摘要：package com.example.thymeleafdemo.controller;import org.springframework.stereotype.Controller;import org.Springframework.ui.Model;
新建一个FileController.java文件，将其作为上传文件的请求入口，代码如下：
package com.example.thymeleafdemo.controller;
import org.springframework.stereotype.Controller;
import org.Springframework.ui.Model;
import org.springframework.util.FileCopyUtils;
import org.springframework.web.bind.annotation.*;
import org.springframework.web.multipart.MultipartFile;
import java.io.*;
@Controller
public class FileController {
/**
* 去文件上传的页面
*/
@GetMapping("toUpload")
public String toUpload {
return "toUpload";
}
/**
* 上传一个文件
*/ @PostMapping("/uploadFile")
public String SingleFileUpLoad(@requestParam("myfile")
MultipartFile File,
Model model) {
//判断文件是否为空
if (file.isEmpty) {
model.addAttribute("result_singlefile", "文件为空");
return "toUpload";
}
//指定上传的位置为 d:/upload/
String path = "d:/upload/";
try {
//获取文件的输入流
InputStream inputStream = file.getInputStream;
//获取上传时的文件名
String fileName = file.getOriginalFilename;
//注意是路径+文件名
File targetFile = new File(path + fileName);
//如果之前的 String path = "d:/upload/" 没有在最后加 / ，那就要
在 path后面 + "/"
//判断文件父目录是否存在
if (!targetFile.getParentFile.exists) {
//不存在就创建一个
targetFile.getParentFile.mkdir;
}
//获取文件的输出流
OutputStream outputStream = new
FileOutputStream(targetFile);
//最后使用资源访问器FileCopyUtils的copy方法拷贝文件
FileCopyUtils.copy(inputStream, outputStream);
//告诉页面上传成功了
model.addAttribute("uploadResult", "上传成功");
} catch (Exception e) {
model.addAttribute("uploadResult", "上传失败");
}
return "toUpload";
}
}
完成上传文件的非空校验后，把上传文件复制到本地存放文件的目录下，最后返回保存文件的URI，数据库只保存文件的路径。
在templates目录下新建upUpload.html文件，将其作为上传文件的页面，代码如下：
文件上传页面
enctype="multipart/form-data">
#strings.
isEmpty(uploadResult)}">
修改application.properties，设置请求文件的大小。
#文件上传的配置
spring.servlet.multipart.max-file-size=10MB
spring.servlet.multipart.max-request-size=10MB
启动服务器，访问http://localhost:8080/toUpload，将显示上传文件的页面，如图4.3所示。
单击“选择文件”按钮，弹出“选择文件”对话框，选择文件之后，单击“上传”按钮就能将文件上传到本地服务器的D:/upload中。上传完成后，页面会提示“上传成功”字样，如图4.4所示。
前面完成了文件上传的功能，下面对文件上传功能的源码进行简单的解析。Spring Boot项目启动时会加载上传的配置文件，其配置类为MultipartAutoConfiguration，源码如下：
@Configuration(proxyBeanMethods = false)
@ConditionalOnClass({Servlet.class,StandardServletMultipartResolver.class, MultipartConfigElement.class })
@ConditionalOnProperty(prefix = "spring.servlet.multipart", name =
"enabled", matchIfMissing = true)
@ConditionalOnWebApplication(type = Type.Servlet)
@EnableConfigurationProperties(MultipartProperties.class)
public class MultipartAutoConfiguration {
private final MultipartProperties multipartProperties;
public MultipartAutoConfiguration(MultipartProperties multipart
Properties) {
this.multipartProperties = multipartProperties;
}
@Bean
@ConditionalOnMissingBean({MultipartConfigElement.class, Commons
MultipartResolver.class })
public MultipartConfigElement multipartConfigElement {
return this.multipartProperties.createMultipartConfig;
}
@Bean(name = DispatcherServlet.MULTIPART_RESOLVER_BEAN_NAME)
@ConditionalOnMissingBean(MultipartResolver.class)
public StandardServletMultipartResolver multipartResolver {
StandardServletMultipartResolver multipartResolver=new Standard
ServletMultipartResolver;
multipartResolver.setResolveLazily(this.multipartProperties.isResolveL
azily);
return multipartResolver;
}
}
MultipartAutoConfiguration类中的
@ConditionalOnClass({Servlet.class, Standard
ServletMultipartResolver.class, MultipartConfigElement.class })
表示只有在特定的类加载后才使用，通过@EnableConfigurationProperties开启自动配置的文件，如果配置文件缺失时默认spring.http.multipart.enabled为true，则说明在Spring中文件上传功能是默认启用的。通过@EnableConfigurationProperties(MultipartProperties.class)可以查看自定义的上传配置，查看MultipartProperties类就可以看到部分源码如下：
@ConfigurationProperties(prefix = "spring.servlet.multipart", ignore
UnknownFields = false)
public class MultipartProperties {
/**
* Whether to enable support of multipart uploads.
*/
private boolean enabled = true;
/**
* Intermediate location of uploaded files.
*/
private String location;
/**
* Max file size.
*/
private DataSize maxFileSize = DataSize.ofMegabytes(1);
/**
* Max request size.
*/
private DataSize maxRequestSize = DataSize.ofMegabytes(10);
/**
* Create a new {@link MultipartConfigElement} using the properties.
* @return a new {@link MultipartConfigElement} configured using
there
properties
*/
public MultipartConfigElement createMultipartConfig {
MultipartConfigFactory factory = new MultipartConfigFactory;
PropertyMapper map =
PropertyMapper.get.alwaysApplyingWhenNonnull;
map.from(this.fileSizeThreshold).to(factory::setFileSizeThreshold);
map.from(this.location).whenHasText.to(factory::setLocation);
map.from(this.maxRequestSize).to(factory::setMaxRequestSize);
map.from(this.maxFileSize).to(factory::setMaxFileSize);
return factory.createMultipartConfig;
}
}
这里就是把在自定义的application.properties中配置的值注入当前配置类中，查看createMultipartConfig方法，该方法是通过MultipartConfigFactory 来设置上传的配置项。当配置类加载完成后，会创建处理的组件，就是MultipartAutoConfiguration这个类的
multipartResolver方法，代码如下：
@Bean(name = DispatcherServlet.MULTIPART_RESOLVER_BEAN_NAME)
@ConditionalOnMissingBean(MultipartResolver.class)
public StandardServletMultipartResolver multipartResolver {
StandardServletMultipartResolver multipartResolver = new Standard
ServletMultipartResolver;
multipartResolver.setResolveLazily(this.multipartProperties.isResolveL
azily);
return multipartResolver;
}
multipartResolver方法用于选择哪一个实现类，默认情况下使用的是StandardServlet-MultipartResolver类。可以通过MultiparResolver来了解一下有哪些部件。在Servlet 3.0之前Spring提供的默认附件解析器是MultipartResoler，实现类是CommonsMultipartResolver，它是基于CommonsFile Upload的第三方实现的，在目前最新的Servlet版本中使用的是StandardServletMultipartResolver，其实现源码如下：
public class StandardServletMultipartResolver implements Multipart
Resolver {
private boolean resolveLazily = false;
/**
* Set whether to resolve the multipart request lazily at the time
of
* file or parameter access.
*
Default is "false", resolving the multipart elements
immediately, throwing
* corresponding exceptions at the time of the {@link
#resolveMultipart} call.
* Switch this to "true" for lazy multipart parsing, throwing parse
exceptions
* once the application attempts to obtain multipart files or
parameters.
* @since 3.2.9
*/
public void setResolveLazily(boolean resolveLazily) {
this.resolveLazily = resolveLazily;
}
@Override
public boolean isMultipart(HttpServletRequest request) {
return
StringUtils.startsWithIgnoreCase(request.getContentType,"multipart/"
);
}
@Override
public MultipartHttpServletRequest
resolveMultipart(HttpServletRequest request) throws MultipartException
{
return new StandardMultipartHttpServletRequest(request,
this.resolveLazily);
}
@Override
public void cleanupMultipart(MultipartHttpServletRequest request) {
if (!(request instanceof AbstractMultipartHttpServletRequest) ||
((AbstractMultipartHttpServletRequest)
request).isResolved) { // To be on the safe side: explicitly delete the parts,
// but only actual file parts (for Resin compatibility)
try {
for (Part part : request.getParts) {
if (request.getFile(part.getName) != null) {
part.delete;
}
}
}
catch (Throwable ex) {
LogFactory.getLog(getClass).warn("Failed to perform
cleanup of multipart items", ex);
}
}
}
}
至此完成了文件上传的代码解析，这里只是简单的讲解，如果读者有兴趣，可以在类上设置断点，一步一步地跟踪代码，最后完成整个代码流程的跟踪。
注意：在跟踪代码时可以忽略一些不重要的点，有目的性地查看，多跟踪几遍后就会对源码非常熟悉了。
来源：程序员高级码农II一点号
