用java编写一个上传下载代码示例

### 回答1：
下面是一个用 Java 编写的上传文件的示例代码：

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

public class FileUpload {
  public static void main(String[] args) throws Exception {
    String fileName = "test.txt";
    String urlString = "http://www.example.com/upload";

    // Create a URL object
    URL url = new URL(urlString);

    // Open a connection to the URL
    HttpURLConnection connection = (HttpURLConnection) url.openConnection();

    // Set the request method to "POST"
    connection.setRequestMethod("POST");

    // Set the request properties
    connection.setRequestProperty("Content-Type", "multipart/form-data");
    connection.setRequestProperty("Content-Length", String.valueOf(new File(fileName).length()));

    // Enable output
    connection.setDoOutput(true);

    // Open a output stream to the server
    DataOutputStream outputStream = new DataOutputStream(connection.getOutputStream());

    // Open an input stream to the file
    FileInputStream inputStream = new FileInputStream(fileName);

    // Read the contents of the file into a byte array
    byte[] buffer = new byte[1024];
    int bytesRead;
    while ((bytesRead = inputStream.read(buffer)) != -1) {
      // Write the bytes to the output stream
      outputStream.write(buffer, 0, bytesRead);
    }

    // Close the input and output streams
    inputStream.close();
    outputStream.close();

    // Get the response code from the server
    int responseCode = connection.getResponseCode();
    System.out.println("Response code: " + responseCode);

    // Read the response from the server
    BufferedReader reader = new BufferedReader(new InputStreamReader(connection.getInputStream()));
    String line;
    while ((line = reader.readLine()) != null) {
      System.out.println(line);
    }
    reader.close();
  }
}

下面是一个用 Java 编写的下载文件的示例代码：

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

public class FileDownload {
  public static void main(String[] args) throws Exception {
    String fileUrl = "http://www.example.com/download/test.txt";
    String fileName = "test.txt";

    // Open a connection to the URL
    URL url = new URL(fileUrl);
    HttpURLConnection connection = (HttpURLConnection) url.openConnection();

    // Set the request method to "GET"
    connection.setRequestMethod("GET");

    // Open an input stream to the file
    InputStream inputStream = connection.getInputStream();

    // Create a file output stream
    FileOutputStream outputStream = new FileOutputStream(fileName);

    // Read the contents of the file into a byte array
    byte[] buffer = new byte[1024];
    int  

### 回答2：
下面是一个使用Java编写的简单的上传和下载代码示例：

上传代码示例：

import java.io.File;
import java.io.FileInputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import java.net.HttpURLConnection;
import java.net.URL;

public class UploadExample {
public static void main(String[] args) {
String fileUrl = "路径/文件名"; // 文件路径和名称
String uploadUrl = "上传目标URL"; // 上传目标的URL

try {
File file = new File(fileUrl);
URL url = new URL(uploadUrl);
HttpURLConnection conn = (HttpURLConnection) url.openConnection();
conn.setDoOutput(true);
conn.setDoInput(true);
conn.setRequestMethod("POST");
conn.setRequestProperty("Content-Type", "multipart/form-data;boundary=" + "*****");

OutputStream outputStream = conn.getOutputStream();

FileInputStream fileInputStream = new FileInputStream(file);

byte[] buffer = new byte[1024];
int bytesRead;

while ((bytesRead = fileInputStream.read(buffer)) != -1) {
outputStream.write(buffer, 0, bytesRead);
}

fileInputStream.close();
outputStream.flush();
outputStream.close();

InputStream inputStream = conn.getInputStream();

// 处理上传结果

inputStream.close();
conn.disconnect();
System.out.println("文件上传成功！");
} catch (IOException e) {
e.printStackTrace();
}
}
}

下载代码示例：

import java.io.FileOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import java.net.HttpURLConnection;
import java.net.URL;

public class DownloadExample {
public static void main(String[] args) {
String downloadUrl = "下载文件URL"; // 下载文件的URL
String savePath = "保存路径/文件名"; // 下载文件保存路径和名称

try {
URL url = new URL(downloadUrl);
HttpURLConnection conn = (HttpURLConnection) url.openConnection();
conn.setRequestMethod("GET");

InputStream inputStream = conn.getInputStream();
byte[] buffer = new byte[1024];

OutputStream outputStream = new FileOutputStream(savePath);

int bytesRead;

while ((bytesRead = inputStream.read(buffer)) != -1) {
outputStream.write(buffer, 0, bytesRead);
}

inputStream.close();
outputStream.flush();
outputStream.close();
conn.disconnect();

System.out.println("文件下载成功！");
} catch (IOException e) {
e.printStackTrace();
}
}
}

以上示例代码可以实现简单的上传和下载功能，具体使用时，需要修改路径、URL等相关参数来适应实际的应用场景。  

### 回答3：
以下是一个使用Java编写的上传下载代码示例：

1.上传文件：

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

public class FileUploader {
public static void main(String[] args) {
try {
File file = new File("path/to/file"); // 要上传的文件路径
String url = "http://localhost/upload.php"; // 上传文件的URL

HttpURLConnection connection = (HttpURLConnection) new URL(url).openConnection();
connection.setDoOutput(true);
connection.setRequestMethod("POST");

OutputStream outputStream = connection.getOutputStream();
FileInputStream fileInputStream = new FileInputStream(file);
byte[] buffer = new byte[4096];
int bytesRead;
while ((bytesRead = fileInputStream.read(buffer)) != -1) {
outputStream.write(buffer, 0, bytesRead);
}

outputStream.close();
fileInputStream.close();

int responseCode = connection.getResponseCode();
if (responseCode == HttpURLConnection.HTTP_OK) {
// 文件上传成功
System.out.println("文件上传成功");
} else {
// 文件上传失败
System.out.println("文件上传失败");
}
} catch (IOException e) {
e.printStackTrace();
}
}
}

2.下载文件：

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

public class FileDownloader {
public static void main(String[] args) {
try {
String fileUrl = "http://localhost/files/sample.txt"; // 要下载的文件URL
String savePath = "path/to/save"; // 保存文件的路径

HttpURLConnection connection = (HttpURLConnection) new URL(fileUrl).openConnection();
int responseCode = connection.getResponseCode();
if (responseCode == HttpURLConnection.HTTP_OK) {
InputStream inputStream = connection.getInputStream();
FileOutputStream fileOutputStream = new FileOutputStream(savePath);

byte[] buffer = new byte[4096];
int bytesRead;
while ((bytesRead = inputStream.read(buffer)) != -1) {
fileOutputStream.write(buffer, 0, bytesRead);
}

fileOutputStream.close();
inputStream.close();

// 文件下载成功
System.out.println("文件下载成功");
} else {
// 文件下载失败
System.out.println("文件下载失败");
}
} catch (IOException e) {
e.printStackTrace();
}
}
}

注意：上述示例中的URL和文件路径需要根据实际情况进行修改。上传文件示例中的`upload.php`可以是后端处理上传文件的接口，也可以是其他符合需求的URL。下载文件示例中的`sample.txt`是要下载的文件名，也可以是其他文件名。