如何使用多线程更快地使Java for循环更快？

Question

假设我有一个非常缓慢且大的for循环.

如何在多个线程中拆分它以便它运行得更快？

for (int a = 0; a < 30000000; a++) {
    for (int b = 0; b < 30000000; b++) {
        for (int c = 0; c < 30000000; c++) {
           slowMethod();
        }
    }
}

Answer 1

这有点宽泛,但是ExecutorService当处理器数量大于1并且它slowMethod是独立的时,使用具有固定线程数的将使其更快.如果slowMethod是I/O密集型,您可以增加线程数以获得更高的性能.

import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;

public class Main {

    public static void main(String[] args) {
        ExecutorService service = Executors.newFixedThreadPool(
                   Runtime.getRuntime().availableProcessors() + 1);

        for (int a = 0; a < 30000000; a++) {
            for (int b = 0; b < 30000000; b++) {
                for (int c = 0; c < 30000000; c++) {
                    final int a_copy = a;
                    final int b_copy = b;
                    final int c_copy = c;
                    service.execute(() -> {
                            slowMethod(a_copy, b_copy, c_copy);
                    });
                }
            }
        }
    }

    public static void slowMethod(int a, int b, int c) {

    }
}

更新

正如评论中所说,它可能导致任务中止,因为队列的容量是Integer.MAX_VALUE.您可以slowMethod在队列已满时让主线程执行.要实现此目的,您需要手动创建池:

BlockingQueue<Runnable> queue = new LinkedBlockingQueue<>();
int threads = Runtime.getRuntime().availableProcessors();
ThreadPoolExecutor executor = new ThreadPoolExecutor(threads, threads,
        0, TimeUnit.MILLISECONDS,
        queue, Executors.defaultThreadFactory(),
        new ThreadPoolExecutor.CallerRunsPolicy());