为什么jdk源码推荐ThreadLocal使用static

ThreadLocal是线程私有变量&＃xff0c;本身是解决多线程环境线程安全&＃xff0c;可以说单线程实际上没必要使用。

既然多线程环境本身不使用static&＃xff0c;那么又怎么会线程不安全。所以这个问题本身并不是问题&＃xff0c;只是有人没有理解ThreadLocal的真正使用场景&＃xff0c;所以有此疑问。

看看jdk源码推荐ThreadLocal使用static吧&＃xff1a;

/*** This class provides thread-local variables. These variables differ from* their normal counterparts in that each thread that accesses one (via its* {&＃64;code get} or {&＃64;code set} method) has its own, independently initialized* copy of the variable. {&＃64;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).**

For example, the class below generates unique identifiers local to each* thread.* A thread&＃39;s id is assigned the first time it invokes {&＃64;code 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 &＃61; new AtomicInteger(0);**     // Thread local variable containing each thread&＃39;s ID*     private static final ThreadLocal threadId &＃61;*         new ThreadLocal() {*             &＃64;Override protected Integer initialValue() {*                 return nextId.getAndIncrement();*         }*     };**     // Returns the current thread&＃39;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 {&＃64;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).** &＃64;author Josh Bloch and Doug Lea* &＃64;since 1.2*/
public class ThreadLocal {/*** 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.*/

这个是Josh Bloch和Doug Lea写的&＃xff0c;再看看他们的著作&＃xff1a;《Java并发编程实战》

再看看jdk中的官方文档&＃xff1a; 

已经说的很明白了&＃xff0c;再看看其他答案吧&＃xff1a;

按照ThreadLocal类的定义

此类提供线程局部变量。这些变量与普通变量不同&＃xff0c;每个访问一个线程&＃xff08;通过其get或set方法&＃xff09;的线程都有其自己的&＃xff0c;独立初始化的变量副本。ThreadLocal实例通常是希望将状态与线程关联的类中的私有静态字段&＃xff08;例如&＃xff0c;用户ID或事务ID&＃xff09;。

这意味着说2个线程t1并t2执行someBMethod()&＃xff0c;它们分别结束设置x1&＃xff06;x2&＃xff08;的实例X&＃xff09;。现在&＃xff0c;当t1come并执行时&＃xff0c;someCMethod()它会获取x1&＃xff08;它是由它自己设置的&＃xff09;和gets 。t2x2

换句话说&＃xff0c;只有一个静态实例是安全的ThreadLocal&＃xff0c;因为在内部调用时它会执行类似的操作set

Java 源代码&＃xff0c;

1. java.lang.Thread Class包含一个实例变量&＃xff0c;如下所示。

   ThreadLocal.ThreadLocalMap threadLocals &＃61; null;

因为threadLocals变量是非静态的&＃xff0c;所以应用程序中的每个线程&＃xff08;即Thread Class的每个实例&＃xff09;将拥有它自己的threadLocals映射副本。

1. 该映射的键是当前 ThreadLocal实例&＃xff0c;而value是您作为参数传递给ThreadLocal.set&＃xff08;&＃xff09;的值。

2. 当您尝试在内部获取 as值时ThreadLocal.get()&＃xff0c;它将从Current Thread的ThreadLocalMap获取。

简而言之&＃xff0c;您是从当前线程对象获取&＃xff06;设置值&＃xff0c;而不是从ThreadLocal对象获取&＃xff06;设置值。