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首先我們要知道什么是Spring Boot，這里簡單說一下，Spring Boot可以看作是一個框架中的框架--->集成了各種框架，像security、jpa、data、cloud等等，它無須關心配置可以快速啟動開發，有興趣可以了解下自動化配置實現原理,本質上是 spring 4.0的條件化配置實現，深拋下注解，就會看到了。

　　說Spring Boot 文件上傳原理其實就是Spring MVC，因為這部分工作是Spring MVC做的而不是Spring Boot，那么，SpringMVC又是怎么處理文件上傳這個過程的呢？

　　圖:

Spring Boot 文件上傳原理解析

　　首先項目啟動相關配置，再執行上述第二步的時候 DispatcherServlet會去查找id為multipartResolver的Bean，在配置中看到Bean指向的是CommonsMultipartResolve，其中實現了MultipartResolver接口。

　　第四步驟這里會判斷是否multipart文件即isMultipart方法，返回true：就會調用 multipartResolver 方法，傳遞HttpServletRequest會返回一個MultipartHttpServletRequest對象，再有DispatcherServlet進行處理到Controller層；返回false：會忽略掉，繼續傳遞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已經自動配置好，直接用就行，做個test沒什么問題。有默認的上傳限制大小，不過在實際開發中我們還是做一些配置的，

如下在application.properties中：

									# multipart config

									#默認支持文件上傳

									spring.http.multipart.enabled=true

									#文件上傳目錄

									spring.http.multipart.location=/tmp/xunwu/images/

									#最大支持文件大小

									spring.http.multipart.max-file-size=4Mb

									#最大支持請求大小

									spring.http.multipart.max-request-size=20MB

當然也可以寫配置類來實現，具體的就不做展示了。

　　看完上述你肯定有個大概的了解了，這里再啰嗦下，Spring提供Multipart的解析器：MultipartResolver，上述說的是CommonsMultipartResolver，它是基于Commons File Upload第三方來實現，這也是在Servlet3.0之前的東西，3.0+之后也可以不需要依賴第三方庫，可以用StandardServletMultipartResolver，同樣也是實現了MultipartResolver接口，我們可以看下它的實現：

									* 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);

									    }

									  }

									}

這里是之前寫的test的后者實現配置類，可以簡單看下，作為了解：

									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;

									  }

									  /**

									   * 注冊解析器

									   * @return

									   */

									  @Bean(name= DispatcherServlet.MULTIPART_RESOLVER_BEAN_NAME)

									  @ConditionalOnMissingBean(MultipartResolver.class)

									  public StandardServletMultipartResolver multipartResolver(){

									    StandardServletMultipartResolver multipartResolver = new StandardServletMultipartResolver();

									    multipartResolver.setResolveLazily(multipartProperties.isResolveLazily());

									    return multipartResolver;

									  }

									  /**

									   * 上傳配置

									   * @return

									   */

									  @Bean

									  @ConditionalOnMissingBean

									  public MultipartConfigElement multipartConfigElement(){

									    return this.multipartProperties.createMultipartConfig();

									  }

									}