相關係列文章：
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，提高系統的響應度。
但是光有Async Processing是不夠的，因爲整個請求-響應過程的速度快慢還牽涉到了客戶端的網絡情況，如果客戶端網絡情況糟糕，其上傳和下載速度都很慢，那麼同樣也會長時間佔用Http Thread使其不能被釋放出來。
於是Servlet 3.1提供了Async IO機制，使得從Request中讀、往Response裏寫變成異步動作。
Async Read
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
然後觀察服務端的打印輸出：
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和第一次進入onDataAvailable是同一個Http thread之外，後面的read動作都發生在另外的Http thread裏。 這是因爲客戶端的數據推送速度太慢了，容器先將Http thread收回，當容器發現可以讀取到新數據的時候，再分配一個Http thread去讀InputStream，如此循環直到全部讀完爲止。
注意：HttpServletRequest.getInputStream()和getParameter*()不能同時使用。
Async Write
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. 後發現即使沒有添加--limit-rate參數，也會出現類似於上面的結果。
Jmeter
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文件所在目錄啓動Jmeter，因爲這個文件裏提供了模擬慢速連接的參數httpclient.socket.http.cps=5120。然後利用Jmeter打開benchmark.xml。