ThreadLocal summary (to be continued)

1. Java Memory model ：

1. Java All variables are stored in main memory
2. Each thread has its own working memory , It stores the copy of variables used by the thread （ This copy is a copy of this variable in main memory ）
3. All operations of a thread on a shared variable must be performed in its own working memory , Can't read and write directly in main memory , After the thread modifies the variables, it should refresh them into the main memory ;
4. Variables in the working memory of other threads cannot be accessed directly between different threads , The transfer of variable values between threads needs to pass through Main memory To complete .

Threads 1 Changes to shared variables , To be threaded 2 See in time , It has to go through the following 2 A process ：
① Put the working memory 1 The shared variables updated in are refreshed to main memory
② Update the value of the latest shared variable in main memory to working memory 2 in

2. background

When multiple threads write to a variable, it is easy to cause thread safety problems , General users need additional synchronization measures to ensure thread safety when accessing shared variables ;
ThreadLocal It is a way to avoid thread insecurity in multi-threaded access in addition to locking ;
When we create a variable , If each thread accesses it All the variables accessed are the thread's own variables , In this way, there will be no thread safety problems .（ Ensure thread safety by data isolation ）

3. summary

ThreadLocal Provide an exclusive copy of variables for each thread , Each thread can change its own copy independently , It doesn't conflict with copies of other threads ;

If you create ⼀ individual ThreadLocal Variable , Then each thread accessing this variable will have a local copy of this variable ;
You can make ⽤ get() and set() ⽅ 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 ;

example ：

ThreadLocal The data of is isolated , Each thread can get a copy of the variable , Threads 2 Still got it null !

4. structure

JDK8 Before ： key yes Thread, from Threadlocal To maintain the ThreadlocalMap

JDK8 The benefits of changing the structure ：
1.Thread As Key, When there are many threads Entry It's going to be a lot , and ThreadLocal As Key,Entry The number will be reduced a lot ;key Less hash conflicts can be avoided
2. When Thread At the time of destruction ,ThreadLocalMap It will be destroyed with it , Reduce memory overhead ; Early stage ThreadLocalMap from ThreadLocal maintain , Thread termination does not destroy ThreadLocalMap;

JDK8 Middle structure ： key yes threadLocal object , from Thread Thread to maintain threadLocalMap

Structure description ：
Every Thread Maintain a ThreadLocalMap,
Key yes Threadlocal object ,value Namely Threadlocal Of set Method stored value （ Copies of variables ）;
For different threads , Have their own different ThreadLocalMap, So we can isolate the data copies ;

5. set() 、get()、remove() Method source code

5.1 set()

1. Get the current thread object first ;
2. Gets the name of the thread object threadLocals attribute , That is to say threadLocalMap;
3. If map Existence will be the current threadLocal Object as key, The value passed in as value Pass in map; If map If it is empty, a map

5.2 get()

1. Get current Thread Thread object ;
2. Get threaded threadLocals namely threadLocalMap;
3. When map There is , With the current threadLocal The object is key, call map.getEntry How to get Entry Entity , if Entry If the entity is not empty, take out Entry In the question value;
4.① If map non-existent or ② Not with the current threadLocal Object associated Entry call setInitiavalue() Set initialization method ：
Judge map, If empty, create map;
If map If it is not empty, the current ThreadLocal Object as key, and value Deposit in map

5.3 remove()

1. Get the current thread object ;
2. Get threaded threadLocals namely threadLocalMap;
3. If map If it is not empty, it will overflow the current threadLocal Object associated Entry

6. Memory leak problem

ThreadLocalMap Chinese envoy ⽤ Of key by ThreadLocal Of Weak reference ,⽽ value yes Strong citation . therefore , If
ThreadLocal Not forced by the outside ⽤ Under the circumstances , When it comes to recycling ,key It's going to be cleaned up ,⽽ value It won't be cleaned up ;

such ⼀ Come on , ThreadLocalMap Will appear in key by null Of Entry, If we don't do anything ,value forever ⽆ Dharma quilt GC Recycling , At this time, it may produce ⽣ Memory leak .
ThreadLocalMap This situation has been considered in the implementation , In tune ⽤ 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 ;