Java并行处理的实现

final Collection< Task > tasks = Arrays.asList(
    new Task( Status.OPEN, 5 ),
    new Task( Status.OPEN, 13 ),
    new Task( Status.CLOSED, 8 ) 
);

// 并行执行多个任务，并 求和
final double totalPoints = tasks
   .stream()
   .parallel()
   .map( task -> task.getPoints() ) // or map( Task::getPoints ) 
   .reduce( 0, Integer::sum );
 
System.out.println( "Total points (all tasks): " + totalPoints );

public class ExecutorServiceTest {
 
    public static final int THRESHOLD = 10_000;
    public static long[] numbers;
 
    public static void main(String[] args) throws Exception {
        numbers = LongStream.rangeClosed(1, 10_000_000).toArray();
        ExecutorService executor = Executors.newFixedThreadPool(Runtime.getRuntime().availableProcessors() + 1);
        CompletionService<Long> completionService = new ExecutorCompletionService<Long>(executor);
        int taskSize = (int) (numbers.length / THRESHOLD);
        for (int i = 1; i <= taskSize; i++) {
            final int key = i;
            completionService.submit(new Callable<Long>() {
 
                @Override
                public Long call() throws Exception {
                    return sum((key - 1) * THRESHOLD, key * THRESHOLD);
                }
            });
        }
        long sumValue = 0;
        for (int i = 0; i < taskSize; i++) {
            sumValue += completionService.take().get();
        }
        // 所有任务已经完成,关闭线程池
        System.out.println("sumValue = " + sumValue);
        executor.shutdown();
    }
 
    private static long sum(int start, int end) {
        long sum = 0;
        for (int i = start; i < end; i++) {
            sum += numbers[i];
        }
        return sum;
    }
}

public class ForkJoinTest extends java.util.concurrent.RecursiveTask<Long> {
    
    private static final long serialVersionUID = 1L;
    private final long[] numbers;
    private final int start;
    private final int end;
    public static final long THRESHOLD = 10_000;
 
    public ForkJoinTest(long[] numbers) {
        this(numbers, 0, numbers.length);
    }
 
    private ForkJoinTest(long[] numbers, int start, int end) {
        this.numbers = numbers;
        this.start = start;
        this.end = end;
    }
 
    @Override
    protected Long compute() {
        int length = end - start;
        if (length <= THRESHOLD) {
            return computeSequentially();
        }
        ForkJoinTest leftTask = new ForkJoinTest(numbers, start, start + length / 2);
        leftTask.fork();
        ForkJoinTest rightTask = new ForkJoinTest(numbers, start + length / 2, end);
        Long rightResult = rightTask.compute();
        // 注：join方法会阻塞，因此有必要在两个子任务的计算都开始之后才执行join方法
        Long leftResult = leftTask.join();
        return leftResult + rightResult;
    }
 
    private long computeSequentially() {
        long sum = 0;
        for (int i = start; i < end; i++) {
            sum += numbers[i];
        }
        return sum;
    }
 
    public static void main(String[] args) {
        System.out.println(forkJoinSum(10_000_000));
    }
 
    public static long forkJoinSum(long n) {
        long[] numbers = LongStream.rangeClosed(1, n).toArray();
        ForkJoinTask<Long> task = new ForkJoinTest(numbers);
        return new ForkJoinPool().invoke(task);
    }
}