Java多线程之利用线程池并行计算例子

import java.util.ArrayList;
import java.util.List;
import java.util.concurrent.*;
import java.util.concurrent.Callable;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.Future;

/**
 * 并行计算例子
 * nasa
 */
public class ParallelComputing {

    public static void main(String[] args) {

        long startTime = System.currentTimeMillis();
        List<Integer> results = getPrime(1, 200000);
        long endTime = System.currentTimeMillis();
        System.out.println("耗时：" + (endTime -startTime));

        final int cpuCoreNum = 4;

        ExecutorService service = Executors.newFixedThreadPool(cpuCoreNum);

        MyTask t1 = new MyTask(1, 80000);
        MyTask t2 = new MyTask(80001, 130000);
        MyTask t3 = new MyTask(130001, 170000);
        MyTask t4 = new MyTask(170001, 200000); //为什么不平均分 和质数计算速度有关

        Future<List<Integer>> s1 = service.submit(t1);
        Future<List<Integer>> s2 = service.submit(t2);
        Future<List<Integer>> s3 = service.submit(t3);
        Future<List<Integer>> s4 = service.submit(t4);

        startTime = System.currentTimeMillis();
        try {
            System.out.println(s1.get());
            System.out.println(s2.get());
            System.out.println(s3.get());
            System.out.println(s4.get());
        } catch (InterruptedException e) {
            e.printStackTrace();
        } catch (ExecutionException e) {
            e.printStackTrace();
        }
        endTime = System.currentTimeMillis();
        System.out.println("耗时：" + (endTime - startTime));
        service.shutdown();
    }

    static class MyTask implements Callable<List<Integer>> {

        int startPos, endPos;

        MyTask(int start, int end) {
            this.startPos = start;
            this.endPos = end;
        }

        @Override
        public List<Integer> call() throws Exception {
            List<Integer> result = getPrime(startPos, endPos);
            return result;
        }
    }

    /**
     * 判断是不是素数或质数
     * @param num
     * @return
     */
    static boolean isPrime(int num) {
        for (int i=2; i<=num/2; i++) {
            if(num % i == 0) return false;
        }
        return true;
    }

    /**
     * 给一个范围判断这个范围内有多少个质数
     * @param start
     * @param end
     * @return
     */
    static List<Integer> getPrime(int start, int end) {
        List<Integer> results = new ArrayList<>();
        for (int i=start; i<=end; i++) {
            if(isPrime(i)) results.add(i);
        }
        return results;
    }

}