为什么我可以在没有Eventlistener的情况下使用Java中的BufferedReader进行读取

Question

I am comfortable with actionscript. When I wantet to have network access for example in air, I coded the request and waited for the EventListener to kick in.

At the moment I'm doing a java tutorial with the following android code (wrapped in a try/catch):

url = new URL("http://www.google.com");
URLConnection conn = url.openConnection();
BufferedReader reader = new BufferedReader(new InputStreamReader(conn.getInputStream());
String line = '';
while((line = reader.readLine()) != null) {
     // do something with the line
}

I don't get how this works.

Why is there no eventListener needed? If the code proceeds immidiately after the URL is requested, the URL may not been retrieved when the while-Construct kicks in.

Is the app just paused in the meantime? If yes, is it possible to realise this with an eventlistner?

Thank you for help!

Answer 1

What happens is that the BufferedReader will request its underlying InputStream for data every time it is requested to provide data (when you call reader.readLine() or any other readXXX method on the BufferedReader ). It's thus the InputStream 's responsibility to provide it at that time - which might be possible or not (if the connection was closed, for instance).

In this particular case, the InputStream provided by the URLConnection will either return data that was cached in a previous read request or request more data from the URL it's pointing to.

As such, no actual data is read until you invoke readLine on the BufferedReader . You've just opened a connection to the server.

Answer 2

Is the app just paused in the meantime?

Yes. The regular Java IO classes are synchronous. The call to readLine() will block until either it can return a line or there's an Exception.

If yes, is it possible to realise this with an eventlistner?

Yes, by using classes in the java.nio package . But only with a somewhat lower-level interface.

In general, Java applications use multithreading instead of event qeueues to deal with such sitatuations.