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[Java] Executor

23 Jun 2020

Reading time ~4 minutes

Java에서는 Executor을 통해서 Thread Pool을 직접 구현하지 않고 사용할 수 있다.

+) 참조 : Thread Pool 설명이 잘 되어 있는 블로그 글 : https://hamait.tistory.com/612

Executor Service

ExecutorService_Structure

  • 작업 Queue + 재사용 위해 준비된 Thread들 (Thread Pool)

Executor 사용법

1. Executors 클래스를 통해 ExecutorService를 생성

// 2개의 Thread를 가지는 Thread Pool을 가지고 있는 ExecutorService 생성
ExecutorService service = Executors.newFixedThreadPool(2);
  • CachedThreadPool, FixedThreadPool, ScheduledThreadPool, SingleThreadExecutor … 등과 같은 다양한 ExecutorService가 있다.
    +) 참조 : https://docs.oracle.com/javase/8/docs/api/java/util/concurrent/Executors.html

2. Thread를 작업 Queue에 제출

  • execute 메소드

    service.execute(new Runnable() {
	@Override
	public void run() {
		System.out.println(Thread.currentThread().getName());
	}
});

      // 람다식
  service.execute(() -> {
  	System.out.println(Thread.currentThread().getName());
  });

  • submit 메소드

    service.submit(new Runnable() {
	@Override
	public void run() {
		System.out.println(Thread.currentThread().getName());
	}
});

    // 람다식
service.submit(() -> {
	System.out.println(Thread.currentThread().getName());
});


💡 execute, submit 어떤 차이일까?

  execute submit
메소드 Executor.execute(Runnable command) ExecutorService.submit(Callable Task)
    ExecutorService.submit(Runnable task, T result)
    ExecutorService.submit(Runnable task)
리턴 값 O O / X
Exception 발생시 Thread 종료 후 재생성 Thread 재사용
  1. 인자
    • execute : Runnable
    • submit : Callable, Runnable
  2. 리턴값 유무
    Runnable 실행시 리턴 값이 없고, Callable 실행시 Future 형태의 리턴 값을 받을 수 있다. (실행 Thread의 결과 값을 받을 수 있다.)
    • execute : 리턴 값 X
    • submit : 리턴 값 O / X
  3. Exception
    • execute 실행 중 Exception이 발생하는 경우
      • 해당 Thread는 stacktrace 출력 후 종료
      • 다음 Task 작업시 Thread Pool에는 새로운 Thread 생성
    • submit 실행 중 Exception이 발생하는 경우
      • 해당 Thread는 종료되지 않고 재사용
      • submit 메소드로 리턴되는 Future의 get메소드를 호출시 Exception이 리턴됨
      • 즉, submit 메소드를 통해서 호출된 Thread는 Exception이 발생하더라도 Future.get()을 호출하지 않는다면 Exception의 stacktrace는 출력되지 않음
    • 예제코드: Code Example 3


Executor 구조 (feat. class diagram)

  • newFixedThreadPool 메소드를 호출하면 ThreadPoolExecutor 인스턴스를 리턴 
    public class Executors {
  public static ExecutorService newFixedThreadPool(int nThreads) {
      return new ThreadPoolExecutor(nThreads, nThreads,
                                    0L, TimeUnit.MILLISECONDS,
                                    new LinkedBlockingQueue<Runnable>());
  }
  ...
}
  • ThreadPoolExecutor 클래스는 AbstractExecutorService를 상속 
    public class ThreadPoolExecutor extends AbstractExecutorService { ... }
  • AbstractExecutorService 클래스는 ExecutorService를 구현 
    public abstract class AbstractExecutorService implements ExecutorService { ... }
  • ExecutorService 인터페이스는 Executor를 상속 
    public interface ExecutorService extends Executor { ... }
  • Executor 인터페이스 
    public interface Executor {
  void execute(Runnable command);
}

Executor Class diagram

Executor_Class_Diagram

  • Executors 클래스를 통해 생성된 ExecutorServie들은 Executor의 execute 메소드, ExecutorService의 submit 메소드를 구현하여 사용함


Code Example

Code Example 1 - (return 없음)

  • FixedThreadPool 을 사용하여 3개의 ThreadPool 생성
  • 10개의 Task를 execute
  • 각 Task는 3초의 시간 소요
public class ExecutorTest {
	public static void testFixedThreadPool() {
		ExecutorService service = Executors.newFixedThreadPool(3);
		for(int i=0; i<10; i++) {
			service.execute(() -> {
				System.out.println(Thread.currentThread().getName());
				try {
					Thread.sleep(3000);
				} catch (InterruptedException e) {
					e.printStackTrace();
				}
			});
		}
		System.out.println("----------------------------------");
	}

	public static void main(String[] args) {
		testFixedThreadPool();
	}
}

결과

pool-1-thread-1
pool-1-thread-2
pool-1-thread-3
----------------------------------
pool-1-thread-2
pool-1-thread-3
pool-1-thread-1
pool-1-thread-1
pool-1-thread-2
pool-1-thread-3
pool-1-thread-1
  • thread1, thread2, thread3이 3초마다 출력되는 것을 확인

Code Example 2 - (return 있음)

  • SingleThreadExecutor 을 사용하여 1개의 ThreadPool 생성
  • 작업의 결과를 리턴받음 (true)
public class ExecutorTest {
	public static Future<Boolean> testSingleThreadExecutor() throws InterruptedException, ExecutionException {
		ExecutorService service = Executors.newSingleThreadExecutor();
		return service.submit(() -> {
			System.out.println(Thread.currentThread().getName());
			return true;
		});
	}

	public static void main(String[] args) {
		try {
			Future<Boolean> result = testSingleThreadExecutor();
			System.out.println("RESULT : " + result.get());
		} catch (InterruptedException | ExecutionException e) {
			e.printStackTrace();
		}
	}
}

결과

pool-1-thread-1
RESULT : true
  • 결과 값을 리턴받음을 확인

Code Example 3 - (execute, submit Exception 비교)

(1) execute

  • Code Example 1 코드에서 RuntimeException 발생만 추가
public class ExecutorTest {
	public static void testFixedThreadPool() {
		ExecutorService service = Executors.newFixedThreadPool(3);
		for(int i=0; i<10; i++) {
			service.execute(() -> {
				System.out.println(Thread.currentThread().getName());
				try {
					Thread.sleep(3000);
				} catch (InterruptedException e) {
					e.printStackTrace();
				}
				throw new RuntimeException(); // 추가
			});
		}
		System.out.println("----------------------------------");
	}

	public static void main(String[] args) {
		testFixedThreadPool();
	}
}

결과

pool-1-thread-1
----------------------------------
pool-1-thread-3
pool-1-thread-2
Exception in thread "pool-1-thread-3"
pool-1-thread-4
Exception in thread "pool-1-thread-2" java.lang.RuntimeException
	at ...
Exception in thread "pool-1-thread-1" java.lang.RuntimeException
	at ...
java.lang.RuntimeException
	at ...
pool-1-thread-6
pool-1-thread-5
Exception in thread "pool-1-thread-5"
pool-1-thread-7
java.lang.RuntimeException
	at ...
Exception in thread "pool-1-thread-6"
Exception in thread "pool-1-thread-4" java.lang.RuntimeException
	at ...
java.lang.RuntimeException
	at ...
pool-1-thread-8
pool-1-thread-9
Exception in thread "pool-1-thread-7" java.lang.RuntimeException
pool-1-thread-10
	at ...
Exception in thread "pool-1-thread-8" java.lang.RuntimeException
	at ...
Exception in thread "pool-1-thread-9" java.lang.RuntimeException
	at ...
Exception in thread "pool-1-thread-10" java.lang.RuntimeException
	at ...
  • 순서가 좀 섞여서 나오지만 thread들이 재사용 되지 않고 재생성됨을 확인

(2) submit

  • Code Example 1 코드에서 execute -> sumbit 수정, RuntimeException 발생 추가
public class ExecutorTest {
	public static void testFixedThreadPool() {
		ExecutorService service = Executors.newFixedThreadPool(3);
		for(int i=0; i<10; i++) {
			service.submit(() -> { // 수정
				System.out.println(Thread.currentThread().getName());
				try {
					Thread.sleep(3000);
				} catch (InterruptedException e) {
					e.printStackTrace();
				}
				throw new RuntimeException(); // 추가
			});
		}
		System.out.println("----------------------------------");
	}

	public static void main(String[] args) {
		testFixedThreadPool();
	}
}

결과

pool-1-thread-1
pool-1-thread-3
pool-1-thread-2
----------------------------------
pool-1-thread-2
pool-1-thread-1
pool-1-thread-3
pool-1-thread-3
pool-1-thread-1
pool-1-thread-2
pool-1-thread-3
  • thread 재사용하는 것을 확인
  • Future.get() 호출하지 않음으로 Exception의 stacktrace가 출력되지 않음을 확인


+) 참조 :

https://hamait.tistory.com/612

https://codechacha.com/ko/java-executors/

https://www.callicoder.com/java-executor-service-and-thread-pool-tutorial/

https://javarevisited.blogspot.com/2016/04/difference-between-ExecutorServie-submit-vs-Executor-execute-method-in-Java.html


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