Principle and analysis of java thread pool

itread01 2020-11-06 01:28:57
principle analysis java thread pool


Thread pool is a very common concurrency framework , Almost all programs that require asynchronous and concurrent processing of tasks are available to thread pools . ** The benefits of using thread pools are as follows **: 1. Reduce resource consumption : Reusing established thread pools , Reduce the cost of building and destroying ; 1. Improve response time : When the mission arrives , Can be executed immediately , You don't have to wait for the thread to build ; 1. Improve thread manageability : Thread pool can allocate threads uniformly 、 Tuning and monitoring . ## principle The principle of thread pool is very simple , Here we use the processing flow to summarize : 1. The thread pool determines whether all threads in the core pool are executing tasks , If not , Create a new thread to execute the task ; 1. If the core thread pool is full , The new task will exist ; 1. If the job queue is full , If the number of threads does not reach the maximum thread pool limit , Create a new thread to execute the task ; 1. The number of threads reached the limit , The saturation strategy is triggered to handle the task ; Using work queues , Is to minimize the overhead of thread creation . Work queues are implemented by blocking queues . ### Blocking queues Blocking queues (BlockingQueue) A queue that supports blocking insert and remove elements . - Blocked insertion : When the queue is full , Block the thread that inserts the element , Until the queue is dissatisfied ; - Removal of obstruction : When the queue is empty , Block the line layer of the removed element , Until the queue is not empty ; principle :** Use the notifier pattern ** Realize . When the producer adds elements to a full queue , It's blocking producers . When the consumer removes the element , The producer is informed that the current queue is available . There are three types of blocking queues , The difference is : - Bounded blocking queues :ArrayBlockingQueue( Array ),LinkedBlockingQueue( Link string ) - Unbounded blocking queue :LinkedTransferQueue( Link string ),PriorityBlockingQueue( Support prioritization ),DelayQueue( Supports unbounded blocking queues for delayed fetch elements ) - Synchronous handover queue :SynchronousQueue #### Bounded blocking queues It mainly includes ArrayBlockingQueue( Array ),LinkedBlockingQueue( Link string ) Two kinds of . The bounded queue size matches the number of threads , The queue capacity is large and the number of threads is hours , It can reduce context switching and reduce cpu Usage rate , But it reduces throughput . #### Unbounded blocking queue What is more commonly used is LinkedTransferQueue.FixedThreadPool That's how it works . Unbounded blocking queue should be used with caution , Because in some cases , It may cause a large number of tasks to pile up in the queue , It's causing memory to soar . #### Synchronous handover queue SynchronousQueue. Block queues that do not store elements , Every one of them put The operation must wait for a take operation , Otherwise, you cannot continue to add new elements . Used to implement CachedThreadPool Thread pool . The task queue mapping relationships used by each thread pool are as follows : Thread pool | Blocking queues ---|--- FixedThreadPool | LinkedBlockingQueue SingleThreadExecutor | LinkedBlockingQueue CachedThreadExecutor | SynchronousQueue ScheduledThreadPoolExecutor | LinkedBlockingQueue ## Implementation class analysis ThreadPoolExecutor yes Java Implementation class of thread pool , yes Executor Interface derived from the core class . The dependency diagram is as follows : ![image](https://qiniu.debrisflow.cn/20201104Executor.png) It has to be mentioned here that Executor frame , The framework consists of three parts , as follows : - Mission . The interface to be implemented by the task to be executed :Runnable and Callable; - Mission execution . This is the core interface Executor And inherited ExecutorService.ExecutorService The following two classes are derived : - ThreadPoolExecutor: Thread pool core implementation class ; - ScheduledThreadPoolExecutor: For regular tasks ; - The result of asynchronous computing . Interface Future And realize Future Interface FutureTask Class . ### Thread pool creation ``` new ThreadPoolExecutor(corePoolSize, maximumPoolSize, keepAliveTime, milliseconds, runnableTaskQueue, handler) ``` The construction method is as follows : ``` public ThreadPoolExecutor(int corePoolSize, int maximumPoolSize, long keepAliveTime, TimeUnit unit, Block
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