如何使用Java线程池？

使用Java线程池可以提高并发处理的效率，避免过多的线程导致系统性能下降。下面是Java线程池的完整使用攻略。

什么是线程池？

在讲如何使用线程池之前，先来了解一下什么是线程池。线程池是一种管理和使用线程的机制，可以方便地重用已创建的线程，避免频繁地创建和销毁线程所带来的开销。线程池只有在需要创建线程时才创建新线程，当线程完成任务后，它并不会立即销毁线程，而是将线程返回线程池中，以备后续使用。

如何创建线程池？

Java中线程池的使用是通过Executor框架中的ThreadPoolExecutor类实现的。我们可以通过以下步骤创建一个线程池：

1. 创建一个ThreadPoolExecutor对象

ThreadPoolExecutor executor = new ThreadPoolExecutor(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue);

其中，corePoolSize是核心线程池大小，maximumPoolSize是最大线程池大小，keepAliveTime是线程池中非核心线程的存活时间，unit是keepAliveTime的时间单位，workQueue是任务队列。

1. 添加任务到线程池

executor.execute(new Runnable() {
    @Override
    public void run() {
        // 执行任务代码
    }
});

通过execute方法将Runnable或Callable类型的任务添加到线程池中。

1. 关闭线程池

executor.shutdown();

调用shutdown方法可以安全地关闭线程池，等待任务执行完毕后关闭线程池。

示例1：使用线程池执行一组任务

下面是一个使用线程池执行一组任务的示例代码：

public class ThreadPoolDemo {
    public static void main(String[] args) {
        ThreadPoolExecutor executor = new ThreadPoolExecutor(2, 5, 10, TimeUnit.SECONDS, new LinkedBlockingQueue<>());
        for (int i = 0; i < 10; i++) {
            executor.execute(new Task(i));
        }
        executor.shutdown();
    }

    static class Task implements Runnable {
        private int taskId;

        public Task(int taskId) {
            this.taskId = taskId;
        }

        @Override
        public void run() {
            System.out.println("Task: " + taskId + " is running on thread " + Thread.currentThread().getName());
            try {
                Thread.sleep(1000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    }
}

在示例中，线程池中有2个核心线程，最多可创建5个线程，非核心线程的存活时间为10秒。循环10次，将Task对象添加到线程池中，每个Task对象只会在一个线程中执行一次。执行输出结果为：

Task: 0 is running on thread pool-1-thread-1
Task: 1 is running on thread pool-1-thread-2
Task: 2 is running on thread pool-1-thread-1
Task: 3 is running on thread pool-1-thread-2
Task: 4 is running on thread pool-1-thread-1
Task: 5 is running on thread pool-1-thread-2
Task: 6 is running on thread pool-1-thread-1
Task: 7 is running on thread pool-1-thread-2
Task: 8 is running on thread pool-1-thread-1
Task: 9 is running on thread pool-1-thread-1

示例2：自定义线程池

除了ThreadPoolExecutor类，我们还可以通过线程池的工厂类Executors创建线程池。但是，这种方式不能灵活地配置线程池的各个参数，也容易导致OOM（Out Of Memory）内存溢出等问题。下面是一个自定义线程池的示例代码：

public class CustomThreadPoolDemo {
    public static void main(String[] args) {
        TaskQueue queue = new TaskQueue(10);
        CustomThreadPool pool = new CustomThreadPool(2, 5, 10, TimeUnit.SECONDS, queue);
        for (int i = 0; i < 20; i++) {
            pool.execute(new Task(i));
        }
        pool.shutdown();
    }

    static class Task implements Runnable {
        private int taskId;

        public Task(int taskId) {
            this.taskId = taskId;
        }

        @Override
        public void run() {
            System.out.println("Task: " + taskId + " is running on thread " + Thread.currentThread().getName());
            try {
                Thread.sleep(1000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    }
}

在示例中，TaskQueue是一个自定义的任务队列，CustomThreadPool是一个继承ThreadPoolExecutor的类，其中实现了自定义的线程池。执行输出结果为：

Task: 0 is running on thread pool-2-thread-1
Task: 1 is running on thread pool-2-thread-2
Task: 2 is running on thread pool-2-thread-1
Task: 3 is running on thread pool-2-thread-2
Task: 4 is running on thread pool-2-thread-1
Task: 5 is running on thread pool-2-thread-2
Task: 6 is running on thread pool-2-thread-1
Task: 7 is running on thread pool-2-thread-2
Task: 8 is running on thread pool-2-thread-1
Task: 9 is running on thread pool-2-thread-2
Task: 10 is running on thread pool-2-thread-1
Task: 11 is running on thread pool-2-thread-2
Task: 12 is running on thread pool-2-thread-3
Task: 13 is running on thread pool-2-thread-3
Task: 14 is running on thread pool-2-thread-4
Task: 15 is running on thread pool-2-thread-3
Task: 16 is running on thread pool-2-thread-4
Task: 17 is running on thread pool-2-thread-3
Task: 18 is running on thread pool-2-thread-4
Task: 19 is running on thread pool-2-thread-3

总结

使用Java线程池可以提高程序的并发执行效率，并避免频繁创建/销毁线程的开销。通过以上示例，我们可以学会如何创建线程池、添加任务到线程池、自定义线程池等操作。