暂停线程（如何优雅地停止线程）
使用 cancelled 标志，当标志状态为true的时候，停止线程。
示例：
public class Test0701CancelThread implements Runnable { public static void main(String[] args) {
Test0701CancelThread cancelThread = new Test0701CancelThread(); try {
cancelThread.aSecondOfPrimes();
} catch (InterruptedException e) {
e.printStackTrace();
}
} private final List<BigInteger> primes = new ArrayList<>(); private volatile boolean cancelled; @Override
public void run() {
System.out.println("执行线程");
BigInteger p = BigInteger.ONE; while (!cancelled) {
p = p.nextProbablePrime(); synchronized (this) {
primes.add(p);
}
}
} public void cancel() {
System.out.println("取消线程");
cancelled = true;
} public synchronized List<BigInteger> get() { return new ArrayList<>(primes);
}
public List<BigInteger> aSecondOfPrimes() throws InterruptedException {
Test0701CancelThread cancelThread = new Test0701CancelThread(); new Thread(cancelThread).start(); try {
SECONDS.sleep(1);
} finally {
cancelThread.cancel();
} return cancelThread.get();
}
}问题：
1、如果调用阻塞方法，可能会导致永远也不会检查取消标志，导致线程不能被终止。
通过 Thread 类的中断方法 interrupt() 方法，中断线程。这是停止线程的最佳实践，它不会马上停止线程，而会先保留必要的内容，然后安全地停止线程。
示例：
public class Test0705CancelThread extends Thread {
private final BlockingQueue<BigInteger> queue; public Test0705CancelThread(BlockingQueue<BigInteger> queue) { this.queue = queue;
}
@Override public void run() { try {
BigInteger p = BigInteger.ONE; while (!Thread.currentThread().isInterrupted()) { queue.put(p = p.nextProbablePrime());
}
} catch (InterruptedException e) {
e.printStackTrace();
}
} public void cancel() {
interrupt();
}
}注意：
Future 类，提供的 cancel() 方法，可以停止 Future 任务。
示例：
public class Test0710FutureCancelThread {
private static final ScheduledExecutorService cancelExec = Executors.newScheduledThreadPool(5); public static void timeRun(final Runnable r, long timeout, TimeUnit unit) {
Future<?> task = cancelExec.submit(r); try {
task.get();
} catch (InterruptedException e) { // 取消线程
task.cancel(true);
} catch (ExecutionException e) { // 如果任务已经执行完毕，那么执行取消不会有什么影响
// 如果任务正在执行，那么将被中断
task.cancel(true);
}
}
}也是取消线程的正确姿势：通过Future来取消线程。当Future.get抛出 InterruptedException或者TimeoutException时，如果你知道不再需要结果，那么就可以调用Future.cancel来取消任务
线程池的停止，可以调用 ExecutorService 类提供的 shutdown() 方法进行停止。
示例：
public class Test0716CloseExecutorService { private static final long TIMEOUT = 1000L; private static final TimeUnit UNIT = TimeUnit.MILLISECONDS; private final ExecutorService exec = Executors.newScheduledThreadPool(5); private final PrintWriter writer; public Test0716CloseExecutorService(PrintWriter writer) { this.writer = writer;
} public void start() {
} public void stop() throws InterruptedException { try {
exec.shutdown(); // TODO 问：为什么停止线程，需要调用以下语句
// 这个方法就是调用shutdown() 之后等待任务执行完毕的方法，可以查看源码的注释
// Blocks until all tasks have completed execution after a shutdown
// request, or the timeout occurs, or the current thread is
// interrupted, whichever happens first.
exec.awaitTermination(TIMEOUT, UNIT);
} finally {
writer.close();
}
} public void log(String msg) { try {
exec.execute(new WriteTask(msg));
} catch (RejectedExecutionException ignored) {
}
}
}核心语句如下：
// 不接收新任务exec.shutdown();// 停止之前提交的任务exec.awaitTermination(TIMEOUT, UNIT);
在生产者-消费者模式中，通过在队列中增加毒丸对象类停止线程。
示例：
public class Test0717PoisonPillCancelThread { private static final File POSION = new File(""); private final IndexerThread consumer = new IndexerThread(); private final CrawlerThread producer = new CrawlerThread(); private final BlockingQueue<File> queue; private final FileFilter fileFilter; private final File root; public Test0717PoisonPillCancelThread(BlockingQueue<File> queue, FileFilter fileFilter, File root) { this.queue = queue; this.fileFilter = fileFilter; this.root = root;
} public void start() {
producer.start();
consumer.start();
} public void stop() {
producer.interrupt();
} public void awaitTermination() throws InterruptedException {
consumer.join();
}
public class IndexerThread extends Thread { @Override
public void run() { try { while (true) {
File file = queue.take(); // 毒丸对象 停止消费者
if (file == POSION) { break;
} else {
indexFile(file);
}
}
} catch (InterruptedException e) { // deal with exception
} finally {
}
} private void indexFile(File file) {
} private void crawl(File root) throws InterruptedException {
}
}
public class CrawlerThread extends Thread { @Override
public void run() { try {
crawl(root);
} catch (InterruptedException e) { // deal with exception
} finally { while (true) { try { // 队列中存入毒丸
queue.put(POSION); break;
} catch (InterruptedException e1) { // deal with exception
}
}
}
} private void crawl(File root) throws InterruptedException {
}
}
}毒丸对象是指放在队列上的对象，其含义是：当得到这个对象时，立即停止。
只有在生产者和消费者的数量都已知的情况下，才可以使用毒丸对象。有多少个生产者，就需要多少个毒丸对象，这样才能停止所有的生产者服务。
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