首先,从set方法入手,
// ThreadLocal
public void set(T value) {
Thread t = Thread.currentThread();
ThreadLocalMap map = getMap(t);//这里可以看出,从Thread对象获取了一个ThreadLocalMap
if (map != null)
map.set(this, value);
else
createMap(t, value);
}
ThreadLocalMap getMap(Thread t) {
return t.threadLocals;
}
//Thread
ThreadLocal.ThreadLocalMap threadLocals = null;
接下来,看如何创建的 这个特殊的Map
首次创建
ThreadLocalMap(ThreadLocal<?> firstKey, Object firstValue) {
table = new Entry[INITIAL_CAPACITY];//初始化空间为16
int i = firstKey.threadLocalHashCode & (INITIAL_CAPACITY - 1);//机损索引
table[i] = new Entry(firstKey, firstValue);//赋值
size = 1;
setThreshold(INITIAL_CAPACITY);//计算扩容的临界点
}
注意点:Entry 的 key 采用弱引用,在内存不足时候,会被回收
static class Entry extends WeakReference<ThreadLocal<?>> {
/** The value associated with this ThreadLocal. */
Object value;
Entry(ThreadLocal<?> k, Object v) {
super(k);// k 采用弱引用,在内存不足时候,会被回收
value = v;
}
}
到这里,细节不说 应该了解大体的数据结构了
每个线程独有一个 Map,Map里的存储结构为 Entry <ThreadLocal,Object> 数组
如果已存在,
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)]) {//循环,直到i位置无数据
ThreadLocal<?> k = e.get();
if (k == key) {//同一个ThreadLocal 替换 value
e.value = value;
return;
}
if (k == null) {// k 为空,替换
replaceStaleEntry(key, value, i);
return;
}
}
tab[i] = new Entry(key, value);
int sz = ++size;
if (!cleanSomeSlots(i, sz) && sz >= threshold)//清除无效Entry 并扩容
rehash();
}
扩容
private void resize() {
Entry[] oldTab = table;
int oldLen = oldTab.length;
int newLen = oldLen * 2;// 双倍空间
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;
}
基于逻辑完成,
get就比较简单了
先获取 Map ,然后根据 key 获取 value
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();
}
关于ThreadLocal 内存溢出问题,一般只会出现在使用 线程池的时候,ThreadLocalMap 一直得不到释放,即使 key 因为软引用,但value没有释放造成的,但可以通过执行 remove 方法来主动释放
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