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 InheritableThreadLoca

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 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 parent i 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 parent i 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 parent
i am a transmittable parent
i am a new transmittable parent

执行代码后发现，使用TransmittableThreadLocalTtlExecutors.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(capture()); this.runnable = runnable; this.releaseTtlValueReferenceAfterRun = releaseTtlValueReferenceAfterRun; }

 

com.alibaba.ttl.TtlRunnable#run /**

• wrap method {@link Runnable#run()}. */ @Override public 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, Object>) captured; Map<TransmittableThreadLocal, Object> backup = new HashMap, Object>();

for (Iterator, ?>> 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 threadLocal = (TransmittableThreadLocal) 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, Object>) backup; // call afterExecute callback doExecuteCallback(false);

for (Iterator, ?>> 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 threadLocal = (TransmittableThreadLocal) 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。

@Bean
public 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。

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