前言
ThreadLocal是JDK包提供的，它提供了線程本地變量，也就是如果你創(chuàng)建了一個(gè)ThreadLocal變量，那么訪問這個(gè)變量的每個(gè)線程都會(huì)有這個(gè)變量的一個(gè)本地副本。當(dāng)多線程操作這個(gè)變量時(shí)，實(shí)際操作的是自己本地內(nèi)存里面的變量，從而避免了線程安全問題。
ThreadLocal的應(yīng)用
列舉個(gè)使用ThreadLocal的例子
static ThreadLocal<String> localV = new ThreadLocal<>();
public static void main(String[] args) {
Thread threadOne = new Thread(new Runnable() {
@Override
public void run() {
localV.set("thradOne local V");
System.out.println("getV = " + localV.get());
}
});
Thread threadTwo = new Thread(new Runnable() {
@Override
public void run() {
localV.set("thradTwo local V");
System.out.println("getV = " + localV.get());
}
});
threadOne.start();
threadTwo.start();
}
Out:
getV = thradTwo local V
getV = thradOne local V
threadOne通過ThreadLocal的set方法設(shè)置的值，其實(shí)設(shè)置的是線程本地內(nèi)存中的一個(gè)副本，這個(gè)副本是threadTwo訪問不了的。
ThreadLocal是如何實(shí)現(xiàn)線程之間數(shù)據(jù)隔離
ThreadLocal其實(shí)就是一個(gè)工具殼，真正實(shí)現(xiàn)存儲(chǔ)的還是ThreadLocalMap，相應(yīng)的變量存儲(chǔ)在Thread類中
/* ThreadLocal values pertaining to this thread. This map is maintained
* by the ThreadLocal class. */
ThreadLocal.ThreadLocalMap threadLocals = null;
/*
* InheritableThreadLocal values pertaining to this thread. This map is
* maintained by the InheritableThreadLocal class.
*/
ThreadLocal.ThreadLocalMap inheritableThreadLocals = null;
ThreadLocalMap是一個(gè)定制化的HashMap,默認(rèn)情況下這兩個(gè)變量都為空。
inheritableThreadLocals:init方法初始化，同一個(gè)ThreadLocal變量在父進(jìn)程中被設(shè)置值后，使用inheritableThreadLocals可以在子線程中獲取
private void init(ThreadGroup g, Runnable target, String name,
long stackSize, AccessControlContext acc,
boolean inheritThreadLocals) {
//獲取當(dāng)前的線程，這里指的是main函數(shù)所在的線程，也就是父線程
Thread parent = currentThread();
if (inheritThreadLocals && parent.inheritableThreadLocals != null) //因?yàn)閙ain線程set/get方法操作的是inheritableThreadLocals，一定不為空
this.inheritableThreadLocals =
ThreadLocal.createInheritedMap(parent.inheritableThreadLocals);
}
static ThreadLocalMap createInheritedMap(ThreadLocalMap parentMap) {
return new ThreadLocalMap(parentMap);
}
threadLocals:只有當(dāng)線程第一次調(diào)用ThreadLocal的set/get方法時(shí)才會(huì)創(chuàng)建
public void set(T value) {
Thread t = Thread.currentThread(); //獲取當(dāng)前線程
ThreadLocalMap map = getMap(t); //查找當(dāng)前線程threadlocals變量
if (map != null)
map.set(this, value);
else
createMap(t, value);//第一次調(diào)用就創(chuàng)建當(dāng)前線程對(duì)應(yīng)的HashMap
}
ThreadLocalMap getMap(Thread t) {
return t.threadLocals;
}
void createMap(Thread t, T firstValue) {
t.threadLocals = new ThreadLocalMap(this, firstValue);
}
最后我們分析一下ThreadLocalMap類是如何實(shí)現(xiàn)線程本地化存儲(chǔ)
ThreadLocalMap類
static class ThreadLocalMap {
//table中每個(gè)結(jié)點(diǎn)，ThreadLocal為弱引用 當(dāng)沒有引用的時(shí)候會(huì)直接回收
static class Entry extends WeakReference<ThreadLocal<?>> {
//ThreadLocal關(guān)聯(lián)的值
Object value;
Entry(ThreadLocal<?> k, Object v) {
super(k);
value = v;
}
}
//數(shù)組的初始化大小
private static final int INITIAL_CAPACITY = 16;
//存儲(chǔ)ThreadLocal的數(shù)組
private Entry[] table;
//默認(rèn)為數(shù)組大小的三分之二
private int threshold; // Default to 0
/**
* Set the resize threshold to maintain at worst a 2/3 load factor.
*/
private void setThreshold(int len) {
threshold = len * 2 / 3;
}
tip:
- ThreadLocalMap初始化大小為16 大于數(shù)組三分之二便進(jìn)行擴(kuò)容
- ThreadLocal為弱引用
存值調(diào)用的ThreadLocalMap.set方法
private void set(ThreadLocal<?> key, Object value) {
Entry[] tab = table;
int len = tab.length;
//計(jì)算key在數(shù)據(jù)table的下標(biāo) 這是是ThreadLocal的hashCode和長(zhǎng)度-1取余
int i = key.threadLocalHashCode & (len-1);
//查看i索引位置有沒有值，有值的話，索引位置+1,直到找到?jīng)]有值的位置
for (Entry e = tab[i];
e != null;
//nextIndex在不超過數(shù)組長(zhǎng)度索引值+1 超過則從0開始
e = tab[i = nextIndex(i, len)]) {
ThreadLocal<?> k = e.get();
//找到內(nèi)存地址一樣的ThreadLocal，則替換
if (k == key) {
e.value = value;
return;
}
//因?yàn)門hreadLocal為weakReference所以當(dāng)無(wú)引用便會(huì)清理掉，這里直接替換
if (k == null) {
replaceStaleEntry(key, value, i);
return;
}
}‘
//當(dāng)前i位置是無(wú)值的，存入threadlocal
tab[i] = new Entry(key, value);
int sz = ++size;
//當(dāng)數(shù)組大小等于擴(kuò)容閾值（數(shù)組大小的三分之二），進(jìn)行擴(kuò)容
if (!cleanSomeSlots(i, sz) && sz >= threshold)
rehash();
}
set方法使用的沖突策略，也將對(duì)get方法產(chǎn)生影響
public T get() {
//獲取當(dāng)前線程
Thread t = Thread.currentThread();
//從當(dāng)前線程取出ThreadLocalMap
ThreadLocalMap map = getMap(t);
if (map != null) {
//set的策略不同導(dǎo)致get策略也不同
ThreadLocalMap.Entry e = map.getEntry(this);
if (e != null) {//如果不為空，讀取ThreadLocal中保存的值
@SuppressWarnings("unchecked")
T result = (T)e.value;
return result;
}
}
//否則給當(dāng)前線程ThreadLocal初始化 并返回null
return setInitialValue();
}
private Entry getEntry(ThreadLocal<?> key) {
//和set方法取索引值相同
int i = key.threadLocalHashCode & (table.length - 1);
Entry e = table[i];
//ThreadLocal的內(nèi)存地址相同則返回，否則getEntryAfterMiss繼續(xù)查找
if (e != null && e.get() == key)
return e;
else
return 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) //內(nèi)存地址一樣，找到了
return e;
if (k == null) //刪除沒用的key
expungeStaleEntry(i);
else //索引位置 + 1
i = nextIndex(i, len);
e = tab[i];
}
return null;
}
最后在看下擴(kuò)容邏輯
private void resize() {
Entry[] oldTab = table;
int oldLen = oldTab.length;
//新數(shù)組大小為老數(shù)組的兩倍
int newLen = oldLen * 2;
Entry[] newTab = new Entry[newLen];
int count = 0;
//老數(shù)組的值拷貝到新數(shù)組上
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 {
//計(jì)算ThreadLocal在新數(shù)組中的位置
int h = k.threadLocalHashCode & (newLen - 1);
//如果索引值h的位置不為空，則往后繼續(xù)尋找，直到找到為空的索引位置
while (newTab[h] != null)
h = nextIndex(h, newLen);
newTab[h] = e;
count++;
}
}
}
//給新數(shù)組初始化下次初始化的閾值，為數(shù)組長(zhǎng)度的三分之二
setThreshold(newLen);
size = count;
table = newTab;
}
總結(jié)
通過學(xué)習(xí)get到
- ThreadLocal如何實(shí)現(xiàn)線程之間數(shù)據(jù)隔離
- inheritableThreadLocals和threadLocals初始化時(shí)機(jī)
- ThreadLocal為弱引用 當(dāng)沒有引用的時(shí)候會(huì)直接回收
- ThreadLocalMap類初始化大小為16 大于數(shù)組三分之二便進(jìn)行擴(kuò)容,擴(kuò)容為原來(lái)的兩倍
