精通SpringBoot——第一篇：DispatcherServlet和Multipart配置

2022-02-14 03:06:48

如果大家搭建过SpringMVC应用，那么一定会写个几个xml配置文件，如
application.xml, spring-mvc.xml 等。一般来说，我们搭建项目的初始步骤如下：

初始化Spring MVC 的DispatcherServlet;

添加转码过滤器(HttpMessageConverter)，保证客户端请求都能正确的编码

搭建视图解析器（view resolver)，告诉Spring去哪里查找视图，以及视图方言（如Freemarker, Thymeleaf等）。

配置静态资源（css,js,image等）

配置mulpart解析器，保证文件上传正常。

配置一些错误处理，AOP切面日志等。

当我们开始使用SpringBoot来搭建我们的Web项目的时候，你会发现，这些事情SpringBoot默认都帮你处理了。
SpringBoot原则是约定优于配置，并且默认情况下，会在你的项目中使用这些约定。

接下来，让我们来了解下，幕后发生了什么？
首先让我们新建一个SpringBoot项目，并在src/main/resources/中的application.properties增加一行：debug=true。
OK，现在我们启动下项目看看控制台的输出。

============================
CONDITIONS EVALUATION REPORT
============================


Positive matches:
-----------------

   CodecsAutoConfiguration matched:
      - @ConditionalOnClass found required class 'org.springframework.http.codec.CodecConfigurer'; @ConditionalOnMissingClass did not find unwanted class (OnClassCondition)


···（中间就省略了）

Negative matches:
-----------------

   ActiveMQAutoConfiguration:
      Did not match:
         - @ConditionalOnClass did not find required classes 'javax.jms.ConnectionFactory', 'org.apache.activemq.ActiveMQConnectionFactory' (OnClassCondition)

我们可以看到Spring Boot 的自动配置报告。它分为两部分：一部分是匹配上的（positive matches），列出了应用中所有的自动配置。另一部分是没有匹配上的（negative matches），这部分是应用在启动的时候，需求没有满足的Spring Boot自动配置。

接下来重点看下DispatcherServletAutoConfiguration这个类的源码(想想还是贴一下源码吧...)

@AutoConfigureOrder(Ordered.HIGHEST_PRECEDENCE)
@Configuration
@ConditionalOnWebApplication(type = Type.SERVLET)
@ConditionalOnClass(DispatcherServlet.class)
@AutoConfigureAfter(ServletWebServerFactoryAutoConfiguration.class)
@EnableConfigurationProperties(ServerProperties.class)
public class DispatcherServletAutoConfiguration {

    /*
     * The bean name for a DispatcherServlet that will be mapped to the root URL "/"
     */
    public static final String DEFAULT_DISPATCHER_SERVLET_BEAN_NAME = "dispatcherServlet";

    /*
     * The bean name for a ServletRegistrationBean for the DispatcherServlet "/"
     */
    public static final String DEFAULT_DISPATCHER_SERVLET_REGISTRATION_BEAN_NAME = "dispatcherServletRegistration";

    @Configuration
    @Conditional(DefaultDispatcherServletCondition.class)
    @ConditionalOnClass(ServletRegistration.class)
    @EnableConfigurationProperties(WebMvcProperties.class)
    protected static class DispatcherServletConfiguration {

        private final WebMvcProperties webMvcProperties;

        private final ServerProperties serverProperties;

        public DispatcherServletConfiguration(WebMvcProperties webMvcProperties,
                ServerProperties serverProperties) {
            this.webMvcProperties = webMvcProperties;
            this.serverProperties = serverProperties;
        }

        @Bean(name = DEFAULT_DISPATCHER_SERVLET_BEAN_NAME)
        public DispatcherServlet dispatcherServlet() {
            DispatcherServlet dispatcherServlet = new DispatcherServlet();
            dispatcherServlet.setDispatchOptionsRequest(
                    this.webMvcProperties.isDispatchOptionsRequest());
            dispatcherServlet.setDispatchTraceRequest(
                    this.webMvcProperties.isDispatchTraceRequest());
            dispatcherServlet.setThrowExceptionIfNoHandlerFound(
                    this.webMvcProperties.isThrowExceptionIfNoHandlerFound());
            return dispatcherServlet;
        }

        @Bean
        @ConditionalOnBean(MultipartResolver.class)
        @ConditionalOnMissingBean(name = DispatcherServlet.MULTIPART_RESOLVER_BEAN_NAME)
        public MultipartResolver multipartResolver(MultipartResolver resolver) {
            // Detect if the user has created a MultipartResolver but named it incorrectly
            return resolver;
        }

        @Bean
        public DispatcherServletPathProvider mainDispatcherServletPathProvider() {
            return () -> DispatcherServletConfiguration.this.serverProperties.getServlet()
                    .getPath();
        }

    }

    @Configuration
    @Conditional(DispatcherServletRegistrationCondition.class)
    @ConditionalOnClass(ServletRegistration.class)
    @EnableConfigurationProperties(WebMvcProperties.class)
    @Import(DispatcherServletConfiguration.class)
    protected static class DispatcherServletRegistrationConfiguration {

        private final ServerProperties serverProperties;

        private final WebMvcProperties webMvcProperties;

        private final MultipartConfigElement multipartConfig;

        public DispatcherServletRegistrationConfiguration(
                ServerProperties serverProperties, WebMvcProperties webMvcProperties,
                ObjectProvider<MultipartConfigElement> multipartConfigProvider) {
            this.serverProperties = serverProperties;
            this.webMvcProperties = webMvcProperties;
            this.multipartConfig = multipartConfigProvider.getIfAvailable();
        }

        @Bean(name = DEFAULT_DISPATCHER_SERVLET_REGISTRATION_BEAN_NAME)
        @ConditionalOnBean(value = DispatcherServlet.class, name = DEFAULT_DISPATCHER_SERVLET_BEAN_NAME)
        public ServletRegistrationBean<DispatcherServlet> dispatcherServletRegistration(
                DispatcherServlet dispatcherServlet) {
            ServletRegistrationBean<DispatcherServlet> registration = new ServletRegistrationBean<>(
                    dispatcherServlet,
                    this.serverProperties.getServlet().getServletMapping());
            registration.setName(DEFAULT_DISPATCHER_SERVLET_BEAN_NAME);
            registration.setLoadOnStartup(
                    this.webMvcProperties.getServlet().getLoadOnStartup());
            if (this.multipartConfig != null) {
                registration.setMultipartConfig(this.multipartConfig);
            }
            return registration;
        }

    }

    @Order(Ordered.LOWEST_PRECEDENCE - 10)
    private static class DefaultDispatcherServletCondition extends SpringBootCondition {

        @Override
        public ConditionOutcome getMatchOutcome(ConditionContext context,
                AnnotatedTypeMetadata metadata) {
            ConditionMessage.Builder message = ConditionMessage
                    .forCondition("Default DispatcherServlet");
            ConfigurableListableBeanFactory beanFactory = context.getBeanFactory();
            List<String> dispatchServletBeans = Arrays.asList(beanFactory
                    .getBeanNamesForType(DispatcherServlet.class, false, false));
            if (dispatchServletBeans.contains(DEFAULT_DISPATCHER_SERVLET_BEAN_NAME)) {
                return ConditionOutcome.noMatch(message.found("dispatcher servlet bean")
                        .items(DEFAULT_DISPATCHER_SERVLET_BEAN_NAME));
            }
            if (beanFactory.containsBean(DEFAULT_DISPATCHER_SERVLET_BEAN_NAME)) {
                return ConditionOutcome
                        .noMatch(message.found("non dispatcher servlet bean")
                                .items(DEFAULT_DISPATCHER_SERVLET_BEAN_NAME));
            }
            if (dispatchServletBeans.isEmpty()) {
                return ConditionOutcome
                        .match(message.didNotFind("dispatcher servlet beans").atAll());
            }
            return ConditionOutcome.match(message
                    .found("dispatcher servlet bean", "dispatcher servlet beans")
                    .items(Style.QUOTE, dispatchServletBeans)
                    .append("and none is named " + DEFAULT_DISPATCHER_SERVLET_BEAN_NAME));
        }

    }

    @Order(Ordered.LOWEST_PRECEDENCE - 10)
    private static class DispatcherServletRegistrationCondition
            extends SpringBootCondition {

        @Override
        public ConditionOutcome getMatchOutcome(ConditionContext context,
                AnnotatedTypeMetadata metadata) {
            ConfigurableListableBeanFactory beanFactory = context.getBeanFactory();
            ConditionOutcome outcome = checkDefaultDispatcherName(beanFactory);
            if (!outcome.isMatch()) {
                return outcome;
            }
            return checkServletRegistration(beanFactory);
        }

        private ConditionOutcome checkDefaultDispatcherName(
                ConfigurableListableBeanFactory beanFactory) {
            List<String> servlets = Arrays.asList(beanFactory
                    .getBeanNamesForType(DispatcherServlet.class, false, false));
            boolean containsDispatcherBean = beanFactory
                    .containsBean(DEFAULT_DISPATCHER_SERVLET_BEAN_NAME);
            if (containsDispatcherBean
                    && !servlets.contains(DEFAULT_DISPATCHER_SERVLET_BEAN_NAME)) {
                return ConditionOutcome
                        .noMatch(startMessage().found("non dispatcher servlet")
                                .items(DEFAULT_DISPATCHER_SERVLET_BEAN_NAME));
            }
            return ConditionOutcome.match();
        }

        private ConditionOutcome checkServletRegistration(
                ConfigurableListableBeanFactory beanFactory) {
            ConditionMessage.Builder message = startMessage();
            List<String> registrations = Arrays.asList(beanFactory
                    .getBeanNamesForType(ServletRegistrationBean.class, false, false));
            boolean containsDispatcherRegistrationBean = beanFactory
                    .containsBean(DEFAULT_DISPATCHER_SERVLET_REGISTRATION_BEAN_NAME);
            if (registrations.isEmpty()) {
                if (containsDispatcherRegistrationBean) {
                    return ConditionOutcome
                            .noMatch(message.found("non servlet registration bean").items(
                                    DEFAULT_DISPATCHER_SERVLET_REGISTRATION_BEAN_NAME));
                }
                return ConditionOutcome
                        .match(message.didNotFind("servlet registration bean").atAll());
            }
            if (registrations
                    .contains(DEFAULT_DISPATCHER_SERVLET_REGISTRATION_BEAN_NAME)) {
                return ConditionOutcome.noMatch(message.found("servlet registration bean")
                        .items(DEFAULT_DISPATCHER_SERVLET_REGISTRATION_BEAN_NAME));
            }
            if (containsDispatcherRegistrationBean) {
                return ConditionOutcome
                        .noMatch(message.found("non servlet registration bean").items(
                                DEFAULT_DISPATCHER_SERVLET_REGISTRATION_BEAN_NAME));
            }
            return ConditionOutcome.match(message.found("servlet registration beans")
                    .items(Style.QUOTE, registrations).append("and none is named "
                            + DEFAULT_DISPATCHER_SERVLET_REGISTRATION_BEAN_NAME));
        }

        private ConditionMessage.Builder startMessage() {
            return ConditionMessage.forCondition("DispatcherServlet Registration");
        }

    }

}

先看类注解@Configuration，很明显这是一个典型的SpringBoot配置类。
@AutoConfigurerOrder来声明优先级。
@AutoConfigureAfter 或@AutoConfigureBefore，从而进一步细化配置处理的顺序。
@ConditionalOnClass (DispatcherServlet.class)这个特殊的配置，能够确保我们的类路径下包含 DispatcherServlet。
@Conditional(DefaultDispatcherServletCondition.class)条件满足的情况下，ServletRegistrationBean 函数才会启用，这有些复杂，但是能够检查在你的配置中，是否已经注册了分发器 Servlet
@ConditionalOnMissingBean(name=DispatcherServlet.MULTIPART_RESOLVER_ BEAN_NAME)条件的情况下，MultipartResolver 函数才会处于激活状态，例如，当我们自己还没有注册的时候。

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