. map若为null则创建ThreadLocalMap对象
createMap(t, value);
return value;
}
/**
*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.
*/
ThreadLocalMap(ThreadLocal<?> firstKey, Object firstValue) {
// 1. 初始化entry数组，size: 16
table = new Entry[INITIAL_CAPACITY];
// 2. 计算value的index
int i = firstKey.threadLocalHashCode & (INITIAL_CAPACITY - 1);
// 3. 在对应index位置赋值
table[i] = new Entry(firstKey, firstValue);
// 4. entry size size = 1;
// 5. 设置threshold: threshold = len * 2 / 3; setThreshold(INITIAL_CAPACITY);
}
/**
*Set the resize threshold to maintain at worst a 2/3 load factor.
*/
private void setThreshold(int len) { threshold = len * 2 / 3;
}