Detailed explanation of ThreadLocal

Catalog

Preface

One 、 What is? ThreadLocal?

Two 、 Bottom source

3、 ... and 、 Why? Map Of key To set as weak reference ？

Four 、 Value How to release ？

5、 ... and 、ThreadLocal How to realize isolation ？

summary

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Preface

I'm reading the algorithm book today hash Table time , solve hash The conflict section mentioned this concept , So I just want to record

One 、 What is? ThreadLocal?

ThreadLocal yes Thread Local variables on , It can realize the isolation between threads , Sharing of resources within threads , Threads are private

ps: Here, the private and public threads are described as follows

  

Two 、 Bottom source

because ThreadLocal Is attached to Thread Of , Then we can take a look here first Thread For in the ThreadLocal The definition of

    ThreadLocalMap threadLocals;
    ThreadLocalMap inheritableThreadLocals;

Thread One was maintained internally ThreadLocalMap Properties of , So what is ThreadLocalMap? Let's take a look at the underlying source code

ThreadLocalMap Is its static inner class , And it is default , So we cannot get this internal object externally , Only through ThreadLocal To control . And the interior is made of Entry constitute , Every Entry Array of key Is a weak reference ,value It is not .

About Entry You can see ：

And then let's see ThreadLocal Internal method of ： 

We found that if we go get Words , No, get To the current thread map perhaps map There is no current ThreadLocal Key words , It initializes a map, also take ThreadLocal As key,null As value Initialize it ( And others map Make a difference , its get Method if get If it doesn't arrive, it will set Current TheadLocal go in , Other map The implementation has not been modified map The material in that ); If you get the map Words , Will key=ThreadLocal Of value It's worth taking out

 

ThreadLocalMap Some characteristics of

The initial capacity is 16, The expansion factor is  2/3, Double the expansion capacity

Use open addressing to resolve conflicts

3、 ... and 、 Why? Map Of key To set as weak reference ？

Soft citation 、 Strong quotation and weak quotation

Strong citation 、 Weak reference 、 Soft citation 、 Virtual reference _young_man2 The blog of -CSDN Blog

Four 、 Value How to release ？

5、 ... and 、ThreadLocal How to realize isolation ？

ThreadLocal The reason why we can achieve variable thread isolation , In fact, each thread has its own ThreadLocalMap Object to store the same threadLocal example set Value , And the value is also based on the same threadLocal Examples go to their own ThreadLocalMap Inside looking for , Nature doesn't affect each other , So as to achieve the goal of thread isolation ！
 

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summary

ThreadLocal Belong to Thread Private ！

ThreadLocalMap If it does not exist, it will be automatically created 【 Use get Method time 】