Stream#parallel() によるスレッド処理がどのようなものかを眺めてみる。

ParallelDemo.java

import java.util.stream.IntStream;
 
/**
 * parallel() によるスレッド処理がどのようなものかを眺めてみる。
 */
public class ParallelDemo {
    public static void main(String[] args) {
        IntStream.range(0, 10)
                .parallel()
                .map(v -> {
                    showThreadNameAndSleep("add +1");
                    return v + 1;
                })
                .mapToObj(v -> {
                    showThreadNameAndSleep("int to String");
                    return String.valueOf(v);
                })
                .map(s -> {
                    showThreadNameAndSleep("String to Double");
                    return Double.valueOf(s);
                })
                .forEach(d -> {
                    showThreadNameAndSleep("print Double value");
                    System.out.println(d);
                });
    }
 
    static void showThreadNameAndSleep(String jobName) {
        try {
            String threadName = Thread.currentThread().getName();
            String message = String.format("threadName: '%s', jobName: '%s'", threadName, jobName);
            System.out.println(message);
            Thread.sleep(100);
 
        } catch (InterruptedException e) {
            throw new RuntimeException(e);
        }
 
    }
}

result.txt

threadName: 'main', jobName: 'add +1'
threadName: 'ForkJoinPool.commonPool-worker-2', jobName: 'add +1'
threadName: 'ForkJoinPool.commonPool-worker-3', jobName: 'add +1'
threadName: 'ForkJoinPool.commonPool-worker-1', jobName: 'add +1'
threadName: 'main', jobName: 'int to String'
threadName: 'ForkJoinPool.commonPool-worker-3', jobName: 'int to String'
threadName: 'ForkJoinPool.commonPool-worker-2', jobName: 'int to String'
threadName: 'ForkJoinPool.commonPool-worker-1', jobName: 'int to String'
threadName: 'ForkJoinPool.commonPool-worker-2', jobName: 'String to Double'
threadName: 'ForkJoinPool.commonPool-worker-3', jobName: 'String to Double'
threadName: 'ForkJoinPool.commonPool-worker-1', jobName: 'String to Double'
threadName: 'main', jobName: 'String to Double'
threadName: 'ForkJoinPool.commonPool-worker-1', jobName: 'print Double value'
threadName: 'ForkJoinPool.commonPool-worker-3', jobName: 'print Double value'
threadName: 'ForkJoinPool.commonPool-worker-2', jobName: 'print Double value'
threadName: 'main', jobName: 'print Double value'
7.0
3.0
9.0
threadName: 'main', jobName: 'add +1'
8.0
threadName: 'ForkJoinPool.commonPool-worker-2', jobName: 'add +1'
threadName: 'ForkJoinPool.commonPool-worker-1', jobName: 'add +1'
threadName: 'ForkJoinPool.commonPool-worker-3', jobName: 'add +1'
threadName: 'ForkJoinPool.commonPool-worker-2', jobName: 'int to String'
threadName: 'ForkJoinPool.commonPool-worker-3', jobName: 'int to String'
threadName: 'main', jobName: 'int to String'
threadName: 'ForkJoinPool.commonPool-worker-1', jobName: 'int to String'
threadName: 'ForkJoinPool.commonPool-worker-3', jobName: 'String to Double'
threadName: 'main', jobName: 'String to Double'
threadName: 'ForkJoinPool.commonPool-worker-1', jobName: 'String to Double'
threadName: 'ForkJoinPool.commonPool-worker-2', jobName: 'String to Double'
threadName: 'ForkJoinPool.commonPool-worker-3', jobName: 'print Double value'
threadName: 'main', jobName: 'print Double value'
threadName: 'ForkJoinPool.commonPool-worker-1', jobName: 'print Double value'
threadName: 'ForkJoinPool.commonPool-worker-2', jobName: 'print Double value'
5.0
2.0
10.0
6.0
threadName: 'ForkJoinPool.commonPool-worker-1', jobName: 'add +1'
threadName: 'ForkJoinPool.commonPool-worker-3', jobName: 'add +1'
threadName: 'ForkJoinPool.commonPool-worker-1', jobName: 'int to String'
threadName: 'ForkJoinPool.commonPool-worker-3', jobName: 'int to String'
threadName: 'ForkJoinPool.commonPool-worker-1', jobName: 'String to Double'
threadName: 'ForkJoinPool.commonPool-worker-3', jobName: 'String to Double'
threadName: 'ForkJoinPool.commonPool-worker-3', jobName: 'print Double value'
threadName: 'ForkJoinPool.commonPool-worker-1', jobName: 'print Double value'
1.0
4.0