AsyncN.java

package async;

import java.io.IOException;

import javax.servlet.AsyncContext;
import javax.servlet.AsyncEvent;
import javax.servlet.AsyncListener;
import javax.servlet.ServletException;
import javax.servlet.ServletInputStream;
import javax.servlet.ServletOutputStream;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;


@WebServlet(urlPatterns = "/asyncn", asyncSupported = true)
public class AsyncN extends HttpServlet {

	private static final long serialVersionUID = 1L;

	@Override
	protected void service(final HttpServletRequest request, final HttpServletResponse response) throws ServletException, IOException {

		println("Before starting job");
		
		// cannot call getWriter here

		final AsyncContext actx = request.startAsync();
		actx.setTimeout(Long.MAX_VALUE);
		
		// set up async listener for fun
		actx.addListener(new AsyncListener() {
            @Override
            public void onComplete(AsyncEvent event) throws IOException {
                println("AsyncServletNIO onComplete() called");
            }

            @Override
            public void onError(AsyncEvent event) {
                println("AsyncServletNIO onError() " + event.getThrowable());
            }

            @Override
            public void onStartAsync(AsyncEvent event) {
                println("AsyncServletNIO onStartAsync()");
            }

            @Override
            public void onTimeout(AsyncEvent event) {
                println("AsyncServletNIO onTimeout()");
            }
        }, request, response);

        final ServletInputStream is = request.getInputStream();
        final ServletOutputStream os = response.getOutputStream();

        // Start NIO Mode!! Can not use regular servlet input stream read/write after this
        // AsyncReadListener for clients that are slow to write data, 
        // and an AsyncWriteListener for clients that are slow to read data.
        is.setReadListener(new AsyncReadListener(actx, is, os));
        
		println("After starting job");

	}


	public static void println(String output) {
		System.out.println("[" + Thread.currentThread().getName() + "]" + output);
	}
}

AsyncReadListener.java

package async;

import java.io.IOException;
import java.util.Queue;
import java.util.concurrent.LinkedBlockingQueue;

import javax.servlet.AsyncContext;
import javax.servlet.ReadListener;
import javax.servlet.ServletInputStream;
import javax.servlet.ServletOutputStream;

public class AsyncReadListener implements ReadListener {

    private final AsyncContext ac;
    private final ServletInputStream is;
    private final ServletOutputStream os;

    final Queue<String> queue = new LinkedBlockingQueue<String>();

    public AsyncReadListener(AsyncContext ac, ServletInputStream is, ServletOutputStream os) {
        this.ac = ac;
        this.is = is;
        this.os = os;
    }


    @Override
    public void onDataAvailable() throws IOException {
        println("AsyncReadListener: data available ");

        int len = -1;
        byte[] b = new byte[32];
        StringBuilder sb = new StringBuilder();

        // The ReadListener will be invoked again when
        // the input#isReady is changed from false to true
        while (is.isReady() && (len = is.read(b)) != -1) {
            String data = new String(b, 0, len);
            println("Reading data: "+data);
            sb.append(data);
        }
        queue.add(sb.toString());
    }

    @Override
    public void onAllDataRead() throws IOException {
        println("AsyncReadListener: All data read.. ");

        // now all data are read, set up a WriteListener to write
        os.setWriteListener(new AsyncWriteListener(ac, queue, os));
    }



    @Override
    public void onError(Throwable t) {
        println("AsyncReadListener onError() " + t);
        t.printStackTrace();
        ac.complete();
    }
    public static void println(String output) {
		System.out.println("[" + Thread.currentThread().getName() + "]" + output);
	}
}

AsyncWriteListener.java

package async;

import java.io.IOException;
import java.util.Queue;

import javax.servlet.AsyncContext;
import javax.servlet.ServletOutputStream;
import javax.servlet.WriteListener;

public class AsyncWriteListener implements WriteListener {
    
    private final AsyncContext ac;
    private final Queue<String> queue;
    private final ServletOutputStream os;
    private static final String header = "<html lang=\"en-US\" xmlns=\"http://www.w3.org/1999/xhtml\"><body>Hello";
    private static final String footer = "</body></html>";
    private boolean writeHeader;
    
    public AsyncWriteListener(AsyncContext ac, Queue<String> queue, ServletOutputStream os) {
        this.ac = ac;
        this.queue = queue;
        this.os = os;
        writeHeader = true;
    }
    
    @Override
    public void onWritePossible() throws IOException {
        println("AsyncWriteListener: onWritePossible.. ");
        if ( writeHeader ) {
            os.println(header);
            writeHeader = false;
        }
        
        while (queue.peek() != null && os.isReady()) {
            String data = queue.poll();
            os.print(data);
        }
        
        // complete the async process when there is no more data to write
        if (queue.peek() == null) {
            os.println(footer);
            ac.complete();
        }
    }
    
    @Override
    public void onError(Throwable t) {
        println("AsyncWriteListener onError() " + t);
        t.printStackTrace();
        ac.complete();
    }
    
    public static void println(String output) {
		System.out.println("[" + Thread.currentThread().getName() + "]" + output);
	}
}