Java8采用stream、parallelStream迭代的區別

我們都知道在Java 8 API添加了一個新的抽象稱為流Stream，可以讓你以一種聲明的方式處理數據。Stream 使用一種類似用 SQL 語句從數據庫查詢數據的直觀方式來提供一種對 Java 集合運算和表達的高階抽象。Stream API可以極大提高Java程序員的生產力，讓程序員寫出高效率、干凈、簡潔的代碼。這種風格將要處理的元素集合看作一種流， 流在管道中傳輸， 并且可以在管道的節點上進行處理， 比如篩選， 排序，聚合等。元素流在管道中經過中間操作（intermediate operation）的處理，最后由最終操作(terminal operation)得到前面處理的結果。

通過查看API能夠看到Java8 API為我們提供了Stream和parallelStream兩個不同的方法，那么同樣是流處理，這兩個方法又有什么區別呢？首先我們來看看以下的代碼：

public static void main(String[] args) {

??????? List<Integer> numberList = Arrays.asList(1,2,3,4,5,6,7,8,9);

??????? System.out.println("運行結果：");

??????? // 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");

}

通過多次運行上述代碼，我們可以發現，通過parallelStream方法迭代集合，每次輸出的結果都不一樣，而通過steam方法或parallelStream方法并以forEachOrdered方式，每次執行輸出的結果都是一樣的，并且順序符合集合元素的存放順序。
那么，為什么會造成這樣的結果差異呢，難道parallelStram是采用多線程并行的方式運行？于是，我們進一步修改下我們的代碼來驗證一下猜測。

public static void main(String[] args) {

??????? System.out.println("運行結果：");

??????? 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");

}

修改后代碼運行結果如下：

運行結果：

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

通過上面的運行結果，我們可以看到通過ParallelStream方法迭代的方法，是采用多線程的，可以看過每次輸出都是不同的線程ID，而ParallelStream(). forEach Ordered是在多線程的基礎上，保證了數據的順序輸出。到此，我們驗證了我們的猜測ParallelStream方法是多線程的，而關于線程是否并行的驗證，我們需進一步修改下我們的代碼，于是有了下面的代碼：

?public static void main(String[] args) throws InterruptedException {

??????? System.out.println("運行結果：");

??????? 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");

}

這里我們加入了傳統的for循環迭代方式，加入一起比較，由于要體現多線程并行的優勢，這里我們將每次循環里加入線程休眠1秒鐘，運行后的結果如下：

運行結果：

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

通過運行結果，我們可以看到parallelStream().forEach方式耗時最短，而另外其他3種方式運行的耗時都幾乎接近。因此，我們可以斷定我們的猜測是正確的，parallelStream().forEach是通過多線程并行的方式來執行我們的代碼，而parallelStream(). forEachOrdered也是采用多線程，但由于加入了順序執行約束，故程序是采用多線程同步的方式運行的，最終耗時與for、stream兩種單線程執行的耗時接近，但parallelStream(). forEachOrdered由于是多線程，與for、stream兩種單線程的方式相比，優勢在于很好的利用了CPU多核的資源。感興趣的同學可以通過以下代碼查看CPU的核數，并通過jstack dump出堆棧來查看線程對CPU使用的情況。

System.out.println("系統一共有"+Runtime.getRuntime().availableProcessors()+"個cpu");

來源：Java8采用stream、parallelStream迭代的區別 - 你我一路隨行 - 博客園

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