Java检查HttpResponse是否仍然有效

Question

Is there a way from a java servlet to check if the httpresponse is still "alive?" For instance, in my situation I send an ajax request from the browser over to a servlet. In this case its a polling request so it may poll for up to 5 minutes, when the servlet is ready to respond with data i'd like to check if the user has closed the browser window, or moved to another page etc. In other words, check to see if sending the data to the response will actually do anything.

Answer 1

Java Servlet Response doesn't have any such method as it is based on request and response behavior. If you wish to check the status, then probably you need to work at lower level eg TCP/IP Sockets , which has several status check methods as below:

boolean isBound()
Returns the binding state of the socket.

boolean isClosed()
Returns the closed state of the socket.

boolean isConnected()
Returns the connection state of the socket.

boolean isInputShutdown()
Returns whether the read-half of the socket connection is closed.

boolean isOutputShutdown()
Returns whether the write-half of the socket connection is closed.

Answer 2

Generally, this problem can be solved by sending a dummy payload before the actual message.

If the socket was severed, an IOException or a SocketException or something similar is thrown (depending on the library). Technically, browsers are supposed to sever a connection whenever you navigate away from a page or close the browser (or anything similar), but I've found out that the implementation details can vary. Older versions of FF, for example, appropriately close a connection when navigating away from a page, but newer versions (especially when using AJAX) tend to leave connections open.

That's the main reason you may use a dummy packet before the actual message. Another important consideration is the timeout. I've done polling before and you either need to implement some sort of heartbeat to keep a connection alive or increase the server timeout (although keep in mind that some browsers may have timeouts as well - timeouts that you have no control over).

Instead of polling or pushing over AJAX, I strongly suggest trying to support (at least in part) a Websocket solution.