多執行緒程式設計ExecutorService用法

以下內容均來自ChatGPT提供的示例，用於自學

ExecutorService 是 Java 中用於管理和控制執行緒池的介面，通常用來簡化多執行緒程式設計。它提供了一組方法，允許我們在非同步任務執行完畢後關閉執行緒池、排程任務等操作。以下是幾個常見的使用場景和示例程式碼：

1. 使用 ExecutorService 執行簡單任務

透過 Executors 建立一個固定大小的執行緒池，然後提交任務執行。

public static void main(String[] args) {
    ExecutorService executorService = Executors.newFixedThreadPool(3);

public static void main(String[] args) {
    ExecutorService executorService = Executors.newFixedThreadPool(3);
    for(int i= 0; i < 5; i ++){
        int taskId = i;
        executorService.submit(()-> {
            System.out.println("start executing task " + taskId + ". Thread:" + Thread.currentThread().getName());
            try {
                Thread.sleep(1000);  //Simulate task execution time
                System.out.println("end executing task " + taskId + ". Thread:" + Thread.currentThread().getName());
            } catch (InterruptedException e) {
                Thread.currentThread().interrupt();
            }
        });
    }
    executorService.shutdown();
}

列印輸出

> Task :ExecutorServiceExample.main()
start executing task 2. Thread:pool-1-thread-3
start executing task 0. Thread:pool-1-thread-1
start executing task 1. Thread:pool-1-thread-2
end executing task 0. Thread:pool-1-thread-1
end executing task 1. Thread:pool-1-thread-2
end executing task 2. Thread:pool-1-thread-3
start executing task 3. Thread:pool-1-thread-2
start executing task 4. Thread:pool-1-thread-1
end executing task 3. Thread:pool-1-thread-2
end executing task 4. Thread:pool-1-thread-1

2. 使用 Callable 和 Future 獲取任務結果

Callable 是一種可以返回結果的任務，而 Future 則用來獲取該任務的執行結果。

public static void main(String[] args) {
    ExecutorService executorService = Executors.newFixedThreadPool(3);
    //Create Callable task
    Callable<String> task = () -> {
        System.out.println("start task : "+ Thread.currentThread().getName());
        Thread.sleep(2000);  //Simulate task execution time
        System.out.println("end task : "+ Thread.currentThread().getName());
        return "Task completed";
    };
    //Submit task and get future
    Future<String> feature = executorService.submit(task);
    System.out.println("start feature : "+ Thread.currentThread().getName());
    try {
        //waiting for task results
        String result = feature.get();
        System.out.println("task result: " + result);
    } catch (Exception e) {
        e.printStackTrace();
    } finally {
        executorService.shutdown();
    }
}

> Task :ExecutorServiceExample.main()
start task : pool-1-thread-1
start feature : main
end task : pool-1-thread-1
task result: Task completed

3. 批次提交任務並等待所有任務完成

invokeAll 方法可以一次性提交多個任務，並等待所有任務完成。

public static void main(String[] args) {
    ExecutorService executorService = Executors.newFixedThreadPool(3);
    List<Callable<String>> taskList = new ArrayList<>();
    //Create multiple tasks
    for(int i= 0; i < 5; i ++){
        int taskId = i;
        taskList.add(() -> {
            System.out.println("start task " + taskId + ". Thread:" + Thread.currentThread().getName());
            Thread.sleep(1000);
            System.out.println("end task " + taskId + ". Thread:" + Thread.currentThread().getName());
            return "Task " + taskId + " completed";
        });
    }
    try {
        //Execute all tasks
        List<Future<String>> results = executorService.invokeAll(taskList);
        //get task results
        for(Future<String> future:results){
            System.out.println(future.get());
        }
    } catch (Exception e) {
        e.printStackTrace();
    }finally {
        executorService.shutdown();
    }
}

> Task :ExecutorServiceExample.main()
start task 1. Thread:pool-1-thread-2
start task 0. Thread:pool-1-thread-1
start task 2. Thread:pool-1-thread-3
end task 0. Thread:pool-1-thread-1
start task 3. Thread:pool-1-thread-1
end task 2. Thread:pool-1-thread-3
end task 1. Thread:pool-1-thread-2
start task 4. Thread:pool-1-thread-3
end task 4. Thread:pool-1-thread-3
end task 3. Thread:pool-1-thread-1
Task 0 completed
Task 1 completed
Task 2 completed
Task 3 completed
Task 4 completed

4. 定時任務

可以使用 ScheduledExecutorService 來排程定時任務，以下示例展示瞭如何每隔一段時間執行一次任務。

public static void main(String[] args) {
    ScheduledExecutorService scheduler = Executors.newScheduledThreadPool(2);
    // Execute tasks every 2 seconds
    scheduler.scheduleAtFixedRate(() -> {
        System.out.println("Execute scheduled tasks, time:" + System.currentTimeMillis());
    }, 0, 2, TimeUnit.SECONDS);

    // Close after running for 10 seconds
    scheduler.schedule(() -> {
        scheduler.shutdown();
    }, 10, TimeUnit.SECONDS);
}

> Task :ExecutorServiceExample.main()
Execute scheduled tasks, time:1730271628977
Execute scheduled tasks, time:1730271630985
Execute scheduled tasks, time:1730271632983
Execute scheduled tasks, time:1730271634984
Execute scheduled tasks, time:1730271636984
Execute scheduled tasks, time:1730271638980