ThreadLocal Local thread

ThreadLocal Local thread , That is, each thread can have its own copy of variables , Other threads cannot access their own internal Copy variable .ThreadLocal Is a weak reference .

1.ThreadLoca It's a thread local variable , In each thread that accesses the variable , An independent variable copy is initialized inside the thread , Only this thread can access 【get() or set()】 Access your own internal Copy variable ,ThreadLocal Instances are usually declared as private static.

2. The thread is alive and ThreadLocal When an instance can be accessed , Each thread implicitly holds a copy of the thread local variable , When the thread life cycle ends ,ThreadLocal A copy of the instance of is lost along with the thread , By GC Garbage collection （ Unless there are other references to these copies ）

ThreadLocal effect ： Be sure to run under the same thread , Multiple methods called , They can share the same data .

ThreadLocal Of API

• set(T value)： Set the value in the current copy of the thread local variable to the specified value

• get()： Returns the value in the current copy of this thread local variable

• initialValue()： Returns the initial value in the current copy of this thread local variable

• remove()： Remove the value from the current copy of this thread local variable

ThreadLocal Of set()

 ThreadLocal Of get()

 ThreadLocal Of initialValue()

 ThreadLocal Of remove()

 ThreadLocalMap Of Entry Entity class

1. Every Thread There is a... Inside the thread Map(ThreadLocalMap)

2.Map Stored in the collection Entry Entity class ：ThreadLocal object （key） and The variable copy of the thread （value） ,   （key yes Current thread ThreadLocal,value The value is that we go to ThreadLocal Value stored inside ）.

3.Thread Inside thread Map By ThreadLocal Maintenance of , from ThreadLocal Responsible for providing Map Get and set the variable value of thread .

4. For different threads , Every time you get a copy value , Other threads cannot get the copy value of the current thread , It forms the isolation of copies , Mutual interference .

  Why use ThreadLocal Weak reference ？

If you find that entry Of key by null, I'll use a new one entry Cover up the old entry.

Under what circumstances key It will be equal to null？ Namely ThreadLocal There's no after use remove, Weak references will be automatically cleaned up at the next garbage collection , This is the time key Is equal to null 了 . Then through the above code , Can overwrite these useless garbage objects , Increase security .

// ThreadLocal  Local thread 

private static ThreadLocal<T> threadLocal= new ThreadLocal<T>() 

ThreadLocal Two main application areas of ：

1. Solve the concurrency problem ：

         The essence of solving concurrency problems is ： A kind of The idea of exchanging space for time , Time efficiency has improved , But there is also a potential overflow risk when using memory .

2. Solve the problem of data storage ：

         The problem of data storage mainly refers to ： How to realize data exchange and sharing among multiple components in the system , The thread as the executor is the most suitable data carrier . Although various components can realize data sharing , But the data is isolated from each other .

Thread synchronization mechanism is that multiple threads share the same variable , and ThreadLocal Is to create a separate copy of the variable for each thread , Each thread can change its own copy of variables without affecting the corresponding copies of other threads .

synchronized Time for space , Queue different threads to access ;

ThreadLocal It's space for time , A variable is set for each thread .