I have a simple echo servlet which uses the Servlet 3.1 Non-Blocking IO API.
It creates a ReadListener, that reads the request's input, buffers it, creates
a WriteListener that takes the buffered input and echoes it back to the
response.
Most of the time this works OK, but when I send a request with no input data I
get the following error.
6-Aug-2013 11:18:09.523 INFO [main] org.apache.catalina.startup.Catalina.start
Server startup in 1452 ms
java.lang.IllegalStateException: The non-blocking write listener has already
been set
at org.apache.coyote.Response.setWriteListener(Response.java:583)
at
org.apache.catalina.connector.OutputBuffer.setWriteListener(OutputBuffer.java:665)
at
org.apache.catalina.connector.CoyoteOutputStream.setWriteListener(CoyoteOutputStream.java:162)
at
com.pivotal.demos.nbio.EchoNbioServlet$1.onAllDataRead(EchoNbioServlet.java:77)
at
org.apache.catalina.connector.CoyoteAdapter.asyncDispatch(CoyoteAdapter.java:384)
at
org.apache.coyote.http11.AbstractHttp11Processor.asyncDispatch(AbstractHttp11Processor.java:1607)
at
org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:622)
at
org.apache.coyote.http11.Http11NioProtocol$Http11ConnectionHandler.process(Http11NioProtocol.java:223)
at
org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1592)
at
org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.run(NioEndpoint.java:1550)
at
java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1145)
at
java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:615)
at java.lang.Thread.run(Thread.java:724)
16-Aug-2013 11:20:13.205 SEVERE [http-nio-8080-exec-7]
org.apache.catalina.connector.CoyoteAdapter.asyncDispatch Exception while
processing an asynchronous request
java.lang.NullPointerException
at
org.apache.catalina.connector.CoyoteAdapter.asyncDispatch(CoyoteAdapter.java:429)
at
org.apache.coyote.http11.AbstractHttp11Processor.asyncDispatch(AbstractHttp11Processor.java:1607)
at
org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:622)
at
org.apache.coyote.http11.Http11NioProtocol$Http11ConnectionHandler.process(Http11NioProtocol.java:223)
at
org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1592)
at
org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.run(NioEndpoint.java:1550)
at
java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1145)
at
java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:615)
at java.lang.Thread.run(Thread.java:724)
Looking into it, what appears to be happening is that the
"ReadListener.onAllDataRead()" method is being called twice. Because I'm
setting the WriteListener in the "onAllDataRead" method, the second invocation
of "onAllDataRead" causes the IllegalStateException. I was assuming that
"onAllDataRead" should only be called once and so it would be OK to set the
WriteListener in that method. Can "onAllDataRead" be legitimately called
multiple times?
Thanks
Dan
Code below…
Servlet
----------
protected void doPost(HttpServletRequest req, HttpServletResponse resp)
throws ServletException, IOException {
// 1. Start Async
final AsyncContext asyncContext = req.startAsync();
final ServletInputStream servletInputStream =
asyncContext.getRequest().getInputStream();
// 2. Add Read Listener to get user's input
ReadListener listener = new ReadListener() {
private StringBuilder sb = new StringBuilder(); //
buffer user's request
public void onDataAvailable() throws IOException {
// 4. Read all the data that is available, may
be called multiple times
try {
byte[] b = new byte[8192];
int read = 0;
do {
read = servletInputStream.read(b);
if (read == -1) {
break;
}
sb.append(new String(b, 0, read));
System.out.println("Buffer is now [" +
sb.length() + "] characters");
} while (servletInputStream.isReady());
} catch (Exception ex) {
ex.printStackTrace(System.err);
asyncContext.complete();
}
}
// 5. Called when all data has been read
public void onAllDataRead() throws IOException {
final ServletOutputStream outputStream =
asyncContext.getResponse().getOutputStream();
final String output = sb.toString();
// 6. Configure a write listener to echo the
response
WriteListener listener = new WriteListener() {
public void onWritePossible() throws
IOException {
// 7. Write output
if (outputStream.isReady()) {
outputStream.print(output);
}
// 8. Call complete, to signal
we are done
asyncContext.complete();
}
public void onError(Throwable
throwable) {
throwable.printStackTrace(System.err);
asyncContext.complete();
}
};
outputStream.setWriteListener(listener);
}
public void onError(Throwable throwable) {
throwable.printStackTrace(System.err);
asyncContext.complete();
}
};
// 3. Add listener, starts Non-blocking IO support
servletInputStream.setReadListener(listener);
}
Client
--------
public static void main(String[] args) throws Exception {
URL url = new
URL("http://localhost:8080/tomcat-8-demos/non-blocking-io/EchoNbioServlet";);
HttpURLConnection connection =
(HttpURLConnection) url.openConnection();
connection.setDoOutput(true);
connection.setReadTimeout(1000000);
connection.connect();
OutputStream os = null;
try {
os = connection.getOutputStream();
// do nothing
} finally {
if (os != null) {
try {
os.close();
} catch (IOException ioe) {
// Ignore
}
}
}
int rc = connection.getResponseCode();
InputStream is;
if (rc < 400) {
is = connection.getInputStream();
} else {
is = connection.getErrorStream();
}
StringBuilder sb = new StringBuilder();
BufferedInputStream bis = null;
try {
bis = new BufferedInputStream(is);
byte[] buf = new byte[2048];
int rd = 0;
while((rd = bis.read(buf)) > 0) {
sb.append(new String(buf, "utf-8"));
}
} finally {
if (bis != null) {
try {
bis.close();
} catch (IOException e) {
// Ignore
}
}
}
System.out.println("Resp: [" + sb.toString() + "]");
}
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