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Java ExecutorService introduction

Java ExecutorService example


public class ExecutorServiceExample {

    public static void main(String[] args) {
        ExecutorService es = Executors.newFixedThreadPool(10);

        es.execute(newRunnable("Task 1"));
        es.execute(newRunnable("Task 2"));
        es.execute(newRunnable("Task 3"));

				es.shutdown();
    }

    private static Runnable newRunnable(String message) {
        return () -> System.out.println(Thread.currentThread().getName() + " : " + message);
    }
}


pool-1-thread-3 : Task 3
pool-1-thread-2 : Task 2
pool-1-thread-1 : Task 1

Process finished with exit code 0

Java ExecutorService implementations

ExecutorService 는 인터페이스임

ThreadPoolExecutor


int  corePoolSize  =    5;
int  maxPoolSize   =   10;
long keepAliveTime = 5000;

ExecutorService threadPoolExecutor =
        new ThreadPoolExecutor(
                corePoolSize,
                maxPoolSize,
                keepAliveTime,
                TimeUnit.MILLISECONDS,
                new LinkedBlockingQueue<Runnable>()
                );


ExecutorService es= Executors.newFiexedThreadPool(3);

ScheduledExecutorService


ScheduledExecutorService scheduledExecutorService =
        Executors.newScheduledThreadPool(5);

ScheduledFuture scheduledFuture =
    scheduledExecutorService.schedule(new Callable() {
        public Object call() throws Exception {
            System.out.println("Executed!");
            return "Called!";
        }
    },
    5,
    TimeUnit.SECONDS);


ScheduledExecutorService scheduledExecutorService =
        Executors.newScheduledThreadPool(5);

Java ThreadPoolExecutor constructor explained

corePoolSize - 시작 쓰레드 사이즈

maxPoolSize - 최대 쓰레드 사이즈

keepAliveTime - 쓰레드가 일을 안하고 있을때 살아있는 시간 (지나면 종료됨)

TimeUnit

작업 큐

Executors 의 팩터리 메서드를 활용하는것이 좋다.

Java ExecutorService methods Java ExecutorService submit(Runnable) method

void execute(Runnable runnable)

Future<?> submit(Runnable runnable)

void shutdown( )


public class ExecutorsServiceExample3 {

    public static void main(String[] args) {
        ExecutorService es = Executors.newSingleThreadExecutor();

        Future<?> future = es.submit(newRunnable("Task 1"));

        System.out.println(future.isDone());

        try {
            future.get();
        } catch (ExecutionException e) {
        } catch (InterruptedException e) {
        }
        System.out.println(future.isDone());

				es.shutdown();
    }
}


false
pool-1-thread-1 : Task 1
true

Process finished with exit code 130

Future future- 작업 상태 정보 저장 객체

future.isDone( ) - 작업 끝낱는지 확인

future.get( ) - 값은 무적권 null 이다. 근데 실행 될 때까지 쓰레드를 block하는 효과로 사용 가능, 동기적으로 작업 실행을 강요할 수 있다.

Java ExecutorService submit(Callable) method


public class ExecutorsServiceExample4 {

    public static void main(String[] args) {
        ExecutorService es = Executors.newSingleThreadExecutor();

        Future<String> future = es.submit(newCallable("Task 1"));

        System.out.println(future.isDone());

        try {
            String message = future.get();
            System.out.println("message = " + message);
        } catch (ExecutionException | InterruptedException e) {
        }
        System.out.println(future.isDone());

        es.shutdown();
    }

    private static Callable<String> newCallable(String message){
        return () -> Thread.currentThread().getName() + " : " + message;
    }
}


false
message = pool-1-thread-1 : Task 1
true

Process finished with exit code 0

How a Future object is connected to a task

Java ExecutorService invokeAny() method


public class ExecutorsServiceExample5 {

    public static void main(String[] args) {
        ExecutorService es = Executors.newSingleThreadExecutor();
        List<Callable<String>> callables = new ArrayList<>();
        callables.add(newCallable("Task 1.1"));
        callables.add(newCallable("Task 1.1"));
        callables.add(newCallable("Task 1.1"));

        try {
            System.out.println(es.invokeAny(callables));
        } catch (ExecutionException | InterruptedException e) {
        }

				es.shutdown();
    }
}

Callable 콜렉션 중 아무거나 호출하고 결과를 바로 반환

Java ExecutorService invokeAll() method



public class ExecutorsServiceExample5 {

    public static void main(String[] args) {
        ExecutorService es = Executors.newSingleThreadExecutor();
        List<Callable<String>> callables = new ArrayList<>();
        callables.add(newCallable("Task 1.1"));
        callables.add(newCallable("Task 1.2"));
        callables.add(newCallable("Task 1.3"));

        try {
            List<Future<String>> futures = es.invokeAll(callables);
            for (Future<String> future : futures) {
                System.out.println(future.get());
            }
        } catch (ExecutionException | InterruptedException e) {
        }

        es.shutdown();
    }

}

Java ExecutorService - Part 1