java ignores proxy settings

问题

I have set up a local proxy server for request logging but my java code ignores it and connects directly (Windows XP, JDK 1.7). Web browsers work with it. So I wrote test code for discussion that seems to connect directly even if a (bogus) proxy is specified. With the bogus proxy, I would expect connection failure but the code succeeds, connecting directly:

System.setProperty("http.proxyHost", "localhost");
System.setProperty("http.proxyPort", "12345");
System.setProperty("http.nonProxyHosts", "noNonProxyHost.com");
URL url = new URL("http://docs.oracle.com/javase/7/docs/technotes/guides/net/proxies.html");
InputStream in = url.openStream();
System.out.println("Connection via bogus proxy succeeded");

The code is run as standalone Java, no Maven, no applet, no container. I have a direct internet connection.

回答1:

In your case using java.net.URL(), if the proxy server cannot be reached at http.proxyHost and http.proxyPort then it simply falls back and tries to do a direct connect. If that succeeds, you'll see no exception thrown which is why your code works without error. You should see a pause while it tries to find the proxy though.
This sample code below happily fetches the URL and displays it, without error, even when run with bogus proxy settings. -Dhttp.proxyHost=bogus -Dhttp.proxyPort=2345 but will talk to my local proxy localhost port 8888 if set correctly

import java.io.*;
import java.net.URL;
import java.util.*;
public class URLClient {
  private static String sUrl = "http://www.apache.org/";
  public static void main(String[] args) {
    try {
    URL url = new URL(sUrl);
    InputStream is = url.openStream();
    java.util.Scanner s = new java.util.Scanner(is).useDelimiter("\\A");
    String output =  s.hasNext() ? s.next() : "";
    System.out.println(output);
    } catch(Throwable e) {
            System.err.println("exception");
    }
  }
}

The problem I was originally having with http.proxyHost and http.proxyPort being ignored (Google led me to your question) was that those settings are completely ignored by apache.commons.httpClient because it uses its own sockets, as described here. http://cephas.net/blog/2007/11/14/java-commons-http-client-and-http-proxies/

回答2:

I have faced a similar problem recently. First of all, one part of the above answer from Daemon42 explains pretty well, why the bogus proxy server didn't lead to a failure of the program:

if the proxy server cannot be reached at http.proxyHost and http.proxyPort then it simply falls back and tries to do a direct connect. If that succeeds, you'll see no exception thrown which is why your code works without error. You should see a pause while it tries to find the proxy though.

Still, your actual question was, why the proxy server configured via the operating system is not used by the Java application. As stated in the Oracle documentation (https://docs.oracle.com/javase/8/docs/technotes/guides/net/proxies.html), the system proxy settings are not evaluated by Java by default. To do so, you have to set the value of the system property "java.net.useSystemProxies" to the value "true".

You can set that system property on the command line, or you can edit the JRE installation file jre/lib/net.properties, that way you have to change it only once on a given system.

来源：https://stackoverflow.com/questions/28004151/java-ignores-proxy-settings

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java

proxy