ThreadLocal是線程私有變量，本身是解決多線程環境線程安全，可以說單線程實際上沒必要使用。

既然多線程環境本身不使用static，那么又怎么會線程不安全。所以這個問題本身并不是問題，只是有人沒有理解ThreadLocal的真正使用場景，所以有此疑問。

看看jdk源碼推薦ThreadLocal使用static吧：

/*** This class provides thread-local variables.  These variables differ from* their normal counterparts in that each thread that accesses one (via its* {@code get} or {@code set} method) has its own, independently initialized* copy of the variable.  {@code ThreadLocal} instances are typically private* static fields in classes that wish to associate state with a thread (e.g.,* a user ID or Transaction ID).** <p>For example, the class below generates unique identifiers local to each* thread.* A thread's id is assigned the first time it invokes {@code ThreadId.get()}* and remains unchanged on subsequent calls.* <pre>* import java.util.concurrent.atomic.AtomicInteger;** public class ThreadId {*     // Atomic integer containing the next thread ID to be assigned*     private static final AtomicInteger nextId = new AtomicInteger(0);**     // Thread local variable containing each thread's ID*     private static final ThreadLocal&lt;Integer&gt; threadId =*         new ThreadLocal&lt;Integer&gt;() {*             &#64;Override protected Integer initialValue() {*                 return nextId.getAndIncrement();*         }*     };**     // Returns the current thread's unique ID, assigning it if necessary*     public static int get() {*         return threadId.get();*     }* }* </pre>* <p>Each thread holds an implicit reference to its copy of a thread-local* variable as long as the thread is alive and the {@code ThreadLocal}* instance is accessible; after a thread goes away, all of its copies of* thread-local instances are subject to garbage collection (unless other* references to these copies exist).** @author  Josh Bloch and Doug Lea* @since   1.2*/
public class ThreadLocal<T> {/*** ThreadLocals rely on per-thread linear-probe hash maps attached* to each thread (Thread.threadLocals and* inheritableThreadLocals).  The ThreadLocal objects act as keys,* searched via threadLocalHashCode.  This is a custom hash code* (useful only within ThreadLocalMaps) that eliminates collisions* in the common case where consecutively constructed ThreadLocals* are used by the same threads, while remaining well-behaved in* less common cases.*/

這個是Josh Bloch和Doug Lea寫的，再看看他們的著作：《Java并發編程實戰》

再看看jdk中的官方文檔：?

已經說的很明白了，再看看其他答案吧：

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按照ThreadLocal類的定義

此類提供線程局部變量。這些變量與普通變量不同，每個訪問一個線程（通過其get或set方法）的線程都有其自己的，獨立初始化的變量副本。ThreadLocal實例通常是希望將狀態與線程關聯的類中的私有靜態字段（例如，用戶ID或事務ID）。

這意味著說2個線程t1并t2執行someBMethod()，它們分別結束設置x1＆x2（的實例X）。現在，當t1come并執行時，someCMethod()它會獲取x1（它是由它自己設置的）和gets?。t2x2

換句話說，只有一個靜態實例是安全的ThreadLocal，因為在內部調用時它會執行類似的操作set

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Java?源代碼，

1. java.lang.Thread Class包含一個實例變量，如下所示。

   ThreadLocal.ThreadLocalMap threadLocals = null;

因為threadLocals變量是非靜態的，所以應用程序中的每個線程（即Thread Class的每個實例）將擁有它自己的threadLocals映射副本。

1. 該映射的鍵是當前?ThreadLocal實例，而value是您作為參數傳遞給ThreadLocal.set（）的值。

2. 當您嘗試在內部獲取?as值時ThreadLocal.get()，它將從Current Thread的ThreadLocalMap獲取。

簡而言之，您是從當前線程對象獲取＆設置值，而不是從ThreadLocal對象獲取＆設置值。

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