最近项目遇到需要从http请求下载文件到服务器，下载前需要判断下http中的文件是否存在。如果判断本地服务器上文件是否存在，用file.exists来判断。但是这个方法却无法判断http中文件是否存在。

        如果要判断http文件是否存在，用如下代码：

import java.io.IOException;
import java.net.HttpURLConnection;
import java.net.URL;
 
public class Main {
    public static void main(String[] args) {
        String urlString = "http://example.com/file.txt";
        try {
            URL url = new URL(urlString);
            HttpURLConnection connection = (HttpURLConnection) url.openConnection();
            connection.setRequestMethod("HEAD");
            int responseCode = connection.getResponseCode();
            if (responseCode == HttpURLConnection.HTTP_OK) {
                System.out.println("文件存在");
            } else {
                System.out.println("文件不存在");
            }
        } catch (IOException e) {
            System.out.println("连接失败");
        }
    }
}

如果对方系统需要身份验证，那么需要加如下代码。我们系统是需要token验证。加权限验证的代码如下：

import java.io.IOException;
import java.net.HttpURLConnection;
import java.net.URL;
 
public class Main {
    public static void main(String[] args) {
        String urlString = "http://example.com/file.txt";
        try {
            URL url = new URL(urlString);
            HttpURLConnection connection = (HttpURLConnection) url.openConnection();
            
            //权限校验
            connection.setRequestProperty("X-JFrog-Art-Api","cmv125VmedaeDAFdafLFAF2ed");

            //HEAD请求，不返回响应体，但是有些服务器可能不支持,则改成GET请求
            connection.setRequestMethod("HEAD");
            int responseCode = connection.getResponseCode();
            if (responseCode == HttpURLConnection.HTTP_OK) {
                System.out.println("文件存在");
            } else {
                System.out.println("文件不存在");
            }
        } catch (IOException e) {
            System.out.println("连接失败");
        }
    }
}

判断http中文件是否存在可以利用java.nio的方法，代码如下：

import java.io.IOException;
import java.net.URL;
import java.nio.file.Files;
import java.nio.file.Path;
import java.nio.file.Paths;
 
public class Main {
    public static void main(String[] args) {
        String urlString = "http://example.com/file.txt";
        try {
            URL url = new URL(urlString);
            Path path = Paths.get(url.toURI());
            if (Files.exists(path)) {
                System.out.println("文件存在");
            } else {
                System.out.println("文件不存在");
            }
        } catch (IOException e) {
            System.out.println("连接失败");
        }
    }
}