How does java.net.URL open a connection? Does it use a socket?

I'm using the following code to get a JSON string from a URL:

public static String getStringFromURL(String addr) throws IOException {
    ByteArrayOutputStream output = new ByteArrayOutputStream();
    URL url = new URL(addr);
    org.apache.commons.io.IOUtils.copy(url.openStream(), output);
    return output.toString();
}

I want to make sure this doesn't hang if the page at "addr" fails for any reason. I don't want it to bring our server down or anything. We started looking into how java.net.URL opens the connection and couldn't tell much from the Javadoc (we are 开发者_如何学Pythonusing 1.5). Any thoughts or inside knowledge would be appreciated. If you can cite sources, so much the better. Thanks!

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Technically, that depends on the protocol. For HTTP, it uses TCP/IP sockets. The openStream() will throw an exception if an I/O error occurs. Just put it in a try/catch. However, if the server returns for example a HTTP 404 (not found) or 500 (internal error), you will get this plain into the string unawarely. You may want to use HttpURLConnection instead for more fine-grained control.

HttpURLConnection connection = (HttpURLConnection) url.openConnection();

if (connection.getResponseStatus() == 200) {
    // All OK, convert connection.getInputStream() to string.
    // Don't forget to take character encoding into account!
} else {
    // Possible server error. Throw exception yourself? Or return some default?
}

Further you can set the timeout URLConnection#setConnectTimeout(). I believe, it defaults to 3 seconds or something. You may want to tweak it to make it all faster. Set with 1000 for 1 second.

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Yes, it will hang.

There are two timeouts to consider:

• The connection timeout: The server may neither accept (ACK) nor reject (RST) the connection because it is firewalled. This is rather short and can be set using setConnectTimeout();
• The timeout the connection waits for data. This one is rather long (5 minutes) and is the usual thing that fails, for example if the webapplication is waiting for a database connection from a pool. It can be set using setReadTimeout()