分叉并加入Java 7 – JSR 166并发实用程序

Java 7最有趣的改进之一是对并发的更好支持。使用JSR 166并发实用程序，我们可以对并发进行一些非常有用的改进。在我看来，fork-join库在软件工程中具有很高的实际应用潜力。 Fork and join为算法提供了非常简单的编程模型，可以将其实现为递归任务。有很多算法可以使用分治法来实现。

在未来几年中，我们将看到标准台式机，笔记本电脑和服务器计算机中内核的数量不断增加。原因很简单：添加额外的内核比构建更快的单个处理器便宜。因此，我们将不得不编写更多支持并发的软件以利用更好的硬件。

老实说，我不喜欢并发。我的个人规则是“您需要一个充分的理由来实现并发，并且如果需要做的话必须非常小心。”在过去的几年中，我看到的错误实现多于工作。这就是为什么我喜欢fork＆join库的原因。清晰的编程模型可实现样板代码，可防止您出错。但是，如果您打算使用fork和join，请花一些时间来了解其行为。

文件＃1和＃2中的示例与Java 7文档中的示例代码非常相似。通常，使用递归算法的斐波那契数不是一个好主意，因为存在更好的线性解决方案（请比较http://nayuki.eigenstate.org/page/fast-fibonacci-algorithms ），但它更易于实现和理解相对于其它的。因此，让我们看一下示例：

// File #1: FibonacciTask.java  [error handling, parameter validation and asserts removed] package com.sprunck.sample;import java.util.concurrent.RecursiveTask;public class FibonacciTask extends RecursiveTask<Long> {private static final long serialVersionUID = 1L;private final long inputValue;public FibonacciTask(long inputValue) {this.inputValue = inputValue;}@Overridepublic Long compute() {if (inputValue == 0L) {return 0L;} else if (inputValue <= 2L) {return 1L;} else {final FibonacciTask firstWorker = new FibonacciTask(inputValue - 1L);firstWorker.fork();final FibonacciTask secondWorker = new FibonacciTask(inputValue - 2L);return secondWorker.compute() + firstWorker.join();}}
}

// File #2: FibonacciTaskTest.java package com.sprunck.sample;import java.util.concurrent.ForkJoinPool;
import junit.framework.Assert;
import org.junit.Test;public class FibonacciTaskTest {// it makes no sense to create more threads than available cores (no speed improvement here)private static final int AVAILABLE_PROCESSORS = Runtime.getRuntime().availableProcessors();// create thread poolprivate final ForkJoinPool pool = new ForkJoinPool(AVAILABLE_PROCESSORS);@Testpublic void testFibonacciArray() {// more test data: http://www.maths.surrey.ac.uk/hosted-sites/R.Knott/Fibonacci/fibtable.htmllong results[] = { 0L, 1L, 1L, 2L, 3L, 5L, 8L, 13L, 21L, 34L, 55L, 89L, 144L, 233L, 377L, 610L, 987L, 1597L,2584L, 4181L, 6765L };for (int inputValue = 0; inputValue < results.length; inputValue++) {final FibonacciTask task = new FibonacciTask(inputValue);System.out.print("Fibonacci(" + inputValue + ") = ");final long result = pool.invoke(task);System.out.println(result);Assert.assertEquals(results[inputValue], result);}}
}

// FibonacciTaskTest.java的输出

Fibonacci(0) = 0
Fibonacci(1) = 1
Fibonacci(2) = 1
Fibonacci(3) = 2
Fibonacci(4) = 3
Fibonacci(5) = 5
Fibonacci(6) = 8
Fibonacci(7) = 13
Fibonacci(8) = 21
Fibonacci(9) = 34
Fibonacci(10) = 55
Fibonacci(11) = 89
Fibonacci(12) = 144
Fibonacci(13) = 233
Fibonacci(14) = 377
Fibonacci(15) = 610
Fibonacci(16) = 987
Fibonacci(17) = 1597
Fibonacci(18) = 2584
Fibonacci(19) = 4181
Fibonacci(20) = 6765

到目前为止，这是一个简单明了的解决方案。没有用于并行的样板代码，例如线程同步。

但我想鼓励您更深入地了解解决方案中发生的情况。在文件＃3和＃4中，您可以找到同一程序的增强版本。第一个版本和第二个版本之间的唯一区别是，一些代码可以跟踪执行期间发生的事情，而较小的slowTask（）可以模拟更实际的行为。

// File #3: FibonacciTaskTraces.java package com.sprunck.sample;import java.util.concurrent.RecursiveTask;public class FibonacciTaskTraces extends RecursiveTask<Long> {private static final long serialVersionUID = 1L;// just needed to format debug outputpublic static final String OUTPUT_PREFIX = " | ";private final String prefix;private final long inputValue;public FibonacciTaskTraces(long inputValue, final String prefix) {this.inputValue = inputValue;this.prefix = prefix;}@Overridepublic Long compute() {if (inputValue == 0L) {slowTask();return 0L;} else if (inputValue <= 2L) {slowTask();return 1L;} else {final long firstValue = inputValue - 1L;System.out.println(prefix + " - Fibonacci(" + firstValue + ") <- " + Thread.currentThread().getName()+ " (fork) ");final FibonacciTaskTraces firstWorker = new FibonacciTaskTraces(firstValue, prefix + OUTPUT_PREFIX);firstWorker.fork();final long secondValue = inputValue - 2L;System.out.println(prefix + " - Fibonacci(" + secondValue + ") <- " + Thread.currentThread().getName());final FibonacciTaskTraces secondWorker = new FibonacciTaskTraces(secondValue, prefix + OUTPUT_PREFIX);long result = secondWorker.compute() + firstWorker.join();System.out.println(prefix + " - Fibonacci(" + inputValue + ") = " + result + " <- "+ Thread.currentThread().getName() + " (join)");slowTask();return result;}}/** just simulate a longer running task (with out disturbing the other threads) */private void slowTask() {for (int k = 0, i = 0; i < 1000 * 1000 * 100; i++) {i = i + k;}}
}

// File #4: FibonacciTaskTracesTask.javapackage com.sprunck.sample;import java.util.concurrent.ForkJoinPool;
import junit.framework.Assert;
import org.junit.Test;public class  FibonacciTaskTracesTest {// it makes no sense to create more threads than available cores (no speed improvement here)private static final int AVAILABLE_PROCESSORS = Runtime.getRuntime().availableProcessors();// create thread poolprivate final ForkJoinPool pool = new ForkJoinPool(AVAILABLE_PROCESSORS);@Testpublic void testFibonacciArrayTraces() {// more test data: http://www.maths.surrey.ac.uk/hosted-sites/R.Knott/Fibonacci/fibtable.htmllong results[] = { 0L, 1L, 1L, 2L, 3L, 5L, 8L, 13L };for (int inputValue = 0; inputValue < results.length; inputValue++) {final FibonacciTaskTraces task = new FibonacciTaskTraces(inputValue, " | ");System.out.println("invoke Fibonacci(" + inputValue + ")  <- " + Thread.currentThread().getName());final long result = pool.invoke(task);System.out.println("result = " + result + "\n");Assert.assertEquals(results[inputValue], result);}}
}

// FibonacciTaskTracesTest.java的输出

invoke Fibonacci(0)  <- mainresult = 0invoke Fibonacci(1)  <- mainresult = 1invoke Fibonacci(2)  <- mainresult = 1invoke Fibonacci(3)  <- main|  - Fibonacci(2) <- ForkJoinPool-1-worker-1 (fork) |  - Fibonacci(1) <- ForkJoinPool-1-worker-1|  - Fibonacci(3) = 2 <- ForkJoinPool-1-worker-1 (join)result = 2invoke Fibonacci(4)  <- main|  - Fibonacci(3) <- ForkJoinPool-1-worker-1 (fork) |  - Fibonacci(2) <- ForkJoinPool-1-worker-1|  |  - Fibonacci(2) <- ForkJoinPool-1-worker-2 (fork) |  |  - Fibonacci(1) <- ForkJoinPool-1-worker-2|  |  - Fibonacci(3) = 2 <- ForkJoinPool-1-worker-2 (join)|  - Fibonacci(4) = 3 <- ForkJoinPool-1-worker-1 (join)result = 3invoke Fibonacci(5)  <- main|  - Fibonacci(4) <- ForkJoinPool-1-worker-1 (fork) |  - Fibonacci(3) <- ForkJoinPool-1-worker-1|  |  - Fibonacci(2) <- ForkJoinPool-1-worker-1 (fork) |  |  - Fibonacci(1) <- ForkJoinPool-1-worker-1|  |  - Fibonacci(3) <- ForkJoinPool-1-worker-2 (fork) |  |  - Fibonacci(2) <- ForkJoinPool-1-worker-2|  |  |  - Fibonacci(2) <- ForkJoinPool-1-worker-2 (fork) |  |  |  - Fibonacci(1) <- ForkJoinPool-1-worker-2|  |  - Fibonacci(3) = 2 <- ForkJoinPool-1-worker-1 (join)|  |  |  - Fibonacci(3) = 2 <- ForkJoinPool-1-worker-2 (join)|  |  - Fibonacci(4) = 3 <- ForkJoinPool-1-worker-2 (join)|  - Fibonacci(5) = 5 <- ForkJoinPool-1-worker-1 (join)result = 5invoke Fibonacci(6)  <- main|  - Fibonacci(5) <- ForkJoinPool-1-worker-1 (fork) |  - Fibonacci(4) <- ForkJoinPool-1-worker-1|  |  - Fibonacci(3) <- ForkJoinPool-1-worker-1 (fork) |  |  - Fibonacci(2) <- ForkJoinPool-1-worker-1|  |  - Fibonacci(4) <- ForkJoinPool-1-worker-2 (fork) |  |  - Fibonacci(3) <- ForkJoinPool-1-worker-2|  |  |  - Fibonacci(2) <- ForkJoinPool-1-worker-2 (fork) |  |  |  - Fibonacci(1) <- ForkJoinPool-1-worker-2|  |  |  - Fibonacci(2) <- ForkJoinPool-1-worker-1 (fork) |  |  |  - Fibonacci(1) <- ForkJoinPool-1-worker-1|  |  |  - Fibonacci(3) = 2 <- ForkJoinPool-1-worker-2 (join)|  |  |  - Fibonacci(3) = 2 <- ForkJoinPool-1-worker-1 (join)|  |  |  - Fibonacci(3) <- ForkJoinPool-1-worker-2 (fork) |  |  |  - Fibonacci(2) <- ForkJoinPool-1-worker-2|  |  - Fibonacci(4) = 3 <- ForkJoinPool-1-worker-1 (join)|  |  |  |  - Fibonacci(2) <- ForkJoinPool-1-worker-2 (fork) |  |  |  |  - Fibonacci(1) <- ForkJoinPool-1-worker-2|  |  |  |  - Fibonacci(3) = 2 <- ForkJoinPool-1-worker-2 (join)|  |  |  - Fibonacci(4) = 3 <- ForkJoinPool-1-worker-2 (join)|  |  - Fibonacci(5) = 5 <- ForkJoinPool-1-worker-2 (join)|  - Fibonacci(6) = 8 <- ForkJoinPool-1-worker-1 (join)result = 8invoke Fibonacci(7)  <- main|  - Fibonacci(6) <- ForkJoinPool-1-worker-1 (fork) |  - Fibonacci(5) <- ForkJoinPool-1-worker-1|  |  - Fibonacci(4) <- ForkJoinPool-1-worker-1 (fork) |  |  - Fibonacci(3) <- ForkJoinPool-1-worker-1|  |  |  - Fibonacci(2) <- ForkJoinPool-1-worker-1 (fork) |  |  |  - Fibonacci(1) <- ForkJoinPool-1-worker-1|  |  - Fibonacci(5) <- ForkJoinPool-1-worker-2 (fork) |  |  - Fibonacci(4) <- ForkJoinPool-1-worker-2|  |  |  - Fibonacci(3) <- ForkJoinPool-1-worker-2 (fork) |  |  |  - Fibonacci(2) <- ForkJoinPool-1-worker-2|  |  |  |  - Fibonacci(2) <- ForkJoinPool-1-worker-2 (fork) |  |  |  |  - Fibonacci(1) <- ForkJoinPool-1-worker-2|  |  |  - Fibonacci(3) = 2 <- ForkJoinPool-1-worker-1 (join)|  |  |  - Fibonacci(3) <- ForkJoinPool-1-worker-1 (fork) |  |  |  - Fibonacci(2) <- ForkJoinPool-1-worker-1|  |  |  |  - Fibonacci(3) = 2 <- ForkJoinPool-1-worker-2 (join)|  |  |  |  - Fibonacci(2) <- ForkJoinPool-1-worker-1 (fork) |  |  |  |  - Fibonacci(1) <- ForkJoinPool-1-worker-1|  |  |  - Fibonacci(4) = 3 <- ForkJoinPool-1-worker-2 (join)|  |  |  - Fibonacci(4) <- ForkJoinPool-1-worker-2 (fork) |  |  |  - Fibonacci(3) <- ForkJoinPool-1-worker-2|  |  |  |  - Fibonacci(2) <- ForkJoinPool-1-worker-2 (fork) |  |  |  |  - Fibonacci(1) <- ForkJoinPool-1-worker-2|  |  |  |  - Fibonacci(3) = 2 <- ForkJoinPool-1-worker-1 (join)|  |  |  - Fibonacci(4) = 3 <- ForkJoinPool-1-worker-1 (join)|  |  |  |  - Fibonacci(3) = 2 <- ForkJoinPool-1-worker-2 (join)|  |  - Fibonacci(5) = 5 <- ForkJoinPool-1-worker-1 (join)|  |  |  |  - Fibonacci(3) <- ForkJoinPool-1-worker-2 (fork) |  |  |  |  - Fibonacci(2) <- ForkJoinPool-1-worker-2|  |  |  |  |  - Fibonacci(2) <- ForkJoinPool-1-worker-1 (fork) |  |  |  |  |  - Fibonacci(1) <- ForkJoinPool-1-worker-1|  |  |  |  |  - Fibonacci(3) = 2 <- ForkJoinPool-1-worker-1 (join)|  |  |  |  - Fibonacci(4) = 3 <- ForkJoinPool-1-worker-2 (join)|  |  |  - Fibonacci(5) = 5 <- ForkJoinPool-1-worker-2 (join)|  |  - Fibonacci(6) = 8 <- ForkJoinPool-1-worker-2 (join)|  - Fibonacci(7) = 13 <- ForkJoinPool-1-worker-1 (join)result = 13

现在，输出使您可以更深入地了解程序的处理。出现以下斐波纳契数计算方法：