以下内容引用自http://wiki.jikexueyuan.com/project/servlet/file-uploading.html：
Servlet可以与HTML form标签一起使用允许用户将文件上传到服务器。上传的文件可以是文本文件或图像文件或任何文档。
一、创建一个文件上传表单
下述HTML代码创建了一个文件上传表单。以下是需要注意的几点：
-
表单method属性应该设置为POST方法且不能使用GET方法。
-
表单enctype属性应该设置为multipart/form-data。
-
表单action属性应该设置为Servlet文件，能够在后端服务器处理文件上传。下面的例子是使用UploadServlet Servlet来上传文件的。
- 要上传单个文件，应该使用单个带有属性type=“file”的<input .../>标签。为了允许多个文件上传，要包含多个带有name属性不同值的输入标签。浏览器将把一个浏览按钮和每个输入标签关联起来。
<html> <head> <title>File Uploading Form</title> </head> <body> <h3>File Upload:</h3> Select a file to upload: <br /> <form action="UploadServlet" method="post" enctype="multipart/form-data"> <input type="file" name="file" size="50" /> <br /> <input type="submit" value="Upload File" /> </form> </body> </html>
这将显示如下所示的结果，允许从本地计算机中选择一个文件，当用户点击“上传文件”时，表单会和选择的文件一起提交：
二、编写后台Servlet
以下是Servlet UploadServlet，会接受上传的文件并把它储存在目录<Tomcat-installation-directory>/webapps/data中。使用外部配置，如web.xml中的context-param元素，这个目录名也可以被添加，如下所示：
<web-app>
....
<context-param>
<description>Location to store uploaded file</description>
<param-name>file-upload</param-name>
<param-value>
c:\apache-tomcat-5.5.29\webapps\data\
</param-value>
</context-param>
....
</web-app>
以下是UploadServlet的源代码，可以一次处理多个文件的上传。在继续操作之前，请确认下列各项：
-
下述例子依赖于FileUpload，所以一定要确保在classpath中有最新版本的commons-fileupload.x.x.jar 文件。可以从http://commons.apache.org/fileupload/中下载。
-
FileUpload依赖于Commons IO，所以一定要确保在classpath中有最新版本的commons-io-x.x.jar文件。可以从http://commons.apache.org/io/中下载。
-
在测试下面实例时，上传的文件大小不能大于maxFileSize，否则文件将无法上传。
- 请确保已经提前创建好目录c:\temp and c:\apache-tomcat-5.5.29\webapps\data。
POM项目可以添加如下依赖：
<!-- commons-fileupload -->
<!-- https://mvnrepository.com/artifact/commons-fileupload/commons-fileupload -->
<dependency>
<groupId>commons-fileupload</groupId>
<artifactId>commons-fileupload</artifactId>
<version>1.3.2</version>
</dependency>
<!-- commons-io -->
<!-- https://mvnrepository.com/artifact/commons-io/commons-io -->
<dependency>
<groupId>commons-io</groupId>
<artifactId>commons-io</artifactId>
<version>2.5</version>
</dependency>
编写Servlet：
//Import required java libraries
import java.io.*;
import java.util.*;
import javax.servlet.ServletConfig;
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.FileUploadException;
import org.apache.commons.fileupload.disk.DiskFileItemFactory;
import org.apache.commons.fileupload.servlet.ServletFileUpload;
import org.apache.commons.io.output.*;
public class UploadServlet extends HttpServlet {
private boolean isMultipart;
private String filePath;
private int maxFileSize = 50 * 1024;
private int maxMemSize = 4 * 1024;
private File file;
public void init() {
// Get the file location where it would be stored.
filePath = getServletContext().getInitParameter("file-upload");
}
public void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, java.io.IOException {
// Check that we have a file upload request
isMultipart = ServletFileUpload.isMultipartContent(request);
response.setContentType("text/html");
java.io.PrintWriter out = response.getWriter();
if (!isMultipart) {
out.println("<html>");
out.println("<head>");
out.println("<title>Servlet upload</title>");
out.println("</head>");
out.println("<body>");
out.println("<p>No file uploaded</p>");
out.println("</body>");
out.println("</html>");
return;
}
DiskFileItemFactory factory = new DiskFileItemFactory();
// maximum size that will be stored in memory
factory.setSizeThreshold(maxMemSize);
// Location to save data that is larger than maxMemSize.
factory.setRepository(new File("c:\\temp"));
// Create a new file upload handler
ServletFileUpload upload = new ServletFileUpload(factory);
// maximum file size to be uploaded.
upload.setSizeMax(maxFileSize);
try {
// Parse the request to get file items.
List fileItems = upload.parseRequest(request);
// Process the uploaded file items
Iterator i = fileItems.iterator();
out.println("<html>");
out.println("<head>");
out.println("<title>Servlet upload</title>");
out.println("</head>");
out.println("<body>");
while (i.hasNext()) {
FileItem fi = (FileItem) i.next();
if (!fi.isFormField()) {
// Get the uploaded file parameters
String fieldName = fi.getFieldName();
String fileName = fi.getName();
String contentType = fi.getContentType();
boolean isInMemory = fi.isInMemory();
long sizeInBytes = fi.getSize();
// Write the file
if (fileName.lastIndexOf("\\") >= 0) {
file = new File(filePath + fileName.substring(fileName.lastIndexOf("\\")));
} else {
file = new File(filePath + fileName.substring(fileName.lastIndexOf("\\") + 1));
}
fi.write(file);
out.println("Uploaded Filename: " + fileName + "<br>");
}
}
out.println("</body>");
out.println("</html>");
} catch (Exception ex) {
System.out.println(ex);
}
}
public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, java.io.IOException {
throw new ServletException("GET method used with " + getClass().getName() + ": POST method required.");
}
}
三、编译和运行Servlet
编译上述Servlet UploadServlet并在web.xml文件中创建所需的条目，如下所示：
<servlet> <servlet-name>UploadServlet</servlet-name> <servlet-class>UploadServlet</servlet-class> </servlet> <servlet-mapping> <servlet-name>UploadServlet</servlet-name> <url-pattern>/UploadServlet</url-pattern> </servlet-mapping>
现在尝试使用上面创建的HTML表单来上传文件。当访问http://localhost:8080/UploadFile.htm时，它会显示如下所示的结果，这将有助于从本地计算机中上传任何文件。
如果Servelt脚本能正常工作，那么文件会被上传到c:\apache-tomcat-5.5.29\webapps\data\目录中。
测试工程：https://github.com/easonjim/5_java_example/tree/master/servletbasics/test11
来源：https://www.cnblogs.com/EasonJim/p/6977597.html