ThreadLocal 實(shí)踐與源碼解析
在多線(xiàn)程編程中，共享資源的管理和同步一直是開(kāi)發(fā)人員面臨的挑戰(zhàn)之一。ThreadLocal 是 Java 提供的一種簡(jiǎn)單而強(qiáng)大的機(jī)制，用于實(shí)現(xiàn)線(xiàn)程局部變量，即每個(gè)線(xiàn)程都有自己的獨(dú)立副本，互不干擾。這種機(jī)制不僅簡(jiǎn)化了并發(fā)編程中的數(shù)據(jù)管理，還提高了代碼的可讀性和可維護(hù)性。
一、詳解ThreadLocal
1.什么是ThreadLocal?它有什么用？
為了保證特定變量對(duì)當(dāng)前線(xiàn)程可見(jiàn)，我們就可以使用ThreadLocal關(guān)鍵字，ThreadLocal可以為每個(gè)線(xiàn)程創(chuàng)建這個(gè)變量的副本并存到每個(gè)線(xiàn)程的存儲(chǔ)空間中(關(guān)于這個(gè)存儲(chǔ)空間后文會(huì)展開(kāi)講述)，從而確保共享變量對(duì)每個(gè)線(xiàn)程隔離：
2.ThreadLocal基礎(chǔ)使用示例
如上文所說(shuō)ThreadLocal最典型的用法就是維護(hù)各個(gè)線(xiàn)程各自需要獨(dú)享變量，基于ThreadLocal為每個(gè)將每個(gè)線(xiàn)程的id存到線(xiàn)程內(nèi)部，彼此之間互不影響。
ThreadLocal<String> THREAD_LOCAL = new ThreadLocal<>();
Thread t1 = new Thread(() -> {
log.info("t1往THREAD_LOCAL存入變量:[{}]", Thread.currentThread().getName());
THREAD_LOCAL.set(Thread.currentThread().getName());
log.info("t1獲取THREAD_LOCAL的值為:[{}]", THREAD_LOCAL.get());
}, "t1");
Thread t2 = new Thread(() -> {
log.info("t2往THREAD_LOCAL存入變量:[{}]", Thread.currentThread().getName());
THREAD_LOCAL.set(Thread.currentThread().getName());
log.info("t2獲取THREAD_LOCAL的值為:[{}]", THREAD_LOCAL.get());
THREAD_LOCAL.remove();
log.info("t2刪除THREAD_LOCAL的后值為:[{}]", THREAD_LOCAL.get());
}, "t2");
t1.start();
t2.start();
ThreadUtil.sleep(1,TimeUnit.DAYS);從輸出結(jié)果可以看出，兩個(gè)線(xiàn)程都用THREAD_LOCAL 在自己的內(nèi)存空間中存儲(chǔ)了變量的副本，彼此互相隔離的使用
21:59:51.351 [t2] INFO MultiApplication - t2往THREAD_LOCAL存入變量:[t2]
21:59:51.351 [t1] INFO MultiApplication - t1往THREAD_LOCAL存入變量:[t1]
21:59:51.358 [t1] INFO MultiApplication - t1獲取THREAD_LOCAL的值為:[t1]
21:59:51.359 [t2] INFO MultiApplication - t2獲取THREAD_LOCAL的值為:[t2]
21:59:51.359 [t2] INFO MultiApplication - t2刪除THREAD_LOCAL的后值為:[null]二、從兩種應(yīng)用場(chǎng)景來(lái)介紹一下ThreadLocal
1.日期格式化工具類(lèi)
我們創(chuàng)建100個(gè)線(xiàn)程使用同一個(gè)dateFormat完成日期格式化：
private static Logger logger = LoggerFactory.getLogger(MyThreadLocalDemo3.class);
static SimpleDateFormat dateFormat=new SimpleDateFormat("yyyy-MM-dd HH:mm:ss:SS");
public static void main(String[] args) throws InterruptedException {
ExecutorService threadPool = Executors.newFixedThreadPool(100);
for (int i = 0; i < 100; i++) {
int finalI = i;
//線(xiàn)程池中的線(xiàn)程
threadPool.submit(()->{
new MyThreadLocalDemo3().caclData(finalI);
});
}
threadPool.shutdown();
}
/**
* 計(jì)算second后的日期
* @param second
* @return
*/
public String caclData(int second){
Date date=new Date(1000*second);
String dateStr = dateFormat.format(date);
logger.info("{}得到的時(shí)間字符串為:{}",Thread.currentThread().getId(),dateStr);
return dateStr;
}從輸出結(jié)果可以看出，間隔幾毫秒的線(xiàn)程出現(xiàn)相同結(jié)果：
基于該問(wèn)題我們使用ThreadLocal為線(xiàn)程分配SimpleDateFormat副本：
static ThreadLocal<SimpleDateFormat> threadLocal=ThreadLocal.withInitial(()->new SimpleDateFormat("yyyy-MM-dd HH:mm:ss:SS"));
public static void main(String[] args) throws InterruptedException {
ExecutorService threadPool = Executors.newFixedThreadPool(100);
for (int i = 0; i < 100; i++) {
int finalI = i;
//線(xiàn)程池中的線(xiàn)程
threadPool.submit(()->{
new MyThreadLocalDemo3().caclData(finalI);
});
}
threadPool.shutdown();
}
/**
* 計(jì)算second后的日期
* @param second
* @return
*/
public String caclData(int second){
Date date=new Date(1000*second);
SimpleDateFormat simpleDateFormat = threadLocal.get();
String dateStr = simpleDateFormat.format(date);
logger.info("{}得到的時(shí)間字符串為:{}",Thread.currentThread().getId(),dateStr);
return dateStr;
}2.服務(wù)間調(diào)用的線(xiàn)程變量共享
我們?nèi)粘eb開(kāi)發(fā)都會(huì)涉及到各種service的調(diào)用，例如某個(gè)controller需要調(diào)用完service1之后再調(diào)用service2。因?yàn)槲覀兊腸ontroller和service都是單例的，所以如果我們希望多線(xiàn)程調(diào)用這些controller和service保證共享變量的隔離，也可以用到ThreadLocal。
為了實(shí)現(xiàn)這個(gè)示例，我們編寫(xiě)了線(xiàn)程獲取共享變量的工具類(lèi)：
public class MyUserContextHolder {
private static ThreadLocal<User> holder = new ThreadLocal<>();
public static ThreadLocal<User> getHolder() {
return holder;
}
}service調(diào)用鏈?zhǔn)纠缦?，筆者創(chuàng)建service1之后，所有線(xiàn)程復(fù)用這個(gè)service完成了調(diào)用，并且在服務(wù)間調(diào)用直接通過(guò)ThreadLocal完成了線(xiàn)程副本共享：
public class MyThreadLocalGetUserId {
private static Logger logger = LoggerFactory.getLogger(MyThreadLocalGetUserId.class);
private static ExecutorService threadPool = Executors.newFixedThreadPool(10);
public static void main(String[] args) {
for (int i = 0; i < 10; i++) {
int finalI = i;
MyService1 service1 = new MyService1();
threadPool.submit(() -> {
service1.doWork1("username" + (finalI+1));
});
}
}
}
class MyService1 {
private static Logger logger = LoggerFactory.getLogger(MyThreadLocalGetUserId.class);
public void doWork1(String name) {
logger.info("service1 存儲(chǔ)userName:" + name);
ThreadLocal<String> holder = MyUserContextHolder.getHolder();
holder.set(name);
MyService2 service2 = new MyService2();
service2.doWork2();
}
}
class MyService2 {
private static Logger logger = LoggerFactory.getLogger(MyThreadLocalGetUserId.class);
public void doWork2() {
ThreadLocal<String> holder = MyUserContextHolder.getHolder();
logger.info("service2 獲取userName:" + holder.get());
MyService3 service3 = new MyService3();
service3.doWork3();
}
}
class MyService3 {
private static Logger logger = LoggerFactory.getLogger(MyThreadLocalGetUserId.class);
public void doWork3() {
ThreadLocal<String> holder = MyUserContextHolder.getHolder();
logger.info("service3獲取 userName:" + holder.get());
// 避免oom問(wèn)題
holder.remove();
}
}從輸出結(jié)果來(lái)看，在單例對(duì)象情況下，既保證了同一個(gè)線(xiàn)程間變量共享。
也保證了不同線(xiàn)程之間變量的隔離。
三、基于源碼了解ThreadlLocal工作原理
1.ThreadlLocal如何做到線(xiàn)程隔離的？
我們下面這段代碼為例進(jìn)行分析，本質(zhì)上ThreadLocal的withInitial指明了每個(gè)線(xiàn)程初始化時(shí)設(shè)置默認(rèn)值：
ThreadLocal<SimpleDateFormat> threadLocal = ThreadLocal.withInitial(() -> new SimpleDateFormat("yyyy-MM-dd HH:mm:ss:SS"));當(dāng)我們執(zhí)行g(shù)et操作時(shí)，threadLocal 就會(huì)為當(dāng)前線(xiàn)程完成內(nèi)部map的初始化，然后通過(guò)initialValue獲取上一步聲明的SimpleDateFormat實(shí)例，由此保證每個(gè)線(xiàn)程內(nèi)部都有一個(gè)獨(dú)有的SimpleDateFormat：
對(duì)應(yīng)的我們給出ThreadlLocal的get的源碼，整體邏輯與上述差不多，即初始化線(xiàn)程內(nèi)部的map，然后通過(guò)setInitialValue調(diào)用initialValue創(chuàng)建初始值存到線(xiàn)程的map中：
public T get() {
//獲取當(dāng)前線(xiàn)程
Thread t = Thread.currentThread();
//拿到當(dāng)前線(xiàn)程中的map
ThreadLocalMap map = getMap(t);
//如果map不為空則取用當(dāng)前這個(gè)ThreadLocal作為key取出值，否則通過(guò)setInitialValue完成ThreadLocal初始化
if (map != null) {
ThreadLocalMap.Entry e = map.getEntry(this);
if (e != null) {
@SuppressWarnings("unchecked")
T result = (T)e.value;
return result;
}
}
return setInitialValue();
}
private T setInitialValue() {
//執(zhí)行initialValue為當(dāng)前線(xiàn)程創(chuàng)建變量value，在這里也就是我們要用的SimpleDateFormat
T value = initialValue();
//獲取當(dāng)前線(xiàn)程map，有則直接以ThreadLocal為key將SimpleDateFormat 設(shè)置進(jìn)去，若沒(méi)有先創(chuàng)建再設(shè)置
Thread t = Thread.currentThread();
ThreadLocalMap map = getMap(t);
if (map != null)
map.set(this, value);
else
createMap(t, value);
//返回SimpleDateFormat
return value;
}2.ThreadLocalMap有什么特點(diǎn)？和HashMap有什么區(qū)別
我們通過(guò)源碼查看到這個(gè)map為T(mén)hreadLocalMap，它是由一個(gè)個(gè)Entry 構(gòu)成的數(shù)組：
private Entry[] table;并且每個(gè)Entry 的key是弱引用，這就意味著當(dāng)觸發(fā)GC時(shí)，Entry 的key也就是ThreadLocal就會(huì)被回收。
static class Entry extends WeakReference<ThreadLocal<?>> {
/** The value associated with this ThreadLocal. */
Object value;
Entry(ThreadLocal<?> k, Object v) {
super(k);
value = v;
}
}除上面所說(shuō)，thread中的map和hashmap還有一個(gè)不同點(diǎn)就是數(shù)據(jù)結(jié)構(gòu)，因?yàn)閠hreadLocal的適用場(chǎng)景特殊，所以大部分情況下其內(nèi)部存儲(chǔ)空間不會(huì)存儲(chǔ)太多元素，所以出于簡(jiǎn)單的考慮，線(xiàn)程中的map本質(zhì)上就是一個(gè)數(shù)組，一旦發(fā)生沖突則直接通過(guò)線(xiàn)性探測(cè)法找到數(shù)組中空閑的位置將值存入：
private void set(ThreadLocal<?> key, Object value) {
//......
Entry[] tab = table;
int len = tab.length;
//定位鍵值對(duì)存儲(chǔ)的索引位置
int i = key.threadLocalHashCode & (len-1);
//通過(guò)線(xiàn)性探測(cè)法循環(huán)找到空閑位置存入元素
for (Entry e = tab[i];
e != null;
e = tab[i = nextIndex(i, len)]) {
ThreadLocal<?> k = e.get();
//......
}
//找到合適的位置將元素存入
tab[i] = new Entry(key, value);
//更新一下容量信息
int sz = ++size;
//......
}四、ThreadLocal使用注意事項(xiàng)
1.內(nèi)存泄漏問(wèn)題
我們有下面這樣一段web代碼，每次請(qǐng)求test0就會(huì)像線(xiàn)程池中的線(xiàn)程存一個(gè)4M的byte數(shù)組：
RestController
public class TestController {
final static ThreadPoolExecutor poolExecutor = new ThreadPoolExecutor(100, 100, 1, TimeUnit.MINUTES,
new LinkedBlockingQueue<>());// 創(chuàng)建線(xiàn)程池，通過(guò)線(xiàn)程池，保證創(chuàng)建的線(xiàn)程存活
final static ThreadLocal<Byte[]> localVariable = new ThreadLocal<Byte[]>();// 聲明本地變量
@RequestMapping(value = "/test0")
public String test0(HttpServletRequest request) {
poolExecutor.execute(() -> {
Byte[] c = new Byte[4* 1024* 1024];
localVariable.set(c);// 為線(xiàn)程添加變量
});
return "success";
}
}我們將這個(gè)代碼打成jar包部署到服務(wù)器上并啟動(dòng)
java -jar -Xms100m -Xmx100m # 調(diào)整堆內(nèi)存大小
-XX:+HeapDumpOnOutOfMemoryError -XX:HeapDumpPath=/tmp/heapdump.hprof # 表示發(fā)生OOM時(shí)輸出日志文件，指定path為/tmp/heapdump.hprof
-XX:+PrintGCTimeStamps -XX:+PrintGCDetails -Xloggc:/tmp/heapTest.log # 打印日志、gc時(shí)間以及指定gc日志的路徑
demo-0.0.1-SNAPSHOT.jar只需頻繁調(diào)用幾次，就會(huì)輸出OutOfMemoryError
Exception in thread "pool-1-thread-5" java.lang.OutOfMemoryError: Java heap space
at com.example.jstackTest.TestController.lambda$test0$0(TestController.java:25)
at com.example.jstackTest.TestController$$Lambda$582/394910033.run(Unknown Source)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
at java.lang.Thread.run(Thread.java:748)問(wèn)題的根本原因是我們沒(méi)有及時(shí)回收Thread從ThreadLocal中得到的變量副本。因?yàn)槲覀兊氖褂玫木€(xiàn)程是來(lái)自線(xiàn)程池中，所以線(xiàn)程使用結(jié)束后并不會(huì)被銷(xiāo)毀，這就使得ThreadLocal中的變量副本會(huì)一直存儲(chǔ)與線(xiàn)程池中的線(xiàn)程中，導(dǎo)致OOM。
可能你會(huì)問(wèn)了，不是說(shuō)Java有GC回收機(jī)制嘛？為什么還會(huì)出現(xiàn)Thread中的ThreadLocalMap的value不會(huì)被回收呢？
我們上文提到ThreadLocal得到值，都會(huì)以ThreadLocal為key，ThreadLocal的initialValue方法得到的value作為值生成一個(gè)entry對(duì)象，存到當(dāng)前線(xiàn)程的ThreadLocalMap中。 而我們的Entry的key是一個(gè)弱引用，一旦我們使用的threadLocal臨時(shí)變量用完被垃圾回收之后，這個(gè)key就會(huì)因?yàn)槿跻玫脑虮换厥?，而我們這個(gè)key所對(duì)應(yīng)的value仍然被線(xiàn)程池中的線(xiàn)程的強(qiáng)引用引用著，所以就遲遲無(wú)法回收，隨著時(shí)間推移每個(gè)線(xiàn)程都出現(xiàn)這種情況導(dǎo)致OOM。
所以我們每個(gè)線(xiàn)程使用完ThreadLocal之后，一定要使用remove方法清楚ThreadLocalMap中的value：
localVariable.remove()從源碼中可以看到remove方法會(huì)遍歷當(dāng)前線(xiàn)程map然后將強(qiáng)引用之間的聯(lián)系切斷，確保下次GC可以回收掉可以無(wú)用對(duì)象。
private void remove(ThreadLocal<?> key) {
Entry[] tab = table;
int len = tab.length;
int i = key.threadLocalHashCode & (len-1);
//定位，并將entry清除
for (Entry e = tab[i];
e != null;
e = tab[i = nextIndex(i, len)]) {
if (e.get() == key) {
e.clear();
expungeStaleEntry(i);
return;
}
}
}2.空指針問(wèn)題
使用ThreadLocal存放包裝類(lèi)的時(shí)候也需要注意添加初始化方法，否則在拆箱時(shí)可能會(huì)出現(xiàn)空指針問(wèn)題。
private static ThreadLocal<Long> threadLocal = new ThreadLocal<>();
public static void main(String[] args) {
Long num = threadLocal.get();
long sum=1+num;
}
輸出錯(cuò)誤:
Exception in thread "main" java.lang.NullPointerException
at com.guide.base.MyThreadLocalNpe.main(MyThreadLocalNpe.java:11)解決方式：
private static ThreadLocal<Long> threadLocal = ThreadLocal.withInitial(()->new Long(0));3.線(xiàn)程重用問(wèn)題
這個(gè)問(wèn)題和OOM問(wèn)題類(lèi)似，在線(xiàn)程池中服用同一個(gè)線(xiàn)程未及時(shí)清理，導(dǎo)致下一次HTTP請(qǐng)求時(shí)得到上一次ThreadLocal存儲(chǔ)的結(jié)果。
ThreadLocal<String> threadLocal = ThreadLocal.withInitial(() -> null);
* 線(xiàn)程池中使用threadLocal示例
*
* @param accountCode
* @return
*/
@GetMapping("/account/getAccountByCode/{accountCode}")
@SentinelResource(value = "getAccountByCode")
ResultData<Map<String, Object>> getAccountByCode(@PathVariable(value = "accountCode") String accountCode) throws InterruptedException {
Map<String, Object> result = new HashMap<>();
CountDownLatch countDownLatch = new CountDownLatch(1);
threadPool.submit(() -> {
String before = Thread.currentThread().getName() + ":" + threadLocal.get();
log.info("before：" + before);
result.put("before", before);
log.info("調(diào)用getByCode，請(qǐng)求參數(shù):{}", accountCode);
QueryWrapper<Account> queryWrapper = new QueryWrapper<>();
queryWrapper.eq("account_code", accountCode);
Account account = accountService.getOne(queryWrapper);
String after = Thread.currentThread().getName() + ":" + account.getAccountName();
result.put("after", account.getAccountName());
log.info("after：" + after);
threadLocal.set(account.getAccountName());
//完成計(jì)算后，使用countDown按下倒計(jì)時(shí)門(mén)閂，通知主線(xiàn)程可以執(zhí)行后續(xù)步驟
countDownLatch.countDown();
});
//等待上述線(xiàn)程池完成
countDownLatch.await();
return ResultData.success(result);
}從輸出結(jié)果可以看出，我們第二次進(jìn)行HTTP請(qǐng)求時(shí)，threadLocal第一get獲得了上一次請(qǐng)求的值，出現(xiàn)臟數(shù)據(jù)。
C:\Users\xxx>curl http://localhost:9000/account/getAccountByCode/demoData
{"status":100,"message":"操作成功","data":{"before":"pool-2-thread-1:null","after":"pool-2-thread-1:demoData"},"success":true,"timestamp":1678410699943}
C:\Users\xxx>curl http://localhost:9000/account/getAccountByCode/Zsy
{"status":100,"message":"操作成功","data":{"before":"pool-2-thread-1:demoData","after":"pool-2-thread-1:zsy"},"success":true,"timestamp":1678410707473}解決方法也很簡(jiǎn)單，手動(dòng)添加一個(gè)threadLocal的remove方法即可：
@GetMapping("/account/getAccountByCode/{accountCode}")
@SentinelResource(value = "getAccountByCode")
ResultData<Map<String, Object>> getAccountByCode(@PathVariable(value = "accountCode") String accountCode) throws InterruptedException {
Map<String, Object> result = new HashMap<>();
CountDownLatch countDownLatch = new CountDownLatch(1);
try {
threadPool.submit(() -> {
String before = Thread.currentThread().getName() + ":" + threadLocal.get();
log.info("before：" + before);
result.put("before", before);
log.info("調(diào)用getByCode，請(qǐng)求參數(shù):{}", accountCode);
QueryWrapper<Account> queryWrapper = new QueryWrapper<>();
queryWrapper.eq("account_code", accountCode);
Account account = accountService.getOne(queryWrapper);
String after = Thread.currentThread().getName() + ":" + account.getAccountName();
result.put("after", after);
log.info("after：" + after);
threadLocal.set(account.getAccountName());
//完成計(jì)算后，使用countDown按下倒計(jì)時(shí)門(mén)閂，通知主線(xiàn)程可以執(zhí)行后續(xù)步驟
countDownLatch.countDown();
});
} finally {
threadLocal.remove();
}
//等待上述線(xiàn)程池完成
countDownLatch.await();
return ResultData.success(result);
}五、ThreadLocal的不可繼承性
1.通過(guò)代碼證明ThreadLocal的不可繼承性
如下代碼所示，ThreadLocal子線(xiàn)程無(wú)法拿到主線(xiàn)程維護(hù)的內(nèi)部變量
/**
* ThreadLocal 不具備可繼承性
*/
public class ThreadLocalInheritTest {
private static ThreadLocal<String> THREAD_LOCAL = new ThreadLocal<>();
private static Logger logger = LoggerFactory.getLogger(ThreadLocalInheritTest.class);
public static void main(String[] args) {
THREAD_LOCAL.set("mainVal");
logger.info("主線(xiàn)程的值為: " + THREAD_LOCAL.get());
new Thread(() -> {
try {
//睡眠3s確保上述邏輯運(yùn)行
Thread.sleep(3000);
} catch (InterruptedException e) {
e.printStackTrace();
}
logger.info("子線(xiàn)程獲取THREAD_LOCAL的值為:[{}]", THREAD_LOCAL.get());
}).start();
}
}2.使用InheritableThreadLocal實(shí)現(xiàn)主線(xiàn)程內(nèi)部變量繼承
如下所示，我們將THREAD_LOCAL 改為InheritableThreadLocal類(lèi)即可解決問(wèn)題。
/**
* ThreadLocal 不具備可繼承性
*/
public class ThreadLocalInheritTest {
private static ThreadLocal<String> THREAD_LOCAL = new InheritableThreadLocal<>();
private static Logger logger = LoggerFactory.getLogger(ThreadLocalInheritTest.class);
public static void main(String[] args) {
THREAD_LOCAL.set("mainVal");
logger.info("主線(xiàn)程的值為: " + THREAD_LOCAL.get());
new Thread(() -> {
try {
//睡眠3s確保上述邏輯運(yùn)行
Thread.sleep(3000);
} catch (InterruptedException e) {
e.printStackTrace();
}
logger.info("子線(xiàn)程獲取THREAD_LOCAL的值為:[{}]", THREAD_LOCAL.get());
}).start();
}
}3.基于源碼剖析原因
因?yàn)?nbsp;ThreadLocal會(huì)將變量存儲(chǔ)在線(xiàn)程的 ThreadLocalMap中，所以我們先看看InheritableThreadLocal的getMap方法，從而定位到了inheritableThreadLocals:
ThreadLocalMap getMap(Thread t) {
return t.inheritableThreadLocals;
}然后我們到Thread類(lèi)去定位這個(gè)變量的使用之處，所以我們?cè)趧?chuàng)建線(xiàn)程的地方打了個(gè)斷點(diǎn):
從而定位到這段初始化，它會(huì)獲取主線(xiàn)程的ThreadLocalMap并將主線(xiàn)程ThreadLocalMap中的值存到子線(xiàn)程的ThreadLocalMap中。
private void init(ThreadGroup g, Runnable target, String name,
long stackSize, AccessControlContext acc,
boolean inheritThreadLocals) {
if (name == null) {
throw new NullPointerException("name cannot be null");
}
this.name = name;
//獲取當(dāng)前線(xiàn)程的主線(xiàn)程
Thread parent = currentThread();
if (inheritThreadLocals && parent.inheritableThreadLocals != null)
this.inheritableThreadLocals =
//將主線(xiàn)程的map的值存到子線(xiàn)程中
ThreadLocal.createInheritedMap(parent.inheritableThreadLocals);
//......
}createInheritedMap內(nèi)部就會(huì)調(diào)用ThreadLocalMap方法將主線(xiàn)程的ThreadLocalMap的值存到子線(xiàn)程的ThreadLocalMap中。
private ThreadLocalMap(ThreadLocalMap parentMap) {
Entry[] parentTable = parentMap.table;
int len = parentTable.length;
setThreshold(len);
table = new Entry[len];
//遍歷父線(xiàn)程數(shù)據(jù)復(fù)制到子線(xiàn)程map中
for (int j = 0; j < len; j++) {
Entry e = parentTable[j];
if (e != null) {
@SuppressWarnings("unchecked")
ThreadLocal<Object> key = (ThreadLocal<Object>) e.get();
if (key != null) {
//......
//定位當(dāng)前子線(xiàn)程bucket位置將value存入
while (table[h] != null)
h = nextIndex(h, len);
table[h] = c;
size++;
}
}
}
}六、ThreadLocal在Spring中的運(yùn)用
其實(shí)針對(duì)日期格式化問(wèn)題，Spring已經(jīng)為我們內(nèi)置好了相應(yīng)的工具類(lèi)即DateTimeContextHolder：
private static final ThreadLocal<DateTimeContext> dateTimeContextHolder =
new NamedThreadLocal<>("DateTimeContext");該工具類(lèi)和simpledateformate差不多，使用示例如下所示，是spring封裝的，使用起來(lái)也很方便:
public class DateTimeContextHolderTest {
protected static final Logger logger = LoggerFactory.getLogger(DateTimeContextHolderTest.class);
private final static DateTimeFormatter formatter = DateTimeFormatter.ofPattern("yyyy-MM-dd");
private Set<String> set = new ConcurrentHashSet<String>();
@Test
public void test_withLocale_same() throws Exception {
ExecutorService threadPool = Executors.newFixedThreadPool(30);
for (int i = 0; i < 30; i++) {
int finalI = i;
threadPool.execute(() -> {
LocalDate currentdate = LocalDate.now();
int year = currentdate.getYear();
int month = currentdate.getMonthValue();
int day = 1 + finalI;
LocalDate date = LocalDate.of(year, month, day);
DateTimeFormatter fmt = DateTimeContextHolder.getFormatter(formatter, null);
String text = date.format(fmt);
set.add(text);
logger.info("轉(zhuǎn)換后的時(shí)間為" + text);
});
}
threadPool.shutdown();
while (!threadPool.isTerminated()) {
}
logger.info("查看去重后的數(shù)量"+set.size());
}
}七、為什么JDK建議將ThreadLocal設(shè)置為static
我們都知道使用static是屬于類(lèi)，存在于方法區(qū)中，即修飾的變量是全局共享的，這意味著當(dāng)前ThreadLocal在通過(guò)static之后，即所有的實(shí)例對(duì)象都共享一個(gè)ThreadLocal。從而避免重復(fù)創(chuàng)建TSO(Thread Specific Object)即ThreadLocal所關(guān)聯(lián)的對(duì)象的創(chuàng)建的開(kāi)銷(xiāo)。以及這種方案使得即使出現(xiàn)內(nèi)存泄漏也是O(1)級(jí)別的內(nèi)存泄露，場(chǎng)景如下：
- 假設(shè)使用線(xiàn)程非線(xiàn)程池模式，即線(xiàn)程結(jié)束后threadLocalMap就會(huì)被回收，這種情況下也只有在threadLocal第一次調(diào)用get到線(xiàn)程銷(xiāo)毀之間的時(shí)間段存在內(nèi)存泄漏的情況。
- 如果使用的是全局線(xiàn)程池，因?yàn)榫€(xiàn)程池的線(xiàn)程并不會(huì)被回收，所以threadLocalMap中的entry一直存在于堆內(nèi)存中，但由于該ThreadLocal屬于全局共享，所以大量線(xiàn)程進(jìn)行操作時(shí)一定概率觸發(fā)expungeStaleEntry清除過(guò)期對(duì)象，一定程度上避免了內(nèi)存泄漏的情況。
- 極端情況下，如果threadLocal創(chuàng)建之后只有線(xiàn)程池中的一個(gè)線(xiàn)程get或初始化后完全沒(méi)有線(xiàn)程再去使用，這就會(huì)導(dǎo)致threadLocalMap存在強(qiáng)引用而導(dǎo)致無(wú)法被回收，O(1)級(jí)別的內(nèi)存泄漏由此誕生。
對(duì)應(yīng)的實(shí)例變量的ThreadLocal的O(n)內(nèi)存泄漏，這就不必多說(shuō)。
八、小結(jié)
- ThreadLocal通過(guò)在將共享變量拷貝一份到每個(gè)線(xiàn)程內(nèi)部的ThreadLocalMap保證線(xiàn)程安全。
- ThreadLocal使用完成后記得使用remove方法手動(dòng)清理線(xiàn)程中的ThreadLocalMap過(guò)期對(duì)象，避免OOM和一些業(yè)務(wù)上的錯(cuò)誤。
- ThreadLocal是不可被繼承了，如果想使用主線(xiàn)的的ThreadLocal，就必須使用InheritableThreadLocal。
