聊聊在Servlet中怎么上传文件

当我们需要上传大文件或者一些二进制文件到服务器上的时候，需要使用文件上传功能。下面是在Servlet中实现文件上传的完整攻略：

1. 导入依赖

在pom.xml文件中加入以下依赖：

<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. 创建Servlet

创建一个Java类，继承HttpServlet，用于处理文件上传的请求。在doPost方法中，读取文件数据，并保存到指定的目录中。文件上传的主要过程如下：

创建DiskFileItemFactory对象，用于在内存中处理文件数据和获取FileItem对象
创建ServletFileUpload对象，并设置文件的大小限制、请求编码、DiskFileItemFactory对象等处理数据的参数
调用parseRequest方法，获取所有的文件项列表
遍历文件项列表，判断是否是上传文件，并获取文件的原始名称和数据
将文件数据写入指定的目录中

示例代码如下：

import java.io.File;
import java.io.IOException;
import java.io.PrintWriter;
import java.util.List;
import javax.servlet.ServletException;
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;

public class FileUploadServlet extends HttpServlet {

    private static final long serialVersionUID = 1L;

    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        response.setContentType("text/html;charset=UTF-8");
        PrintWriter out = response.getWriter();

        String uploadPath = "/path/to/your/upload/folder";
        int maxFileSize = 1024 * 1024 * 10; // 10MB
        int maxMemSize = 1024 * 1024 * 1; // 1MB
        String characterEncoding = "UTF-8";

        // 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(maxMemSize);
        // 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.setFileSizeMax(maxFileSize);
        // Set overall request size constraint
        upload.setSizeMax(1024 * 1024 * 50); //50MB
        // Parse the request
        List<FileItem> items = upload.parseRequest(request);
        for (FileItem item : items) {
            // Check if the item is a file upload
            if (!item.isFormField()) {
                // Get the file name
                String fileName = item.getName();
                // Get the uploaded file's content type
                String contentType = item.getContentType();
                // Get the uploaded file data
                byte[] fileData = item.get();
                // Write the file data to the specified location
                File uploadedFile = new File(uploadPath + File.separator + fileName);
                item.write(uploadedFile);
                out.println("File " + fileName + " has been uploaded successfully.");
                out.println("<br>");
            }
        }
        out.close();
    }
}

3. 创建页面

在web.xml文件中，配置Servlet和它的映射路径。然后创建一个html页面，用于选择要上传的文件。

示例代码如下：

<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title>File Upload</title>
<style>
    body {
        font-family: Arial, sans-serif;
        margin: 20px;
    }
    h1, h2 {
        text-align: center;
    }
    form {
        width: 50%;
        margin: 0 auto;
        border: 2px solid #ccc;
        padding: 20px;
        text-align: center;
    }
    input[type="file"] {
        font-size: 1.5em;
        margin-top: 20px;
    }
    input[type="submit"] {
        font-size: 1.5em;
        margin-top: 20px;
        background: #5cb85c;
        color: #fff;
        border: none;
        padding: 10px 20px;
        border-radius: 5px;
        cursor: pointer;
    }
    input[type="submit"]:hover {
        background: #4cae4c;
    }
</style>
</head>
<body>
    <h1>File Upload Example</h1>
    <form action="upload" method="post" enctype="multipart/form-data">
        <input type="file" name="fileToUpload" id="fileToUpload">
        <br>
        <input type="submit" value="Upload">
    </form>
</body>
</html>

4. 验证文件上传

启动应用，访问页面。在表单中选择要上传的文件，点击“Upload”按钮，即可将文件上传到服务器上。

另外一个示例：实现一个多文件上传的功能。步骤与上面基本相同，主要是在遍历文件项列表的时候，可以获取到多个文件项，并逐一处理。

public class MultipleFileUploadServlet extends HttpServlet {

    private static final long serialVersionUID = 1L;

    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        response.setContentType("text/html;charset=UTF-8");
        PrintWriter out = response.getWriter();

        String uploadPath = "/path/to/your/upload/folder";
        int maxFileSize = 1024 * 1024 * 10; // 10MB
        int maxMemSize = 1024 * 1024 * 1; // 1MB
        String characterEncoding = "UTF-8";

        DiskFileItemFactory factory = new DiskFileItemFactory();
        factory.setSizeThreshold(maxMemSize);
        factory.setRepository(new File(System.getProperty("java.io.tmpdir")));

        ServletFileUpload upload = new ServletFileUpload(factory);
        upload.setFileSizeMax(maxFileSize);
        upload.setSizeMax(1024 * 1024 * 50); //50MB

        try {
            List<FileItem> items = upload.parseRequest(request);
            for (FileItem item : items) {
                if (!item.isFormField()) {
                    String fileName = item.getName();
                    fileName = fileName.substring(fileName.lastIndexOf("\\") + 1);
                    String contentType = item.getContentType();
                    byte[] fileData = item.get();
                    File uploadedFile = new File(uploadPath + File.separator + fileName);
                    item.write(uploadedFile);
                    out.println("File " + fileName + " has been uploaded successfully.");
                    out.println("<br>");
                }
            }
            out.println("<a href=\"multiple.html\">Upload more files</a>");
        } catch (Exception ex) {
            out.println("Error: " + ex.getMessage());
            out.println("<br>");
            out.println("<a href=\"multiple.html\">Try again</a>");
        } finally {
            out.close();
        }
    }

}

页面示例代码如下：

<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title>Multiple File Upload</title>
<style>
    body {
        font-family: Arial, sans-serif;
        margin: 20px;
    }
    h1, h2 {
        text-align: center;
    }
    form {
        width: 50%;
        margin: 0 auto;
        border: 2px solid #ccc;
        padding: 20px;
        text-align: center;
    }
    input[type="file"] {
        font-size: 1.5em;
        margin-top: 20px;
    }
    input[type="submit"] {
        font-size: 1.5em;
        margin-top: 20px;
        background: #5cb85c;
        color: #fff;
        border: none;
        padding: 10px 20px;
        border-radius: 5px;
        cursor: pointer;
    }
    input[type="submit"]:hover {
        background: #4cae4c;
    }
</style>
</head>
<body>
    <h1>Multiple File Upload Example</h1>
    <form action="multiple" method="post" enctype="multipart/form-data">
        <input type="file" name="fileToUpload1" id="fileToUpload1">
        <br>
        <input type="file" name="fileToUpload2" id="fileToUpload2">
        <br>
        <input type="file" name="fileToUpload3" id="fileToUpload3">
        <br>
        <input type="submit" value="Upload">
    </form>
</body>
</html>

要注意的是，在HTML页面中要添加一个enctype属性，值为multipart/form-data，以便能够正确接收文件上传的数据。另外，Servlet中需要解析请求的body部分，因此需要使用commons-fileupload提供的API方法来读取数据。

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聊聊在Servlet中怎么上传文件

1. 导入依赖

2. 创建Servlet

3. 创建页面

4. 验证文件上传

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