Java 文件上传的实现

Java 文件上传的实现可以通过以下步骤：

1. 创建一个表单，允许用户选择文件并提交表单。

<form action="/upload" method="POST" enctype="multipart/form-data">
  <input type="file" name="file">
  <button type="submit">Submit</button>
</form>

2. 在后台处理文件上传的请求。你可以使用 Apache Commons FileUpload 库来处理文件上传请求。

// Create a factory for disk-based file items
DiskFileItemFactory factory = new DiskFileItemFactory();

// Set the size threshold, above which content will be stored on disk.
factory.setSizeThreshold(MAX_MEMORY_SIZE);

// Set the temporary directory to store the uploaded files of size above threshold.
factory.setRepository(new File(System.getProperty("java.io.tmpdir")));

// Create a new file upload handler
ServletFileUpload upload = new ServletFileUpload(factory);

// Set overall request size constraint
upload.setSizeMax(MAX_REQUEST_SIZE);

try {
    // Parse the request
    List<FileItem> items = upload.parseRequest(request);
    for (FileItem item : items) {
        // Process uploaded files
        if (!item.isFormField()) {
            String fileName = new File(item.getName()).getName();
            String filePath = uploadPath + File.separator + fileName;
            File uploadedFile = new File(filePath);
            item.write(uploadedFile);
        }
    }
    request.setAttribute("message", "File uploaded successfully!");
} catch (Exception ex) {
    request.setAttribute("message", "An error occurred: " + ex.getMessage());
}

在这个例子中，我们使用了 `DiskFileItemFactory` 来创建文件上传的文件项，然后使用 `ServletFileUpload` 来解析请求。接下来，我们遍历每个上传的文件项，检查是否为文件，如果是，则将其保存到指定的上传路径中。

请注意，我们还可以设置请求大小限制和内存大小限制，以确保上传的文件不会过大导致内存溢出或拒绝服务攻击。

3. 返回上传结果给用户。

request.getRequestDispatcher("/result.jsp").forward(request, response);

在这个例子中，我们将上传结果保存在 `request` 中，并将其发送到一个结果页面 `result.jsp`，以便用户可以查看上传结果。

完整的 Java 文件上传代码示例：

import java.io.File;
import java.util.List;

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

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

    // maximum file size to be uploaded.
    private static final int MAX_FILE_SIZE = 1024 * 1024 * 10; // 10MB

    // maximum size of request that can be accepted.
    private static final int MAX_REQUEST_SIZE = 1024 * 1024 * 50; // 50MB

    // directory to store uploaded files.
    private static final String UPLOAD_DIRECTORY = "uploads";

    /**
     * Upon receiving file upload submission, parses the request to read
     * upload data and saves the file on disk.
     */
    protected void doPost(HttpServletRequest request,
            HttpServletResponse response) throws ServletException, IOException {
        // checks if the request actually contains upload file
        if (!ServletFileUpload.isMultipartContent(request)) {
            // if not, we stop here
            PrintWriter writer = response.getWriter();
            writer.println("Error: Form must have enctype=multipart/form-data.");
            writer.flush();
            return;
        }

        // configures upload settings
        DiskFileItemFactory factory = new DiskFileItemFactory();
        // sets memory threshold - beyond which files are stored in disk
        factory.setSizeThreshold(MAX_FILE_SIZE);
        // sets temporary location to store files
        factory.setRepository(new File(System.getProperty("java.io.tmpdir")));

        ServletFileUpload upload = new ServletFileUpload(factory);

        // sets maximum size of upload file
        upload.setFileSizeMax(MAX_FILE_SIZE);

        // sets maximum size of request (include file + form data)
        upload.setSizeMax(MAX_REQUEST_SIZE);

        // constructs the directory path to store upload file
        String uploadPath = getServletContext().getRealPath("")
                + File.separator + UPLOAD_DIRECTORY;

        // creates the directory if it does not exist
        File uploadDir = new File(uploadPath);
        if (!uploadDir.exists()) {
            uploadDir.mkdir();
        }

        try {
            // parses the request's content to extract file data
            List<FileItem> formItems = upload.parseRequest(request);

            if (formItems != null && formItems.size() > 0) {
                // iterates over form's fields
                for (FileItem item : formItems) {
                    // processes only fields that are not form fields
                    if (!item.isFormField()) {
                        String fileName = new File(item.getName()).getName();
                        String filePath = uploadPath + File.separator + fileName;
                        File storeFile = new File(filePath);

                        // saves the file on disk
                        item.write(storeFile);
                        request.setAttribute("message",
                                "File uploaded successfully!");
                    }
                }
            }
        } catch (Exception ex) {
            request.setAttribute("message",
                    "An error occurred while uploading the file: "
                            + ex.getMessage());
        }
        // redirects client to message page
        getServletContext().getRequestDispatcher("/result.jsp").forward(
                request, response);
    }
}