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。