为什么 ProxySelector 在 Java 中返回不一致的结果？

Question

8 months ago I used this stack overflow post to automatically use a proxy server in a Java project. It worked beautifully.

Here is the code I came up with at the time:

    System.setProperty("java.net.useSystemProxies", "true");

    List<Proxy> proxyServers = null;
    try {
        proxyServers = ProxySelector.getDefault().select(new URI("http://www.twitter.com"));
    } catch (URISyntaxException e) {
      System.out.println("Error using system proxy");
    }
    if (proxyServers != null) {
        for (Iterator<Proxy> iter = proxyServers.iterator(); iter.hasNext();) {
            Proxy proxy = iter.next();
            System.out.println("Found Proxy: " + proxy);
            InetSocketAddress addr = (InetSocketAddress) proxy.address();

            if (addr == null) {
              System.out.println("No Proxy");
            } else {

                System.setProperty("http.proxyHost", addr.getHostName());
                System.setProperty("http.proxyPort", Integer.toString(addr.getPort()));

                System.out.println("proxy hostname : " + addr.getHostName());
                System.out.println("proxy port : " + addr.getPort());

            }
        }
    }

I tried adding this exact same code to another project recently and I get different results. I have my computer setup to use a test proxy server and ran both projects to compare the results. If I run the project from 8 months ago the following is printed out:

Found Proxy: HTTP @ 192.168.1.100:8000
proxy hostname : 192.168.1.100
proxy port : 8000

If I run my current project on the same machine with the same Proxy server set up, the following is printed out.

Found Proxy: DIRECT 
No Proxy

The only proxy found is a "Direct" and proxy.address() is null.

What would cause this to find the proxy settings in one case but not the other?

Edit:

I fixed this, by moving the code sooner in the startup process, but I don't know why this fixed it.

Answer 1

Probably due to this sentence in http://docs.oracle.com/javase/7/docs/api/java/net/doc-files/net-properties.html#Proxies

java.net.useSystemProxies (default: false) On recent Windows systems and on Gnome 2.x systems it is possible to tell the java.net stack, setting this property to true, to use the system proxy settings (both these systems let you set proxies globally through their user interface). Note that this property is checked only once at startup.

You can guarantee it working if you put the property in the java command: java -Djava.net.useSystemProxies=true yourclass

Regards

Answer 2

If you will use the code along with secure page, it will not consider the proxy hence it would display Direct and No Proxy.

Try the below code:

proxyServers = ProxySelector.getDefault().select(new URI("http://www.twitter.com"));
proxyServers = ProxySelector.getDefault().select(new URI("https://www.twitter.com"));

you will get the difference.