Spring Boot处理Multipart文件上传

Spring Boot采用Servlet 3 javax.servlet.http.Part API来支持文件上传。
默认情况下，Spring Boot配置Spring MVC在单个请求中只处理每个文件最大1Mb，最多10Mb的文件数据。你可以覆盖那些值，也可以设置临时文件存储的位置（比如，存储到/tmp文件夹下）及传递数据刷新到磁盘的阀值（通过使用MultipartProperties类暴露的属性）。
如果你需要设置文件不受限制，可以设置spring.http.multipart.max-file-size属性值为-1。

当你想要接收multipart编码文件数据作为Spring MVC控制器（controller）处理方法中被@RequestParam注解的MultipartFile类型的参数时，multipart支持就非常有用了。

更多详情可参考MultipartAutoConfiguration：

//
// Source code recreated from a .class file by IntelliJ IDEA
// (powered by Fernflower decompiler)
//

package org.springframework.boot.autoconfigure.web.servlet;

import javax.servlet.MultipartConfigElement;
import javax.servlet.Servlet;
import org.springframework.boot.autoconfigure.condition.ConditionalOnClass;
import org.springframework.boot.autoconfigure.condition.ConditionalOnMissingBean;
import org.springframework.boot.autoconfigure.condition.ConditionalOnProperty;
import org.springframework.boot.autoconfigure.condition.ConditionalOnWebApplication;
import org.springframework.boot.autoconfigure.condition.ConditionalOnWebApplication.Type;
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.commons.CommonsMultipartResolver;
import org.springframework.web.multipart.support.StandardServletMultipartResolver;

@Configuration
@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 multipartProperties) {
        this.multipartProperties = multipartProperties;
    }

    @Bean
    @ConditionalOnMissingBean({MultipartConfigElement.class, CommonsMultipartResolver.class})
    public MultipartConfigElement multipartConfigElement() {
        return this.multipartProperties.createMultipartConfig();
    }

    @Bean(
        name = {"multipartResolver"}
    )
    @ConditionalOnMissingBean({MultipartResolver.class})
    public StandardServletMultipartResolver multipartResolver() {
        StandardServletMultipartResolver multipartResolver = new StandardServletMultipartResolver();
        multipartResolver.setResolveLazily(this.multipartProperties.isResolveLazily());
        return multipartResolver;
    }
}

其中，MultipartProperties中可以看到Spring MVC的默认单个请求中只处理每个文件最大1Mb，最多10Mb的文件数据

//
// Source code recreated from a .class file by IntelliJ IDEA
// (powered by Fernflower decompiler)
//

package org.springframework.boot.autoconfigure.web.servlet;

import javax.servlet.MultipartConfigElement;
import org.springframework.boot.context.properties.ConfigurationProperties;
import org.springframework.boot.web.servlet.MultipartConfigFactory;
import org.springframework.util.StringUtils;

@ConfigurationProperties(
    prefix = "spring.servlet.multipart",
    ignoreUnknownFields = false
)
public class MultipartProperties {
    private boolean enabled = true;
    private String location;
    private String maxFileSize = "1MB";
    private String maxRequestSize = "10MB";
    private String fileSizeThreshold = "0";
    private boolean resolveLazily = false;

    public MultipartProperties() {
    }

    public boolean getEnabled() {
        return this.enabled;
    }

    public void setEnabled(boolean enabled) {
        this.enabled = enabled;
    }

    public String getLocation() {
        return this.location;
    }

    public void setLocation(String location) {
        this.location = location;
    }

    public String getMaxFileSize() {
        return this.maxFileSize;
    }

    public void setMaxFileSize(String maxFileSize) {
        this.maxFileSize = maxFileSize;
    }

    public String getMaxRequestSize() {
        return this.maxRequestSize;
    }

    public void setMaxRequestSize(String maxRequestSize) {
        this.maxRequestSize = maxRequestSize;
    }

    public String getFileSizeThreshold() {
        return this.fileSizeThreshold;
    }

    public void setFileSizeThreshold(String fileSizeThreshold) {
        this.fileSizeThreshold = fileSizeThreshold;
    }

    public boolean isResolveLazily() {
        return this.resolveLazily;
    }

    public void setResolveLazily(boolean resolveLazily) {
        this.resolveLazily = resolveLazily;
    }

    public MultipartConfigElement createMultipartConfig() {
        MultipartConfigFactory factory = new MultipartConfigFactory();
        if (StringUtils.hasText(this.fileSizeThreshold)) {
            factory.setFileSizeThreshold(this.fileSizeThreshold);
        }

        if (StringUtils.hasText(this.location)) {
            factory.setLocation(this.location);
        }

        if (StringUtils.hasText(this.maxRequestSize)) {
            factory.setMaxRequestSize(this.maxRequestSize);
        }

        if (StringUtils.hasText(this.maxFileSize)) {
            factory.setMaxFileSize(this.maxFileSize);
        }

        return factory.createMultipartConfig();
    }
}