ThreadLocal源码

This class provides thread-local variables. These variables differ from their normal counterparts in that each thread that accesses one (via its get or set method) has its own, independently initialized copy of the variable. 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).
For example, the class below generates unique identifiers local to each thread. A thread's id is assigned the first time it invokes ThreadId.get() and remains unchanged on subsequent calls.
   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<Integer> threadId =
           new ThreadLocal<Integer>() {
               @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();
       }
   }
   
Each thread holds an implicit reference to its copy of a thread-local variable as long as the thread is alive and the 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).

its get or set method) has its own, independently initialized copy of the variable. 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).
For example, the class below generates unique identifiers local to each thread. A thread's id is assigned the first time it invokes ThreadId.get() and remains unchanged on subsequent calls.

此类提供线程局部变量。 这些变量不同于它们的正常对应变量它的每个访问一个（通过它的 get 或 set 方法）的线程都有它自己的、独立初始化的变量副本。 ThreadLocal 实例通常是希望将状态与线程相关联的类中的私有静态字段（例如，用户 ID 或事务 ID）相关联。
例如，下面的类生成每个线程本地的唯一标识符。 线程的 id 在第一次调用 ThreadId.get() 时被分配，并且在后续调用中保持不变。
只要线程处于活动状态并且 ThreadLocal 实例可访问，每个线程都持有对其线程局部变量副本的隐式引用； 在线程消失后，它的所有线程本地实例副本都将受到垃圾回收（除非存在对这些副本的其他引用）。