Creation of thread pool

Creation of thread pool


To create a thread pool, you need to enter several parameters , as follows .

1）corePoolSize（ The basic size of the thread pool ）： When a task is submitted to the thread pool , The thread pool creates a thread to perform a task , Even if other idle basic threads can perform new tasks, threads will be created , When the number of tasks to be executed is larger than the basic size of the thread pool, it will not be created anymore . If the thread pool is called prestartAllCoreThreads() Method , The thread pool is created ahead of time and starts all the base threads .

2）runnableTaskQueue（ Task queue ）： The blocking queue used to hold tasks waiting to be executed . You can select the following blocking queues .

- ArrayBlockingQueue： It is a bounded blocking queue based on array structure , This queue is FIFO（ fifo ） The principle sorts the elements .
- LinkedBlockingQueue： A blocking queue based on linked list structure , This queue is FIFO Sorting elements , Throughput is usually higher ArrayBlockingQueue. Static factory method Executors.newFixedThreadPool() Using this queue .
- SynchronousQueue： A blocked queue that does not store elements . Each insert must wait until another thread calls the remove operation , Otherwise, the insert operation is always blocked , Throughput is usually higher
- Linked-BlockingQueue, Static factory method Executors.newCachedThreadPool Using this queue .
- PriorityBlockingQueue： An infinite blocking queue with priority .

3）maximumPoolSize（ Maximum number of thread pools ）： The maximum number of threads allowed to be created by the thread pool . If the queue is full , And the number of threads created is less than the maximum number of threads , The thread pool creates a new thread to execute the task . It is worth noting that , If the boundless task queue parameter is used, it has no effect .

4）ThreadFactory： Factory for setting up thread creation , You can set a more meaningful name for each created thread through the thread factory . Using open source framework guava Provided ThreadFactoryBuilder You can quickly set meaningful names for threads in the thread pool , The code is as follows .
```java
 new ThreadFactoryBuilder().setNameFormat("XX-task-%d").build();


5）RejectedExecutionHandler（ Saturated strategy ）： When the queue and thread pool are full , Indicates that the thread pool is saturated , Then a strategy must be adopted to deal with the new tasks submitted . By default, this strategy is AbortPolicy, An exception is thrown when a new task cannot be processed . stay JDK 1.5 in Java The thread pool framework provides the following 4 Strategies .

• AbortPolicy： Throw an exception directly .

• CallerRunsPolicy： Only the thread where the caller is running the task .

• DiscardOldestPolicy： Discard the nearest task in the queue , And carry out the current task .

• DiscardPolicy： Don't deal with , discarded .

Of course , It can also be implemented according to the needs of application scenarios RejectedExecutionHandler Interface customization policy . Such as logging or persistent storage can not handle the task .

keepAliveTime（ Thread activity holds time ）： After the worker thread of thread pool is idle , Time to stay alive . therefore , If there are many tasks , And the execution time of each task is relatively short , You can turn up the time , Improve thread utilization .

TimeUnit（ A unit of time in which thread activity is maintained ）： Optional units have days （DAYS）、 Hours （HOURS）、 minute （MINUTES）、 millisecond （MILLISECONDS）、 Microsecond （MICROSECONDS, A thousandth of a millisecond ） And nanoseconds （NANOSECONDS, Thousandth of a microsecond ）.