一. 注意事项
在Web应用中，文件上传和下载功能是非常常用的功能，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
【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
二. 文件上传步骤
1. 下载jar包
1-1.需要jar的地址：
https://mvnrepository.com/artifact/commons-io/commons-io
https://mvnrepository.com/artifact/commons-fileupload/commons-fileupload
1-2.在JavaWeb项目中导入jar包
1-3.【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. 编写upload.jsp
【注意：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multipart/form-data】
<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
<head>
<title>Title</title>
</head>
<body>
<form action="${pageContext.request.contextPath}/s"
method="post" enctype="multipart/form-data">
<p>上传用户：<input type="text" name="username"></p>
<p>上传文件：<input type="file" name="file"></p>
<p><input type="submit"></p>
</form>
</body>
</html>
3. 代码编写 UploadFileServlet
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.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.File;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.util.List;
import java.util.UUID;
public class ServletUpload extends javax.servlet.http.HttpServlet {
@Override
protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
<span class="hljs-keyword">try</span> {
<span class="hljs-comment">//1.判断上传的文件是普通表单还是带文件的表单</span>
<span class="hljs-keyword">if</span> (!ServletFileUpload.isMultipartContent(req)) {
<span class="hljs-keyword">return</span>;<span class="hljs-comment">//如果是普通文件，直接返回</span>
}<span class="hljs-comment">//通过if就是带文件的表单</span>
<span class="hljs-comment">//2.创建上传文件的保存路径，建议在WEB-INF路径下</span>
<span class="hljs-built_in">String</span> uploadPath = <span class="hljs-keyword">this</span>.getServletContext().getRealPath(<span class="hljs-string">"/WEB-INF/upload"</span>);
File uploadFile = <span class="hljs-keyword">new</span> File(uploadPath);
<span class="hljs-keyword">if</span> (!uploadFile.exists()) {
uploadFile.mkdirs();<span class="hljs-comment">//如果保存目录不存在，就创建一个目录</span>
}
<span class="hljs-comment">//3.创建临时路径</span>
<span class="hljs-built_in">String</span> tmpPath = <span class="hljs-keyword">this</span>.getServletContext().getRealPath(<span class="hljs-string">"/WEB-INF/tmp"</span>);
File tmpFile = <span class="hljs-keyword">new</span> File(tmpPath);
<span class="hljs-keyword">if</span> (!tmpFile.exists()) {
tmpFile.mkdirs();<span class="hljs-comment">//如果临时目录不存在，就创建一个目录</span>
}
<span class="hljs-comment">//4.创建DiskFileItemFactory对象，处理文件上传路径或者大小限制的；</span>
DiskFileItemFactory <span class="hljs-keyword">factory</span> = getDiskFileItemFactory(tmpFile);
<span class="hljs-comment">//5.获取ServletFileUpload</span>
ServletFileUpload upload = getServletFileUpload(<span class="hljs-keyword">factory</span>);
<span class="hljs-comment">//6.处理上传的文件</span>
<span class="hljs-built_in">String</span> msg = uploadParseRequest(upload, req, uploadPath);
<span class="hljs-comment">//7.servlet请求转发消息</span>
req.setAttribute(<span class="hljs-string">"msg"</span>,msg);
req.getRequestDispatcher(<span class="hljs-string">"msg.jsp"</span>).forward(req,resp);
} <span class="hljs-keyword">catch</span> (FileUploadException e) {
e.printStackTrace();
}
}
<span class="hljs-comment">//4.创建DiskFileItemFactory对象，处理文件上传路径或者大小限制的；</span>
public <span class="hljs-keyword">static</span> DiskFileItemFactory getDiskFileItemFactory(File tmpFile) {
DiskFileItemFactory <span class="hljs-keyword">factory</span> = <span class="hljs-keyword">new</span> DiskFileItemFactory();
<span class="hljs-comment">//4-1.设置一个缓冲区，当上传文件大于这个缓冲区的时候，将它放在临时文件中</span>
<span class="hljs-keyword">factory</span>.setSizeThreshold(<span class="hljs-number">1024</span> * <span class="hljs-number">1024</span>);
<span class="hljs-keyword">factory</span>.setRepository(tmpFile);<span class="hljs-comment">//4-2.临时目录的保存目录</span>
<span class="hljs-keyword">return</span> <span class="hljs-keyword">factory</span>;
}
<span class="hljs-comment">//5.获取ServletFileUpload</span>
public <span class="hljs-keyword">static</span> ServletFileUpload getServletFileUpload(DiskFileItemFactory <span class="hljs-keyword">factory</span>) {
ServletFileUpload upload = <span class="hljs-keyword">new</span> ServletFileUpload(<span class="hljs-keyword">factory</span>);
<span class="hljs-comment">//5-1.监听文件上传进度</span>
upload.setProgressListener(<span class="hljs-keyword">new</span> ProgressListener() {
<span class="hljs-meta">@Override</span>
<span class="hljs-comment">//5-2.l:已经读取到文件的大小</span>
<span class="hljs-comment">//5-3.l1:文件大小</span>
public <span class="hljs-keyword">void</span> update(long l, long l1, <span class="hljs-built_in">int</span> i) {
System.out.println(<span class="hljs-string">"总大小:"</span> + l1 + <span class="hljs-string">" "</span> + <span class="hljs-string">"已上传:"</span> + l);
}
});
upload.setHeaderEncoding(<span class="hljs-string">"utf-8"</span>);<span class="hljs-comment">//5-4.处理乱码问题</span>
upload.setFileSizeMax(<span class="hljs-number">1024</span> * <span class="hljs-number">1024</span> * <span class="hljs-number">10</span>);<span class="hljs-comment">//5-5.设置单个文件的最大值</span>
upload.setSizeMax(<span class="hljs-number">1024</span> * <span class="hljs-number">1024</span> * <span class="hljs-number">10</span>);<span class="hljs-comment">//5-6.设置总共能够上传文件的大小</span>
<span class="hljs-keyword">return</span> upload;
}
<span class="hljs-comment">//6.处理上传的文件</span>
public <span class="hljs-keyword">static</span> <span class="hljs-built_in">String</span> uploadParseRequest(ServletFileUpload upload, HttpServletRequest req, <span class="hljs-built_in">String</span> uploadPath)
throws FileUploadException, IOException {
<span class="hljs-built_in">String</span> msg = <span class="hljs-string">""</span>;
<span class="hljs-comment">//6-1.把前端请求解析，封装成一个FileItem对象</span>
<span class="hljs-built_in">List</span><FileItem> fileItems = upload.parseRequest(req);
<span class="hljs-keyword">for</span> (FileItem fileItem : fileItems) {
<span class="hljs-keyword">if</span> (fileItem.isFormField()) {<span class="hljs-comment">//6-2.判断上传的文件是普通的表单还是带文件的表单</span>
<span class="hljs-built_in">String</span> name = fileItem.getFieldName();
<span class="hljs-built_in">String</span> value = fileItem.getString(<span class="hljs-string">"utf-8"</span>);<span class="hljs-comment">//6-3.处理 乱码</span>
} <span class="hljs-keyword">else</span> {<span class="hljs-comment">//6-4.判断它是上传文件</span>
<span class="hljs-comment">//6-5.拿到文件名字</span>
<span class="hljs-built_in">String</span> uploadFileName = fileItem.getName();
System.out.println(<span class="hljs-string">"上传的文件名："</span> + uploadFileName);
<span class="hljs-keyword">if</span> (uploadFileName.trim().equals(<span class="hljs-string">""</span>) || uploadFileName == <span class="hljs-keyword">null</span>) {
<span class="hljs-keyword">continue</span>;
}
<span class="hljs-comment">//6-6.获得上传文件名</span>
<span class="hljs-built_in">String</span> fileName = uploadFileName.substring(uploadFileName.lastIndexOf(<span class="hljs-string">"/"</span>) + <span class="hljs-number">1</span>);
<span class="hljs-comment">//6-7.获得文件的后缀名</span>
<span class="hljs-built_in">String</span> fileExtName = uploadFileName.substring(uploadFileName.lastIndexOf(<span class="hljs-string">"."</span>) + <span class="hljs-number">1</span>);
System.out.println(<span class="hljs-string">"文件信息 [文件名："</span> + fileName + <span class="hljs-string">"---文件类型"</span> + fileExtName + <span class="hljs-string">"]"</span>);
<span class="hljs-comment">//6-8.使用UUID(唯一识别的通用码),保证文件名唯一</span>
<span class="hljs-built_in">String</span> uuidPath = UUID.randomUUID().toString();
<span class="hljs-comment">//6-9.文件真实存在的路径 realPath</span>
<span class="hljs-built_in">String</span> realPath = uploadPath + <span class="hljs-string">"/"</span> + uuidPath;
<span class="hljs-comment">//6-10.给每个文件创建一个对应的文件夹</span>
File realPathFile = <span class="hljs-keyword">new</span> File(realPath);
<span class="hljs-keyword">if</span> (!realPathFile.exists()) {
realPathFile.mkdir();
}
<span class="hljs-comment">//6-11.获得文件上传的流</span>
InputStream inputStream = fileItem.getInputStream();
<span class="hljs-comment">//6-12.创建一个文件输出流</span>
<span class="hljs-comment">//6-13.realPath = 真实的文件夹；</span>
<span class="hljs-comment">//6-14.差了一个文件; 加上输出文件的名字+"/"+uuidFileName</span>
FileOutputStream fos = <span class="hljs-keyword">new</span> FileOutputStream(realPath + <span class="hljs-string">"/"</span> + fileName);
<span class="hljs-comment">//6-15.创建一个缓冲区</span>
byte[] buffer = <span class="hljs-keyword">new</span> byte[<span class="hljs-number">1024</span> * <span class="hljs-number">1024</span>];
<span class="hljs-comment">//6-16.判断是否读取完毕</span>
<span class="hljs-built_in">int</span> len = <span class="hljs-number">0</span>;
<span class="hljs-comment">//6-17.如果大于0说明还存在数据；</span>
<span class="hljs-keyword">while</span> ((len = inputStream.read(buffer)) > <span class="hljs-number">0</span>) {
fos.write(buffer, <span class="hljs-number">0</span>, len);
}
<span class="hljs-comment">//6-18.关闭流</span>
fos.close();
inputStream.close();
msg = <span class="hljs-string">"文件上传成功！"</span>;
fileItem.delete();<span class="hljs-comment">//6-19.上传成功，清除临时文件</span>
}
}
<span class="hljs-keyword">return</span> msg;
}
<span class="hljs-meta">@Override</span>
protected <span class="hljs-keyword">void</span> doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
doGet(req, resp);
}
}
4. 编写msg.jsp
<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
<head>
<title>Title</title>
</head>
<body>
${msg}
</body>
</html>
5. 配置web.xml
<servlet>
<servlet-name>ServletFileUpload</servlet-name>
<servlet-class>com.sz.ServletUpload</servlet-class>
</servlet>
<span class="hljs-tag"><<span class="hljs-name">servlet-mapping</span>></span>
<span class="hljs-tag"><<span class="hljs-name">servlet-name</span>></span>ServletFileUpload<span class="hljs-tag"></<span class="hljs-name">servlet-name</span>></span>
<span class="hljs-tag"><<span class="hljs-name">url-pattern</span>></span>/s<span class="hljs-tag"></<span class="hljs-name">url-pattern</span>></span>
<span class="hljs-tag"></<span class="hljs-name">servlet-mapping</span>></span>