Thread pool in Java

In Java, the thread pool is used to manage tasks such as the creation, maintenance, and destruction of threads. The thread pool contains a group of threads and a task queue. When a task needs to be executed, the threads in the thread pool will automatically obtain the task and execute it. After the task is executed, the thread will also be recycled to the thread pool for reuse.

The thread pool API in Java provides an Executors class to help us create a thread pool, and provides four thread pool implementation methods: FixedThreadPool, CachedThreadPool, SingleThreadExecutor and ScheduledThreadPool.

FixedThreadPool

A fixed-size thread pool will only create new threads to perform tasks when the number of working threads does not reach the thread pool size. The thread pool can specify the maximum number of threads through the constructor. If not specified, it defaults to Integer.MAX_VALUE.

Sample code:

ExecutorService executorService = Executors.newFixedThreadPool(5);
for (int i = 0; i < 10; i++) {
    executorService.execute(()->{
        System.out.println(Thread.currentThread().getName()+" is executing task ");
        try {
            Thread.sleep(1000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    });
}

Running result:

pool-1-thread-1 is executing task 
pool-1-thread-3 is executing task 
pool-1-thread-5 is executing task 
pool-1-thread-2 is executing task 
pool-1-thread-4 is executing task 
pool-1-thread-5 is executing task 
pool-1-thread-3 is executing task 
pool-1-thread-1 is executing task 
pool-1-thread-2 is executing task 
pool-1-thread-4 is executing task 

CachedThreadPool

Cacheable thread pool, when the number of threads exceeds the currently required number, Excess threads will be recycled into the thread pool and automatically destroyed when no longer needed. If the thread pool has no available threads and a new task arrives, the thread pool will create a new thread to execute the task until the thread pool size reaches the limit of Integer.MAX_VALUE.

Sample code:

ExecutorService executorService = Executors.newCachedThreadPool();
for (int i = 0; i < 10; i++) {
    executorService.execute(()->{
        System.out.println(Thread.currentThread().getName()+" is executing task ");
        try {
            Thread.sleep(1000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    });
}

Running result:

pool-1-thread-1 is executing task 
pool-1-thread-2 is executing task 
pool-1-thread-3 is executing task 
pool-1-thread-4 is executing task 
pool-1-thread-5 is executing task 
pool-1-thread-6 is executing task 
pool-1-thread-7 is executing task 
pool-1-thread-8 is executing task 
pool-1-thread-9 is executing task 
pool-1-thread-10 is executing task 

SingleThreadExecutor

Single-threaded thread pool, with only one worker thread, can ensure that all tasks are executed as specified Executed in the order (FIFO, LIFO, priority, etc.), equivalent to a special FixedThreadPool.

Sample code:

ExecutorService executorService = Executors.newSingleThreadExecutor();
for (int i = 0; i < 10; i++) {
    executorService.execute(()->{
        System.out.println(Thread.currentThread().getName()+" is executing task ");
        try {
            Thread.sleep(1000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    });
}

Running results:

pool-1-thread-1 is executing task 
pool-1-thread-1 is executing task 
pool-1-thread-1 is executing task
......

ScheduledThreadPool

The regularly scheduled thread pool can be scheduled according to the specified delay time or periodically. To execute tasks, you can implement scheduled tasks or periodic tasks. The size of the thread pool can be specified. If not specified, it defaults to Integer.MAX_VALUE.

Sample code:

ScheduledExecutorService scheduledExecutorService = Executors.newScheduledThreadPool(3);
ScheduledFuture<?> future = scheduledExecutorService.schedule(()->{
    System.out.println(Thread.currentThread().getName()+" is executing delay task ");
}, 5, TimeUnit.SECONDS);
scheduledExecutorService.scheduleAtFixedRate(()->{
    System.out.println(Thread.currentThread().getName()+" is executing periodic task ");
}, 2, 3, TimeUnit.SECONDS);

Running results:

pool-1-thread-1 is executing periodic task 
pool-1-thread-2 is executing periodic task 
pool-1-thread-3 is executing periodic task 
pool-1-thread-1 is executing periodic task 
pool-1-thread-3 is executing periodic task 
pool-1-thread-2 is executing periodic task 
pool-1-thread-3 is executing periodic task 
pool-1-thread-2 is executing periodic task 
......
pool-1-thread-1 is executing delay task 

Summary

Thread pool is an extremely important concept in multi-threaded development and can effectively Reduce thread creation, destruction, context switching and other overheads, and improve system performance and maintainability. Java provides the Executors class to easily create thread pools, and provides different implementation methods to deal with different application scenarios. When developers use thread pools, they need to choose an appropriate implementation method based on specific needs and load conditions to achieve the best performance and effects.

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