How to fix: Java Network Error: Connection refused

How to solve: Java Network Error: Connection refused

In Java programming, we often encounter the connection refused network error. This error usually occurs when trying to establish a connection with a remote server. Connection refused may be caused by a variety of reasons, including network problems, firewall settings, port being occupied, etc. This article explains how to solve this problem and provides some code examples to help you understand.

1. Check network connection

First, you need to make sure your computer is properly connected to the network. You can use the ping command to test network connectivity to a remote server. If the ping command returns success (no packet loss), it means your network connection is normal.

2. Check firewall settings

A firewall is a network security measure used to protect computers from unauthorized access. However, sometimes firewalls prevent our Java application from establishing a connection with the remote server. Therefore, you need to ensure that the firewall settings allow your Java application to communicate through specific ports.

The following is an example of using Java code to open a port:

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

public class OpenPortExample {
    public static void main(String[] args) {
        try {
            int port = 8080; // 要打开的端口号
            ServerSocket serverSocket = new ServerSocket(port);
            System.out.println("端口 " + port + " 已打开");
            
            // 这里可以添加更多的处理逻辑，比如监听客户端请求等
            
            serverSocket.close();
        } catch (IOException e) {
            System.out.println("无法打开端口：" + e.getMessage());
        }
    }
}

In this example, we use the ServerSocket class to open the specified port. If the port is successfully opened, a corresponding message will be output. If the port cannot be opened, an error message will be output.

3. Check whether the port is occupied

If the port is already occupied by another program, your Java application cannot use the port. You can use command line tools (such as netstat) or other network tools to view currently running programs and connected ports.

In Windows systems, you can use the command netstat -ano to list all listening ports and their corresponding PIDs (process IDs). You can then use Task Manager or the command line command taskkill /F /PID <PID> to shut down the program occupying the port.

In Linux systems, you can use the command sudo netstat -tunlp to view all listening ports and their corresponding programs. Then, you can use the command sudo kill -9 <PID> to shut down the program occupying the port.

Once the port is released, your Java application can use it.

4. Check proxy settings

If you are using a proxy server for network connections, the connection refused problem may be due to incorrect proxy server settings. You need to make sure that your proxy server is configured correctly and that your Java application is correctly using the proxy server for network connections.

You can configure the proxy by setting the system properties under java.net as follows:

System.setProperty("http.proxyHost", "your_proxy_host");
System.setProperty("http.proxyPort", "your_proxy_port");

The above code will direct the HTTP request to the specified proxy server .

5. Sending request interval

Some network servers limit the connection frequency of each client. If your Java application sends connection requests too frequently, the server may reject your connection. In this case, you might consider adding a delay to reduce the frequency of connection requests. For example, use Thread.sleep(1000) to wait 1 second between each connection request.

Summary:

Connection refused is one of the common network errors in Java programming. You can resolve this issue by checking your network connection, firewall settings, port occupancy, proxy settings, and request interval. I hope the solutions and code examples provided in this article can help you successfully complete your Java network programming tasks.

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