java实现文件上传到服务器

下面我来详细讲解Java实现文件上传到服务器的完整攻略。首先，我们需要了解两种上传文件的方式：使用Servlet API和使用第三方库。

使用Servlet API

使用Servlet API实现文件上传需要依赖于HttpServletRequest和Part对象。具体步骤如下：

1. 在html页面中添加文件上传表单

<form method="post" enctype="multipart/form-data" action="upload">
  <input type="file" name="file">
  <input type="submit" value="上传">
</form>

2. 在Servlet中获取上传的文件

// 获取文件
Part filePart = request.getPart("file");
String fileName = filePart.getSubmittedFileName();
InputStream inputStream = filePart.getInputStream();

// 将文件上传到服务器
// 此处可以根据需要自定义上传路径和文件名
String uploadPath = "/path/to/upload/dir/" + fileName;
Files.copy(inputStream, Paths.get(uploadPath), StandardCopyOption.REPLACE_EXISTING);

示例

@WebServlet(name = "FileUploadServlet", urlPatterns = {"/upload"})
@MultipartConfig // 必须添加该注解
public class FileUploadServlet extends HttpServlet {

    @Override
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        Part filePart = request.getPart("file");
        String fileName = filePart.getSubmittedFileName();
        InputStream inputStream = filePart.getInputStream();

        String uploadPath = "/path/to/upload/dir/" + fileName;
        Files.copy(inputStream, Paths.get(uploadPath), StandardCopyOption.REPLACE_EXISTING);

        response.getWriter().println("文件上传成功！");
    }
}

使用第三方库

除了使用Servlet API，还可以使用第三方库来实现文件上传。这里介绍一下常用的两个库：Apache Commons FileUpload和Spring MVC。

Apache Commons FileUpload

Apache Commons FileUpload是一个流行的文件上传库，使用起来非常方便。具体步骤如下：

1. 添加依赖

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

2. 在Servlet中解析上传的文件

// 创建文件上传工厂
DiskFileItemFactory factory = new DiskFileItemFactory();

// 创建文件上传处理器
ServletFileUpload fileUpload = new ServletFileUpload(factory);

// 解析上传的文件
List<FileItem> fileItemList = fileUpload.parseRequest(request);

// 保存文件
for (FileItem fileItem : fileItemList) {
    if (!fileItem.isFormField()) {
        String fileName = fileItem.getName();
        InputStream inputStream = fileItem.getInputStream();

        String uploadPath = "/path/to/upload/dir/" + fileName;
        Files.copy(inputStream, Paths.get(uploadPath), StandardCopyOption.REPLACE_EXISTING);
    }
}

示例

@WebServlet(name = "FileUploadServlet", urlPatterns = {"/upload"})
public class FileUploadServlet extends HttpServlet {

    @Override
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        DiskFileItemFactory factory = new DiskFileItemFactory();
        ServletFileUpload fileUpload = new ServletFileUpload(factory);

        try {
            List<FileItem> fileItemList = fileUpload.parseRequest(request);

            for (FileItem fileItem : fileItemList) {
                if (!fileItem.isFormField()) {
                    String fileName = fileItem.getName();
                    InputStream inputStream = fileItem.getInputStream();

                    String uploadPath = "/path/to/upload/dir/" + fileName;
                    Files.copy(inputStream, Paths.get(uploadPath), StandardCopyOption.REPLACE_EXISTING);
                }
            }

            response.getWriter().println("文件上传成功！");
        } catch (FileUploadException e) {
            e.printStackTrace();
        }
    }
}

Spring MVC

Spring MVC是一个流行的Java Web框架，它也提供了文件上传的功能。具体步骤如下：

1. 添加依赖

<dependency>
    <groupId>org.springframework</groupId>
    <artifactId>spring-web</artifactId>
    <version>5.3.10</version>
</dependency>
<dependency>
    <groupId>org.springframework</groupId>
    <artifactId>spring-webmvc</artifactId>
    <version>5.3.10</version>
</dependency>

2. 配置文件上传处理器

<bean id="multipartResolver" class="org.springframework.web.multipart.commons.CommonsMultipartResolver">
    <property name="maxUploadSize" value="10485760"/> <!-- 最大文件大小，单位是字节，默认为1MB -->
</bean>

3. 在Controller中处理文件上传

@RequestMapping("/upload")
public String upload(@RequestParam("file") MultipartFile file) throws IOException {
    String fileName = file.getOriginalFilename();
    InputStream inputStream = file.getInputStream();

    String uploadPath = "/path/to/upload/dir/" + fileName;
    Files.copy(inputStream, Paths.get(uploadPath), StandardCopyOption.REPLACE_EXISTING);

    return "上传成功！";
}

示例

@Controller
public class FileUploadController {

    @RequestMapping("/upload")
    public String upload(@RequestParam("file") MultipartFile file) throws IOException {
        String fileName = file.getOriginalFilename();
        InputStream inputStream = file.getInputStream();

        String uploadPath = "/path/to/upload/dir/" + fileName;
        Files.copy(inputStream, Paths.get(uploadPath), StandardCopyOption.REPLACE_EXISTING);

        return "上传成功！";
    }
}

至此，Java实现文件上传到服务器的攻略已完整讲解完毕。