本文實例講述了Java ThreadLocal用法。分享給大家供大家參考，具體如下：
ThreadLocal實現了Java中線程局部變量。所謂線程局部變量就是保存在每個線程中獨有的一些數據，我們知道一個進程中的所有線程是共享該進程的資源的，線程對進程中的資源進行修改會反應到該進程中的其他線程上，如果我們希望一個線程對資源的修改不會影響到其他線程，那么就需要將該資源設為線程局部變量的形式。
如下示例所示，定義兩個ThreadLocal變量，然后分別在主線程和子線程中對線程局部變量進行修改，然后分別獲取線程局部變量的值：
public class ThreadLocalTest {
private static ThreadLocal<String> threadLocal1 = ThreadLocal.withInitial(() -> "threadLocal1 first value");
private static ThreadLocal<String> threadLocal2 = ThreadLocal.withInitial(() -> "threadLocal2 first value");
public static void main(String[] args) throws Exception{
Thread thread = new Thread(() -> {
System.out.println("================" + Thread.currentThread().getName() + " enter=================");
// 子線程中打印出初始值
printThreadLocalInfo();
// 子線程中設置新值
threadLocal1.set("new thread threadLocal1 value");
threadLocal2.set("new thread threadLocal2 value");
// 子線程打印出新值
printThreadLocalInfo();
System.out.println("================" + Thread.currentThread().getName() + " exit=================");
});
thread.start();
// 等待新線程執行
thread.join();
// 在main線程打印threadLocal1和threadLocal2，驗證子線程對這兩個變量的修改是否會影響到main線程中的這兩個值
printThreadLocalInfo();
// 在main線程中給threadLocal1和threadLocal2設置新值
threadLocal1.set("main threadLocal1 value");
threadLocal2.set("main threadLocal2 value");
// 驗證main線程中這兩個變量是否為新值
printThreadLocalInfo();
}
private static void printThreadLocalInfo() {
System.out.println(Thread.currentThread().getName() + ": " + threadLocal1.get());
System.out.println(Thread.currentThread().getName() + ": " + threadLocal2.get());
}
}
運行結果如下：
================Thread-0 enter================= Thread-0: threadLocal1 first value Thread-0: threadLocal2 first value Thread-0: new thread threadLocal1 value Thread-0: new thread threadLocal2 value ================Thread-0 exit================= main: threadLocal1 first value main: threadLocal2 first value main: main threadLocal1 value main: main threadLocal2 value
如果子線程對threadLocal1和threadLocal2的修改會影響到main線程中的threadLocal1和threadLocal2，那么在main線程第一次printThreadLocalInfo();打印出的應該是修改后的新值，即為new thread threadLocal1 value和new thread threadLocal2 value和，但實際打印結果并不是這樣，說明在新線程中對threadLocal1和threadLocal2的修改并不會影響到main線程中的這兩個變量，似乎main線程中的threadLocal1和threadLocal2作用域僅局限于main線程，新線程中的threadLocal1和threadLocal2作用域僅局限于新線程，這就是線程局部變量的由來。
如下圖所示
每個線程對象里會持有一個java.lang.ThreadLocal.ThreadLocalMap類型的threadLocals成員變量，而ThreadLocalMap里有一個java.lang.ThreadLocal.ThreadLocalMap.Entry[]類型的table成員，這是一個數組，數組元素是Entry類型，Entry中相當于有一個key和value，key指向所有線程共享的java.lang.ThreadLocal對象，value指向各線程私有的變量，這樣保證了線程局部變量的隔離性，每個線程只是讀取和修改自己所持有的那個value對象，相互之間沒有影響。
源碼包括ThreadLocal和ThreadLocalMap，ThreadLocalMap是ThreadLocal內定義的一個靜態內部類，用于存儲實際的數據。當調用ThreadLocal的get或者set方法時都有可能創建當前線程的threadLocals成員(ThreadLocalMap類型)。
ThreadLocal的get方法定義如下
/**
* Returns the value in the current thread's copy of this
* thread-local variable. If the variable has no value for the
* current thread, it is first initialized to the value returned
* by an invocation of the {@link #initialValue} method.
*
* @return the current thread's value of this thread-local
*/
public T get() {
// 獲取當前線程
Thread t = Thread.currentThread();
// 獲取當前線程的threadLocals成員變量，這是一個ThreadLocalMap
ThreadLocalMap map = getMap(t);
// threadLocals不為null則直接從threadLocals中取出ThreadLocal
// 對象對應的值
if (map != null) {
// 從map中獲取當前ThreadLocal對象對應Entry對象
ThreadLocalMap.Entry e = map.getEntry(this);
if (e != null) {
// 獲取ThreadLocal對象對應的value值
@SuppressWarnings("unchecked")
T result = (T)e.value;
return result;
}
}
// threadLocals為null，則需要創建ThreadLocalMap對象并賦給
// threadLocals，將當前ThreadLocal對象作為key，調用initialValue
// 獲得的初始值作為value，放置到threadLocals的entry中;
// 或者threadLocals不為null，但在threadLocals中未
// 找到當前ThreadLocal對象對應的entry，則需要向threadLocals添加新的
// entry，該entry以當前的ThreadLocal對象作為key，調用initialValue
// 獲得的值作為value
return setInitialValue();
}
/**
* Get the map associated with a ThreadLocal. Overridden in
* InheritableThreadLocal.
*
* @param t the current thread
* @return the map
*/
ThreadLocalMap getMap(Thread t) {
return t.threadLocals;
}
當Thread的threadLocals為null，或者在Thread的threadLocals中未找到當前ThreadLocal對象對應的entry，則進入到setInitialValue方法；否則進入到ThreadLocalMap的getEntry方法。
定義如下：
private T setInitialValue() {
// 獲取初始值,如果我們在定義ThreadLocal對象時實現了ThreadLocal
// 的initialValue方法，就會調用我們自定義的方法來獲取初始值，否則
// 使用initialValue的默認實現返回null值
T value = initialValue();
Thread t = Thread.currentThread();
// 獲取當前線程的threadLocals成員
ThreadLocalMap map = getMap(t);
if (map != null) {
// 若threadLocals存在則將ThreadLocal對象對應的value設置為初始值
map.set(this, value);
} else {
// 否則創建threadLocals對象并設置初始值
createMap(t, value);
}
if (this instanceof TerminatingThreadLocal) {
TerminatingThreadLocal.register((TerminatingThreadLocal<?>) this);
}
return value;
}
createMap方法實現
/**
* Create the map associated with a ThreadLocal. Overridden in
* InheritableThreadLocal.
*
* @param t the current thread
* @param firstValue value for the initial entry of the map
*/
void createMap(Thread t, T firstValue) {
// 創建一個ThreadLocalMap對象，用當前ThreadLocal對象和初始值value來
// 構造ThreadLocalMap中table的第一個entry。ThreadLocalMap對象賦
// 給線程的threadLocals成員
t.threadLocals = new ThreadLocalMap(this, firstValue);
}
ThreadLocalMap的構造方法定義如下：
/**
* 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.
*/
ThreadLocalMap(ThreadLocal<?> firstKey, Object firstValue) {
// 構造table數組，數組大小為INITIAL_CAPACITY
table = new Entry[INITIAL_CAPACITY];
// 計算key(ThreadLocal對象)在table中的索引
int i = firstKey.threadLocalHashCode & (INITIAL_CAPACITY - 1);
// 用ThreadLocal對象和value來構造entry對象，并放到table的第i個位置
table[i] = new Entry(firstKey, firstValue);
size = 1;
// 設置table的閾值，當table中元素個數超過該閾值時需要對table
// 進行resize，通常在調用ThreadLocalMap的set方法時會發生resize
setThreshold(INITIAL_CAPACITY);
}
/**
* Set the resize threshold to maintain at worst a 2/3 load factor.
*/
private void setThreshold(int len) {
threshold = len * 2 / 3;
}
這里firstKey.threadLocalHashCode是ThreadLocal中定義的一個hashcode，使用該hashcode進行hash運算從而找到該ThreadLocal對象對應的entry在table中的索引。
定義如下：
/**
* Get the entry associated with key. This method
* itself handles only the fast path: a direct hit of existing
* key. It otherwise relays to getEntryAfterMiss. This is
* designed to maximize performance for direct hits, in part
* by making this method readily inlinable.
*
* @param key the thread local object
* @return the entry associated with key, or null if no such
*/
private Entry getEntry(ThreadLocal<?> key) {
// 根據ThreadLocal的hashcode計算該ThreadLocal對象在table中的位置
int i = key.threadLocalHashCode & (table.length - 1);
Entry e = table[i];
// e為null則table不存在key對應的entry;
// e.get() != key 可能是由于hash沖突導致key對應的entry在table
// 的另外一個位置，需要繼續查找
if (e != null && e.get() == key)
return e;
else
// e==null或者e.get() != key 繼續查找key對應的entry
return getEntryAfterMiss(key, i, e);
}
getEntryAfterMiss方法定義如下：
/**
* 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;
// 從table的第i個位置一直往后找，直到找到鍵為key的entry為止
while (e != null) {
ThreadLocal<?> k = e.get();
// 若k==key，則找到了entry
if (k == key)
return e;
// k == null 需要刪除該entry
if (k == null)
expungeStaleEntry(i);
// k != key && k != null 繼續往后尋找，nextIndex就是取(i+1)
// 即table中第(i+1)個位置的entry
else
i = nextIndex(i, len);
e = tab[i];
}
return null;
}
expungeStaleEntry方法刪除key為null的entry，刪除后對staleSlot位置的entry和其后第一個為null的entry之間的entry進行一個rehash操作，rehash的目的是降低table發生碰撞的概率:
/**
* 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
// 刪除staleSlot位置的entry
tab[staleSlot].value = null;
tab[staleSlot] = null;
// table中元素個數減一
size--;
// Rehash until we encounter null
// 將table中staleSlot處entry和下一個為null的entry之間的
// entry重新進行hash放置到新的位置
// 遇到的entry的key為null則刪除該entry
Entry e;
int i;
for (i = nextIndex(staleSlot, len);
(e = tab[i]) != null;
i = nextIndex(i, len)) {
// e是下一個entry
ThreadLocal<?> k = e.get();
if (k == null) {
// 若entry的key為null，則刪除
e.value = null;
tab[i] = null;
size--;
} else {
// entry的key不為null，需要將entry放到新的位置
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.
// tab[h]不為null則發生沖突，繼續尋找下一個位置
while (tab[h] != null)
h = nextIndex(h, len);
tab[h] = e;
}
}
}
return i;
}
ThreadLocal的set方法定義如下：
/**
* Sets the current thread's copy of this thread-local variable
* to the specified value. Most subclasses will have no need to
* override this method, relying solely on the {@link #initialValue}
* method to set the values of thread-locals.
*
* @param value the value to be stored in the current thread's copy of
* this thread-local.
*/
public void set(T value) {
Thread t = Thread.currentThread();
// 獲取當前線程的threadLocals
ThreadLocalMap map = getMap(t);
// threadLocals不為null直接設置新值
if (map != null) {
map.set(this, value);
} else {
// threadLocals為null則需要創建ThreadLocalMap對象并賦給
// Thread的threadLocals成員
createMap(t, value);
}
}
createMap前面已經分析過，接下來分析ThreadLocalMap的set方法
ThreadLocalMap的set方法定義如下，將當前的ThreadLocal對象作為key，傳入的value為值，用key和value創建entry，放到table中適當的位置：
/**
* 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;
// 用key計算entry在table中的位置
int i = key.threadLocalHashCode & (len-1);
// tab[i]不為null的話，則第i個位置已經存在有效的entry，需要繼續
// 往后尋找新的位置
for (Entry e = tab[i];
e != null;
e = tab[i = nextIndex(i, len)]) {
ThreadLocal<?> k = e.get();
// 找到與key相同的entry，直接更新value的值
if (k == key) {
e.value = value;
return;
}
// 遇到key為null的entry，刪除該entry
if (k == null) {
replaceStaleEntry(key, value, i);
return;
}
}
// 此時第i個位置entry為null，將新entry放置到這個位置
tab[i] = new Entry(key, value);
int sz = ++size;
// 試圖清除無效的entry，若清除失敗并且table中有效entry個數
// 大于threshold，這進行rehash操作
if (!cleanSomeSlots(i, sz) && sz >= threshold)
rehash();
}
replaceStaleEntry的作用是用set方法傳過來的key和value構造entry，將這個entry放到staleSlot后面的某個位置：
/**
* 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).
// 從staleSlot往前找到第一個key為null的entry的位置
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
// 從staleSlot位置往后尋找
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.
// 若k與key相同，則直接更新value
if (k == key) {
e.value = value;
// 將原來staleSlot位置的entry放置到第i個位置，此時tab[i]處的entry的key為null
tab[i] = tab[staleSlot];
tab[staleSlot] = e;
// Start expunge at preceding stale entry if it exists
// 從staleSlot處往前未找到key為null的entry
if (slotToExpunge == staleSlot)
// tab[i]處entry的key為null，也即tab[slotToExpunge]處entry的key為null
slotToExpunge = i;
// 清除slotToExpunge位置的entry并進行rehash操作.....
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);
}
以下源碼只可意會，不可言傳…不再做說明
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;
}
rehash方法：
/**
* Re-pack and/or re-size the table. First scan the entire
* table removing stale entries. If this doesn't sufficiently
* shrink the size of the table, double the table size.
*/
private void rehash() {
expungeStaleEntries();
// Use lower threshold for doubling to avoid hysteresis
if (size >= threshold - threshold / 4)
resize();
}
expungeStaleEntries方法：
/**
* 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);
}
}
resize方法：
/**
* 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 (Entry e : oldTab) {
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;
}
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