1、两阶段终止模式介绍
有时候，我们希望提前结束线程，但安全可靠地停止线程，并不是一件容易的事情，如果立即停止线程，会使共享的数据结构处于不一致的状态，如目前已经废弃使用的Thread类的stop方法(它会使线程在抛出java.lang.ThreadDeath之后终止线程，即使是在执行synchronized方法的时候)。更好的做法是执行完终止处理，再终止线程，即Two-phase Termination，两阶段终止模式。
该模式有两个角色：
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Terminator，终止者，负责接收终止请求，执行终止处理，处理完成后再终止自己。 -
TerminationRequester：终止请求发出者，用来向Terminator发出终止请求。
2、Terminator代码演示
该模式示例代码如下：
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public class CounterIncrement extends Thread { private volatile boolean terminated = false; private int counter = 0; private Random random = new Random(System.currentTimeMillis()); @Override public void run() { try { while (!terminated) { System.out.println(Thread.currentThread().getName()+" "+counter++); Thread.sleep(random.nextInt(1000)); } } catch (InterruptedException e) { e.printStackTrace(); } finally { this.clean(); } } private void clean() { System.out.println("do some clean work for the second phase,current counter "+counter); } public void close() { this.terminated = true; this.interrupt(); }} |
3、TerminationRequester
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public class CounterTest { public static void main(String[] args) throws InterruptedException { CounterIncrement counterIncrement = new CounterIncrement(); counterIncrement.start(); Thread.sleep(15_000L); //主动清理 counterIncrement.close(); }} |
这段代码可以看出实现两阶段终止模式必须注意的是：
使用线程停止标志和interrupt方法，两者缺一不可
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public void close() { this.terminated = true; this.interrupt(); } |
这里使用了terminated作为线程停止标志，变量采用volatile修饰，避免了使用显式锁的开销，又保证了内存可见性。线程run方法会检查terminated属性，如果属性为true，就停止线程，但线程可能调用了阻塞方法，处于wait状态，任务也就可能永远不会检查terminated标志；线程也有可能处于sleep()状态，等sleep时间过后再执行终止状态，程序的响应性就下降了。你可以把方法改成如下运行，线程停止明显变慢了许多：
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public void close() { terminated = true;} |
4、模拟客户端或者服务端都可能终止服务的例子
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public class AppServer extends Thread { private static final int DEFAULT_PORT = 12722; private final static ExecutorService executor = Executors.newFixedThreadPool(10); private int port; private volatile boolean start = true; private List<ClientHandler> clientHandlers = new ArrayList<>(); private ServerSocket server; public AppServer() { this(DEFAULT_PORT); } public AppServer(int port) { this.port = port; } @Override public void run() { try { server = new ServerSocket(port); while (start) { Socket client = server.accept(); ClientHandler clientHandler = new ClientHandler(client); executor.submit(clientHandler); this.clientHandlers.add(clientHandler); } } catch (IOException e) { //throw new RuntimeException(); } finally { this.dispose(); } } public void dispose() { System.out.println("dispose"); this.clientHandlers.stream().forEach(ClientHandler::stop); this.executor.shutdown(); } public void shutdown() throws IOException { this.start = false; this.interrupt(); this.server.close(); }} |
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public class ClientHandler implements Runnable { private final Socket socket; private volatile boolean running = true; public ClientHandler(Socket socket) { this.socket = socket; } @Override public void run() { try (InputStream inputStream = socket.getInputStream(); OutputStream outputStream = socket.getOutputStream(); BufferedReader br = new BufferedReader(new InputStreamReader(inputStream)); PrintWriter printWriter = new PrintWriter(outputStream)) { while (running) { String message = br.readLine(); if (message == null) { break; } System.out.println("Come from client >" + message); printWriter.write("echo " + message+"\n"); printWriter.flush(); } } catch (IOException e) { //自动关闭的时候 将running this.running = false; }finally { this.stop(); } } public void stop() { if (!running) { return; } this.running = false; try { this.socket.close(); } catch (IOException e) { } }} |
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public class AppServerClient { public static void main(String[] args) throws InterruptedException, IOException { AppServer server = new AppServer(12135); server.start(); Thread.sleep(20_000L); server.shutdown(); }} |
5、mac telnet模拟客户端输入
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bogon:~ kpioneer$ telnet localhost 12135Trying ::1...Connected to localhost.Escape character is '^]'.hello echo hello I love youecho I love youConnection closed by foreign host. |
服务端输出：
Come from client >hello
Come from client >I love you
dispose
总结：
可以看到，在子类使用两阶段终止模式时，其只需要实现各自所需要执行的任务，并且更新当前任务的数量即可。在某些情况下，当前任务的数量也可以不进行更新，比如在进行终止时，不关心当前剩余多少任务需要执行。
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原文链接：https://juejin.cn/post/7023614457683116069
