AsyncContext异步输出报错：java.lang.NullPointerException

@WebServlet(urlPatterns="/asyncPrint",asyncSupported = true)

public class LoginServlet extends HttpServlet 

{

	private static final long serialVersionUID = -1428172087125899118L;

	protected void doPost(HttpServletRequest req, HttpServletResponse resp)

			throws ServletException, IOException 

	{

		resp.setContentType("text/html;charset=UTF-8");

		AsyncContext async = req.startAsync();

		new CounterThread(async).start();

	}

}

class CounterThread extends Thread{

	

	private AsyncContext asyncContext;

    public CounterThread(AsyncContext asyncContext){

       this.asyncContext = asyncContext;

    }

    public void run() {

    	if(asyncContext == null){

    		System.out.println(asyncContext);

    		return;

    	}

    	try

    	{

    		PrintWriter writer= asyncContext.getResponse().getWriter();

    		for( int i=0;i<50;i++)

    		{

    			int rand = new Random().nextInt(10)*1000;

        		printPage(writer,"this print is " + rand);

        		Thread.sleep(rand);

    		}

    		writer.close();

    	}catch(Exception e){

    		e.printStackTrace();

    	}

    }

    private void printPage(PrintWriter writer,String str)

    {

    	writer.write("<h1>" + str + "</h1>");  

    	writer.flush();

    }

}

每次调用，都报错：
java.lang.NullPointerException
at org.apache.coyote.http11.InternalOutputBuffer.realWriteBytes(InternalOutputBuffer.java:215)
at org.apache.tomcat.util.buf.ByteChunk.flushBuffer(ByteChunk.java:480)
at org.apache.coyote.http11.InternalOutputBuffer.flush(InternalOutputBuffer.java:119)
at org.apache.coyote.http11.AbstractHttp11Processor.action(AbstractHttp11Processor.java:799)
at org.apache.coyote.Response.action(Response.java:174)
at org.apache.catalina.connector.OutputBuffer.doFlush(OutputBuffer.java:366)
at org.apache.catalina.connector.OutputBuffer.flush(OutputBuffer.java:333)
at org.apache.catalina.connector.CoyoteWriter.flush(CoyoteWriter.java:98)
at cn.cloud.action.CounterThread.printPage(LoginServlet.java:69)
at cn.cloud.action.CounterThread.run(LoginServlet.java:53)
五月 02, 2014 10:03:16 上午 org.apache.catalina.core.StandardContext reload
信息: Reloading Context with name [/jshop] has started
五月 02, 2014 10:03:16&