[thread pool]

Main parameters of thread pool

7 Big parameters

1. corePoolSize： Number of core threads , Number of resident core threads in the thread pool
   After creating the thread pool , When a request comes , The thread in the pool will be arranged to perform the request task , Approximately understood as the thread on duty today
   When the number of threads in the thread pool reaches corePoolSize after , The arrival queue will be put into the cache queue
2. maximumPoolSize： The thread pool can hold the maximum number of threads executing at the same time , This value must be greater than or equal to 1
   Quite a number of threads after capacity expansion , The maximum number of threads this thread pool can hold
3. keepAliveTime： Extra idle thread lifetime
   When the number of thread pools exceeds corePoolSize when , When the free time reaches keepAliveTime When the value of , Redundant idle threads will be destroyed , Until there is only corePoolSize Up to threads
   By default , Only if the number of threads in the thread pool is greater than corePoolSize when ,keepAliveTime It works
4. unit：keepAliveTime The unit of
5. workQueue： Task queue , Tasks submitted but not executed （ It's like a waiting area in a bank ）
   LinkedBlockingQueue： The linked list blocks the queue
   SynchronousBlockingQueue： Synchronous blocking queue
6. threadFactory： Represents the thread factory that generates the worker threads in the thread pool , Used to create a thread pool Generally, you can use default
7. handler： Refusal strategy , Indicates that when the queue is full and the worker thread is greater than the maximum number of threads in the thread pool （maximumPoolSize3） when , How to refuse the request to execute Runnable The strategy of
   

How the underlying thread pool works

Thread pool operation architecture diagram

The main processing flow of thread pool

A caption

1. After the thread pool is created , Waiting for the task request submitted
2. When calling execute() Method to add a request task , The thread pool will make the following judgment
   2.1 If the number of running thread pools is less than corePoolSize, Now create a thread to run the task
   2.2 If the number of running threads is greater than or equal to corePoolSize, Put the task in the queue
   2.3 If the queue is full at this time , And the number of running threads is less than maximumPoolSize, Then create a non core thread like Run this task ;
   2.4 If the queue is full and the number of running threads is greater than or equal to maximumPoolSize, Then the thread pool will start the saturation rejection policy to execute
3. When a thread completes a task , It takes a task from the queue to execute
4. When a thread has nothing to do for a certain time (keepAliveTime) when , Thread pool will judge ：
   4.1 If the number of threads currently running is greater than corePoolSize, Then the thread is stopped
   4.2 So after all tasks of thread pool are completed , It will eventually shrink to corePoolSize Size

Rejection policy for thread pool

Waiting thread is full ,max Threads have also reached , Unable to service new task , You need a rejection strategy to solve

• AbortPolicy： Default , Direct selling RejectedExcutionException abnormal , Prevent the system from working properly
• DiscardPolicy： Discard tasks directly , No processing and no exception thrown , If the running task is lost , It's a good plan
• CallerRunsPolicy： The strategy will not abandon the task , It doesn't throw an exception , Instead, some tasks are rolled back to the caller
• DiscardOldestPolicy： Discard the task waiting the longest in the queue , Then add the current task to the queue and try to submit the current task again