通过I/O在线程间进行通信通常很有用。提供线程功能的类库以“管道”的形式对线程间的 I/O 提供了支持。它们在Java I/O 类库中的对应物就是PipedWriter(允许任务向管道写)和PipedReader(允许不同的任务从同一个管道中读取)。这个模型可以看做是“生产者-消费者”问题的变体,这里的管道就是一个封装好的解决方案。管道基本上是一个阻塞队列, 存在于多个引入BlockingQueue之前的Java版本中。
下面是一个简单的例子,两个任务使用一个管道进行通信:
import java.io.IOException;
import java.io.PipedReader;
import java.io.PipedWriter;
import java.util.Random;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.TimeUnit;
/**
* 发送端
*/
class Sender implements Runnable {
private Random rand = new Random(47);
private PipedWriter writer = new PipedWriter();
public PipedWriter getWriter() { return writer; }
@Override
public void run() {
try {
while(true) {
for (char c = 'A'; c < 'z'; c++) {
writer.write(c);
TimeUnit.MILLISECONDS.sleep(rand.nextInt(500));
}
}
} catch (IOException e) {
System.out.println(e + " Sender write Exception");
} catch (InterruptedException e) {
System.out.println(e + " Sender sleep Interrupted");
}
}
}
/**
* 接收端
*/
class Receiver implements Runnable {
private PipedReader reader;
public Receiver(Sender sender) throws IOException {
reader = new PipedReader(sender.getWriter());
}
@Override
public void run() {
int count = 0;
try {
while(true) {
//在读取到内容之前,会一直阻塞
char s = (char)reader.read();
System.out.print("Read: " + s + ", ");
if (++count % 5 == 0) {
System.out.println();
}
}
} catch (IOException e) {
System.out.println(e + " Receiver read Exception.");
}
}
}
public class PipedIO {
public static void main(String[] args) throws Exception {
Sender sender = new Sender();
Receiver receiver = new Receiver(sender);
ExecutorService exec = Executors.newCachedThreadPool();
exec.execute(sender);
exec.execute(receiver);
TimeUnit.SECONDS.sleep(5);
exec.shutdownNow();
}
}
执行结果(可能的结果):
Read: A, Read: B, Read: C, Read: D, Read: E,
Read: F, Read: G, Read: H, Read: I, Read: J,
Read: K, Read: L, Read: M, Read: N, Read: O,
Read: P, Read: Q, Read: R, Read: S, Read: T,
Read: U, java.io.InterruptedIOException Receiver read Exception.
java.lang.InterruptedException: sleep interrupted Sender sleep Interrupted
Sender和Receiver代表了需要互相通信的两个任务。Sender创建了一个PipedWriter,它是一个单独的对象;但是对于Receiver,PipedReader的建立必须在构造器中与一个PipedWriter相关联。就是说,PipedReader与PipedWriter的构造可以通过如下两种方式:
//方式一:先构造PipedReader,再通过它构造PipedWriter。
PipedReader reader = new PipedReader();
PipedWriter writer = new PipedWriter(reader);
//方式二:先构造PipedWriter,再通过它构造PipedReader。
PipedWriter writer2 = new PipedWriter();
PipedReader reader2 = new PipedReader(writer2);
Sender把数据放进Writer,然后休眠一段时间(随机数)。然而,Receiver没有sleep()和wait。但当它调用read()时,如果没有更多的数据,****管道将自动阻塞。
注意Sender和Receiver是在main()中启动的,即对象构造彻底完毕之后。如果你启动了一个没有构造完毕的对象,在不同的平台上管道可能会产生不一致的行为(注意,BlockingQueue使用起来更加健壮而容易)。
在shutdownNow()被调用时,可以看到PipedReader与普通I/O之间最重要的差异——PipedReader是可以中断的。如果你将reader.read()替换为System.in.read(),那么interrupt()将不能打断read()调用。