Java 并发编程之任务取消（七）

只运行一次的服务

以检查是否有新邮件为栗

采用AtomicBoolean而不使用Volatile类型的boolean.是因为能从内部的Runnable中访问hasNewMail标志，因此它必须是final类型以免被修改。

boolean checkMain(Set<String> hosts, long timeout, TimeUnit unit) throws InterruptedException {ExecutorService exec = Executors.newCachedThreadPool();final AtomicBoolean hasNewMail = new AtomicBoolean(false);try {for (String host : hosts) {exec.execute(new Runnable() {@Overridepublic void run() {// TODO Auto-generated method stubif (checkMail(host)) {hasNewMail.set(true);}}});}} finally {exec.shutdown();exec.awaitTermination(timeout, unit);}return hasNewMail.get();}

ShutdownNow的局限性

上次说shutdownNow会返回所有尚未启动的Runnable。但是它无法返回的是正在执行的Runnable。通过封装ExecutorService并使得execute记录哪些任务是在关闭后取消的。并且在任务返回时必须保持线程的中断状态

import java.util.ArrayList;import java.util.Collection;import java.util.Collections;import java.util.HashSet;import java.util.List;import java.util.Set;import java.util.concurrent.AbstractExecutorService;import java.util.concurrent.ExecutorService;import java.util.concurrent.TimeUnit;public class TrackingExecutor extends AbstractExecutorService {private final ExecutorService exec;private final Set<Runnable> tasksCancelledAtShutdown = Collections.synchronizedSet(new HashSet<Runnable>());public TrackingExecutor(ExecutorService exec) {super();this.exec = exec;}public List<Runnable> getCancelledTasks() {if (!exec.isTerminated()) {throw new IllegalStateException("terminated");}return new ArrayList<Runnable>(tasksCancelledAtShutdown);}@Overridepublic void execute(final Runnable command) {// TODO Auto-generated method stubexec.execute(new Runnable() {public void run() {try {command.run();} finally {if (isShutdown() && Thread.currentThread().isInterrupted()) {tasksCancelledAtShutdown.add(command);}}}});}@Overridepublic void shutdown() {// TODO Auto-generated method stub}@Overridepublic List<Runnable> shutdownNow() {// TODO Auto-generated method stubreturn null;}@Overridepublic boolean isShutdown() {// TODO Auto-generated method stubreturn false;}@Overridepublic boolean isTerminated() {// TODO Auto-generated method stubreturn false;}@Overridepublic boolean awaitTermination(long timeout, TimeUnit unit)throws InterruptedException {// TODO Auto-generated method stubreturn false;}}

将ExecutorService的其他方法委托给exec。以网页爬虫为例。使用TrackingExecutor的用法

import java.io.IOException;import java.net.MalformedURLException;import java.net.URL;import java.util.ArrayList;import java.util.HashSet;import java.util.List;import java.util.Set;import java.util.concurrent.Executors;import java.util.concurrent.TimeUnit;public abstract class WebCrawler {private volatile TrackingExecutor exec;private final Set<URL> urltoCrawl = new HashSet<URL>();public static void main(String[] args) {WebCrawler webc = new WebCrawler() {@Overrideprotected List<URL> processPage(URL url) {System.out.println(url.getHost());List<URL> url2 = new ArrayList<URL>();try {url2.add(new URL("http://www.baidu.com"));} catch (MalformedURLException e) {// TODO Auto-generated catch blocke.printStackTrace();}return url2;}};try {webc.urltoCrawl.add(new URL("http://www.baidu.com"));} catch (MalformedURLException e) {// TODO Auto-generated catch blocke.printStackTrace();}webc.start();try {Thread.sleep(1000);webc.stop();} catch (InterruptedException e) {// TODO Auto-generated catch blocke.printStackTrace();}}public synchronized void start() {exec = new TrackingExecutor(Executors.newCachedThreadPool());for (URL url : urltoCrawl) {submitCrawlTask(url);}urltoCrawl.clear();}public synchronized void stop() throws InterruptedException {try {saveUncrawled(exec.shutdownNow());if (exec.awaitTermination(2000, TimeUnit.MILLISECONDS)) {saveUncrawled(exec.getCancelledTasks());}} finally {exec = null;}}protected abstract List<URL> processPage(URL url);private void saveUncrawled(List<Runnable> uncrawled) {for (Runnable runnable : uncrawled) {urltoCrawl.add(((CrawlTask) runnable).getPage());}}private void submitCrawlTask(URL u) {exec.execute(new CrawlTask(u));}private class CrawlTask implements Runnable {private final URL url;public CrawlTask(URL url) {super();this.url = url;}@Overridepublic void run() {// TODO Auto-generated method stubfor (URL link : processPage(url)) {if (Thread.currentThread().isInterrupted())return;submitCrawlTask(link);}}public URL getPage() {return url;}}}

上面程序中抽象方法processPage中的代码是测试用的。里面正确的代码目的是获取该url下所有的链接。其实也不难。但是不属于本篇知识点内。只要得到所有<a href就可以了。有兴趣的可以看看元素选择器 还有一个开源的爬虫Crawl研究一下。

执行Stop方法后会调用saveUncrawle方法获取正在执行中的任务。之后要进行保存到文件中还是怎么办只要把代码添加到saveUncrawled内即可。