Spring Boot 文件上传原理解析

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();
  }
}

总结

以上所述是小编给大家介绍的spring boot 文件上传原理解析，希望对大家有所帮助