并发–顺序线程和原始线程

我不久前参与了一个项目，该项目的报告流程如下：

用户会要求举报
报告要求将被翻译成较小的部分
每个零件的报告将基于零件/节的类型由报告生成器生成
组成报告的各个部分将重新组合成最终报告，并返回给用户

我的目标是展示如何从错误的实施过渡到相当好的实施：
单元测试最好地展示了我拥有的一些基本构建基块：这是一个测试助手，它生成示例报告请求，其中包括组成报告请求部分：

public class FixtureGenerator {public static ReportRequest generateReportRequest(){List<ReportRequestPart> requestParts = new ArrayList<ReportRequestPart>();Map<String, String> attributes = new HashMap<String, String>();attributes.put("user","user");Context context = new Context(attributes );ReportRequestPart part1 = new ReportRequestPart(Section.HEADER, context);ReportRequestPart part2 = new ReportRequestPart(Section.SECTION1, context);ReportRequestPart part3 = new ReportRequestPart(Section.SECTION2, context);ReportRequestPart part4 = new ReportRequestPart(Section.SECTION3, context);ReportRequestPart part5 = new ReportRequestPart(Section.FOOTER, context);   requestParts.add(part1);        requestParts.add(part2);requestParts.add(part3);requestParts.add(part4);requestParts.add(part5);ReportRequest reportRequest  = new ReportRequest(requestParts );return reportRequest;}}

以及生成报告的测试：

public class FixtureGenerator {@Testpublic void testSequentialReportGeneratorTime(){long startTime = System.currentTimeMillis();Report report = this.reportGenerator.generateReport(FixtureGenerator.generateReportRequest());long timeForReport = System.currentTimeMillis()-startTime;assertThat(report.getSectionReports().size(), is (5));logger.error(String.format("Sequential Report Generator : %s ms", timeForReport));}

生成报告一部分的组件是一个虚拟实现，具有2秒的延迟以模拟IO密集调用：

public class DummyReportPartGenerator implements ReportPartGenerator{@Overridepublic ReportPart generateReportPart(ReportRequestPart reportRequestPart) {try {//Deliberately introduce a delayThread.sleep(2000);} catch (InterruptedException e) {e.printStackTrace();}return new ReportPart(reportRequestPart.getSection(), "Report for " + reportRequestPart.getSection());}
}

顺序执行

给定这些基本的类集，我的第一个天真的顺序实现如下：

public class SequentialReportGenerator implements ReportGenerator {private ReportPartGenerator reportPartGenerator;@Overridepublic Report generateReport(ReportRequest reportRequest){List<ReportRequestPart> reportRequestParts = reportRequest.getRequestParts();List<ReportPart> reportSections = new ArrayList<ReportPart>();for (ReportRequestPart reportRequestPart: reportRequestParts){reportSections.add(reportPartGenerator.generateReportPart(reportRequestPart));}return new Report(reportSections);}......
}

显然，对于其中包含5个部分的报告请求，每个部分需要2秒钟才能完成，此报告大约需要10秒钟才能返回给用户。

它请求同时进行。

基于原始线程的实现

以下是第一个并发实现，虽然不好，但比顺序的要好，其后是为每个报告请求部分生成一个线程，等待要生成的报告部分（使用thread.join（）方法），并在出现这些块时对其进行汇总在。

public class RawThreadBasedReportGenerator implements ReportGenerator {private static final Logger logger = LoggerFactory.getLogger(RawThreadBasedReportGenerator.class);private ReportPartGenerator reportPartGenerator;@Overridepublic Report generateReport(ReportRequest reportRequest) {List<ReportRequestPart> reportRequestParts = reportRequest.getRequestParts();List<Thread> threads = new ArrayList<Thread>();List<ReportPartRequestRunnable> runnablesList = new ArrayList<ReportPartRequestRunnable>();for (ReportRequestPart reportRequestPart : reportRequestParts) {ReportPartRequestRunnable reportPartRequestRunnable = new ReportPartRequestRunnable(reportRequestPart, reportPartGenerator);runnablesList.add(reportPartRequestRunnable);Thread thread = new Thread(reportPartRequestRunnable);threads.add(thread);thread.start();}for (Thread thread : threads) {try {thread.join();} catch (InterruptedException e) {logger.error(e.getMessage(), e);}}List<ReportPart> reportParts = new ArrayList<ReportPart>();for (ReportPartRequestRunnable reportPartRequestRunnable : runnablesList) {reportParts.add(reportPartRequestRunnable.getReportPart());}return new Report(reportParts);}    .....
}

这种方法的危险在于，将为每个报表部件创建一个新线程，因此在实际情况下，如果同时发出100个请求，并且每个请求都产生5个线程，则可能最终在vm中创建500个代价高昂的线程!!

因此，必须以某种方式限制线程的创建。在下一篇博客文章中，我将介绍另外两种控制线程的方法。

参考： 并发–来自JCG合作伙伴 Biju Kunjummen的all和杂物博客，并发-顺序和原始线程。