秋田君說(shuō)的也很不錯(cuò)， 我這里補(bǔ)充下， Map<Thread, T>這種結(jié)構(gòu)，hash表沖突會(huì)很?chē)?yán)重，舉個(gè)例子。map.put(thread1, 100);map.put(thread1, 200);map.put(thread1, 300);你發(fā)現(xiàn)沒(méi)有，一個(gè)map put了三個(gè)值，那取值的時(shí)候， 怎么辦呢？

同樣的 main 函數(shù)，使用老師的代碼執(zhí)行會(huì)發(fā)現(xiàn)主線程不同 MyThreadLocal 實(shí)例中的數(shù)據(jù)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(yīng)該是為了不同場(chǎng)景使用時(shí)數(shù)據(jù)隔離，同一線程之間不同 MyThreadLocal 實(shí)例間的數(shù)據(jù)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(lái)存儲(chǔ)數(shù)據(jù)，由于 threadLocalMap 是靜態(tài)變量，那么同一線程不同 MyThreadLocal 實(shí)例存儲(chǔ)時(shí)，數(shù)據(jù)會(huì)被覆蓋，讀取數(shù)據(jù)時(shí)，會(huì)得到錯(cuò)亂的值！

應(yīng)該為了解耦