異步I/O模型與Java

異步IO模型原理圖：

Java1.7中在NIO包中增加了Asynchronous I/O操作。

異步I/O類使用教程中講解得很好。

如何使用Asychronous I/O類

• JUC包中Fature
  • Fature#isDone polling模式。輪訓檢查
  • Fature#get wait模式。等待I/O操作完畢，也就是阻塞
• NIO包中CompletionHandler
  • completed I/O執行完畢時的回調
  • failed I/O執行失敗時的回調。好像ajax一樣，有成功與失敗的回調方法。

顯然，如果要實現異步I/O模型（非阻塞模式），需要使用回調的方法，也就是選擇CompletionHandler。

以下代碼片段摘自網絡java2s.com

import static java.nio.file.StandardOpenOption.CREATE;
import static java.nio.file.StandardOpenOption.WRITE;

import java.io.IOException;
import java.nio.ByteBuffer;
import java.nio.channels.AsynchronousFileChannel;
import java.nio.channels.CompletionHandler;
import java.nio.charset.Charset;
import java.nio.file.Path;
import java.nio.file.Paths;
public class Main {
  public static void main(String[] args) throws Exception {
    Path path = Paths.get("test.txt");
    AsynchronousFileChannel afc = AsynchronousFileChannel.open(path, WRITE,
        CREATE);
    WriteHandler handler = new WriteHandler();
    ByteBuffer dataBuffer = getDataBuffer();
    Attachment attach = new Attachment();
    attach.asyncChannel = afc;
    attach.buffer = dataBuffer;
    attach.path = path;

    afc.write(dataBuffer, 0, attach, handler);

    System.out.println("Sleeping for 5  seconds...");
    Thread.sleep(5000);
  }
  public static ByteBuffer getDataBuffer() {
    String lineSeparator = System.getProperty("line.separator");
    StringBuilder sb = new StringBuilder();
    sb.append("test");
    sb.append(lineSeparator);
    sb.append("test");
    sb.append(lineSeparator);
    String str = sb.toString();
    Charset cs = Charset.forName("UTF-8");
    ByteBuffer bb = ByteBuffer.wrap(str.getBytes(cs));
    return bb;
  }
}
class Attachment {
  public Path path;
  public ByteBuffer buffer;
  public AsynchronousFileChannel asyncChannel;
}

class WriteHandler implements CompletionHandler<Integer, Attachment> {
  @Override
  public void completed(Integer result, Attachment attach) {
    System.out.format("%s bytes written  to  %s%n", result,
        attach.path.toAbsolutePath());
    try {
       attach.asyncChannel.close();
    } catch (IOException e) {
      e.printStackTrace();
    }
  }

  @Override
  public void failed(Throwable e, Attachment attach) {
    try {
      attach.asyncChannel.close();
    } catch (IOException e1) {
      e1.printStackTrace();
    }
  }
}

import static java.nio.file.StandardOpenOption.READ;
/*  w ww  .j a v  a 2 s. c  o  m*/
import java.io.IOException;
import java.nio.ByteBuffer;
import java.nio.channels.AsynchronousFileChannel;
import java.nio.channels.CompletionHandler;
import java.nio.charset.Charset;
import java.nio.file.Path;
import java.nio.file.Paths;
public class Main {
  public static void main(String[] args) throws Exception{
    Path path = Paths.get("test.txt");
    AsynchronousFileChannel afc = AsynchronousFileChannel.open(path, READ);
    ReadHandler handler = new ReadHandler();
    int fileSize = (int) afc.size();
    ByteBuffer dataBuffer = ByteBuffer.allocate(fileSize);

    Attachment attach = new Attachment();
    attach.asyncChannel = afc;
    attach.buffer = dataBuffer;
    attach.path = path;

    afc.read(dataBuffer, 0, attach, handler);

    System.out.println("Sleeping for 5  seconds...");
    Thread.sleep(5000);
  }
}
class Attachment {
  public Path path;
  public ByteBuffer buffer;
  public AsynchronousFileChannel asyncChannel;
}

class ReadHandler implements CompletionHandler<Integer, Attachment> {
  @Override
  public void completed(Integer result, Attachment attach) {
    System.out.format("%s bytes read   from  %s%n", result, attach.path);
    System.out.format("Read data is:%n");
    byte[] byteData = attach.buffer.array();
    Charset cs = Charset.forName("UTF-8");
    String data = new String(byteData, cs);
    System.out.println(data);
    try {
      // Close the channel
      attach.asyncChannel.close();
    } catch (IOException e) {
      e.printStackTrace();
    }
  }

  @Override
  public void failed(Throwable e, Attachment attach) {
    System.out.format("Read operation  on  %s  file failed."
        + "The  error is: %s%n", attach.path, e.getMessage());
    try {
      // Close the channel
      attach.asyncChannel.close();
    } catch (IOException e1) {
      e1.printStackTrace();
    }
  }
}