Spring Boot 文件上传原理

　　首先我们要知道什么是Spring Boot，这里简单说一下，Spring Boot可以看作是一个框架中的框架--->集成了各种框架，像security、jpa、data、cloud等等，它无须关心配置可以快速启动开发，有兴趣可以了解下自动化配置实现原理,本质上是 spring 4.0的条件化配置实现，深抛下注解，就会看到了。

　　说Spring Boot 文件上传原理 其实就是Spring MVC，因为这部分工作是Spring MVC做的而不是Spring Boot，那么，SpringMVC又是怎么处理文件上传这个过程的呢？

　　图:

　　首先项目启动相关配置，再执行上述第二步的时候 DispatcherServlet会去查找id为multipartResolver的Bean，在配置中看到Bean指向的是CommonsMultipartResolve，其中实现了MultipartResolver接口。

　　第四步骤这里会判断是否multipart文件即isMultipart方法，返回true：就会调用 multipartResolver 方法，传递HttpServletRequest会返回一个MultipartHttpServletRequest对象，再有DispatcherServlet进行处理到Controller层；返回false：会忽略掉，继续传递HttpServletRequest。

　　在MVC中需要在配置文件webApplicationContext.xml中配置 如下：

    <bean id="multipartResolver" class="org.springframework.web.multipart.commons.CommonsMultipartResolver">
            <property name="defaultEncoding" value="UTF-8"/>
            <property name="maxUploadSize" value="100000000"/>
            <property name="uploadTempDir" value="fileUpload/temp"/>
    </bean>

　　而Spring Boot已经自动配置好，直接用就行，做个test没什么问题。有默认的上传限制大小，不过在实际开发中我们还是做一些配置的，

如下在application.properties中：

# multipart config
#默认支持文件上传
spring.http.multipart.enabled=true
#文件上传目录
spring.http.multipart.location=/tmp/xunwu/images/
#最大支持文件大小
spring.http.multipart.max-file-size=4Mb
#最大支持请求大小
spring.http.multipart.max-request-size=20MB

当然也可以写配置类来实现，具体的就不做展示了。

　　看完上述你肯定有个大概的了解了，这里再啰嗦下，Spring提供Multipart的解析器：MultipartResolver，上述说的是CommonsMultipartResolver，它是基于Commons File Upload第三方来实现，这也是在Servlet3.0之前的东西，3.0+之后也可以不需要依赖第三方库，可以用StandardServletMultipartResolver，同样也是实现了MultipartResolver接口，我们可以看下它的实现：

 * Copyright 2002-2017 the original author or authors.

package org.springframework.web.multipart.support;

import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.Part;

import org.apache.commons.logging.LogFactory;

import org.springframework.web.multipart.MultipartException;
import org.springframework.web.multipart.MultipartHttpServletRequest;
import org.springframework.web.multipart.MultipartResolver;

/**
 * Standard implementation of the {@link MultipartResolver} interface,
 * based on the Servlet 3.0 {@link javax.servlet.http.Part} API.
 * To be added as "multipartResolver" bean to a Spring DispatcherServlet context,
 * without any extra configuration at the bean level (see below).
 *
 * <p><b>Note:</b> In order to use Servlet 3.0 based multipart parsing,
 * you need to mark the affected servlet with a "multipart-config" section in
 * {@code web.xml}, or with a {@link javax.servlet.MultipartConfigElement}
 * in programmatic servlet registration, or (in case of a custom servlet class)
 * possibly with a {@link javax.servlet.annotation.MultipartConfig} annotation
 * on your servlet class. Configuration settings such as maximum sizes or
 * storage locations need to be applied at that servlet registration level;
 * Servlet 3.0 does not allow for them to be set at the MultipartResolver level.
 *
 * @author Juergen Hoeller
 * @since 3.1
 * @see #setResolveLazily
 * @see HttpServletRequest#getParts()
 * @see org.springframework.web.multipart.commons.CommonsMultipartResolver
 */
public class StandardServletMultipartResolver implements MultipartResolver {

    private boolean resolveLazily = false;

    /**
     * Set whether to resolve the multipart request lazily at the time of
     * file or parameter access.
     * <p>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.
     */
    public void setResolveLazily(boolean resolveLazily) {
        this.resolveLazily = resolveLazily;
    }

    @Override
    public boolean isMultipart(HttpServletRequest request) {
        // Same check as in Commons FileUpload...
        if (!"post".equals(request.getMethod().toLowerCase())) {
            return false;
        }
        String contentType = request.getContentType();
        return (contentType != null && contentType.toLowerCase().startsWith("multipart/"));
    }

    @Override
    public MultipartHttpServletRequest resolveMultipart(HttpServletRequest request) throws MultipartException {
        return new StandardMultipartHttpServletRequest(request, this.resolveLazily);
    }

    @Override
    public void cleanupMultipart(MultipartHttpServletRequest request) {
        // 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);
        }
    }

}

这里是之前写的test的后者实现配置类，可以简单看下，作为了解：

package com.bj.config;

import org.springframework.boot.autoconfigure.condition.ConditionalOnMissingBean;
import org.springframework.boot.autoconfigure.web.MultipartProperties;
import org.springframework.boot.context.properties.EnableConfigurationProperties;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
import org.springframework.web.multipart.MultipartResolver;
import org.springframework.web.multipart.support.StandardServletMultipartResolver;
import org.springframework.web.servlet.DispatcherServlet;

import javax.servlet.MultipartConfigElement;

@Configuration
@EnableConfigurationProperties(MultipartProperties.class)
public class FileUploadConfig {

    private final MultipartProperties multipartProperties;

    public FileUploadConfig(MultipartProperties multipartProperties){
        this.multipartProperties=multipartProperties;
    }

    /**
     * 注册解析器
     * @return
     */
    @Bean(name= DispatcherServlet.MULTIPART_RESOLVER_BEAN_NAME)
    @ConditionalOnMissingBean(MultipartResolver.class)
    public StandardServletMultipartResolver multipartResolver(){
        StandardServletMultipartResolver multipartResolver = new StandardServletMultipartResolver();
        multipartResolver.setResolveLazily(multipartProperties.isResolveLazily());
        return multipartResolver;
    }

    /**
     * 上传配置
     * @return
     */
    @Bean
    @ConditionalOnMissingBean
    public MultipartConfigElement multipartConfigElement(){
        return this.multipartProperties.createMultipartConfig();
    }

}