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先看Android源碼（API24）中對ThreadLocal的定義：
public class ThreadLocal<T>
即ThreadLoca是一個泛型類，再看對該類的注釋：
/**
* This class provides thread-local variables. These variables differ from
* their normal counterparts in that each thread that accesses one (via its
* <tt>get</tt> or <tt>set</tt> method) has its own, independently initialized
* copy of the variable. <tt>ThreadLocal</tt> instances are typically private
* static
