Servlet可与HTML表单标记一起使用，以允许用户将文件上载到服务器.上传的文件可以是文本文件或图像文件或任何文档.
以下HTM代码创建一个上传器形成.以下是需要注意的重要事项:
表单方法属性应该是设置为 POST 方法并且无法使用GET方法
应设置表单 enctype 属性到 multipart/form-data .
表单 action 属性应设置为servlet文件这将处理后端服务器上的文件上传.以下示例使用 UploadServlet servlet上传文件.
要上传单个文件，您应该使用单个< input .. ./>标签，属性类型="文件".要允许多个文件上载，请包含多个具有不同name属性值的输入标记.浏览器将"浏览"按钮与每个按钮相关联.
<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>
这将显示以下结果，允许从本地PC选择文件，当用户点击"上传文件"时，表格将随之提交选中的fil :
File Upload: Select a file to upload: NOTE: This is just dummy form and would not work.
以下是servlet UploadServlet ，它将负责接受上传文件并将其存储在目录< Tomcat-installation-directory>/webapps/data中.此目录名也可以使用外部配置添加，例如web.xml中的 context-param 元素，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，它可以一次处理多个文件上传.在继续之前，您必须确保以下内容 :
以下示例取决于FileUpload，因此请确保您拥有最新版本您的类路径中的 commons-fileupload.xxjar 文件.您可以从 https://commons.apache.org/fileupload/下载.
FileUpload依赖于Commons IO，因此请确保您的类路径中包含最新版本的 commons-io-xxjar 文件.您可以从 https://commons.apache.org/io/下载.
在测试以下示例时，您应该上传一个大小小于 maxFileSize 的文件，否则文件将无法上传.
确保您提前创建了目录c:\ temp和c:\ apache-tomcat8.0.28 \webapps \ data.
// 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.");
}
}
}编译上面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>
现在尝试使用您在上面创建的HTML表单上传文件.当您尝试http://localhost:8080/UploadFile.htm时，它会显示以下结果，这将有助于您从本地计算机上传任何文件.
File Upload: Select a file to upload:
如果你的servlet脚本有效好的，您的文件应该上传到c:\ apache-tomcat8.0.28 \webapps \data \目录.
