正文
代码1.3
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);//有没有很熟悉
for (Entry e = tab[i];
e != null;
e = tab[i = nextIndex(i, len)]) {//线性探测法
ThreadLocal k = e.get();
if (k == key) {//key相同 替换值
e.value = value;
return;
}
//Entry 集成自 WeakReference<ThreadLocal> k很有可能为null
if (k == null) {
replaceStaleEntry(key, value, i);//查看代码1.4
return;
}
}
tab[i] = new Entry(key, value);//表里没数据 生成节点加入进去
int sz = ++size;//更改当前size
if (!cleanSomeSlots(i, sz) && sz >= threshold)//判断是否触发阈值 触发则扩容
rehash();//查看代码1.7
}
主要用线性探测法向数组中确定节点位置，与HashMap的链地址法实现方式不一样。
代码1.4
//replaceStaleEntry方法
private void replaceStaleEntry(ThreadLocal key, Object value,
int staleSlot) {
Entry[] tab = table;
int len = tab.length;
Entry e;
int slotToExpunge = staleSlot;//记录删除节点位置起始位置
for (int i = prevIndex(staleSlot, len);
(e = tab[i]) != null;
i = prevIndex(i, len))//从数组往前找 有节点但节点无key值则更新slotToExpunge ，否则停止查找
if (e.get() == null)
slotToExpunge = i;
for (int i = nextIndex(staleSlot, len);
(e = tab[i]) != null;
i = nextIndex(i, len)) {//线性探索查找key相同节点
ThreadLocal k = e.get();
if (k == key) {//如果 k == key 则更新value 讲该节点更新到 staleSlot位置上
e.value = value;
tab[i] = tab[staleSlot];
tab[staleSlot] = e;
// Start expunge at preceding stale entry if it exists
if (slotToExpunge == staleSlot)
slotToExpunge = i;
//清除部分节点expungeStaleEntry()查看代码1.5 cleanSomeSlots()查看代码1.6
cleanSomeSlots(expungeStaleEntry(slotToExpunge), len);
return;
}
if (k == null && slotToExpunge == staleSlot)
slotToExpunge = i;
}
// If key not found, put new entry in stale slot
tab[staleSlot].value = null;
tab[staleSlot] = new Entry(key, value);//未找到 生成新节点放入
// If there are any other stale entries in run, expunge them
if (slotToExpunge != staleSlot)
cleanSomeSlots(expungeStaleEntry(slotToExpunge), len);
}
代码1.5 方法expungeStaleEntry()
// 从删除节点到后边遍历 到第一个为 null节点之间的节点都经过检测 返回第一个null节点位置
private int expungeStaleEntry(int staleSlot) {
Entry[] tab = table;
int len = tab.length;
// expunge entry at staleSlot
tab[staleSlot].value = null;
tab[staleSlot] = null;//删除该位置节点
size--;
// Rehash until we encounter null
Entry e;
int i;
for (i = nextIndex(staleSlot, len);
(e = tab[i]) != null;
i = nextIndex(i, len)) {//从当前节点现象探索遍历
ThreadLocal k = e.get();
if (k == null) {//删除 key为null节点
e.value = null;
tab[i] = null;
size--;
} else {
int h = k.threadLocalHashCode & (len - 1);//根据数组长度计算出新的位置 因为数组有可能扩容
if (h != i) {//不相等的情况需要改变该节点位置
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)//讲h位置节点进行线性探测法确定位置
h = nextIndex(h, len);
tab[h] = e;//讲e节点更新到h位置
}
}
}
return i;
}
代码 1.6 方法cleanSomeSlots() 用于清除线性探索方向上的空节点
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);// 见 2.2.2
}
} while ( (n >>>= 1) != 0);//n = n>>>1 无符号右移动并赋值 这边每次除以2有点不太理解 欢迎大家讨论
return removed;
}
代码 1.7
private void rehash() {
expungeStaleEntries(); //见下边
// Use lower threshold for doubling to avoid hysteresis
if (size >= threshold - threshold / 4)
resize();//见1.8
}
/**
* Expunge all stale entries in the table.
*/
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);//见1.5
}
}
}
代码1.8 resize()
private void resize() {
Entry[] oldTab = table;
int oldLen = oldTab.length;
int newLen = oldLen * 2;//扩容 容量依然是2的幂
Entry[] newTab = new Entry[newLen];//生成新的数组
int count = 0;
for (int j = 0; j < oldLen; ++j) {//遍历旧的数组
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);//计算在新数组中的位置
while (newTab[h] != null)// 冲突后 线性探测法
h = nextIndex(h, newLen);
newTab[h] = e;
count++;
}
}
}
//更新属性
setThreshold(newLen);
size = count;
table = newTab;
}
上边就是ThreadLocal中set()方法的实现，主要： 向数组中插入节点，根据key (ThreadLocal)的threadLocalHashCode&(len-1)决定位置，然后根据线性探索法解决冲突问题，包括如果数组size超过阈值则扩容。
