秋田君說的也很不錯， 我這里補充下， Map<Thread, T>這種結構，hash表沖突會很嚴重，舉個例子。map.put(thread1, 100);map.put(thread1, 200);map.put(thread1, 300);你發現沒有，一個map put了三個值，那取值的時候， 怎么辦呢？

同樣的 main 函數，使用老師的代碼執行會發現主線程不同 MyThreadLocal 實例中的數據是互不影響互不干擾的

public?static?void?main(String[]?args)?{
????MyThreadLocal<Long>?userIdContext?=?new?MyThreadLocal<>()?{
????????@Override
????????protected?Long?initialValue()?{
????????????return?1L;
????????}
????};
????MyThreadLocal<Connection>?connectionContext?=?new?MyThreadLocal<>();

????System.out.println(userIdContext.get());?//?1
????System.out.println(connectionContext.get());?//?null
}

從使用上看應該是為了不同場景使用時數據隔離，同一線程之間不同 MyThreadLocal 實例間的數據隔離

@Deprecated
public?class?MyThreadLocalWrong<T>?{

????private?static?Map<Thread,?Object>?threadLocalMap?=?new?HashMap<>();

????protected?T?initialValue()?{
????????return?null;
????}

????public?synchronized?T?get()?{
????????return?(T)?threadLocalMap.computeIfAbsent(Thread.currentThread(),?t?->?initialValue());
????}

????public?synchronized?void?set(T?t)?{
????????threadLocalMap.put(Thread.currentThread(),?t);
????}

????public?static?void?main(String[]?args)?{
????????MyThreadLocalWrong<Long>?userIdContext?=?new?MyThreadLocalWrong<>()?{
????????????@Override
????????????protected?Long?initialValue()?{
????????????????return?1L;
????????????}
????????};
????????MyThreadLocalWrong<Connection>?connectionContext?=?new?MyThreadLocalWrong<>();

????????System.out.println(userIdContext.get());?//?1
????????System.out.println(connectionContext.get());?//?1
????}
}

如果直接使用 Map<Thread, T> 來存儲數據，由于 threadLocalMap 是靜態變量，那么同一線程不同 MyThreadLocal 實例存儲時，數據會被覆蓋，讀取數據時，會得到錯亂的值！

應該為了解耦