Ajax preguntas y respuestas de entrevista
Pregunta 56. Does Java have support for Comet style server-side push?
Current AJAX applications use polling to communicate changes data between the server and client. Some applications, such as chat applications, stock tickers, or score boards require more immediate notifications of updates to the client. Comet is an event based low latency server side push for AJAX applications. Comet communication keeps one of the two connections available to the browser open to continously communicate events from the server to the client. A Java based solution for Comet is being developed for Glassfish on top of the Grizzly HTTP connector. See Enabling Grizzly by Jean-Francois Arcand for more details.
Pregunta 57. How do I create a thread to do AJAX polling?
JavaScript does not have threads. JavaScript functions are called when an event happens in a page such as the page is loaded, a mouse click, or a form element gains focus. You can create a timer using the setTimeout which takes a function name and time in milliseconds as arguments. You can then loop by calling the same function as can be seen in the JavaScript example below.
function checkForMessage() {
// start AJAX interaction with processCallback as the callback function
}
// callback for the request
function processCallback() {
// do post processing
setTimeout("checkForMessage()", 10000);
}
Notice that the checkForMessage will continue to loop indefinitely. You may want to vary the increment the interval based on activity in the page or your use cases. You may also choose to have logic that would break out of the loop based on some AJAX response processing condition.
Pregunta 58. Is the XMLHttpRequest object part of a W3C standard?
No. Or not yet. It is part of the DOM Level 3 Load and Save Specification proposal.
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