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This article introduces the relevant knowledge of "how to use Executor in Java". Many people will encounter such a dilemma in the operation of actual cases, so let the editor lead you to learn how to deal with these situations. I hope you can read it carefully and be able to achieve something!

In the previous article, we briefly introduced the use of thread pool. although thread pool is more convenient than using the original thread class directly, the parameters need to be carefully considered by developers when creating thread pool objects. otherwise, the thread pool will not achieve our satisfactory results. In actual development, the thread framework we use most often is not thread pool but Executor. It provides a more convenient way for us to create a thread pool. Executor is an interface that is the basis of the Executor framework and separates task submission from task execution. The underlying layer still uses a thread pool. Through the utility class Executors of the Executor framework, you can create three types of thread pools: FixedThreadPool, SingleThreadExecutor, and CachedThreadPool. Let's take a look at the differences between them.

FixedThreadPool: thread pool that can be reused with a fixed number of threads.

We can see from the source code that the newFixedThreadPool () method sets the number of initialized threads and the maximum number of threads allowed in the thread pool to parameter values. In other words, how many threads are executing at the same time as many of our parameters are passed. And it creates a LinkedBlockingQueue to implement the queue service in the thread pool. We know that LinkedBlockingQueue is an unbounded queue, which means that it can store many thread tasks, and tasks are saved to this queue when the number of concurrent tasks in the thread pool is greater than the maximum number allowed in the thread pool. When a thread finishes execution, the thread pool takes the corresponding task out of the queue to execute. And we found that keepAliveTime has been set to 0 in the newFixedThreadPool () method, so that if a thread finishes executing the task, it needs to be terminated immediately.

SingleThreadExecutor: a thread pool for a single thread.

We see that the SingleThreadExecutor () and newFixedThreadPool () methods are almost the same, except that both the number of initialization threads and the maximum number of threads allowed for the thread pool are set to 1. The process of enforcement is the same as the logic of appeal.

CachedThreadPool: create a thread pool for new threads as needed.

Let's see that CachedThreadPool () sets corePool to 0 and maximumPoolSize to the most Integer.MAX_VALUE. This means that when the thread pool is created, the idle thread is not initialized, but if a task is added, it creates a new thread.

Let's take a look at the specific code.

FixedThreadPool

Because we only created 2 threads, the maximum number of concurrency of this thread pool is 2, so task 1, task 2 is output at the same time, task 3 is output later. Because task 3 is added to the queue, the tasks in the queue will not be executed until the other tasks have been executed.

SingleThreadExecutor

Because the maximum concurrency of the SingleThreadExecutor thread pool is 1, when we submit three tasks, only one will execute and the other two will be added to the queue, so the result of execution shows that there is only one thread.

CachedThreadPool

We know that the idle thread initialized in the CachedThreadPool thread pool is 0, but the maximum number of threads it allows is Integer.MAX_VALUE, which is equivalent to infinity. So when we submit the task, the thread pool creates a new thread for us because it does not exceed the maximum number of threads allowed. So task one, task two, task three are all executed in parallel.

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