如果大家搭建過SpringMVC應用，那麼一定會寫個幾個xml配置文件，如 application.xml, spring-mvc.xml 等。一般來說，我們搭建項目的初始步驟如下：
- 初始化Spring MVC 的DispatcherServlet;
- 添加轉碼過濾器(HttpMessageConverter)，保證客戶端請求都能正確的編碼
- 搭建視圖解析器（view resolver)，告訴Spring去哪裏查找視圖，以及視圖方言（如Freemarker, Thymeleaf等）。
- 配置靜態資源（css,js,image等）
- 配置mulpart解析器，保證文件上傳正常。
- 配置一些錯誤處理，AOP切面日誌等。
當我們開始使用SpringBoot來搭建我們的Web項目的時候，你會發現，這些事情SpringBoot默認都幫你處理了。 SpringBoot原則是約定優於配置，並且默認情況下，會在你的項目中使用這些約定。
接下來，讓我們來了解下，幕後發生了什麼？
首先讓我們新建一個SpringBoot項目，並在src/main/resources/中的application.properties增加一行：debug=true。
OK，現在我們啓動下項目看看控制檯的輸出。
============================
CONDITIONS EVALUATION REPORT
============================
Positive matches:
-----------------
CodecsAutoConfiguration matched:
- @ConditionalOnClass found required class 'org.springframework.http.codec.CodecConfigurer'; @ConditionalOnMissingClass did not find unwanted class (OnClassCondition)
···（中間就省略了）
Negative matches:
-----------------
ActiveMQAutoConfiguration:
Did not match:
- @ConditionalOnClass did not find required classes 'javax.jms.ConnectionFactory', 'org.apache.activemq.ActiveMQConnectionFactory' (OnClassCondition)
我們可以看到Spring Boot 的自動配置報告。它分爲兩部分：一部分是匹配上的（positive matches），列出了應用中所有的自動配置。另一部分是沒有匹配上的（negative matches），這部分是應用在啓動的時候，需求沒有滿足的Spring Boot自動配置。
接下來重點看下DispatcherServletAutoConfiguration這個類的源碼(想想還是貼一下源碼吧...)
@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");
}
}
}
-
先看類註解@Configuration，很明顯這是一個典型的SpringBoot配置類。
-
@AutoConfigurerOrder來聲明優先級。
-
@AutoConfigureAfter 或@AutoConfigureBefore， 從而進一步細化配置處理的順序。
-
@ConditionalOnClass (DispatcherServlet.class)這個特殊的配置，能夠確保我們的類路徑下包含 DispatcherServlet。
-
@Conditional(DefaultDispatcherServletCondition.class)條件滿足的情況下，ServletRegistrationBean 函 數纔會啓用，這有些複雜，但是能夠檢查在你的配置中，是否已經註冊了分發器 Servlet
-
@ConditionalOnMissingBean(name=DispatcherServlet.MULTIPART_RESOLVER_ BEAN_NAME)條件的情況下，MultipartResolver 函數纔會處於激活狀態，例如，當我們自己還沒有註冊的時候。
我的公衆號：developlee的瀟灑人生，歡迎來撩~~~
