我的家園

[本文是我對Java Concurrency In Practice 7.1的歸納和總結. ?轉載請注明作者和出處, ?如有謬誤, 歡迎在評論中指正. ]

并不是所有的阻塞都是可中斷的, 比如InputStream.read方法. 在檢測到輸入數據可用, 到達流末尾或者拋出異常前, 該方法一直阻塞. 而且阻塞的時候不會檢查中斷標記, 所以中斷線程無法使read從阻塞狀態返回. 但是關閉流可以使得read方法拋出異常, 從而從阻塞狀態返回.?

public class ReaderThread extends Thread {
	private static final int BUFSZ = 1024;
	private final Socket socket;
	private final InputStream in;

	public ReaderThread(Socket socket) throws IOException {
		this.socket = socket;
		this.in = socket.getInputStream();
	}

	// 覆蓋Thread類的interrupt方法, 加入關閉socket的代碼
	// 如果發生中斷時, 線程阻塞在read方法上, socket的關閉會導致read方法拋出SocketException, 然后run方法運行完畢
	public void interrupt() {
		try {
			socket.close();
		} catch (IOException ignored) {
		} finally {
			super.interrupt();
		}
	}

	public void run() {
		try {
			byte[] buf = new byte[BUFSZ];
			while (true) {
				int count = in.read(buf);
				if (count < 0)
					break;
				else if (count > 0)
					processBuffer(buf, count);
			}
		} catch (IOException e) { /*  Allow thread to exit  */
		}
	}

	private void processBuffer(byte[] buf, int count) {
		//...
	}
}

如果task并非在自己創建的線程里運行, 而是提交給線程池運行的話, 就無法使用上例的方式處理不可中斷阻塞了. 之前有過分析, 對于提交給線程池執行的task, 應該通過Future.cancel方法提前終止task的運行, 所以可以考慮重寫Future.cancel方法, 在其中加入關閉socket的操作. Future對象是由submit方法返回的, 其源代碼如下:

public <T> Future<T> submit(Callable<T> task) {
	if (task == null) 
		throw new NullPointerException();
	RunnableFuture<T> ftask = newTaskFor(task);
	execute(ftask);
	return ftask;
}?

可知submit方法返回的Future對象是調用newTaskFor方法獲得的:

protected <T> RunnableFuture<T> newTaskFor(Callable<T> callable) {
	return new FutureTask<T>(callable);
}?

newTaskFor方法被聲明為protected, 所以我們可以通過繼承覆蓋該方法, 返回自定義的Future對象.

首先將需要覆蓋的2個方法定義在接口中:

public interface CancellableTask<T> extends Callable<T> { 
	void cancel(); 
	RunnableFuture<T> newTask(); 
} ?

然后讓task類實現CancellableTask接口:

public abstract class SocketUsingTask<T> implements CancellableTask<T> {
	private Socket socket;

	protected synchronized void setSocket(Socket s) {
		socket = s;
	}

	public synchronized void cancel() {
		try {
			if (socket != null)
				socket.close();
		} catch (IOException ignored) {
		}
	}

	public RunnableFuture<T> newTask() {
		return new FutureTask<T>(this) {
			// 定義FutureTask的匿名內部類, 并覆蓋cancel方法, 向其中加入關閉socket的操作
			public boolean cancel(boolean mayInterruptIfRunning) {
				try {
					SocketUsingTask.this.cancel();
				} finally {
					return super.cancel(mayInterruptIfRunning);
				}
			}
		};
	}
}

接著繼承ThreadPoolExecutor類并覆蓋newTaskFor方法, 讓該方法返回自定義的FutureTask對象:

public class CancellingExecutor extends ThreadPoolExecutor {
	// ...
	protected <T> RunnableFuture<T> newTaskFor(Callable<T> callable) {
		// 如果callable是CancellableTask對象, 那么就返回自定義的FutureTask(通過調用其newTaskFor方法實現)
		if (callable instanceof CancellableTask)
			return ((CancellableTask<T>) callable).newTask();
		else
			return super.newTaskFor(callable);
	}
}

測試代碼:

public static void main(String[] args) {
	CancellingExecutor executor = new CancellingExecutor();
	SocketUsingTask task = new SocketUsingTask();
	task.setSocket(new Socket("www.baidu.com", 80));
	Future<V> future = executor.submit(task);
	future.cancel(true);
}