ThreadLocal詳解(二)
上一篇文章講了ThreadLocal的get、set、resize等方法的源碼，但是對於一些單獨的方法例如cleanSomeSlots、expungeStaleEntry並沒有講述，這一節就是要講述這些源碼，在講之前首先提幾個ThreadLocal的注意事項，一般來說就是ThreadLocal如果在使用不當的情況下會出現內存泄漏的問題，其原因就在於如果在線程清除時沒有清除ThreadLocal中的變量，因爲ThreadLocal中的key是當前線程的一個弱引用，此時噹噹前線程結束後，該key就變爲null了，此時ThreadLocalMap中就出現了key爲null，但是value仍然有值的情況，此時就出現了內存泄露。
改進
爲了解決因使用不當可能存在的內存泄漏問題，Doug Lea大師還是做了一些改進來儘可能的避免出現內存泄露，就是方法cleanSomeSlots、expungeStaleEntry、replaceStaleEntry，現在就來看看這三個方法做了哪些措施避免出現內存泄露問題。
replaceStaleEntry
原本準備先說cleanSomeSlots方法，但是在set操作源碼中對於key的處理中，有replaceStaleEntry源碼，因此先說replaceStaleEntry方法，先回到set操作源碼中。
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)]) {
ThreadLocal<?> k = e.get();
if (k == key) {
e.value = value;
return;
}
if (k == null) {
replaceStaleEntry(key, value, i);
return;
}
}
tab[i] = new Entry(key, value);
int sz = ++size;
if (!cleanSomeSlots(i, sz) && sz >= threshold)
rehash();
}
上述for循環插入新值的操作中，最後的if判斷是當value存在值，但是key爲null時進行的value替換操作，這就是在hash碰撞時，遇到髒key的操作，進入到方法replaceStaleEntry中。
/**
* Replace a stale entry encountered during a set operation
* with an entry for the specified key. The value passed in
* the value parameter is stored in the entry, whether or not
* an entry already exists for the specified key.
*
* As a side effect, this method expunges all stale entries in the
* "run" containing the stale entry. (A run is a sequence of entries
* between two null slots.)
*
* @param key the key
* @param value the value to be associated with key
* @param staleSlot index of the first stale entry encountered while
* searching for key.
*/
private void replaceStaleEntry(ThreadLocal<?> key, Object value,
int staleSlot) {
Entry[] tab = table;
int len = tab.length;
Entry e;
// Back up to check for prior stale entry in current run.
// We clean out whole runs at a time to avoid continual
// incremental rehashing due to garbage collector freeing
// up refs in bunches (i.e., whenever the collector runs).
int slotToExpunge = staleSlot;
for (int i = prevIndex(staleSlot, len);
(e = tab[i]) != null;
i = prevIndex(i, len))
if (e.get() == null)
slotToExpunge = i;
// Find either the key or trailing null slot of run, whichever
// occurs first
for (int i = nextIndex(staleSlot, len);
(e = tab[i]) != null;
i = nextIndex(i, len)) {
ThreadLocal<?> k = e.get();
// If we find key, then we need to swap it
// with the stale entry to maintain hash table order.
// The newly stale slot, or any other stale slot
// encountered above it, can then be sent to expungeStaleEntry
// to remove or rehash all of the other entries in run.
if (k == key) {
e.value = value;
tab[i] = tab[staleSlot];
tab[staleSlot] = e;
// Start expunge at preceding stale entry if it exists
if (slotToExpunge == staleSlot)
slotToExpunge = i;
cleanSomeSlots(expungeStaleEntry(slotToExpunge), len);
return;
}
// If we didn't find stale entry on backward scan, the
// first stale entry seen while scanning for key is the
// first still present in the run.
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);
// If there are any other stale entries in run, expunge them
if (slotToExpunge != staleSlot)
cleanSomeSlots(expungeStaleEntry(slotToExpunge), len);
}
首先看第一個for循環方法。
int slotToExpunge = staleSlot;
for (int i = prevIndex(staleSlot, len);
(e = tab[i]) != null;
i = prevIndex(i, len))
if (e.get() == null)
slotToExpunge = i;
這個for循環上面有段註解，爲:
// Back up to check for prior stale entry in current run.
// We clean out whole runs at a time to avoid continual
// incremental rehashing due to garbage collector freeing
// up refs in bunches (i.e., whenever the collector runs).
大致意思就是檢查當前髒key的前後元素，這裏認爲髒key也是成聚集在一起的，如果檢查到髒key，則前置slotToExpunge,最終檢查至沒有未遇到髒key爲止。
同時，在下一段代碼中，該for循環的操作,就是填充這些髒key了，可以看看源碼。
// Find either the key or trailing null slot of run, whichever
// occurs first
for (int i = nextIndex(staleSlot, len);
(e = tab[i]) != null;
i = nextIndex(i, len)) {
ThreadLocal<?> k = e.get();
// If we find key, then we need to swap it
// with the stale entry to maintain hash table order.
// The newly stale slot, or any other stale slot
// encountered above it, can then be sent to expungeStaleEntry
// to remove or rehash all of the other entries in run.
if (k == key) {
e.value = value;
tab[i] = tab[staleSlot];
tab[staleSlot] = e;
// Start expunge at preceding stale entry if it exists
if (slotToExpunge == staleSlot)
slotToExpunge = i;
cleanSomeSlots(expungeStaleEntry(slotToExpunge), len);
return;
}
這裏做了一個事情，就是向後環形查找過程中發現key相同的entry就覆蓋並且和髒entry進行交換。
if (slotToExpunge == staleSlot)
slotToExpunge = i;
cleanSomeSlots(expungeStaleEntry(slotToExpunge), len);
上述代碼，如果沒有查找到髒key，就以當前位置作爲起點執行cleanSomeSlots方法。
expungeStaleEntry
在看cleanSomeSlots方法前先看expungeStaleEntry方法，這個方法就比較容易理解了，就是清除髒key。
/**
* Expunge a stale entry by rehashing any possibly colliding entries
* lying between staleSlot and the next null slot. This also expunges
* any other stale entries encountered before the trailing null. See
* Knuth, Section 6.4
*
* @param staleSlot index of slot known to have null key
* @return the index of the next null slot after staleSlot
* (all between staleSlot and this slot will have been checked
* for expunging).
*/
private int expungeStaleEntry(int staleSlot) {
Entry[] tab = table;
int len = tab.length;
// expunge entry at staleSlot
tab[staleSlot].value = null;
tab[staleSlot] = null;
size--;
// Rehash until we encounter null
Entry e;
int i;
for (i = nextIndex(staleSlot, len);
(e = tab[i]) != null;
i = nextIndex(i, len)) {
ThreadLocal<?> k = e.get();
if (k == null) {
e.value = null;
tab[i] = null;
size--;
} else {
int h = k.threadLocalHashCode & (len - 1);
if (h != i) {
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)
h = nextIndex(h, len);
tab[h] = e;
}
}
}
return i;
}
將當前節點key與value均置爲null，便於當前位置放置新的entry，同時該方法還會做一個操作就是往後尋找直到tab[i]爲null爲止，如果在搜索過程中繼續遇到髒key，則繼續執行清除操作。
cleanSomeSlots
現在回頭來看cleanSomeSlots方法。
/**
* Heuristically scan some cells looking for stale entries.
* This is invoked when either a new element is added, or
* another stale one has been expunged. It performs a
* logarithmic number of scans, as a balance between no
* scanning (fast but retains garbage) and a number of scans
* proportional to number of elements, that would find all
* garbage but would cause some insertions to take O(n) time.
*
* @param i a position known NOT to hold a stale entry. The
* scan starts at the element after i.
*
* @param n scan control: {@code log2(n)} cells are scanned,
* unless a stale entry is found, in which case
* {@code log2(table.length)-1} additional cells are scanned.
* When called from insertions, this parameter is the number
* of elements, but when from replaceStaleEntry, it is the
* table length. (Note: all this could be changed to be either
* more or less aggressive by weighting n instead of just
* using straight log n. But this version is simple, fast, and
* seems to work well.)
*
* @return true if any stale entries have been removed.
*/
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);
}
} while ( (n >>>= 1) != 0);
return removed;
}
cleanSomeSlots操作就比較簡單了，cleanSomeSlots中n這個參數就是當前tab的長度，然後while循環中進行n >>>= 1,就是一個log2(n)，就行log2(n)次循環後，當前清理髒key就結束了。
綜上
threadLocal這一套清理髒key還是挺複雜的，另外也和我們平常認識的有些不同，平常在用時一直以爲此處沒有髒key的處理操作，但現在知道前人對threadLocal還是做了許多優化，而且對以上髒key的處理操作中，並非是能將所有髒key都清理掉，而是在性能和髒key處理上做了一定的折中的選擇，因此在自己平時使用時仍然要避免髒key的出現。