3、ThreadLocal最主要的實(shí)現(xiàn)在于ThreadLocalMap這個(gè)內(nèi)部類里面，我們重點(diǎn)關(guān)注ThreadLocalMap這個(gè)類的用法，看看兩位大師Josh Bloch and Doug Lea是什么設(shè)計(jì)出如此好的類。
static class Entry extends WeakReference<ThreadLocal<?>> {/** The value associated with this ThreadLocal. */Object value;Entry(ThreadLocal<?> k, Object v) {super(k);value = v;}}
/*** 初始容量，必須為2的冪.*/private static final int INITIAL_CAPACITY = 16;/*** 根據(jù)需要調(diào)整大小。* 長度必須總是2的冪。*/private Entry[] table;/*** 表中條目的數(shù)量。*/private int size = 0;/*** 要調(diào)整大小的下一個(gè)大小值。默認(rèn)為0*/private int threshold; // Default to 0/*** 將調(diào)整大小閾值設(shè)置維持最壞2/3的負(fù)載因子。*/private void setThreshold(int len) {threshold = len * 2 / 3;}/***上一個(gè)索引*/private static int nextIndex(int i, int len) {return ((i + 1 < len) ? i + 1 : 0);}/*** 下一個(gè)索引*/private static int prevIndex(int i, int len) {return ((i - 1 >= 0) ? i - 1 : len - 1);}
/*** 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.* 構(gòu)造一個(gè)最初包含(firstKey, firstValue)的新映射threadlocalmap是延遲構(gòu)造的，因 * 此當(dāng)我們至少有一個(gè)元素可以放進(jìn)去的時(shí)候才去創(chuàng)建。*/ThreadLocalMap(ThreadLocal<?> firstKey, Object firstValue) {table = new Entry[INITIAL_CAPACITY];int i = firstKey.threadLocalHashCode & (INITIAL_CAPACITY - 1);table[i] = new Entry(firstKey, firstValue);size = 1;setThreshold(INITIAL_CAPACITY);}
重點(diǎn)說下這個(gè)hash函數(shù)int i = firstKey.threadLocalHashCode & (INITIAL_CAPACITY - 1); ThreadLocal類中有一個(gè)被final修飾的類型為int的threadLocalHashCode，它在該ThreadLocal被構(gòu)造的時(shí)候就會(huì)生成，相當(dāng)于一個(gè)ThreadLocal的ID，而它的值來源于
private final int threadLocalHashCode = nextHashCode();/*** The next hash code to be given out. Updated atomically. Starts at* zero.*/private static AtomicInteger nextHashCode =new AtomicInteger();/*** 連續(xù)生成的哈希碼之間的區(qū)別——循環(huán)隱式順序線程本地id以近乎最優(yōu)的方式展開* 用于兩倍大小表的乘法哈希值。*/private static final int HASH_INCREMENT = 0x61c88647;/*** 返回下一個(gè)hashcode*/private static int nextHashCode() {return nextHashCode.getAndAdd(HASH_INCREMENT);}
private Entry getEntry(ThreadLocal<?> key) {int i = key.threadLocalHashCode & (table.length - 1);Entry e = table[i];if (e != null && e.get() == key)return e;elsereturn getEntryAfterMiss(key, i, e);}/*** Version of getEntry method for use when key is not found in* its direct hash slot.** @param key the thread local object* @param i the table index for key's hash code* @param e the entry at table[i]* @return the entry associated with key, or null if no such*/private Entry getEntryAfterMiss(ThreadLocal<?> key, int i, Entry e) {Entry[] tab = table;int len = tab.length;while (e != null) {ThreadLocal<?> k = e.get();if (k == key)return e;if (k == null)expungeStaleEntry(i);elsei = nextIndex(i, len);e = tab[i];}return null;}private int expungeStaleEntry(int staleSlot) {Entry[] tab = table;int len = tab.length;// expunge entry at staleSlottab[staleSlot].value = null;tab[staleSlot] = null;size--;// Rehash until we encounter nullEntry 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;} private int expungeStaleEntry(int staleSlot) {Entry[] tab = table;int len = tab.length;// expunge entry at staleSlottab[staleSlot].value = null;tab[staleSlot] = null;size--;// Rehash until we encounter nullEntry 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;}
/*** Set the value associated with key.** @param key the thread local object* @param value the value to be set*/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();}/*** 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 firstfor (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 existsif (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 slottab[staleSlot].value = null;tab[staleSlot] = new Entry(key, value);// If there are any other stale entries in run, expunge themif (slotToExpunge != staleSlot)cleanSomeSlots(expungeStaleEntry(slotToExpunge), len);}/*** 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;}private void rehash() {expungeStaleEntries();// Use lower threshold for doubling to avoid hysteresisif (size >= threshold - threshold / 4)resize();}/*** Double the capacity of the table.*/private void resize() {Entry[] oldTab = table;int oldLen = oldTab.length;int newLen = oldLen * 2;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;}/*** 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);}}
