文件上传和下载
在Web应用中，文件上传和下载功能是非常常用的功能，今天来实现一下JavaWeb中的文件上传和下载功能
1. 准备工作
对于文件上传，浏览器在上传的过程中是将文件以流的形式提交到服务器端的。
一般选择采用apache的开源工具common-fileupload这个文件上传组件。
common-fileupload是依赖于common-io这个包的，所以还需要下载这个包。
下载最新的jar包
https://mvnrepository.com/artifact/commons-io/commons-io
https://mvnrepository.com/artifact/commons-fileupload/commons-fileupload
【注意】：使用IDEA导包需要注意修复路径，将lib添加到项目输出目录
2. 使用类介绍
【文件上传的注意事项】
- 为保证服务器安全，上传文件应该放在外界无法直接访问的目录下，比如放于WEB-INF目录下。
- 为防止文件覆盖的现象发生，要为上传文件产生一个唯一的文件名
- 要限制上传文件的最大值。
- 可以限制上传文件的类型，在收到上传文件名时，判断后缀名是否合法。
【需要用到的类详解】
ServletFileUpload负责处理上传的文件数据,并将表单中每个输入项封装成一个FileItem对象， 在使用ServletFileUpload对象解析请求时需要DiskFileItemFactory对象。所以，我们需要在进行解析工作前构造好DiskFileItemFactory对象，通过ServletFileUpload对象的构造方法或setFileItemFactory()方法设置ServletFileUpload对象的fileItemFactory属性。
FileItem类
在HTML页面input 必须有 name <input type="file" name="filename">
表单如果包含一个文件上传输入项的话，这个表单的enctype属性就必须设置为multipart/form-data
<form action="${pageContext.request.contextPath}/upload.do" enctype="multipart/form-data" method="post">
上传用户：<input type="text" name="username"><br/>
上传文件1：<input type="file" name="file1"><br/>
上传文件2：<input type="file" name="file2"><br/>
<input type="submit" value="提交">
</form>
浏览器表单的类型如果为multipart/form-data , 在服务器端想获取数据就要通过流。
【常用方法介绍】
//isFormField方法用于判断FileItem类对象封装的数据是一个普通文本表单
//还是一个文件表单，如果是普通表单字段则返回true，否则返回false
boolean isFormField();
//getFieldName方法用于返回表单标签name属性的值。
String getFieldName();
//getString方法用于将FileItem对象中保存的数据流内容以一个字符串返回
String getString();
//getName方法用于获得文件上传字段中的文件名。
String getName();
//以流的形式返回上传文件的数据内容。
InputStream getInputStream()
//delete方法用来清空FileItem类对象中存放的主体内容
//如果主体内容被保存在临时文件中，delete方法将删除该临时文件。
void delete();
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ServletFileUpload 类
ServletFileUpload负责处理上传的文件数据,并将表单中每个输入项封装成一个FileItem对象中 . 使用其parseRequest(HttpServletRequest) 方法可以将通过表单中每一个HTML标签提交的数据封装成一个FileItem对象，然后以List列表的形式返回。使用该方法处理上传文件简单易用。
3. 代码编写
UploadServlet
package servlet;
import org.apache.commons.fileupload.FileItem;
import org.apache.commons.fileupload.FileUploadException;
import org.apache.commons.fileupload.ProgressListener;
import org.apache.commons.fileupload.disk.DiskFileItemFactory;
import org.apache.commons.fileupload.servlet.ServletFileUpload;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.*;
import java.util.List;
import java.util.UUID;
public class UploadServlet extends HttpServlet {
protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
doGet(request, response);
}
<span class="token keyword">protected</span> <span class="token keyword">void</span> <span class="token function">doGet</span><span class="token punctuation">(</span>HttpServletRequest request<span class="token punctuation">,</span> HttpServletResponse response<span class="token punctuation">)</span> <span class="token keyword">throws</span> ServletException<span class="token punctuation">,</span> IOException <span class="token punctuation">{<!-- --></span>
<span class="token keyword">try</span> <span class="token punctuation">{<!-- --></span>
<span class="token comment">//判断上传的文件是普通的表单还是带文件的表单</span>
<span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token operator">!</span>ServletFileUpload<span class="token punctuation">.</span><span class="token function">isMultipartContent</span><span class="token punctuation">(</span>request<span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token punctuation">{<!-- --></span>
<span class="token keyword">return</span><span class="token punctuation">;</span><span class="token comment">//如果是普通文件，我们可以直接返回</span>
<span class="token punctuation">}</span> <span class="token comment">//如果通过了这个if，说明我们的表单是带文件上传的；</span>
<span class="token comment">//创建上传文件的保存路径，建议在WEB-INF路径下，安全，用户无法直接访问上传的文件；</span>
String uploadPath <span class="token operator">=</span> <span class="token keyword">this</span><span class="token punctuation">.</span><span class="token function">getServletContext</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">getRealPath</span><span class="token punctuation">(</span><span class="token string">"/WEB-INF/upload"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
File uploadFile <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">File</span><span class="token punctuation">(</span>uploadPath<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token operator">!</span>uploadFile<span class="token punctuation">.</span><span class="token function">exists</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token punctuation">{<!-- --></span><span class="token comment">//如果目录不存在，创建这样一个目录；</span>
uploadFile<span class="token punctuation">.</span><span class="token function">mkdir</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token comment">//临时路径，假如文件超过了预期的大小，我们就把他放到一个临时文件中，过几天自动删除，或者提醒用户转存为永久</span>
String tmpPath <span class="token operator">=</span> <span class="token keyword">this</span><span class="token punctuation">.</span><span class="token function">getServletContext</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">getRealPath</span><span class="token punctuation">(</span><span class="token string">"/WEB-INF/tmp"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
File file <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">File</span><span class="token punctuation">(</span>tmpPath<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token operator">!</span>file<span class="token punctuation">.</span><span class="token function">exists</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token punctuation">{<!-- --></span><span class="token comment">//如果目录不存在，创建这样一个目录；</span>
file<span class="token punctuation">.</span><span class="token function">mkdir</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token comment">//处理上传的文件，一般都需要通过流来获取，我们可以使用request.getInputStream(),原生态的文件上传流获取，十分麻烦</span>
<span class="token comment">//但是我们都建议使用 Apache的文件上传组件来实现，common-fileupload，它需要依赖于 commons-io组件；</span>
<span class="token comment">//1.创建DiskFileItemFactory对象，处理文件上传路径或者大小限制的；</span>
DiskFileItemFactory factory <span class="token operator">=</span> <span class="token function">getDiskFileItemFactory</span><span class="token punctuation">(</span>file<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token comment">//2.获取ServletFileUpload</span>
ServletFileUpload upload <span class="token operator">=</span> <span class="token function">getServletFileUpload</span><span class="token punctuation">(</span>factory<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token comment">//3.处理上传的文件</span>
String msg <span class="token operator">=</span> <span class="token function">uploadParseRequest</span><span class="token punctuation">(</span>upload<span class="token punctuation">,</span> request<span class="token punctuation">,</span> uploadPath<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token comment">//servlet请求转发消息</span>
request<span class="token punctuation">.</span><span class="token function">setAttribute</span><span class="token punctuation">(</span><span class="token string">"msg"</span><span class="token punctuation">,</span>msg<span class="token punctuation">)</span><span class="token punctuation">;</span>
request<span class="token punctuation">.</span><span class="token function">getRequestDispatcher</span><span class="token punctuation">(</span><span class="token string">"msg.jsp"</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">forward</span><span class="token punctuation">(</span>request<span class="token punctuation">,</span>response<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span> <span class="token keyword">catch</span> <span class="token punctuation">(</span><span class="token class-name">FileUploadException</span> e<span class="token punctuation">)</span> <span class="token punctuation">{<!-- --></span>
e<span class="token punctuation">.</span><span class="token function">printStackTrace</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token punctuation">}</span>
<span class="token keyword">public</span> <span class="token keyword">static</span> DiskFileItemFactory <span class="token function">getDiskFileItemFactory</span><span class="token punctuation">(</span>File file<span class="token punctuation">)</span> <span class="token punctuation">{<!-- --></span>
DiskFileItemFactory factory <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">DiskFileItemFactory</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token comment">//通过这个工厂设置一个缓冲区，当上传的文件大于这个缓冲区的时候，将他放到临时文件中；</span>
factory<span class="token punctuation">.</span><span class="token function">setSizeThreshold</span><span class="token punctuation">(</span><span class="token number">1024</span> <span class="token operator">*</span> <span class="token number">1024</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">//缓存区大小为1M</span>
factory<span class="token punctuation">.</span><span class="token function">setRepository</span><span class="token punctuation">(</span>file<span class="token punctuation">)</span><span class="token punctuation">;</span><span class="token comment">//临时目录的保存目录，需要一个File</span>
<span class="token keyword">return</span> factory<span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token keyword">public</span> <span class="token keyword">static</span> ServletFileUpload <span class="token function">getServletFileUpload</span><span class="token punctuation">(</span>DiskFileItemFactory factory<span class="token punctuation">)</span> <span class="token punctuation">{<!-- --></span>
ServletFileUpload upload <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">ServletFileUpload</span><span class="token punctuation">(</span>factory<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token comment">//监听文件上传进度；</span>
upload<span class="token punctuation">.</span><span class="token function">setProgressListener</span><span class="token punctuation">(</span><span class="token keyword">new</span> <span class="token class-name">ProgressListener</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{<!-- --></span>
<span class="token annotation punctuation">@Override</span>
<span class="token comment">//pBytesRead:已经读取到的文件大小</span>
<span class="token comment">//pContentLength ： 文件大小</span>
<span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token function">update</span><span class="token punctuation">(</span><span class="token keyword">long</span> pBytesRead<span class="token punctuation">,</span> <span class="token keyword">long</span> pContentLength<span class="token punctuation">,</span> <span class="token keyword">int</span> pItems<span class="token punctuation">)</span> <span class="token punctuation">{<!-- --></span>
System<span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token string">"总大小："</span> <span class="token operator">+</span> pContentLength <span class="token operator">+</span> <span class="token string">"已上传："</span> <span class="token operator">+</span> pBytesRead<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token comment">//处理乱码问题</span>
upload<span class="token punctuation">.</span><span class="token function">setHeaderEncoding</span><span class="token punctuation">(</span><span class="token string">"UTF-8"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token comment">//设置单个文件的最大值</span>
upload<span class="token punctuation">.</span><span class="token function">setFileSizeMax</span><span class="token punctuation">(</span><span class="token number">1024</span> <span class="token operator">*</span> <span class="token number">1024</span> <span class="token operator">*</span> <span class="token number">10</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token comment">//设置总共能够上传文件的大小</span>
<span class="token comment">//1024 = 1kb * 1024 = 1M * 10 = 10M</span>
upload<span class="token punctuation">.</span><span class="token function">setSizeMax</span><span class="token punctuation">(</span><span class="token number">1024</span> <span class="token operator">*</span> <span class="token number">1024</span> <span class="token operator">*</span> <span class="token number">10</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">return</span> upload<span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token keyword">public</span> <span class="token keyword">static</span> String <span class="token function">uploadParseRequest</span><span class="token punctuation">(</span>ServletFileUpload upload<span class="token punctuation">,</span>HttpServletRequest request<span class="token punctuation">,</span>String uploadPath<span class="token punctuation">)</span>
<span class="token keyword">throws</span> FileUploadException<span class="token punctuation">,</span> IOException <span class="token punctuation">{<!-- --></span>
String msg <span class="token operator">=</span> <span class="token string">""</span><span class="token punctuation">;</span>
<span class="token comment">//3.把前端请求解析，封装成一个FileItem对象</span>
List<span class="token generics function"><span class="token punctuation"><</span>FileItem<span class="token punctuation">></span></span> fileItems <span class="token operator">=</span> upload<span class="token punctuation">.</span><span class="token function">parseRequest</span><span class="token punctuation">(</span>request<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">for</span> <span class="token punctuation">(</span>FileItem fileItem <span class="token operator">:</span> fileItems<span class="token punctuation">)</span> <span class="token punctuation">{<!-- --></span>
<span class="token keyword">if</span> <span class="token punctuation">(</span>fileItem<span class="token punctuation">.</span><span class="token function">isFormField</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">{<!-- --></span> <span class="token comment">//判断上传的文件是普通的表单还是带文件的表单</span>
<span class="token comment">//getFieldName指的是前端表单控件的name；</span>
String name <span class="token operator">=</span> fileItem<span class="token punctuation">.</span><span class="token function">getFieldName</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
String value <span class="token operator">=</span> fileItem<span class="token punctuation">.</span><span class="token function">getString</span><span class="token punctuation">(</span><span class="token string">"UTF-8"</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">//处理乱码</span>
System<span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span>name<span class="token operator">+</span><span class="token string">":"</span><span class="token operator">+</span>value<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span><span class="token keyword">else</span> <span class="token punctuation">{<!-- --></span> <span class="token comment">//判断它是上传的文件</span>
<span class="token comment">//=======================处理文件===============================//</span>
<span class="token comment">//拿到文件名字</span>
String uploadFileName <span class="token operator">=</span> fileItem<span class="token punctuation">.</span><span class="token function">getName</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
System<span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token string">"上传的文件名："</span><span class="token operator">+</span>uploadFileName<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">if</span> <span class="token punctuation">(</span>uploadFileName<span class="token punctuation">.</span><span class="token function">trim</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">equals</span><span class="token punctuation">(</span><span class="token string">""</span><span class="token punctuation">)</span><span class="token operator">||</span>uploadFileName<span class="token operator">==</span>null<span class="token punctuation">)</span><span class="token punctuation">{<!-- --></span>
<span class="token keyword">continue</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token comment">//获得上传的文件名 /images/girl/paojie.png</span>
String fileName <span class="token operator">=</span> uploadFileName<span class="token punctuation">.</span><span class="token function">substring</span><span class="token punctuation">(</span>uploadFileName<span class="token punctuation">.</span><span class="token function">lastIndexOf</span><span class="token punctuation">(</span><span class="token string">"\\"</span><span class="token punctuation">)</span> <span class="token operator">+</span> <span class="token number">1</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token comment">//获得文件的后缀名</span>
String fileExtName <span class="token operator">=</span> uploadFileName<span class="token punctuation">.</span><span class="token function">substring</span><span class="token punctuation">(</span>uploadFileName<span class="token punctuation">.</span><span class="token function">lastIndexOf</span><span class="token punctuation">(</span><span class="token string">"."</span><span class="token punctuation">)</span> <span class="token operator">+</span> <span class="token number">1</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token comment">/*
如果文件后缀名 fileExtName 不是我们所需要的
就直接return，不处理，告诉用户文件类型不对。
*/</span>
System<span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token string">"文件信息 [件名："</span><span class="token operator">+</span>fileName<span class="token operator">+</span><span class="token string">"---文件类型"</span><span class="token operator">+</span>fileExtName<span class="token operator">+</span><span class="token string">"]"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token comment">//可以使用UUID（唯一识别的通用码），保证文件名唯一；</span>
<span class="token comment">//UUID.randomUUID()，随机生一个唯一识别的通用码；</span>
String uuidPath <span class="token operator">=</span> UUID<span class="token punctuation">.</span><span class="token function">randomUUID</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">toString</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token comment">//=======================处理文件完毕===============================//</span>
<span class="token comment">//存到哪？ uploadPath</span>
<span class="token comment">//文件真实存在的路径 realPath</span>
String realPath <span class="token operator">=</span> uploadPath<span class="token operator">+</span><span class="token string">"\\"</span><span class="token operator">+</span>uuidPath<span class="token punctuation">;</span>
<span class="token comment">//给每个文件创建一个对应的文件夹</span>
File realPathFile <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">File</span><span class="token punctuation">(</span>realPath<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token operator">!</span>realPathFile<span class="token punctuation">.</span><span class="token function">exists</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">{<!-- --></span>
realPathFile<span class="token punctuation">.</span><span class="token function">mkdir</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token comment">//=======================存放地址完毕===============================//</span>
<span class="token comment">//获得文件上传的流</span>
InputStream inputStream <span class="token operator">=</span> fileItem<span class="token punctuation">.</span><span class="token function">getInputStream</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token comment">//创建一个文件输出流</span>
<span class="token comment">//realPath = 真实的文件夹；</span>
<span class="token comment">//差了一个文件; 加上输出文件的名字+"/"+uuidFileName</span>
FileOutputStream fos <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">FileOutputStream</span><span class="token punctuation">(</span>realPath<span class="token operator">+</span><span class="token string">"\\"</span><span class="token operator">+</span>fileName<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token comment">//创建一个缓冲区</span>
<span class="token keyword">byte</span><span class="token punctuation">[</span><span class="token punctuation">]</span> buffer <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">byte</span><span class="token punctuation">[</span><span class="token number">1024</span><span class="token operator">*</span><span class="token number">1024</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
<span class="token comment">//判断是否读取完毕</span>
<span class="token keyword">int</span> len <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span>
<span class="token comment">//如果大于0说明还存在数据；</span>
<span class="token keyword">while</span> <span class="token punctuation">(</span><span class="token punctuation">(</span>len<span class="token operator">=</span>inputStream<span class="token punctuation">.</span><span class="token function">read</span><span class="token punctuation">(</span>buffer<span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token operator">></span><span class="token number">0</span><span class="token punctuation">)</span><span class="token punctuation">{<!-- --></span>
fos<span class="token punctuation">.</span><span class="token function">write</span><span class="token punctuation">(</span>buffer<span class="token punctuation">,</span><span class="token number">0</span><span class="token punctuation">,</span>len<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token comment">//关闭流</span>
fos<span class="token punctuation">.</span><span class="token function">close</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
inputStream<span class="token punctuation">.</span><span class="token function">close</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
msg <span class="token operator">=</span> <span class="token string">"文件上传成功！"</span><span class="token punctuation">;</span>
fileItem<span class="token punctuation">.</span><span class="token function">delete</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">//上传成功，清除临时文件</span>
<span class="token comment">//=======================文件传输完毕===============================//</span>
<span class="token punctuation">}</span>
<span class="token punctuation">}</span>
<span class="token keyword">return</span> msg<span class="token punctuation">;</span>
<span class="token punctuation">}</span>
}
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upload.jsp
<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
<head>
<title>文件上传</title>
</head>
<body>
<form action="${pageContext.request.contextPath}/upload.do" enctype="multipart/form-data" method="post">
上传用户：<input type="text" name="username"><br/>
上传文件1：<input type="file" name="file1"><br/>
上传文件2：<input type="file" name="file2"><br/>
<input type="submit" value="提交">
</form>
</body>
</html>
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msg.jsp
<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
<head>
<title>消息提示</title>
</head>
<body>
${msg}
</body>
</html>
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web.xml
<?xml version="1.0" encoding="UTF-8"?> <web-app xmlns="http://xmlns.jcp.org/xml/ns/javaee" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/javaee http://xmlns.jcp.org/xml/ns/javaee/web-app_4_0.xsd" version="4.0"> <servlet> <servlet-name>upload</servlet-name> <servlet-class>com.kuang.servlet.UploadFileServlet</servlet-class> </servlet><span class="token tag"><span class="token tag"><span class="token punctuation"><</span>servlet-mapping</span><span class="token punctuation">></span></span> <span class="token tag"><span class="token tag"><span class="token punctuation"><</span>servlet-name</span><span class="token punctuation">></span></span>upload<span class="token tag"><span class="token tag"><span class="token punctuation"></</span>servlet-name</span><span class="token punctuation">></span></span> <span class="token tag"><span class="token tag"><span class="token punctuation"><</span>url-pattern</span><span class="token punctuation">></span></span>/upload.do<span class="token tag"><span class="token tag"><span class="token punctuation"></</span>url-pattern</span><span class="token punctuation">></span></span> <span class="token tag"><span class="token tag"><span class="token punctuation"></</span>servlet-mapping</span><span class="token punctuation">></span></span>
</web-app>
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上传成功
【注意】其中的servlet代码真的很容易写错，一点点错误浏览器动不动就报404、500，而且都是小的问题，不仔细很难发现，心态都崩了。。。写的时候还是要细心。。。
文件上传和下载
在Web应用中，文件上传和下载功能是非常常用的功能，今天来实现一下JavaWeb中的文件上传和下载功能
1. 准备工作
对于文件上传，浏览器在上传的过程中是将文件以流的形式提交到服务器端的。
一般选择采用apache的开源工具common-fileupload这个文件上传组件。
common-fileupload是依赖于common-io这个包的，所以还需要下载这个包。
下载最新的jar包
https://mvnrepository.com/artifact/commons-io/commons-io
https://mvnrepository.com/artifact/commons-fileupload/commons-fileupload
【注意】：使用IDEA导包需要注意修复路径，将lib添加到项目输出目录
2. 使用类介绍
【文件上传的注意事项】
- 为保证服务器安全，上传文件应该放在外界无法直接访问的目录下，比如放于WEB-INF目录下。
- 为防止文件覆盖的现象发生，要为上传文件产生一个唯一的文件名
- 要限制上传文件的最大值。
- 可以限制上传文件的类型，在收到上传文件名时，判断后缀名是否合法。
【需要用到的类详解】
ServletFileUpload负责处理上传的文件数据,并将表单中每个输入项封装成一个FileItem对象， 在使用ServletFileUpload对象解析请求时需要DiskFileItemFactory对象。所以，我们需要在进行解析工作前构造好DiskFileItemFactory对象，通过ServletFileUpload对象的构造方法或setFileItemFactory()方法设置ServletFileUpload对象的fileItemFactory属性。
FileItem类
在HTML页面input 必须有 name <input type="file" name="filename">
表单如果包含一个文件上传输入项的话，这个表单的enctype属性就必须设置为multipart/form-data
<form action="${pageContext.request.contextPath}/upload.do" enctype="multipart/form-data" method="post">
上传用户：<input type="text" name="username"><br/>
上传文件1：<input type="file" name="file1"><br/>
上传文件2：<input type="file" name="file2"><br/>
<input type="submit" value="提交">
</form>
浏览器表单的类型如果为multipart/form-data , 在服务器端想获取数据就要通过流。
【常用方法介绍】
//isFormField方法用于判断FileItem类对象封装的数据是一个普通文本表单
//还是一个文件表单，如果是普通表单字段则返回true，否则返回false
boolean isFormField();