Servlets - 文件上传( File Uploading)
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。
创建文件上载表单
以下HTM代码创建了一个上传器表单。 以下是需要注意的重点 -
表单method属性应设置为POST方法，并且不能使用GET方法
表单enctype属性应设置为multipart/form-data 。
表单action属性应设置为servlet文件，该文件将处理后端服务器上的文件上载。 以下示例使用UploadServlet servlet上传文件。
要上传单个文件，您应该使用带有属性type =“file”的单个标记。 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，表格将与选定的文件一起提交 -
<b class="notranslate">File Upload:</b>
Select a file to upload:
<input type="file" name="file" size="50">
<input type="button" value="Upload File">
NOTE: This is just dummy form and would not work.
编写后端Servlet
以下是servlet UploadServlet ，它负责接受上传的文件并将其存储在“Tomcat-installation-directory”/ webapps/data目录中。 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，您必须确保以下事项 -
以下示例依赖于FileUpload，因此请确保您的类路径中包含最新版本的commons-fileupload.xxjar文件。 您可以从https://commons.apache.org/fileupload/下载它。
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，您应该上传大小小于maxFileSize的文件，否则将无法上载文件。
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.");
}
}
}
编译和运行Servlet
编译上面的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，它将显示以下结果，这将有助于您从本地计算机上载任何文件。
<b class="notranslate">File Upload:</b>
<p>Select a file to upload:</p>
<input type="file" name="file" size="50">
<input type="button" value="Upload File">
如果您的servlet脚本正常工作，您的文件应该上传到c:\apache-tomcat8.0.28\webapps\data \目录。
