Java服务器小程序如何处理文件上传

在Java服务器端处理文件上传，通常会使用Servlet API或者一些流行的第三方库，如Apache Commons FileUpload。以下是使用Servlet API和Apache Commons FileUpload库处理文件上传的基本步骤：

使用Servlet API处理文件上传

1. 添加依赖：确保你的项目中包含了Servlet API的依赖。

2. 配置web.xml：在web.xml中配置一个multipart-config元素，指定上传文件的大小限制等信息。

<servlet>
    <servlet-name>FileUploadServlet</servlet-name>
    <servlet-class>com.example.FileUploadServlet</servlet-class>
    <multipart-config>
        <max-file-size>20848820</max-file-size>
        <max-request-size>418018841</max-request-size>
        <file-size-threshold>1048576</file-size-threshold>
    </multipart-config>
</servlet>

3. 编写Servlet：创建一个继承自HttpServlet的类，并重写doPost方法来处理文件上传。

import javax.servlet.ServletException;
import javax.servlet.annotation.MultipartConfig;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.Part;
import java.io.IOException;
import java.io.InputStream;
import java.nio.file.Files;
import java.nio.file.Paths;
import java.nio.file.StandardCopyOption;

@MultipartConfig
public class FileUploadServlet extends HttpServlet {
    @Override
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        for (Part part : request.getParts()) {
            String fileName = Paths.get(part.getSubmittedFileName()).getFileName().toString();
            InputStream fileContent = part.getInputStream();
            Files.copy(fileContent, Paths.get("uploadDir", fileName), StandardCopyOption.REPLACE_EXISTING);
        }
        response.getWriter().println("File uploaded successfully!");
    }
}

使用Apache Commons FileUpload库处理文件上传

1. 添加依赖：在你的项目中添加Apache Commons FileUpload和Apache Commons IO的依赖。

<dependency>
    <groupId>commons-fileupload</groupId>
    <artifactId>commons-fileupload</artifactId>
    <version>1.4</version>
</dependency>
<dependency>
    <groupId>commons-io</groupId>
    <artifactId>commons-io</artifactId>
    <version>2.8.0</version>
</dependency>

2. 编写Servlet：创建一个Servlet来处理文件上传请求。

import org.apache.commons.fileupload.FileItem;
import org.apache.commons.fileupload.disk.DiskFileItemFactory;
import org.apache.commons.fileupload.servlet.ServletFileUpload;

import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.File;
import java.io.IOException;
import java.util.List;

@WebServlet("/upload")
public class FileUploadServlet extends HttpServlet {
    private static final long serialVersionUID = 1L;
    private static final String UPLOAD_DIRECTORY = "uploadDir";

    @Override
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        if (ServletFileUpload.isMultipartContent(request)) {
            try {
                List<FileItem> multiparts = new ServletFileUpload(new DiskFileItemFactory()).parseRequest(request);
                for (FileItem item : multiparts) {
                    if (!item.isFormField()) {
                        String name = new File(item.getName()).getName();
                        item.write(new File(UPLOAD_DIRECTORY + File.separator + name));
                    }
                }
                response.getWriter().println("File uploaded successfully!");
            } catch (Exception ex) {
                response.getWriter().println("There was an error: " + ex.getMessage());
            }
        } else {
            response.getWriter().println("Request does not contain upload data");
        }
    }
}

在这两个例子中，我们都假设上传的文件将被保存在服务器上的uploadDir目录中。你需要确保这个目录存在，并且服务器有足够的权限写入文件。

请注意，这些代码示例仅用于演示目的，实际部署时需要考虑更多的安全性和错误处理措施，例如验证上传文件的类型和大小，防止目录遍历攻击等。