ThreadLocal integration

[driventokill]

In the user code, ThreadLocal can be passed as a cross-thread transmit capability by defining it as TransmittableThreadLocal. However, if the user relies on a library that uses the ThreadLocal implementation, it cannot modify its code at this time, so it cannot be easily used in asynchronous. The ability to get context transmit in the code.

One possible solution is the integration transmit for ThreadLocal.

Users can register the ThreadLocal that needs to be passed, and can provide two methods:

// ThreadLocal will be transmitted after this
<T> boolean TransmittableThreadLocal.register(ThreadLocal<T> threadLocal);

// Then this ThreadLocal will not be transmitted
<T> boolean TransmittableThreadLocal.unregister(ThreadLocal<T> threadLocal);

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ThreadLocal集成支持

用户代码中 ThreadLocal 通过定义为 TransmittableThreadLocal 即可获得跨线程传递能力，但是如果用户依赖的库中使用的是 ThreadLocal 实现，这个时候又无法修改其代码，就不能很方便的在异步化的代码中获得上下文传递的能力了。

一个可行的解决方式是开放集成的能力。

用户可以将需要传递的 ThreadLocal 注册进来，可以提供两个方法：

// 接下来这个 ThreadLocal 会被传递
<T> boolean TransmittableThreadLocal.register(ThreadLocal<T> threadLocal);

// 接下来这个 ThreadLocal 不会被传递
<T> boolean TransmittableThreadLocal.unregister(ThreadLocal<T> threadLocal);

[oldratlee]

The integration transmit for ThreadLocal cause the potential break of semantics of ThreadLocal:

• the Thread Confinement from ThreadLocal.
  • aka. the thread-safe relied on the ThreadLocal does no longer established. 💣

Integrate ThreadLocal with a copy lambda can express this risk explicitly, just like the copy method in TransmittableThreadLocal.

📔 Of course, the potential break of semantics of ThreadLocal also exists in java std lib class InheritableThreadLocal, and the method childValue(T) express this risk explicitly.

// ThreadLocal will be transmitted after this
<T> boolean Transmitter.register(
    ThreadLocal<T> threadLocal, Function<T, T> copyLambda);


// Then this ThreadLocal will not be transmitted
<T> boolean Transmitter.unregister(ThreadLocal<T> threadLocal);

cc @driventokill

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ThreadLocal的集成传输 会导致 潜在打破ThreadLocal的语义：

• ThreadLocal保证的线程封闭。
  • 即，依赖于ThreadLocal的线程安全不再成立。 💣

集成ThreadLocal，提供一个用于拷贝的Lambda可以明确地表达出此风险，就像TransmittableThreadLocal的copy方法一样。

📔 当然，Java标准类库的InheritableThreadLocal也一样潜在打破了ThreadLocal语义，而方法childValue(T)明确表达了此风险。

// 接下来这个 ThreadLocal 会被传递
<T> boolean Transmitter.register(
    ThreadLocal<T> threadLocal, Function<T, T> copyLambda);

// 接下来这个 ThreadLocal 不会被传递
<T> boolean Transmitter.unregister(ThreadLocal<T> threadLocal);

[oldratlee]

released in v2.11.0.