Non-blocking I/O using Servlet 3.1: Scalable applications using Java EE 7 (TOTD #188)

Servlet 3.0 allowed asynchronous request processing but only traditional I/O was permitted. This can restrict scalability of your applications. In a typical application, ServletInputStream is read in a while loop.

public class TestServlet extends HttpServlet {
    protected void doGet(HttpServletRequest request, HttpServletResponse response)
         throws IOException, ServletException {     
 ServletInputStream input = request.getInputStream();
       byte[] b = new byte[1024];
       int len = -1;
       while ((len = input.read(b)) != -1) {
          . . . 
       }
   }
}

If the incoming data is blocking or streamed slower than the server can read then the server thread is waiting for that data. The same can happen if the data is written to ServletOutputStream.

This is resolved in Servet 3.1 (JSR 340, to be released as part Java EE 7) by adding event listeners - ReadListener and WriteListener interfaces. These are then registered using ServletInputStream.setReadListener and ServletOutputStream.setWriteListener. The listeners have callback methods that are invoked when the content is available to be read or can be written without blocking.

The updated doGet in our case will look like:

AsyncContext context = request.startAsync();
ServletInputStream input = request.getInputStream();
input.setReadListener(new MyReadListener(input, context));

Invoking setXXXListener methods indicate that non-blocking I/O is used instead of the traditional I/O. At most oneReadListener can be registered on ServletIntputStream and similarly at most one WriteListener can be registered on ServletOutputStream. ServletInputStream.isReady and ServletInputStream.isFinished are new methods to check the status of non-blocking I/O read. ServletOutputStream.canWrite is a new method to check if data can be written without blocking.

 MyReadListener implementation looks like:

@Override
public void onDataAvailable() {
 try {
 StringBuilder sb = new StringBuilder();
 int len = -1;
 byte b[] = new byte[1024];
 while (input.isReady()
&& (len = input.read(b)) != -1) {
 String data = new String(b, 0, len);
 System.out.println("--> " + data);
 }
 } catch (IOException ex) {
 Logger.getLogger(MyReadListener.class.getName()).log(Level.SEVERE, null, ex);
 }
}

@Override
public void onAllDataRead() {
 System.out.println("onAllDataRead");
 context.complete();
}

@Override
public void onError(Throwable t) {
 t.printStackTrace();
 context.complete();
}

This implementation has three callbacks:

• onDataAvailable callback method is called whenever data can be read without blocking
  
• onAllDataRead callback method is invoked data for the current request is completely read.
• onError callback is invoked if there is an error processing the request.
  

Notice, context.complete() is called in onAllDataRead and onError to signal the completion of data read.

For now, the first chunk of available data need to be read in the doGet or service method of the Servlet. Rest of the data can be read in a non-blocking way using ReadListener after that. This is going to get cleaned up where all data read can happen in ReadListener only.

The sample explained above can be downloaded from here and works with GlassFish 4.0 build 64 and onwards.

The slides and a complete re-run of What's new in Servlet 3.1: An Overview session at JavaOne is available here.

Here are some more references for you:

• Java EE 7 Specification Status
• Servlet Specification Project
• JSR Expert Group Discussion Archive
• Servlet 3.1 Javadocs