Examples of ThreadLocal usage scenarios under multithreading

In business development ,threadLocal There are two business scenarios that are used more .

1、threadlocal Ensure that each thread has its own object , Create a copy of the local thread , Ensure that each thread has its own copy , This ensures thread safety .

2、threadlocal It can act like a global variable , In one of the methods, use set In the data , Other methods can be used get Direct access to , Avoid passing parameters , The function is to save information independently for each thread .

Scene one ：

result ： Three threads are opened , Because in the threadlocal, Each thread has its own local copy , It won't add to 15, It is n Maximum output 5.

And that is simpledateformat It is also applicable to scenario 1 , When thread pool is on , Submit 10000 When there are more tasks , Each thread creates its own simpledateformat It's going to cost a lot , And it takes up memory , Destroying objects is also costly ： In this case , Our first thought is that all threads share one simpledataformate, But it's not thread safe , How to solve it , Add... To him synchrolized lock , This will solve the problem , But when the thread is running , There will be a blockage , It affects efficiency very much , At the moment threadlocal Is a very good solution .

threadlocal Will make every thread simpledateformat Independent , If the thread pool Executors.newFixedThreadPool(15) Turn on 15 Threads , It will only create 15 individual simpledateformat, And thread safe , It won't affect each other before .

Scene two ：

Scenario 2 is similar to global variables , It avoids the trouble of transferring parameters , Ensure that the object values obtained by the method are the same , First store the object in localthread Inside , And then get , Finally, remember remove fall .

ask ： Used in thread pool threadlocal Why is it easy to cause memory leakage ？

Because the thread lifetime in the thread pool is too long , It is often destroyed after the program stops , such threadlocal Held threadlocalmap Will never be recycled , terms of settlement , Can be manually in finally Block calls its remove() Method .