java.net.UnknownHostException解决方法

遇到这个问题是因为无法找到合适的DNS解析，有一下几种方法解决。

1. 在windows\system32\drivers\etc\hosts中添加一行

202.181.254.101       http://xxx.xxx.com

这种方法只是临时解决问题，可以让程序运行通过，但前提是你需要知道这个网址的ip地址。

 

2. 在程序中设置代理

System.setProperty("http.proxyHost", "myProxyServer.com");System.setProperty("http.proxyPort", "80");

 

Since Java 1.5 you can also pass a java.net.Proxy instance to the openConnection() method:

//Proxy instance, proxy ip = 123.0.0.1 with port 8080

Proxy proxy = new Proxy(Proxy.Type.HTTP, new InetSocketAddress("123.0.0.1", 8080));

URL url = new URL(http://www.yahoo.com);

HttpURLConnection uc = (HttpURLConnection)url.openConnection(proxy);

uc.connect();        

String page;StringBuffer tmp = new StringBuffer();

BufferedReader in = new BufferedReader(new InputStreamReader(uc.getInputStream()));

while ((line = in.readLine()) != null){   page.append(line + "\n");}System.out.println(page);

 

so you don't need to set system properties.

You can use the default PROXY as defined by your networking settings.

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

List l = null;

try {  l = ProxySelector.getDefault().select(new URI("http://www.yahoo.com"));}catch (URISyntaxException e) {  e.printStackTrace();}if (l != null) {   

	for (Iterator iter = l.iterator(); iter.hasNext() {      

	java.net.Proxy proxy = (java.net.Proxy) iter.next();      

	System.out.println("proxy hostname : " + proxy.type());      

	InetSocketAddress addr = (InetSocketAddress) proxy.address();      

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

	else {        

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

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

	}   

}

}

To bypass the PROXY,

URL url = new URL("http://internal.server.local/");URLConnection conn = url.openConnection(Proxy.NO_PROXY);

 

Proxy and Username/Password

You might need to identify yourself to the proxy server.

One way is to use the HTTP property "Proxy-Authorization" with a username:password base64 encoded.

System.setProperty("http.proxyHost", "myProxyServer.com");

System.setProperty("http.proxyPort", "80");

URL url=new URL(http://someserver/somepage);

URLConnection uc = url.openConnection ();

String encoded = new String      (Base64.base64Encode(new String("username:password").getBytes()));

uc.setRequestProperty("Proxy-Authorization", "Basic " + encoded);uc.connect();

NOTE: For a base64 function, see this How-to.

 

The following example dumps the content of a URL but before we identify ourself to the proxy.

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

public class URLUtils {  

	public static void main(String s[]) {    

		URLUtils.dump("http://www.yahoo.com");    

		System.out.println("**************");    

		URLUtils.dump("https://www.paypal.com");    

		System.out.println("**************");  

	}  

public static void dump(String URLName){    

try {      

	DataInputStream di = null;      

	FileOutputStream fo = null;      

	byte [] b = new byte[1];      // PROXY      

	System.setProperty("http.proxyHost","proxy.mydomain.local") ;      

	System.setProperty("http.proxyPort", "80") ;      

	URL u = new URL(URLName);      

	HttpURLConnection con = (HttpURLConnection) u.openConnection();      //      // it's not the greatest idea to use a sun.misc.* class      // Sun strongly advises not to use them since they can      // change or go away in a future release so beware.      //      

	sun.misc.BASE64Encoder encoder = new sun.misc.BASE64Encoder();      

	String encodedUserPwd =         encoder.encode("mydomain\\MYUSER:MYPASSWORD".getBytes());      

	con.setRequestProperty         ("Proxy-Authorization", "Basic " + encodedUserPwd);      // PROXY ----------      

	di = new DataInputStream(con.getInputStream());      

	while(-1 != di.read(b,0,1)) {         System.out.print(new String(b));      }    

}    catch (Exception e) {      e.printStackTrace();    }  

}

}

With JDK1.2, the java.net.Authenticator can be used to send the credentials when needed.

public static void dump(String URLName){  try {    DataInputStream di = null;    FileOutputStream fo = null;    byte [] b = new byte[1];     // PROXY    System.setProperty("http.proxyHost","proxy.mydomain.local") ;    System.setProperty("http.proxyPort", "80") ;     Authenticator.setDefault(new Authenticator() {      protected PasswordAuthentication getPasswordAuthentication() {        return new           PasswordAuthentication("mydomain\\username","password".toCharArray());    }});     URL u = new URL(URLName);    HttpURLConnection con = (HttpURLConnection) u.openConnection();    di = new DataInputStream(con.getInputStream());    while(-1 != di.read(b,0,1)) {       System.out.print(new String(b));    }  }  catch (Exception e) {          e.printStackTrace();  }}

Bypass a Proxy

In intranet environment, you may need to bypass the proxy server and go directly to the http server.

The http.nonProxyHosts property indicates the hosts which should be connected too directly and not through the proxy server. The value can be a list of hosts, each seperated by a |, and in addition a wildcard character (*) can be used for matching.

java.exe     -Dhttp.nonProxyHosts="*.mycompany.com|*.mycompany.local|localhost"       MyClass