如何故意产生ConnectException？

Question

I am using java HttpURLConnection class to connect to a url, and I want to produce ConnectException to monitor and record some data. i am just trying to connect random websites but I couldn't be able to produce ConnectionException even though when I am trying to visit the url which doesn't exist but it is catching MalformedURLException

from Java Docs of ConnectException ,

http://docs.oracle.com/javase/7/docs/api/java/net/ConnectException.html#ConnectException()

Signals that an error occurred while attempting to connect a socket to a remote address and port. Typically, the connection was refused remotely (eg, no process is listening on the remote address/port).

It means that the connection needed to be refused from the remote server, but I don't know what does that mean and how to produce it from my java code ?

Answer 1

When you try to connect to an URL that does not exist, two kinds of error can occur:

1. The DNS entry for the domain itself does not exist. Think www.argh-nonexistant.com vs. www.google.com
2. The domain exists, and a webserver is listening, but the path does not exist. ( http://www.google.de/please-give-my-an-error/ )

In both cases, no ConnectException will be thrown, because this exception requires that

1. The DNS entry for the domain is valid and name resolution succeeds, and
2. no websever is running on the given port.

The ConnectException and the socket basically reside on layers 3 or 4 of the ISO/OSI stack (ie, TCP/IP), whereas your malformed URL is an application layer problem.

Your solution: Find a server that has a valid DNS entry, but no listening webserver. ::1 should be a safe bet, unless you host a webserver on port 80 yourself. Here's a minimal working example:

package Foo;

import java.io.IOException;
import java.net.HttpURLConnection;
import java.net.URL;

public class TestClass {
    public static void main(String[] args) throws IOException {
        URL url = new URL("http://localhost");
        HttpURLConnection c = (HttpURLConnection)url.openConnection();
        c.connect();
    }
}