Thread pools are the core mechanism used to manage threads in Java concurrent programming. Their role is to avoid the performance overhead caused by frequent creation and destruction of threads. 1. It improves response speed and resource utilization by pre-creating a set of threads and waiting for task allocation; 2. It is suitable for handling a large number of short life cycle and highly repetitive tasks, such as network requests or timing tasks; 3. Java provides a variety of thread pool types, including FixedThreadPool (suitable for heavy load systems), CachedThreadPool (suitable for short-term asynchronous tasks), SingleThreadExecutor (suitable for task serial execution) and ScheduledThreadPool (suitable for timing and periodic tasks); 4. When using correctly, pay attention to setting the number of threads, selecting the appropriate task queue, closing the thread pool in time, and monitoring the running status; 5. Custom thread pools can more flexibly control core parameters, such as the number of core threads, maximum number of threads, idle timeout time, queue capacity and rejection strategies, thereby improving system stability and observability.

Java's ExecutorService is an important tool in concurrent programming, and thread pool (Thread Pool) is the core mechanism behind it. Using thread pools can avoid the performance overhead caused by frequent creation and destruction of threads, and improve program response speed and resource utilization.

What is a thread pool?

Simply put, a thread pool is a set of pre-created threads that are waiting for tasks to be executed. You just need to submit the task to the thread pool, which will automatically assign an idle thread to run the task instead of creating a new thread every time.

This method is particularly suitable for handling a large number of tasks with short life cycles and strong repetitiveness, such as network requests, timing tasks, etc. Java provides java.util.concurrent.Executors factory class to quickly create common thread pool types, such as fixed-size thread pools, cache thread pools, single-thread pools, etc.

Common thread pool types and applicable scenarios

1. FixedThreadPool
   Fixed-size thread pool. Suitable for systems with heavy load and stable tasks, such as back-end services continuously processing user requests.

   

2. CachedThreadPool
   A cacheable thread pool, the number of threads is not fixed, and a new thread will be created as needed, but the idle thread will be reused. Suitable for scenarios where many short-term asynchronous tasks are performed.

3. SingleThreadExecutor
   A single-threaded thread pool ensures that all tasks are executed in order. Suitable for scenarios where tasks need to be executed in serial, such as log writing or queue consumption.

4. ScheduledThreadPool
   Supports timing and periodic task execution. It is often used for regular refresh of caches, regular check of status and other requirements.

You can choose the appropriate thread pool type according to the actual business situation, or you can customize the thread pool to control core parameters more flexibly.

How to use thread pool correctly?

There are several key points to pay attention to when using thread pools:

• Set the number of threads reasonably : Too many threads will cause context switching overhead, and too few may cause tasks to wait in line. Generally, it can be set according to the number of CPU cores and task type (CPU-intensive or IO-intensive).

• Pay attention to the selection of task queues : the default is to use an unbounded queue (such as LinkedBlockingQueue ), which may cause memory overflow. In high concurrency scenarios, it is recommended to use bounded queues and use them in conjunction with rejection policies.

• Remember to close the thread pool : When the thread pool is no longer needed, shutdown() method should be called to close gracefully to ensure that the submitted tasks can be executed. If you want to stop immediately, you can call shutdownNow() .

• Monitor thread pool status : You can monitor the number of active threads, task queue length and other indicators through subclasses of ThreadPoolExecutor or third-party tools (such as Prometheus Micrometer), so as to facilitate timely discover performance bottlenecks.

What are the advantages of custom thread pools?

Although Executors provides a convenient factory approach, it is generally recommended to manually create thread pools with new ThreadPoolExecutor(...) in production environments. This allows for more precise control of the following parameters:

• Number of core threads (corePoolSize)
• Maximum number of threads (maximumPoolSize)
• Idle thread timeout time (keepAliveTime)
• Task Queue Capacity (workQueue)
• RejectedExecutionHandler

For example, if you want to log and notify the monitoring system when there are too many tasks, you can customize the rejection policy instead of throwing exceptions directly by default.

Basically that's it. Thread pools are not a black technology, but they can significantly improve concurrency performance when used properly, and may also cause serious problems if configured improperly, such as OOM or task accumulation. Therefore, understanding its principles and behavior is very important in actual development.

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