使用FileUpload组件实现文件上传

廖雪峰 / 文章 / ... / Reads: 27350 Edit

文件上传在Web应用中非常普遍,要在Java Web环境中实现文件上传功能非常容易,因为网上已经有许多用Java开发的组件用于文件上传,本文以使用最普遍的commons-fileupload组件为例,演示如何为Java Web应用添加文件上传功能。

commons-fileupload组件是Apache的一个开源项目之一,可以从http://commons.apache.org/fileupload/下载。该组件简单易用,可实现一次上传一个或多个文件,并可限制文件大小。

下载后解压zip包,将commons-fileupload-1.x.jar复制到tomcat的webapps/你的webapp/WEB-INF/lib/下,如果目录不存在请自建目录。

新建一个UploadServlet.java用于文件上传:

package com.liaoxuefeng.web;

public class FileUploadServlet extends HttpServlet {

    private String uploadDir = "C:\\temp";

    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp)
            throws ServletException, IOException
    {
        // TODO:
    }
}

当servlet收到浏览器发出的Post请求后,在doPost()方法中实现文件上传,我们需要遍历FileItemIterator,获得每一个FileItemStream:

@Override
protected void doPost(HttpServletRequest req, HttpServletResponse resp)
        throws ServletException, IOException
{
    try {
        ServletFileUpload upload = new ServletFileUpload();
        // set max file size to 1 MB:
        upload.setFileSizeMax(1024 * 1024);
        FileItemIterator it = upload.getItemIterator(req);
        // handle with each file:
        while (it.hasNext()) {
            FileItemStream item = it.next();
            if (! item.isFormField()) {
                // it is a file upload:
                handleFileItem(item);
            }
        }
        req.getRequestDispatcher("success.jsp").forward(req, resp);
    }
    catch(FileUploadException e) {
        throw new ServletException("Cannot upload file.", e);
    }
}

在handleFileItem()方法中读取上传文件的输入流,然后写入到uploadDir中,文件名通过UUID随机生成:

void handleFileItem(FileItemStream item) throws IOException {
    System.out.println("upload file: " + item.getName());
    File newUploadFile = new File(uploadDir + "/" + UUID.randomUUID().toString());
    byte[] buffer = new byte[4096];
    InputStream input = null;
    OutputStream output = null;
    try {
        input = item.openStream();
        output = new BufferedOutputStream(new FileOutputStream(newUploadFile));
        for (;;) {
            int n = input.read(buffer);
            if (n==(-1))
                break;
            output.write(buffer, 0, n);
        }
    }
    finally {
        if (input!=null) {
            try {
                input.close();
            }
            catch (IOException e) {}
        }
        if (output!=null) {
            try {
                output.close();
            }
            catch (IOException e) {}
        }
    }
}

如果要在web.xml配置文件中读取指定的上传文件夹,可以在init()方法中初始化:

@Override
public void init(ServletConfig config) throws ServletException {
    super.init(config);
    this.uploaddir = config.getInitParameter("dir");
}

最后在web.xml中配置Servlet:

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE web-app
    PUBLIC "-//Sun Microsystems, Inc.//DTD Web Application 2.3//EN"
    "http://java.sun.com/dtd/web-app_2_3.dtd">
<web-app>
    <servlet>
        <servlet-name>UploadServlet</servlet-name>
        <servlet-class>com.liaoxuefeng.web.FileUploadServlet</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>UploadServlet</servlet-name>
        <url-pattern>/upload</url-pattern>
    </servlet-mapping>
</web-app>

配置好Servlet后,启动Tomcat或Resin,写一个简单的index.htm测试:

<html>
<body>
<p>FileUploadServlet Demo</p>
<form name="form1" action="upload" method="post" enctype="multipart/form-data">
    <input type="file" name="file" />
    <input type="submit" name="button" value="Submit" />
</form>
</body>
</html>

注意action="upload"指定了处理上传文件的FileUploadServlet的映射URL。

当上传成功后,显示success.jsp,否则,抛出异常。如果上传的文件大小超过了我们设定的1MB,就会得到一个FileSizeLimitExceededException。

下载完整的源代码:

http://javaeedev.googlecode.com/files/FileUpload.zip

Comments

Make a comment

Author: 廖雪峰

Publish at: ...

关注公众号不定期领红包:

关注微博获取实时动态: