JavaThreadLocal用法的實(shí)例分析

JavaThreadLocal用法的實(shí)例分析，針對(duì)這個(gè)問(wèn)題，這篇文章詳細(xì)介紹了相對(duì)應(yīng)的分析和解答，希望可以幫助更多想解決這個(gè)問(wèn)題的小伙伴找到更簡(jiǎn)單易行的方法。

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ThreadLocal實(shí)現(xiàn)了Java中線程局部變量。所謂線程局部變量就是保存在每個(gè)線程中獨(dú)有的一些數(shù)據(jù)，我們知道一個(gè)進(jìn)程中的所有線程是共享該進(jìn)程的資源的，線程對(duì)進(jìn)程中的資源進(jìn)行修改會(huì)反應(yīng)到該進(jìn)程中的其他線程上，如果我們希望一個(gè)線程對(duì)資源的修改不會(huì)影響到其他線程，那么就需要將該資源設(shè)為線程局部變量的形式。

ThreadLocal的基本使用

如下示例所示，定義兩個(gè)ThreadLocal變量，然后分別在主線程和子線程中對(duì)線程局部變量進(jìn)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(shè)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(zhí)行 thread.join();

// 在main線程打印threadLocal1和threadLocal2，驗(yàn)證子線程對(duì)這兩個(gè)變量的修改是否會(huì)影響到main線程中的這兩個(gè)值 printThreadLocalInfo();

// 在main線程中給threadLocal1和threadLocal2設(shè)置新值 threadLocal1.set("main threadLocal1 value"); threadLocal2.set("main threadLocal2 value");

// 驗(yàn)證main線程中這兩個(gè)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(yùn)行結(jié)果如下：

================Thread-0 enter=================Thread-0: threadLocal1 first valueThread-0: threadLocal2 first valueThread-0: new thread threadLocal1 valueThread-0: new thread threadLocal2 value================Thread-0 exit=================main: threadLocal1 first valuemain: threadLocal2 first valuemain: main threadLocal1 valuemain: main threadLocal2 value

如果子線程對(duì)threadLocal1和threadLocal2的修改會(huì)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(yīng)該是修改后的新值，即為new thread threadLocal1 value和new thread threadLocal2 value和，但實(shí)際打印結(jié)果并不是這樣，說(shuō)明在新線程中對(duì)threadLocal1和threadLocal2的修改并不會(huì)影響到main線程中的這兩個(gè)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(lái)。

ThreadLocal實(shí)現(xiàn)原理

如下圖所示每個(gè)線程對(duì)象里會(huì)持有一個(gè)java.lang.ThreadLocal.ThreadLocalMap類(lèi)型的threadLocals成員變量，而ThreadLocalMap里有一個(gè)java.lang.ThreadLocal.ThreadLocalMap.Entry[]類(lèi)型的table成員，這是一個(gè)數(shù)組，數(shù)組元素是Entry類(lèi)型，Entry中相當(dāng)于有一個(gè)key和value，key指向所有線程共享的java.lang.ThreadLocal對(duì)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(gè)線程只是讀取和修改自己所持有的那個(gè)value對(duì)象，相互之間沒(méi)有影響。

源碼分析(基于openjdk11)

源碼包括ThreadLocal和ThreadLocalMap，ThreadLocalMap是ThreadLocal內(nèi)定義的一個(gè)靜態(tài)內(nèi)部類(lèi)，用于存儲(chǔ)實(shí)際的數(shù)據(jù)。當(dāng)調(diào)用ThreadLocal的get或者set方法時(shí)都有可能創(chuàng)建當(dāng)前線程的threadLocals成員(ThreadLocalMap類(lèi)型)。

get方法：

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(dāng)前線程 Thread t = Thread.currentThread();

// 獲取當(dāng)前線程的threadLocals成員變量，這是一個(gè)ThreadLocalMap ThreadLocalMap map = getMap(t);

// threadLocals不為null則直接從threadLocals中取出ThreadLocal

// 對(duì)象對(duì)應(yīng)的值 if (map != null) {

// 從map中獲取當(dāng)前ThreadLocal對(duì)象對(duì)應(yīng)Entry對(duì)象 ThreadLocalMap.Entry e = map.getEntry(this);

if (e != null) {

// 獲取ThreadLocal對(duì)象對(duì)應(yīng)的value值 @SuppressWarnings("unchecked") T result = (T)e.value; return result; } }

// threadLocals為null，則需要?jiǎng)?chuàng)建ThreadLocalMap對(duì)象并賦給

// threadLocals，將當(dāng)前ThreadLocal對(duì)象作為key，調(diào)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(dāng)前ThreadLocal對(duì)象對(duì)應(yīng)的entry，則需要向threadLocals添加新的

// entry，該entry以當(dāng)前的ThreadLocal對(duì)象作為key，調(diào)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(dāng)Thread的threadLocals為null，或者在Thread的threadLocals中未找到當(dāng)前ThreadLocal對(duì)象對(duì)應(yīng)的entry，則進(jìn)入到setInitialValue方法；否則進(jìn)入到ThreadLocalMap的getEntry方法。

setInitialValue方法

定義如下：

private T setInitialValue() { // 獲取初始值,如果我們?cè)诙xThreadLocal對(duì)象時(shí)實(shí)現(xiàn)了ThreadLocal // 的initialValue方法，就會(huì)調(diào)用我們自定義的方法來(lái)獲取初始值，否則 // 使用initialValue的默認(rèn)實(shí)現(xiàn)返回null值 T value = initialValue(); Thread t = Thread.currentThread(); // 獲取當(dāng)前線程的threadLocals成員 ThreadLocalMap map = getMap(t); if (map != null) { // 若threadLocals存在則將ThreadLocal對(duì)象對(duì)應(yīng)的value設(shè)置為初始值 map.set(this, value); } else { // 否則創(chuàng)建threadLocals對(duì)象并設(shè)置初始值 createMap(t, value); } if (this instanceof TerminatingThreadLocal) { TerminatingThreadLocal.register((TerminatingThreadLocal<?>) this); } return value; }

createMap方法實(shí)現(xiàn)

/** * 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(chuàng)建一個(gè)ThreadLocalMap對(duì)象，用當(dāng)前ThreadLocal對(duì)象和初始值value來(lái)

// 構(gòu)造ThreadLocalMap中table的第一個(gè)entry。ThreadLocalMap對(duì)象賦

// 給線程的threadLocals成員 t.threadLocals = new ThreadLocalMap(this, firstValue); }

ThreadLocalMap的構(gòu)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(gòu)造table數(shù)組，數(shù)組大小為INITIAL_CAPACITY table = new Entry[INITIAL_CAPACITY];

// 計(jì)算key(ThreadLocal對(duì)象)在table中的索引 int i = firstKey.threadLocalHashCode & (INITIAL_CAPACITY - 1);

// 用ThreadLocal對(duì)象和value來(lái)構(gòu)造entry對(duì)象，并放到table的第i個(gè)位置 table[i] = new Entry(firstKey, firstValue); size = 1;

// 設(shè)置table的閾值，當(dāng)table中元素個(gè)數(shù)超過(guò)該閾值時(shí)需要對(duì)table

// 進(jìn)行resize，通常在調(diào)用ThreadLocalMap的set方法時(shí)會(huì)發(fā)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(gè)hashcode，使用該hashcode進(jìn)行hash運(yùn)算從而找到該ThreadLocal對(duì)象對(duì)應(yīng)的entry在table中的索引。

getEntry方法

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(jù)ThreadLocal的hashcode計(jì)算該ThreadLocal對(duì)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(duì)應(yīng)的entry;

// e.get() != key 可能是由于hash沖突導(dǎo)致key對(duì)應(yīng)的entry在table

// 的另外一個(gè)位置，需要繼續(xù)查找 if (e != null && e.get() == key) return e; else

// e==null或者e.get() != key 繼續(xù)查找key對(duì)應(yīng)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(gè)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?jiǎng)h除該entry if (k == null) expungeStaleEntry(i);

// k != key && k != null 繼續(xù)往后尋找，nextIndex就是取(i+1)

// 即table中第(i+1)個(gè)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(duì)staleSlot位置的entry和其后第一個(gè)為null的entry之間的entry進(jìn)行一個(gè)rehash操作，rehash的目的是降低table發(fā)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(gè)數(shù)減一 size--;

// Rehash until we encounter null

// 將table中staleSlot處entry和下一個(gè)為null的entry之間的

// entry重新進(jìn)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(gè)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(fā)生沖突，繼續(xù)尋找下一個(gè)位置 while (tab[h] != null) h = nextIndex(h, len); tab[h] = e; } } } return i; }

set方法

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(dāng)前線程的threadLocals ThreadLocalMap map = getMap(t);

// threadLocals不為null直接設(shè)置新值 if (map != null) { map.set(this, value); } else {

// threadLocals為null則需要?jiǎng)?chuàng)建ThreadLocalMap對(duì)象并賦給

// Thread的threadLocals成員 createMap(t, value); } }

createMap前面已經(jīng)分析過(guò)，接下來(lái)分析ThreadLocalMap的set方法

ThreadLocalMap的set方法

ThreadLocalMap的set方法定義如下，將當(dāng)前的ThreadLocal對(duì)象作為key，傳入的value為值，用key和value創(chuàng)建entry，放到table中適當(dāng)?shù)奈恢茫?/p>

/** * 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(jì)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(gè)位置已經(jīng)存在有效的entry，需要繼續(xù)// 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(shí)第i個(gè)位置entry為null，將新entry放置到這個(gè)位置

tab[i] = new Entry(key, value);

int sz = ++size;

// 試圖清除無(wú)效的entry，若清除失敗并且table中有效entry個(gè)數(shù)

// 大于threshold，這進(jìn)行rehash操作

if (!cleanSomeSlots(i, sz) && sz >= threshold) rehash();

}

replaceStaleEntry方法

replaceStaleEntry的作用是用set方法傳過(guò)來(lái)的key和value構(gòu)造entry，將這個(gè)entry放到staleSlot后面的某個(gè)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(gè)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(lái)staleSlot位置的entry放置到第i個(gè)位置，此時(shí)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(jìn)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(huì)，不可言傳…不再做說(shuō)明

cleanSomeSlots方法

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

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

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

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;

}adLocal

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