[Interview: Concurrency 38: Multithreading: Thread Pool] Basic concepts of the ThreadPoolExecutor class

[Interview: Concurrency 38: Multithreading: Thread Pool] Basic Concepts of ThreadPoolExecutor Class

00. Foreword

If you have any questions please point them out, thanks.

01. Introduction

The ThreadPoolExecutor class implements the ExecutorService interface. The difference between it and the ScheduledThreadPoolExecutor class is that the ScheduledThreadPoolExecutor isThe thread pool with task scheduling function implements this and we will talk about it later.

02.ThreadPoolExecutor-Pool Status

ThreadPoolExecutor uses the upper 3 bits of int to represent the thread pool status, and the 29th bit represents the number of threads

State Name	High 3	Receive new assignments	Handling blocking queue tasks	Description
RUNNING	111	Y	Y
SHUTDOWN	000	N	Y	Will not receive new tasks, but will process the remaining tasks in the blocking queue
STOP	001			will interrupt the running task and discard the blocking queue task
TIDYING	010			The task is fully executed, and the active thread is 0 and is about to end
TERMINAIED	011			End state
Why the thread pool status and the number of thread pools are represented by one number instead of two, the reason is that these two information are stored in an atomic variable ctl, so that cas can be used only once in the futureAtomic operation for assignment.

03. Constructor

We can see that the constructor of the ThreadPoolExecutor class has many parameters, one by oneExplanation
corePoolSize: number of core threads
maximumPoolSize: maximum number of threads
keepAliveTime: time to live - for rescue threads
br>unit: time unit - for emergency threads
workQueue: blocking queue
threaFactory: thread factory - can be created for threads
hander: Rejection Policy

The difference between ThreadPoolExecutor and our custom thread pool

1. Help us customize the thread pool. There is only one thread in the thread pool, which is the core thread, and our core thread has the take method and the pull method.But the ThreadPoolExecutor thread pool has two types of threads: one is corePoolSize when the number of core threads, and the other is when the number of emergency threads is maximumPoolSize-corePoolSize, and the core thread has only the take method and the emergency thread has only the pull method, which is why keepAlivTime and unit are only for emergency threads..
2. The name of the thread in the thread pool cannot be specified in the custom thread pool, and the thread name in the ThreadPoolExecutor thread pool can be changed through the threadFactory parameter
3. Custom thread pool and ThreadPoolExecutor thread pool rejection policy

How it works

Introduction

We know that there are only core threads in the custom thread pool, so the process of the custom thread pool is:
Create a task -> give it to an idle core thread for execution -> exceed the part of the taskPut into the task queue -> if there are idle core threads at this time, get the task execution from the task queue -> if there are no idle threads and the task queue is full -> the production thread executes the rejection policy
ThreadPoolExecutor thread poolThere are core threads and emergency threads, so the process of the ThreadPoolExecutor thread pool is:
Additional: The core thread of the ThreadPoolExecutor thread pool has no timeout period, but the emergency thread does.
Create a task -> give it to an idle core thread for execution -> put the excess task into the task queue -> if there is an idle core thread at this time -> if there is an idle core thread at this time, get the task execution assignment from the task queueFor the core -> if there are no idle threads and the task queue is full -> the extra tasks created by the emergency thread are handed over to the emergency thread for execution -> if the emergency thread is no longer idle -> the production thread executes the rejection policy

Explanation

It can be seen that the role of the emergency thread is to create an emergency thread when the core thread and the task queue are full. If the emergency thread does not obtain a task within the timeout period, it will be removed from the thread pool.

Deny Policy

If the core thread is not idle, the task queue is full, and the emergency thread is not idle, the rejection policy is executed in this case. The rejection policy jdk provides four implementations.

1. AbortPolicy makes the caller throw a RejectedExecutionException, which is the default policy
2. CallerRunsPolicy lets callers run tasks
3. DiscardPolicy abandons this task
4. DiscardOldestPolicy Discard the oldest task in the queue and replace it with this task