什麼是線程局部變數？
- 零下一度原創
- 2017-07-21 14:34:262335瀏覽
ThreadLocal翻譯成中文比較準確的叫法應該是：線程局部變數。
這個玩意有什麼用處，或者說為什麼要有這麼一個東東？先解釋一下，在並發程式設計的時候，成員變數如果不做任何處理其實是線程不安全的，各個線程都在操作同一個變量，顯然是不行的，並且我們也知道volatile這個關鍵字也是不能保證線程安全的。那麼在有一種情況之下，我們需要滿足這樣一個條件：變數是同一個，但是每個執行緒都使用同一個初始值，也就是使用同一個變數的新的副本。這種情況之下ThreadLocal就非常使用，比如說DAO的資料庫連接，我們知道DAO是單例的，那麼他的屬性Connection就不是線程安全的變數。而我們每個線程都需要使用他，並且各自使用各自的。這種情況，ThreadLocal就比較好的解決了這個問題。
我們從原始碼的角度來分析這個問題。
先定義一個ThreadLocal：
ThreadLocal<Connection> tl = ThreadLocal<Connection> Connection initConn = = DriverManager.getConnection("url, name and password"=( ==
package java.lang;import java.lang.ref.*;import java.util.concurrent.atomic.AtomicInteger;public class ThreadLocal<T> { private final int threadLocalHashCode = nextHashCode(); private static AtomicInteger nextHashCode =new AtomicInteger();private static final int HASH_INCREMENT = 0x61c88647;private static int nextHashCode() {return nextHashCode.getAndAdd(HASH_INCREMENT);
}protected T initialValue() {return null;
}public ThreadLocal() {
}public T get() {
Thread t = Thread.currentThread();
ThreadLocalMap map = getMap(t);if (map != null) {
ThreadLocalMap.Entry e = map.getEntry(this);if (e != null)return (T)e.value;
}return setInitialValue();
}private T setInitialValue() {
T value = initialValue();
Thread t = Thread.currentThread();
ThreadLocalMap map = getMap(t);if (map != null)
map.set(this, value);elsecreateMap(t, value);return value;
}public void set(T value) {
Thread t = Thread.currentThread();
ThreadLocalMap map = getMap(t);if (map != null)
map.set(this, value);elsecreateMap(t, value);
} public void remove() {
ThreadLocalMap m = getMap(Thread.currentThread()); if (m != null)
m.remove(this);
}ThreadLocalMap getMap(Thread t) {return t.threadLocals;
}void createMap(Thread t, T firstValue) {
t.threadLocals = new ThreadLocalMap(this, firstValue);
}static ThreadLocalMap createInheritedMap(ThreadLocalMap parentMap) {return new ThreadLocalMap(parentMap);
}T childValue(T parentValue) {throw new UnsupportedOperationException();
}static class ThreadLocalMap {static class Entry extends WeakReference<ThreadLocal> {/** The value associated with this ThreadLocal. */Object value;
Entry(ThreadLocal k, Object v) {super(k);
value = v;
}
}private static final int INITIAL_CAPACITY = 16;private Entry[] table;private int size = 0;private int threshold; // Default to 0private void setThreshold(int len) {
threshold = len * 2 / 3;
}private static int nextIndex(int i, int len) {return ((i + 1 < len) ? i + 1 : 0);
}private static int prevIndex(int i, int len) {return ((i - 1 >= 0) ? i - 1 : len - 1);
}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);
}private ThreadLocalMap(ThreadLocalMap parentMap) {
Entry[] parentTable = parentMap.table;int len = parentTable.length;
setThreshold(len);
table = new Entry[len];for (int j = 0; j < len; j++) {
Entry e = parentTable[j];if (e != null) {
ThreadLocal key = e.get();if (key != null) {
Object value = key.childValue(e.value);
Entry c = new Entry(key, value);int h = key.threadLocalHashCode & (len - 1);while (table[h] != null)
h = nextIndex(h, len);
table[h] = c;
size++;
}
}
}
}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);
}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 void set(ThreadLocal key, Object value) {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();
}private void remove(ThreadLocal key) {
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)]) {if (e.get() == key) {
e.clear();
expungeStaleEntry(i);return;
}
}
}private void replaceStaleEntry(ThreadLocal key, Object value, int staleSlot) {
Entry[] tab = table;int len = tab.length;
Entry e;int slotToExpunge = staleSlot;for (int i = prevIndex(staleSlot, len);
(e = tab[i]) != null;
i = prevIndex(i, len))if (e.get() == null)
slotToExpunge = i;for (int i = nextIndex(staleSlot, len);
(e = tab[i]) != null;
i = nextIndex(i, len)) {
ThreadLocal k = e.get();if (k == key) {
e.value = value;
tab[i] = tab[staleSlot];
tab[staleSlot] = e;if (slotToExpunge == staleSlot)
slotToExpunge = i;
cleanSomeSlots(expungeStaleEntry(slotToExpunge), len);return;
}if (k == null && slotToExpunge == staleSlot)
slotToExpunge = i;
}tab[staleSlot].value = null;
tab[staleSlot] = new Entry(key, value);if (slotToExpunge != staleSlot)
cleanSomeSlots(expungeStaleEntry(slotToExpunge), len);
}private int expungeStaleEntry(int staleSlot) {
Entry[] tab = table;int len = tab.length;tab[staleSlot].value = null;
tab[staleSlot] = null;
size--; 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;while (tab[h] != null)
h = nextIndex(h, len);
tab[h] = e;
}
}
}return i;
}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();
}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;
}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);
}
}
}
}這樣子，都是用同一個連接，但是每個連接都是新的，是同一個連接的副本。
那麼實現機制是如何的呢？
1、每個Thread物件內部都維護了一個ThreadLocalMap這樣一個ThreadLocal的Map，可以存放若干個ThreadLocal
/* ThreadLocal values pertaining to this thread. This map is maintained * by the ThreadLocal class. */ThreadLocal.ThreadLocalMap threadLocals = null;
2、當我們在呼叫get( )方法的時候，先取得當前線程，然後取得到當前線程的ThreadLocalMap對象，如果非空，那麼取出ThreadLocal的value，否則進行初始化，初始化就是將initialValue的值set到ThreadLocal。
public T get() {
Thread t = Thread.currentThread();
ThreadLocalMap map = getMap(t);if (map != null) {
ThreadLocalMap.Entry e = map.getEntry(this);if (e != null)return (T)e.value;
}return setInitialValue();
}
3、當我們呼叫set()方法的時候，很常規，就是將值設為ThreadLocal。
4、總結：當我們呼叫get方法的時候，其實每個目前執行緒都有一個ThreadLocal。每次取得或設定都是對該ThreadLocal進行的操作，是與其他執行緒分開的。
5、應用程式場景：當很多執行緒需要多次使用同一個對象，並且需要該物件具有相同初始化值的時候最適合使用ThreadLocal。
6、其實說再多也不如看一下原始碼來得清晰。如果要看源碼，其中涉及到一個WeakReference和一個Map，這兩個地方需要了解下，這兩個東西分別是a.Java的弱引用，也就是GC的時候會銷毀該引用所包裹(引用)的對象，這個threadLocal作為key可能被銷毀，但是只要我們定義成他的類不卸載，tl這個強引用就始終引用著這個ThreadLocal的，永遠不會被gc掉。 b.和HashMap差不多。
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