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        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");}}
}