Java8采用stream、parallelStream迭代的区别

我们都知道在Java 8 API添加了一个新的抽象称为流Stream，可以让你以一种声明的方式处理数据。Stream 使用一种类似用 SQL 语句从数据库查询数据的直观方式来提供一种对 Java 集合运算和表达的高阶抽象。Stream API可以极大提高Java程序员的生产力，让程序员写出高效率、干净、简洁的代码。这种风格将要处理的元素集合看作一种流，流在管道中传输，并且可以在管道的节点上进行处理，比如筛选，排序，聚合等。元素流在管道中经过中间操作（intermediate operation）的处理，最后由最终操作(terminal operation)得到前面处理的结果。
通过查看API能够看到Java8 API为我们提供了Stream和parallelStream两个不同的方法，那么同样是流处理，这两个方法又有什么区别呢？首先我们来看看以下的代码：

public static void main(String[] args) {
        List<Integer> numberList = Arrays.asList(1,2,3,4,5,6,7,8,9);
        System.out.println("运行结果：");
        // stream method
        numberList.stream().forEach(number -> {
            System.out.print(String.format("%d ",number));
        });
        System.out.println("\r");
        // parallelStream method
        numberList.parallelStream().forEach(number -> {
            System.out.print(String.format("%d ",number));
        });
        System.out.println("\r");
        // parallelStream method
        numberList.parallelStream().forEachOrdered(number -> {
            System.out.print(String.format("%d ",number));
        });
        System.out.println("\r");
}

通过多次运行上述代码，我们可以发现，通过parallelStream方法迭代集合，每次输出的结果都不一样，而通过steam方法或parallelStream方法并以forEachOrdered方式，每次执行输出的结果都是一样的，并且顺序符合集合元素的存放顺序。
那么，为什么会造成这样的结果差异呢，难道parallelStram是采用多线程并行的方式运行？于是，我们进一步修改下我们的代码来验证一下猜测。

public static void main(String[] args) {
        System.out.println("运行结果：");
        List<Integer> numberList = Arrays.asList(1,2,3,4,5,6,7,8,9);
        // stream method
        numberList.stream().forEach(number -> {
            System.out.println(String.format("Stream The Current Thread's ID is %d and output number %d ",Thread.currentThread().getId(),number));
        });
        System.out.println("\r");
        // parallelStream method
        numberList.parallelStream().forEach(number -> {
            System.out.println(String.format("ParallelStream The Current Thread's ID is %d and output number %d ",Thread.currentThread().getId(),number));
        });
        System.out.println("\r");
        // parallelStream method
        numberList.parallelStream().forEachOrdered(number -> {
            System.out.println(String.format("ParallelStream forEach Ordered The Current Thread's ID is %d and output number %d ",Thread.currentThread().getId(),number));
        });
        System.out.println("\r");
}

修改后代码运行结果如下：

运行结果：
Stream The Current Thread's ID is 1 and output number 1 
Stream The Current Thread's ID is 1 and output number 2 
Stream The Current Thread's ID is 1 and output number 3 
Stream The Current Thread's ID is 1 and output number 4 
Stream The Current Thread's ID is 1 and output number 5 
Stream The Current Thread's ID is 1 and output number 6 
Stream The Current Thread's ID is 1 and output number 7 
Stream The Current Thread's ID is 1 and output number 8 
Stream The Current Thread's ID is 1 and output number 9 

ParallelStream The Current Thread's ID is 1 and output number 6 
ParallelStream The Current Thread's ID is 19 and output number 9 
ParallelStream The Current Thread's ID is 18 and output number 1 
ParallelStream The Current Thread's ID is 15 and output number 2 
ParallelStream The Current Thread's ID is 17 and output number 4 
ParallelStream The Current Thread's ID is 14 and output number 8 
ParallelStream The Current Thread's ID is 13 and output number 3 
ParallelStream The Current Thread's ID is 16 and output number 7 
ParallelStream The Current Thread's ID is 1 and output number 5 

ParallelStream forEach Ordered The Current Thread's ID is 15 and output number 1 
ParallelStream forEach Ordered The Current Thread's ID is 14 and output number 2 
ParallelStream forEach Ordered The Current Thread's ID is 14 and output number 3 
ParallelStream forEach Ordered The Current Thread's ID is 14 and output number 4 
ParallelStream forEach Ordered The Current Thread's ID is 14 and output number 5 
ParallelStream forEach Ordered The Current Thread's ID is 14 and output number 6 
ParallelStream forEach Ordered The Current Thread's ID is 14 and output number 7 
ParallelStream forEach Ordered The Current Thread's ID is 14 and output number 8 
ParallelStream forEach Ordered The Current Thread's ID is 14 and output number 9 

Disconnected from the target VM, address: '127.0.0.1:52976', transport: 'socket'

Process finished with exit code 0

通过上面的运行结果，我们可以看到通过ParallelStream方法迭代的方法，是采用多线程的，可以看过每次输出都是不同的线程ID，而ParallelStream(). forEach Ordered是在多线程的基础上，保证了数据的顺序输出。到此，我们验证了我们的猜测ParallelStream方法是多线程的，而关于线程是否并行的验证，我们需进一步修改下我们的代码，于是有了下面的代码：

 public static void main(String[] args) throws InterruptedException {
        System.out.println("运行结果：");
        List<Integer> numberList = Arrays.asList(1,2,3,4,5,6,7,8,9);
        //for
        Long forBegin = System.currentTimeMillis();
        for(Integer number : numberList){
            //System.out.println(String.format("For The Current Thread's ID is %d and output number %d ",Thread.currentThread().getId(),number));
            Thread.sleep(1000);
        }
        System.out.println(String.format("For execute time cost %d ms",System.currentTimeMillis()-forBegin));
        System.out.println("\r");
        // stream method
        Long streamBegin = System.currentTimeMillis();
        numberList.stream().forEach(number -> {
            //System.out.println(String.format("Stream The Current Thread's ID is %d and output number %d ",Thread.currentThread().getId(),number));
            try {
                Thread.sleep(1000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        });
        System.out.println(String.format("Stream execute time cost %d ms",System.currentTimeMillis()-streamBegin));
        System.out.println("\r");
        // parallelStream method
        Long parallelStreamBegin = System.currentTimeMillis();
        numberList.parallelStream().forEach(number -> {
            //System.out.println(String.format("ParallelStream The Current Thread's ID is %d and output number %d ",Thread.currentThread().getId(),number));
            try {
                Thread.sleep(1000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        });
        System.out.println(String.format("ParallelStream execute time cost %d ms",System.currentTimeMillis()-parallelStreamBegin));
        System.out.println("\r");
        // parallelStream method
        Long parallelStreamForEachOrderBegin = System.currentTimeMillis();
        numberList.parallelStream().forEachOrdered(number -> {
            //System.out.println(String.format("ParallelStream forEachOrdered The Current Thread's ID is %d and output number %d ",Thread.currentThread().getId(),number));
            try {
                Thread.sleep(1000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        });
        System.out.println(String.format("ParallelStream forEachOrdered execute time cost %d ms",System.currentTimeMillis()-parallelStreamForEachOrderBegin));
        System.out.println("\r");
}

这里我们加入了传统的for循环迭代方式，加入一起比较，由于要体现多线程并行的优势，这里我们将每次循环里加入线程休眠1秒钟，运行后的结果如下：

运行结果：
For execute time cost 9032 ms

Stream execute time cost 9079 ms

ParallelStream execute time cost 2011 ms

ParallelStream forEachOrdered execute time cost 9037 ms

通过运行结果，我们可以看到parallelStream().forEach方式耗时最短，而另外其他3种方式运行的耗时都几乎接近。因此，我们可以断定我们的猜测是正确的，parallelStream().forEach是通过多线程并行的方式来执行我们的代码，而parallelStream(). forEachOrdered也是采用多线程，但由于加入了顺序执行约束，故程序是采用多线程同步的方式运行的，最终耗时与for、stream两种单线程执行的耗时接近，但parallelStream(). forEachOrdered由于是多线程，与for、stream两种单线程的方式相比，优势在于很好的利用了CPU多核的资源。感兴趣的同学可以通过以下代码查看CPU的核数，并通过jstack dump出堆栈来查看线程对CPU使用的情况。

System.out.println("系统一共有"+Runtime.getRuntime().availableProcessors()+"个cpu");