摘要：于是提供了機(jī)制，使得從中讀往里寫變成異步動作。這是因為客戶端的數(shù)據(jù)推送速度太慢了，容器先將收回，當(dāng)容器發(fā)現(xiàn)可以讀取到新數(shù)據(jù)的時候，再分配一個去讀，如此循環(huán)直到全部讀完為止。注意和不能同時使用。

Github地址

相關(guān)系列文章：

Servlet 3.0 異步處理詳解

Spring MVC異步處理的幾種方式

Servlet Async Processing提供了一種異步請求處理的手段（見我的另一篇文章Servlet 3.0 異步處理詳解），能夠讓你將Http thread從慢速處理中釋放出來出來其他請求，提高系統(tǒng)的響應(yīng)度。

但是光有Async Processing是不夠的，因為整個請求-響應(yīng)過程的速度快慢還牽涉到了客戶端的網(wǎng)絡(luò)情況，如果客戶端網(wǎng)絡(luò)情況糟糕，其上傳和下載速度都很慢，那么同樣也會長時間占用Http Thread使其不能被釋放出來。

于是Servlet 3.1提供了Async IO機(jī)制，使得從Request中讀、往Response里寫變成異步動作。

我們先來一段客戶端上傳速度慢的例子，AsyncReadServlet.java：

@WebServlet(value = "/async-read", asyncSupported = true)
public class AsyncReadServlet extends HttpServlet {

  @Override
  protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {

    System.out.println("Servlet thread: " + Thread.currentThread().getName());
    AsyncContext asyncCtx = req.startAsync();
    ServletInputStream is = req.getInputStream();
    is.setReadListener(new ReadListener() {
      private int totalReadBytes = 0;

      @Override
      public void onDataAvailable() {
        System.out.println("ReadListener thread: " + Thread.currentThread().getName());

        try {
          byte buffer[] = new byte[1 * 1024];
          int readBytes = 0;
          while (is.isReady() && !is.isFinished()) {
            int length = is.read(buffer);
            if (length == -1 && is.isFinished()) {
              asyncCtx.complete();
              System.out.println("Read: " + readBytes + " bytes");
              System.out.println("Total Read: " + totalReadBytes + " bytes");
              return;
            }
            readBytes += length;
            totalReadBytes += length;

          }
          System.out.println("Read: " + readBytes + " bytes");

        } catch (IOException ex) {
          ex.printStackTrace();
          asyncCtx.complete();
        }
      }

      @Override
      public void onAllDataRead() {
        try {
          System.out.println("Total Read: " + totalReadBytes + " bytes");
          asyncCtx.getResponse().getWriter().println("Finished");
        } catch (IOException ex) {
          ex.printStackTrace();
        }
        asyncCtx.complete();
      }

      @Override
      public void onError(Throwable t) {
        System.out.println(ExceptionUtils.getStackTrace(t));
        asyncCtx.complete();
      }
    });

  }

}

我們利用curl的--limit-rate選項來模擬慢速上傳curl -X POST -F "bigfile=@src/main/resources/bigfile" --limit-rate 5k http://localhost:8080/async-read

然后觀察服務(wù)端的打印輸出：

Servlet thread: http-nio-8080-exec-3
ReadListener thread: http-nio-8080-exec-3
Read: 16538 bytes
ReadListener thread: http-nio-8080-exec-4
Read: 16384 bytes
ReadListener thread: http-nio-8080-exec-5
Read: 16384 bytes
ReadListener thread: http-nio-8080-exec-7
Read: 16384 bytes
ReadListener thread: http-nio-8080-exec-6
Read: 16384 bytes
ReadListener thread: http-nio-8080-exec-8
Read: 16384 bytes
ReadListener thread: http-nio-8080-exec-9
Read: 16384 bytes
ReadListener thread: http-nio-8080-exec-10
Read: 2312 bytes
ReadListener thread: http-nio-8080-exec-1
Read: 48 bytes
Total Read: 117202 bytes

可以從輸出看到除了doGet和第一次進(jìn)入onDataAvailable是同一個Http thread之外，后面的read動作都發(fā)生在另外的Http thread里。
這是因為客戶端的數(shù)據(jù)推送速度太慢了，容器先將Http thread收回，當(dāng)容器發(fā)現(xiàn)可以讀取到新數(shù)據(jù)的時候，再分配一個Http thread去讀InputStream，如此循環(huán)直到全部讀完為止。

注意：HttpServletRequest.getInputStream()和getParameter*()不能同時使用。

再來一段客戶端下載慢的例子，AsyncWriteServlet.java：

@WebServlet(value = "/async-write", asyncSupported = true)
public class AsyncWriteServlet extends HttpServlet {

  @Override
  protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {

    System.out.println("Servlet thread: " + Thread.currentThread().getName());
    AsyncContext asyncCtx = req.startAsync();
    ServletOutputStream os = resp.getOutputStream();
    InputStream bigfileInputStream = ClassLoader.getSystemClassLoader().getResourceAsStream("bigfile");

    os.setWriteListener(new WriteListener() {

      @Override
      public void onWritePossible() throws IOException {

        int loopCount = 0;
        System.out.println("WriteListener thread: " + Thread.currentThread().getName());
        while (os.isReady()) {
          loopCount++;
          System.out.println("Loop Count: " + loopCount);
          byte[] bytes = readContent();
          if (bytes != null) {
            os.write(bytes);
          } else {
            closeInputStream();
            asyncCtx.complete();
            break;
          }
        }
      }

      @Override
      public void onError(Throwable t) {

        try {
          os.print("Error happened");
          os.print(ExceptionUtils.getStackTrace(t));
        } catch (IOException e) {
          e.printStackTrace();
        } finally {
          closeInputStream();
          asyncCtx.complete();
        }

      }

      private byte[] readContent() throws IOException {
        byte[] bytes = new byte[1024];
        int readLength = IOUtils.read(bigfileInputStream, bytes);
        if (readLength <= 0) {
          return null;
        }
        return bytes;
      }

      private void closeInputStream() {
        IOUtils.closeQuietly(bigfileInputStream);
      }
    });

  }

}

同樣利用curl做慢速下載，curl --limit-rate 5k http://localhost:8080/async-write

接下來看以下服務(wù)端打印輸出：

Servlet thread: http-nio-8080-exec-1
WriteListener thread: http-nio-8080-exec-1
Write bytes: 8192
WriteListener thread: http-nio-8080-exec-2
Write bytes: 8192
WriteListener thread: http-nio-8080-exec-3
Write bytes: 8192
WriteListener thread: http-nio-8080-exec-4
Write bytes: 8192
WriteListener thread: http-nio-8080-exec-5
Write bytes: 8192
WriteListener thread: http-nio-8080-exec-6
Write bytes: 8192
WriteListener thread: http-nio-8080-exec-7
Write bytes: 8192
WriteListener thread: http-nio-8080-exec-8
Write bytes: 8192
WriteListener thread: http-nio-8080-exec-9
Write bytes: 8192
WriteListener thread: http-nio-8080-exec-10
Write bytes: 8192
WriteListener thread: http-nio-8080-exec-1
Write bytes: 8192
WriteListener thread: http-nio-8080-exec-2
Write bytes: 8192
WriteListener thread: http-nio-8080-exec-3
Write bytes: 8192
WriteListener thread: http-nio-8080-exec-4
Write bytes: 8192
WriteListener thread: http-nio-8080-exec-5
Write bytes: 2312

PS. 后發(fā)現(xiàn)即使沒有添加--limit-rate參數(shù)，也會出現(xiàn)類似于上面的結(jié)果。

上面兩個例子使用的是curl來模擬，我們也提供了Jmeter的benchmark。

需要注意的是，必須在user.properties文件所在目錄啟動Jmeter，因為這個文件里提供了模擬慢速連接的參數(shù)httpclient.socket.http.cps=5120。然后利用Jmeter打開benchmark.xml。

Java EE 7 Tutorial: Java Servlet Technology - Nonblocking I/O

Slides - Servlet 3.1 Async IO

Non-blocking I/O using Servlet 3.1: Scalable applications using Java EE 7

How to simulate network bandwidth in JMeter?

Configuring JMeter

Servlet 3.1 Asynchronous IO and Jetty-9.1