Meet ThreadPoolExecutor

Creation of thread pool

We don't usually use Executors Creating a thread pool , Because there will be the risk of avoiding resource exhaustion . Illustrate with examples ：

• FixedThreadPool： The allowed request queue length is  Integer.MAX_VALUE, A large number of requests may pile up , Which leads to  OOM.

• SingleThreadPool： and FixedThreadPool Will face the same problem

• CachedThreadPool ： The number of threads allowed to be created is  Integer.MAX_VALUE, A large number of threads may be created , Which leads to  OOM.

• ScheduledThreadPool： and CachedThreadPool Will face the same problem

and  Executors  Medium method Executors.newFixedThreadPool()、Executors.newSingleThreadExecutor()  and  Executors.newCachedThreadPool(), The bottom floor is all through  ThreadPoolExecutor  Realized .

ThreadPoolExecutor Constructor source code ：

public ThreadPoolExecutor(int corePoolSize,
 int maximumPoolSize,
 long keepAliveTime,
 TimeUnit unit,
 BlockingQueue<Runnable> workQueue,
 ThreadFactory threadFactory,
 RejectedExecutionHandler handler) {
 if (corePoolSize < 0 ||
 maximumPoolSize <= 0 ||
 maximumPoolSize < corePoolSize ||
 keepAliveTime < 0)
 throw new IllegalArgumentException();
 if (workQueue == null || threadFactory == null || handler == null)
 throw new NullPointerException();
 this.acc = System.getSecurityManager() == null ?
 null :
 AccessController.getContext();
 this.corePoolSize = corePoolSize;
 this.maximumPoolSize = maximumPoolSize;
 this.workQueue = workQueue;
 this.keepAliveTime = unit.toNanos(keepAliveTime);
 this.threadFactory = threadFactory;
 this.handler = handler;
}


• corePoolSize
    Indicates the number of core threads in the thread pool . If set to 0, The thread pool will be destroyed when there are no tasks ; If it is greater than 0, Even if there are no tasks, the number of threads in the thread pool will be guaranteed to be equal to this value .

• maximumPoolSize
    Indicates that the thread pool increases when tasks increase , The maximum number of threads that can be created .

• keepAliveTime
    Indicates the lifetime of the thread , When the thread pool is idle and exceeds this time , The extra threads will be destroyed , Until the number of threads in the thread pool is equal to  corePoolSize  until , If  maximumPoolSize  be equal to  corePoolSize, Then the thread pool will not destroy any threads when it is idle .

• unit
    A unit of survival time

• workQueue
    Represents the task queue executed by the thread pool , When all threads in the thread pool are processing tasks , If a new task comes, it will be cached in this task queue and queued for execution .

• threadFactory
    Represents the creation factory of the thread

Be careful ： If corePoolSize The settings are relatively small , Threads will be created and destroyed frequently , If the setting is large , It will waste system resources , Therefore, this value needs to be adjusted according to the actual business .

Submit thread tasks

There are two ways to submit thread tasks ：

• submit()

• execute()

among ,submit()  Methods can cooperate with  Futrue  To receive the return value executed by the thread , Belong to  ExecutorService  Interface method ;execute() No return value .

Task rejection policy

• AbortPolicy, Termination strategy , The thread pool throws an exception and terminates execution , It is the default rejection policy ;

• CallerRunsPolicy, Give the task to the current thread to execute ;

• DiscardPolicy, Ignore this task （ The latest mission ）;

• DiscardOldestPolicy, Ignore the earliest tasks （ The first task to join the queue ）.

• Custom reject policy