IOException Java

Question

I tried to run this piece of code without an internet connection, expecting and IOException to trigger:

import java.net.*;
import java.io.*;

public class API_connect {

    public static void main(String[] args) {
        try {
            URL API = new URL("http://api.football-data.org");
            URLConnection API_connection = API.openConnection();
        }
        catch(MalformedURLException exception) {
            System.out.print(exception);
        }
        catch(IOException exception) {
            System.out.print(exception);
            System.out.print("is something going on here?");
        }                   
    }

}

And well... To my surprise nothing was printed, and I can't figure out why. Wouldn't a lack of internet connection be the main reason why an IOException is thrown here?

When printing exceptions I generally prefer to use printStackTrace() — phflack
– phflack, Commented Nov 17, 2017 at 19:03

Roman Puchkovskiy · Accepted Answer · 2017-11-17 19:00:38Z

6

openConnection() does not actually try to connect:

It should be noted that a URLConnection instance does not establish the actual network connection on creation. This will happen only when calling URLConnection.connect().

Try calling connect() on it.

Alternatively, you could try the following:

new URL(...).openStream().read();

That would actually try to read 1 byte from that url and would fail.

answered Nov 17, 2017 at 19:00

Roman Puchkovskiy

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7 Comments

dscham Over a year ago

I would add, he opens a connection, but not getting and outputing anything from that connection. Meaning: It works and you need to get and write something from that connection. Sometimes nothing is the best that can happen. In your code. You can't get any output if it works correctly.

Roman Puchkovskiy Over a year ago

I think, the OP was expecting some exception on a connection attempt. Both connect() and read() would produce an exception.

Janilson Over a year ago

That did infact trigger the exception, thanks. Now I'm curious however as to why openConnection() would throw an IOException. If being offline doesn't trigger it, what would?

dscham Over a year ago

@RomanPuchkovskiy probably should make my own thread for this. But why does connect() and read() throw an exception?

Roman Puchkovskiy Over a year ago

openConnection() is defined in URL class that is not specific to only HTTP (where it may be costly to 'connect right away', but it can be used with file: protocol where 'a connection right away' makes sense. So I believe that the designers of openConnection() method allowed it throw an IOException to give it more flexibility (in regard with different implementations).

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