What the hell is ThreadLocal doing?

Treadlocal brief introduction

Usually , The variables we create can be accessed and modified by any thread . If you want to realize that each thread has its own unique local variables, how to solve it ？ JDK Provided in ThreadLocal Class is to solve this problem . ThreadLocal The main solution of class is to let each thread bind its own value , Can be ThreadLocal A class image is a box for storing data , The box can store the private data of each thread .

If you create a ThreadLocal Variable , Then each thread accessing this variable will have a local copy of this variable , This is also ThreadLocal The origin of variable names . They can use get（） and set（） Method to get the default value or change its value to the value of the copy saved by the current thread , This avoids thread safety issues .

It's like having a public notebook , Anyone can write something on it . But some people are not satisfied , He just wants his own things on his notebook , Everyone has a notebook

Treadlocal principle

The simplest idea is to Threadlocal Create a thread safe map surface ,thread As key,set The value entered as value, Each thread finds its own value. However, the following method is used in actual table creation .
Thread There's a parameter called threadlocals The variable of , He is ThreadLocalMap Type of .ThreadLocalMap It can be regarded as a thread safe hashmap, yes threadlocal A static inner class of . It means that each thread has a name threadlocals Of map,key yes threadlocal,value you set In the object. and threadlocal It's the equivalent of threadlocalmap An encapsulation of . Every time set and get Values are from this thread map find key by threadlocal Of value.

Comparison of the two methods

After this design, each Map Of Entry The number is getting smaller ： It used to be Thread The number of , Now it is ThreadLocal The number of , Can improve performance ;
When Thread After destruction, the corresponding ThreadLocalMap Then it was destroyed , Can reduce memory usage .
One should be built on threadlocal The big watch is divided into many small watches , In every thread . These small tables will die with the death of threads .

ThreadLocal Memory leak problem

ThreadLocalMap Used in key by ThreadLocal The weak references , and value Is a strong quote . therefore , If ThreadLocal Without strong external reference , When it comes to recycling ,key It's going to be cleaned up , and value It won't be cleaned up . thus ,ThreadLocalMap Will appear in key by null Of Entry. If we don't do anything ,value Can never be GC Recycling , At this time, there may be a memory leak .ThreadLocalMap This situation has been considered in the implementation , Calling set()、get()、remove() Method time , Will clean up key by null The record of . Use up ThreadLocal After the method It's better to call... Manually remove() Method