Servlet的文件上传

孤街醉人 提交于 2020-03-09 16:48:04

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

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