Java 多线程上下文传递在复杂场景下的实践

一、引言

海外商城从印度做起，慢慢的会有一些其他国家的诉求，这个时候需要我们针对当前的商城做一个改造，可以支撑多个国家的商城，这里会涉及多个问题，多语言，多国家，多时区，本地化等等。在多国家的情况下如何把识别出来的国家信息传递下去，一层一层直到代码执行的最后一步。甚至还有一些多线程的场景需要处理。

二、背景技术

2.1 ThreadLocal

ThreadLocal是最容易想到了，入口识别到国家信息后，丢进ThreadLocal，这样后续代码、redis、DB等做国家区分的时候都能使用到。

这里先简单介绍一下ThreadLocal：

/** * 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(); ThreadLocalMap map = getMap(t); if (map != null) map.set(this, value); else createMap(t, value);} /** * 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() { Thread t = Thread.currentThread(); ThreadLocalMap map = getMap(t); if (map != null) { ThreadLocalMap.Entry e = map.getEntry(this); if (e != null) { @SuppressWarnings("unchecked") T result = (T)e.value; return result; } } 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;} /** * 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) { int i = key.threadLocalHashCode & (table.length - 1); Entry e = table[i]; if (e != null && e.get() == key) return e; else return getEntryAfterMiss(key, i, e);}

• 每一个Thread线程都有属于自己的threadLocals(ThreadLocalMap),里面有一个弱引用的Entry(ThreadLocal,Object)。

• get方法首先通过Thread.currentThread得到当前线程，然后拿到线程的threadLocals(ThreadLocalMap)，再从Entry中取得当前线程存储的value。

• set值的时候更改当前线程的threadLocals(ThreadLocalMap)中Entry对应的value值。

实际使用中除了同步方法之外，还有起异步线程处理的场景，这个时候就需要把ThreadLocal的内容从父线程传递给子线程，这个怎么办呢？

不急，Java 还有InheritableThreadLocal来帮我们解决这个问题。

2.2 InheritableThreadLocal

public class InheritableThreadLocal<T> extends ThreadLocal<T> { /** * Computes the child's initial value for this inheritable thread-local * variable as a function of the parent's value at the time the child * thread is created. This method is called from within the parent * thread before the child is started. * <p> * This method merely returns its input argument, and should be overridden * if a different behavior is desired. * * @param parentValue the parent thread's value * @return the child thread's initial value */ protected T childValue(T parentValue) { return parentValue; } /** * Get the map associated with a ThreadLocal. * * @param t the current thread */ ThreadLocalMap getMap(Thread t) { return t.inheritableThreadLocals; } /** * Create the map associated with a ThreadLocal. * * @param t the current thread * @param firstValue value for the initial entry of the table. */ void createMap(Thread t, T firstValue) { t.inheritableThreadLocals = new ThreadLocalMap(this, firstValue); }}

• java.lang.Thread#init(java.lang.ThreadGroup, java.lang.Runnable, java.lang.String, long, java.security.AccessControlContext, boolean)

if (inheritThreadLocals && parent.inheritableThreadLocals != null) this.inheritableThreadLocals = ThreadLocal.createInheritedMap(parent.inheritableThreadLocals);

• InheritableThreadLocal操作的是inheritableThreadLocals这个变量，而不是ThreadLocal操作的threadLocals变量。

• 创建新线程的时候会检查父线程中parent.inheritableThreadLocals变量是否为null，如果不为null则复制一份parent.inheritableThreadLocals的数据到子线程的this.inheritableThreadLocals中去。

• 因为复写了getMap(Thread)和CreateMap()方法直接操作inheritableThreadLocals，这样就实现了在子线程中获取父线程ThreadLocal值。

现在在使用多线程的时候，都是通过线程池来做的，这个时候用InheritableThreadLocal可以吗？会有什么问题吗？先看下下面的代码的执行情况：

• test

static InheritableThreadLocal<String> inheritableThreadLocal = new InheritableThreadLocal<>(); public static void main(String[] args) throws InterruptedException { ExecutorService executorService = Executors.newFixedThreadPool(1); inheritableThreadLocal.set("i am a inherit parent"); executorService.execute(new Runnable() { @Override public void run() { System.out.println(inheritableThreadLocal.get()); } }); TimeUnit.SECONDS.sleep(1); inheritableThreadLocal.set("i am a new inherit parent");// 设置新的值 executorService.execute(new Runnable() { @Override public void run() { System.out.println(inheritableThreadLocal.get()); } });} i am a inherit parenti am a inherit parent public static void main(String[] args) throws InterruptedException { ExecutorService executorService = Executors.newFixedThreadPool(1); inheritableThreadLocal.set("i am a inherit parent"); executorService.execute(new Runnable() { @Override public void run() { System.out.println(inheritableThreadLocal.get()); inheritableThreadLocal.set("i am a old inherit parent");// 子线程中设置新的值 } }); TimeUnit.SECONDS.sleep(1); inheritableThreadLocal.set("i am a new inherit parent");// 主线程设置新的值 executorService.execute(new Runnable() { @Override public void run() { System.out.println(inheritableThreadLocal.get()); } });} i am a inherit parenti am a old inherit parent

这里看第一个执行结果，发现主线程第二次设置的值，没有改掉，还是第一次设置的值“i am a inherit parent”，这是什么原因呢？

再看第二个例子的执行结果，发现在第一个任务中设置的“i am a old inherit parent"的值，在第二个任务中打印出来了。这又是什么原因呢？

回过头来看看上面的源码，在线程池的情况下，第一次创建线程的时候会从父线程中copy inheritableThreadLocals中的数据，所以第一个任务成功拿到了父线程设置的”i am a inherit parent“，第二个任务执行的时候复用了第一个任务的线程，并不会触发复制父线程中的inheritableThreadLocals操作，所以即使在主线程中设置了新的值，也会不生效。同时get()方法是直接操作inheritableThreadLocals这个变量的，所以就直接拿到了第一个任务设置的值。

那遇到线程池应该怎么办呢？

2.3 TransmittableThreadLocal

TransmittableThreadLocal（TTL）这个时候就派上用场了。这是阿里开源的一个组件，我们来看看它怎么解决线程池的问题，先来一段代码，在上面的基础上修改一下，使用TransmittableThreadLocal。

static TransmittableThreadLocal<String> transmittableThreadLocal = new TransmittableThreadLocal<>();// 使用TransmittableThreadLocal  public static void main(String[] args) throws InterruptedException { ExecutorService executorService = Executors.newFixedThreadPool(1); executorService = TtlExecutors.getTtlExecutorService(executorService); // 用TtlExecutors装饰线程池 transmittableThreadLocal.set("i am a transmittable parent"); executorService.execute(new Runnable() { @Override public void run() { System.out.println(transmittableThreadLocal.get()); transmittableThreadLocal.set("i am a old transmittable parent");// 子线程设置新的值 } }); System.out.println(transmittableThreadLocal.get()); TimeUnit.SECONDS.sleep(1); transmittableThreadLocal.set("i am a new transmittable parent");// 主线程设置新的值 executorService.execute(new Runnable() { @Override public void run() { System.out.println(transmittableThreadLocal.get()); } });} i am a transmittable parenti am a transmittable parenti am a new transmittable parent

执行代码后发现，使用TransmittableThreadLocal和TtlExecutors.getTtlExecutorService(executorService)装饰线程池之后，在每次调用任务的时，都会将当前的主线程的TransmittableThreadLocal数据copy到子线程里面，执行完成后，再清除掉。同时子线程里面的修改回到主线程时其实并没有生效。这样可以保证每次任务执行的时候都是互不干涉的。这是怎么做到的呢？来看源码。

• TtlExecutors和TransmittableThreadLocal源码

private TtlRunnable(Runnable runnable, boolean releaseTtlValueReferenceAfterRun) { this.capturedRef = new AtomicReference<Object>(capture()); this.runnable = runnable; this.releaseTtlValueReferenceAfterRun = releaseTtlValueReferenceAfterRun;} com.alibaba.ttl.TtlRunnable#run/** * wrap method {@link Runnable#run()}. */@Overridepublic void run() { Object captured = capturedRef.get();// 获取线程的ThreadLocalMap if (captured == null || releaseTtlValueReferenceAfterRun && !capturedRef.compareAndSet(captured, null)) { throw new IllegalStateException("TTL value reference is released after run!"); } Object backup = replay(captured);// 暂存当前子线程的ThreadLocalMap到backup try { runnable.run(); } finally { restore(backup);// 恢复线程执行时被改版的Threadlocal对应的值 }} com.alibaba.ttl.TransmittableThreadLocal.Transmitter#replay /** * Replay the captured {@link TransmittableThreadLocal} values from {@link #capture()}, * and return the backup {@link TransmittableThreadLocal} values in current thread before replay. * * @param captured captured {@link TransmittableThreadLocal} values from other thread from {@link #capture()} * @return the backup {@link TransmittableThreadLocal} values before replay * @see #capture() * @since 2.3.0 */public static Object replay(Object captured) { @SuppressWarnings("unchecked") Map<TransmittableThreadLocal<?>, Object> capturedMap = (Map<TransmittableThreadLocal<?>, Object>) captured; Map<TransmittableThreadLocal<?>, Object> backup = new HashMap<TransmittableThreadLocal<?>, Object>(); for (Iterator<? extends Map.Entry<TransmittableThreadLocal<?>, ?>> iterator = holder.get().entrySet().iterator(); iterator.hasNext(); ) { Map.Entry<TransmittableThreadLocal<?>, ?> next = iterator.next(); TransmittableThreadLocal<?> threadLocal = next.getKey(); // backup backup.put(threadLocal, threadLocal.get()); // clear the TTL value only in captured // avoid extra TTL value in captured, when run task. if (!capturedMap.containsKey(threadLocal)) { iterator.remove(); threadLocal.superRemove(); } } // set value to captured TTL for (Map.Entry<TransmittableThreadLocal<?>, Object> entry : capturedMap.entrySet()) { @SuppressWarnings("unchecked") TransmittableThreadLocal<Object> threadLocal = (TransmittableThreadLocal<Object>) entry.getKey(); threadLocal.set(entry.getValue()); } // call beforeExecute callback doExecuteCallback(true); return backup;} com.alibaba.ttl.TransmittableThreadLocal.Transmitter#restore /** * Restore the backup {@link TransmittableThreadLocal} values from {@link Transmitter#replay(Object)}. * * @param backup the backup {@link TransmittableThreadLocal} values from {@link Transmitter#replay(Object)} * @since 2.3.0 */public static void restore(Object backup) { @SuppressWarnings("unchecked") Map<TransmittableThreadLocal<?>, Object> backupMap = (Map<TransmittableThreadLocal<?>, Object>) backup; // call afterExecute callback doExecuteCallback(false); for (Iterator<? extends Map.Entry<TransmittableThreadLocal<?>, ?>> iterator = holder.get().entrySet().iterator(); iterator.hasNext(); ) { Map.Entry<TransmittableThreadLocal<?>, ?> next = iterator.next(); TransmittableThreadLocal<?> threadLocal = next.getKey(); // clear the TTL value only in backup // avoid the extra value of backup after restore if (!backupMap.containsKey(threadLocal)) { iterator.remove(); threadLocal.superRemove(); } } // restore TTL value for (Map.Entry<TransmittableThreadLocal<?>, Object> entry : backupMap.entrySet()) { @SuppressWarnings("unchecked") TransmittableThreadLocal<Object> threadLocal = (TransmittableThreadLocal<Object>) entry.getKey(); threadLocal.set(entry.getValue()); }}

可以看下整个过程的完整时序图：

OK，既然问题都解决了，来看看实际使用吧，有两种使用，先看第一种，涉及HTTP请求、Dubbo请求和 job，采用的是数据级别的隔离。

三、 TTL 在海外商城的实际应用

3.1 不分库，分数据行 + SpringMVC

用户 HTTP 请求，首先我们要从url或者cookie中解析出国家编号，然后在TransmittableThreadLocal中存放国家信息，在 MyBatis 的拦截器中读取国家数据，进行sql改造，最终操作指定的国家数据，多线程场景下用TtlExecutors包装原有自定义线程池，保障在使用线程池的时候能够正确将国家信息传递下去。

•  HTTP 请求

public class ShopShardingHelperUtil { private static TransmittableThreadLocal<String> countrySet = new TransmittableThreadLocal<>(); /** * 获取threadLocal中设置的国家标志 * @return */ public static String getCountry() { return countrySet.get(); } /** * 设置threadLocal中设置的国家 */ public static void setCountry (String country) { countrySet.set(country.toLowerCase()); } /** * 清除标志 */ public static void clear () { countrySet.remove(); }} /** 拦截器对cookie和url综合判断国家信息，放入到TransmittableThreadLocal中 **/// 设置线程中的国家标志String country = localeContext.getLocale().getCountry().toLowerCase(); ShopShardingHelperUtil.setCountry(country); /** 自定义线程池，用TtlExecutors包装原有自定义线程池 **/public static Executor getExecutor() { if (executor == null) { synchronized (TransmittableExecutor.class) { if (executor == null) { executor = TtlExecutors.getTtlExecutor(initExecutor());// 用TtlExecutors装饰Executor，结合TransmittableThreadLocal解决异步线程threadlocal传递问题 } } } return executor;} /** 实际使用线程池的地方,直接调用执行即可**/TransmittableExecutor.getExecutor().execute(new BatchExeRunnable(param1,param2)); /** mybatis的Interceptor代码, 使用TransmittableThreadLocal的国家信息，改造原有sql，加上国家参数，在增删改查sql中区分国家数据 **/public Object intercept(Invocation invocation) throws Throwable { StatementHandler statementHandler = (StatementHandler) invocation.getTarget(); BoundSql boundSql = statementHandler.getBoundSql(); String originalSql = boundSql.getSql(); Statement statement = (Statement) CCJSqlParserUtil.parse(originalSql); String threadCountry = ShopShardingHelperUtil.getCountry(); // 线程中的国家不为空才进行处理 if (StringUtils.isNotBlank(threadCountry)) { if (statement instanceof Select) { Select selectStatement = (Select) statement; VivoSelectVisitor vivoSelectVisitor = new VivoSelectVisitor(threadCountry); vivoSelectVisitor.init(selectStatement); } else if (statement instanceof Insert) { Insert insertStatement = (Insert) statement; VivoInsertVisitor vivoInsertVisitor = new VivoInsertVisitor(threadCountry); vivoInsertVisitor.init(insertStatement); } else if (statement instanceof Update) { Update updateStatement = (Update) statement; VivoUpdateVisitor vivoUpdateVisitor = new VivoUpdateVisitor(threadCountry); vivoUpdateVisitor.init(updateStatement); } else if (statement instanceof Delete) { Delete deleteStatement = (Delete) statement; VivoDeleteVisitor vivoDeleteVisitor = new VivoDeleteVisitor(threadCountry); vivoDeleteVisitor.init(deleteStatement); } Field boundSqlField = BoundSql.class.getDeclaredField("sql"); boundSqlField.setAccessible(true); boundSqlField.set(boundSql, statement.toString()); } else { logger.error("----------- intercept not-add-country sql.... ---------" + statement.toString()); } logger.info("----------- intercept query new sql.... ---------" + statement.toString()); // 调用方法，实际上就是拦截的方法 Object result = invocation.proceed(); return result;}

对于 Dubbo 接口和无法判断国家信息的 HTTP 接口，在入参部分增加国家信息参数，通过拦截器或者手动set国家信息到TransmittableThreadLocal。

对于定时任务job，因为所有国家都需要执行，所以会把所有国家进行遍历执行，这也可以通过简单的注解来解决。

这个版本的改造，点检测试也基本通过了，自动化脚本验证也是没问题的，不过因为业务发展问题最终没上线。

3.2 分库 + SpringBoot

后续在建设新的国家商城的时候，分库分表方案调整为每个国家独立数据库，同时整体开发框架升级到SpringBoot，我们把这套方案做了升级，总体思路是一样的，只是在实现细节上略有不同。

SpringBoot 里面的异步一般通过@Async这个注解来实现，通过自定义线程池来包装，使用时在 HTTP 请求判断locale信息的写入国家信息，后续完成切DB的操作。

对于 Dubbo 接口和无法判断国家信息的 HTTP 接口，在入参部分增加国家信息参数，通过拦截器或者手动set国家信息到TransmittableThreadLocal。

@Beanpublic ThreadPoolTaskExecutor threadPoolTaskExecutor(){ return TtlThreadPoolExecutors.getAsyncExecutor();}  public class TtlThreadPoolExecutors { private static final String COMMON_BUSINESS = "COMMON_EXECUTOR"; public static final int QUEUE_CAPACITY = 20000; public static ExecutorService getExecutorService() { return TtlExecutorServiceMananger.getExecutorService(COMMON_BUSINESS); } public static ExecutorService getExecutorService(String threadGroupName) { return TtlExecutorServiceMananger.getExecutorService(threadGroupName); } public static ThreadPoolTaskExecutor getAsyncExecutor() { // 用TtlExecutors装饰Executor，结合TransmittableThreadLocal解决异步线程threadlocal传递问题 return getTtlThreadPoolTaskExecutor(initTaskExecutor()); } private static ThreadPoolTaskExecutor initTaskExecutor () { return initTaskExecutor(TtlThreadPoolFactory.DEFAULT_CORE_SIZE, TtlThreadPoolFactory.DEFAULT_POOL_SIZE, QUEUE_CAPACITY); } private static ThreadPoolTaskExecutor initTaskExecutor (int coreSize, int poolSize, int executorQueueCapacity) { ThreadPoolTaskExecutor taskExecutor = new ThreadPoolTaskExecutor(); taskExecutor.setCorePoolSize(coreSize); taskExecutor.setMaxPoolSize(poolSize); taskExecutor.setQueueCapacity(executorQueueCapacity); taskExecutor.setKeepAliveSeconds(120); taskExecutor.setAllowCoreThreadTimeOut(true); taskExecutor.setThreadNamePrefix("TaskExecutor-ttl"); taskExecutor.initialize(); return taskExecutor; } private static ThreadPoolTaskExecutor getTtlThreadPoolTaskExecutor(ThreadPoolTaskExecutor executor) { if (null == executor || executor instanceof ThreadPoolTaskExecutorWrapper) { return executor; } return new ThreadPoolTaskExecutorWrapper(executor); }} /** * @ClassName : LocaleContextHolder * @Description : 本地化信息上下文holder */public class LocalizationContextHolder { private static TransmittableThreadLocal<LocalizationContext> localizationContextHolder = new TransmittableThreadLocal<>(); private static LocalizationInfo defaultLocalizationInfo = new LocalizationInfo(); private LocalizationContextHolder(){} public static LocalizationContext getLocalizationContext() { return localizationContextHolder.get(); } public static void resetLocalizationContext () { localizationContextHolder.remove(); } public static void setLocalizationContext (LocalizationContext localizationContext) { if(localizationContext == null) { resetLocalizationContext(); } else { localizationContextHolder.set(localizationContext); } } public static void setLocalizationInfo (LocalizationInfo localizationInfo) { LocalizationContext localizationContext = getLocalizationContext(); String brand = (localizationContext instanceof BrandLocalizationContext ? ((BrandLocalizationContext) localizationContext).getBrand() : null); if(StringUtils.isNotEmpty(brand)) { localizationContext = new SimpleBrandLocalizationContext(localizationInfo, brand); } else if(localizationInfo != null) { localizationContext = new SimpleLocalizationContext(localizationInfo); } else { localizationContext = null; } setLocalizationContext(localizationContext); } public static void setDefaultLocalizationInfo(@Nullable LocalizationInfo localizationInfo) { LocalizationContextHolder.defaultLocalizationInfo = localizationInfo; } public static LocalizationInfo getLocalizationInfo () { LocalizationContext localizationContext = getLocalizationContext(); if(localizationContext != null) { LocalizationInfo localizationInfo = localizationContext.getLocalizationInfo(); if(localizationInfo != null) { return localizationInfo; } } return defaultLocalizationInfo; } public static String getCountry(){ return getLocalizationInfo().getCountry(); } public static String getTimezone(){ return getLocalizationInfo().getTimezone(); } public static String getBrand(){ return getBrand(getLocalizationContext()); } public static String getBrand(LocalizationContext localizationContext) { if(localizationContext == null) { return null; } if(localizationContext instanceof BrandLocalizationContext) { return ((BrandLocalizationContext) localizationContext).getBrand(); } throw new LocaleException("unsupported localizationContext type"); }} @Override public LocaleContext resolveLocaleContext(final HttpServletRequest request) { parseLocaleCookieIfNecessary(request); LocaleContext localeContext = new TimeZoneAwareLocaleContext() { @Override public Locale getLocale() { return (Locale) request.getAttribute(LOCALE_REQUEST_ATTRIBUTE_NAME); } @Override public TimeZone getTimeZone() { return (TimeZone) request.getAttribute(TIME_ZONE_REQUEST_ATTRIBUTE_NAME); } }; // 设置线程中的国家标志 setLocalizationInfo(request, localeContext.getLocale()); return localeContext; } private void setLocalizationInfo(HttpServletRequest request, Locale locale) { String country = locale!=null?locale.getCountry():null; String language = locale!=null?(locale.getLanguage() + "_" + locale.getVariant()):null; LocaleRequestMessage localeRequestMessage = localeRequestParser.parse(request); final String countryStr = country; final String languageStr = language; final String brandStr = localeRequestMessage.getBrand(); LocalizationContextHolder.setLocalizationContext(new BrandLocalizationContext() { @Override public String getBrand() { return brandStr; } @Override public LocalizationInfo getLocalizationInfo() { return LocalizationInfoAssembler.assemble(countryStr, languageStr); } }); }

对于定时任务job，因为所有国家都需要执行，所以会把所有国家进行遍历执行，这也可以通过简单的注解和AOP来解决。

四、总结

本文从业务拓展的角度阐述了在复杂业务场景下如何通过ThreadLocal，过渡到InheritableThreadLocal，再通过TransmittableThreadLocal解决实际业务问题。因为海外的业务在不断的探索中前进，技术也在不断的探索中演进，面对这种复杂多变的情况，我们的应对策略是先做国际化，再做本地化，more global才能more local，多国家的隔离只是国际化最基本的起点，未来还有很多业务和技术等着我们去挑战。

END

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