ThreadLocal 源码阅读

作者的类注释：

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).

　　此类提供线程本地变量。这些变量和每一个线程中通过 {@code get} or {@code set} method)访问的普通变量不同，它们都有自己独立初始化的变量副本。

      此类是线程安全的，应用很多。

1、按照几个关键类变量作图如下：

　　

 2、理解实现原理、好处

　　应对其带来的问题，如内存泄漏问题，解决方法