How to Resolve UnknownHostException in Java When Host Resolves with Ping, nslookup, or curl

What should I do if I encounter an UnknownHostException in Java, despite being able to resolve the host using Ping, nslookup, or curl?

The UnknownHostException in Java typically arises when the Java application cannot resolve a host's IP address. This can be puzzling, especially when other tools like Ping, nslookup, or curl are able to resolve the domain name successfully. This discrepancy can be attributed to various factors, including DNS configuration, Java's network settings, or environmental issues.

try {
    InetAddress address = InetAddress.getByName("www.example.com");
    System.out.println("Host Address: " + address.getHostAddress());
} catch (UnknownHostException e) {
    System.err.println("UnknownHostException: " + e.getMessage());
}

Causes

• Mismatch between Java's DNS resolver and system resolver settings.
• Firewall settings or security software blocking Java's DNS requests.
• Java's security policies that restrict network access.
• Using an outdated or incompatible version of the Java Runtime Environment.

Solutions

• Verify that the Java application is using the same DNS resolver as the operating system.
• Check for any firewall rules or security software settings that might be preventing Java from accessing the network.
• Update the Java Runtime Environment to the latest version.
• Modify the Java application's security policy to ensure it has permissions for network access.

Mistake: Ignoring proxy settings in the Java application.

Solution: Check if a proxy is required for network access and configure the Java application accordingly.

Mistake: Hardcoding IP addresses instead of using domain names.

Solution: Always use domain names to reduce the likelihood of encountering resolution issues.

Mistake: Assuming firewall rules are not affecting Java applications.

Solution: Test Java applications with the firewall disabled to confirm if it’s causing the issue.