How to fix: Java Network Error: Unable to resolve hostname

How to solve: Java network error: Unable to resolve host name

When using Java for network programming, you sometimes encounter a common error: Unable to resolve host name name. This error usually occurs when trying to connect to a non-existent host address or an unresolvable domain name. This article explains how to solve this problem and provides some code examples.

1. Check the network connection
First, we need to check whether the network connection is normal. You can use the ping command to verify whether the host address or domain name is reachable. Enter the following command on the command line:

ping www.example.com
If you can receive a reply, the network connection is normal. If you do not receive a reply, there may be a problem with your network environment. You can try to connect to other addresses to confirm whether it is a network problem.

2. Confirm whether the host address or domain name is correct
When an error that the host name cannot be resolved occurs, first make sure that the host address or domain name you entered is correct. You can verify it by entering the address in your browser. If it cannot be accessed, it means the entered address is incorrect.

3. Check the DNS settings
If the network connection is normal and the host address or domain name is correct, then it is most likely a DNS setting problem. DNS (Domain Name System) is used to convert domain names into IP addresses. Java uses the system's default DNS settings. We can use the following code to check and modify the DNS settings:

import java.net.InetAddress;
import java.net.UnknownHostException;

public class DNSExample {
    public static void main(String[] args) {
        try {
            // 获取系统默认的DNS设置
            InetAddress dns = InetAddress.getByName("www.example.com");
            System.out.println("System DNS: " + dns.getHostAddress());

            // 修改DNS设置
            InetAddress alternativeDNS = InetAddress.getByName("8.8.8.8");
            System.setProperty("sun.net.spi.nameservice.provider.1", "dns,sun");
            System.setProperty("sun.net.spi.nameservice.nameservers", alternativeDNS.getHostAddress());
            
            // 再次获取DNS设置并打印
            dns = InetAddress.getByName("www.example.com");
            System.out.println("Modified DNS: " + dns.getHostAddress());
        } catch (UnknownHostException e) {
            e.printStackTrace();
        }
    }
}

Run the above code, it will print the system's default DNS settings and modified DNS settings. If the modified DNS settings are able to resolve the hostname, then the problem is solved.

4. Use a proxy server
In some cases, the problem of being unable to resolve the host name may be because your network has a proxy server and your Java program does not have a proxy configured. You can configure Java's proxy server through the following code:

import java.net.InetSocketAddress;
import java.net.Proxy;
import java.net.URL;
import java.net.URLConnection;

public class ProxyExample {
    public static void main(String[] args) {
        try {
            // 配置代理服务器
            Proxy proxy = new Proxy(Proxy.Type.HTTP, new InetSocketAddress("proxy.example.com", 8080));
            URL url = new URL("http://www.example.com");
            
            // 打开连接
            URLConnection conn = url.openConnection(proxy);
            conn.connect();
            System.out.println("Connected!");
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

In the above code, we use a proxy server to establish a connection with the host. You need to replace the proxy server address and port number in the code with your own proxy server information.

Summary:
When a Java program encounters a network error that cannot resolve the host name, first check whether the network connection is normal and confirm whether the host address or domain name is correct. If there are no problems, you can try to check and modify DNS settings, or configure a proxy server to solve the problem. I hope the solutions and code examples provided in this article can help you solve this problem.

The above is the detailed content of How to fix: Java Network Error: Unable to resolve hostname. For more information, please follow other related articles on the PHP Chinese website!