SpringBoot自動配置原理

引導類

package com.company;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;

@SpringBootApplication
public class BootDemoApplication {

    public static void main(String[] args) {
        SpringApplication.run(BootDemoApplication.class,args);
    }
}

@SpringBootApplication注解

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@SpringBootConfiguration
@EnableAutoConfiguration//啟用自動配置
@ComponentScan(excludeFilters = { @Filter(type = FilterType.CUSTOM, classes = TypeExcludeFilter.class),
      @Filter(type = FilterType.CUSTOM, classes = AutoConfigurationExcludeFilter.class) })
public @interface SpringBootApplication {

追run()方法

/**
 * Static helper that can be used to run a {@link SpringApplication} from the
 * specified source using default settings.
 * @param primarySource the primary source to load
 * @param args the application arguments (usually passed from a Java main method)
 * @return the running {@link ApplicationContext}
 */
public static ConfigurableApplicationContext run(Class<?> primarySource, String... args) {
   return run(new Class<?>[] { primarySource }, args);
}

　　

繼續追run()方法

/**
 * Static helper that can be used to run a {@link SpringApplication} from the
 * specified sources using default settings and user supplied arguments.
 * @param primarySources the primary sources to load
 * @param args the application arguments (usually passed from a Java main method)
 * @return the running {@link ApplicationContext}
 */
public static ConfigurableApplicationContext run(Class<?>[] primarySources, String[] args) {
   return new SpringApplication(primarySources).run(args);
}

追構造方法

　　

/**
 * Create a new {@link SpringApplication} instance. The application context will load
 * beans from the specified primary sources (see {@link SpringApplication class-level}
 * documentation for details. The instance can be customized before calling
 * {@link #run(String...)}.
 * @param primarySources the primary bean sources
 * @see #run(Class, String[])
 * @see #SpringApplication(ResourceLoader, Class...)
 * @see #setSources(Set)
 */
public SpringApplication(Class<?>... primarySources) {
   this(null, primarySources);
}

繼續

/**
 * Create a new {@link SpringApplication} instance. The application context will load
 * beans from the specified primary sources (see {@link SpringApplication class-level}
 * documentation for details. The instance can be customized before calling
 * {@link #run(String...)}.
 * @param resourceLoader the resource loader to use
 * @param primarySources the primary bean sources
 * @see #run(Class, String[])
 * @see #setSources(Set)
 */
@SuppressWarnings({ "unchecked", "rawtypes" })
public SpringApplication(ResourceLoader resourceLoader, Class<?>... primarySources) {
   this.resourceLoader = resourceLoader;
   Assert.notNull(primarySources, "PrimarySources must not be null");
   this.primarySources = new LinkedHashSet<>(Arrays.asList(primarySources));
　　//判斷當前項目的應用類型
   this.webApplicationType = WebApplicationType.deduceFromClasspath();
   setInitializers((Collection) getSpringFactoriesInstances(ApplicationContextInitializer.class));
   setListeners((Collection) getSpringFactoriesInstances(ApplicationListener.class));
   this.mainApplicationClass = deduceMainApplicationClass();
}

追getSpringFactoriesInstances

private <T> Collection<T> getSpringFactoriesInstances(Class<T> type) {
   return getSpringFactoriesInstances(type, new Class<?>[] {});
}

繼續

　　

private <T> Collection<T> getSpringFactoriesInstances(Class<T> type, Class<?>[] parameterTypes, Object... args) {
   ClassLoader classLoader = getClassLoader();
   // Use names and ensure unique to protect against duplicates
   Set<String> names = new LinkedHashSet<>(SpringFactoriesLoader.loadFactoryNames(type, classLoader));
   List<T> instances = createSpringFactoriesInstances(type, parameterTypes, classLoader, args, names);
   AnnotationAwareOrderComparator.sort(instances);
   return instances;
}

追loadFactoryNames

public static List<String> loadFactoryNames(Class<?> factoryType, @Nullable ClassLoader classLoader) {
    String factoryTypeName = factoryType.getName();
    return (List)loadSpringFactories(classLoader).getOrDefault(factoryTypeName, Collections.emptyList());
}

追loadSpringFactories

　　

private static Map<String, List<String>> loadSpringFactories(@Nullable ClassLoader classLoader) {
    MultiValueMap<String, String> result = (MultiValueMap)cache.get(classLoader);
    if (result != null) {
        return result;
    } else {
        try {
　　　　　　　//這里去加載自動配置文件，如果當前項目classpath中沒有，則會去jar包中找。默認的自動配置文件在spring-boot-autoconfigure-2.2.2.RELEASE.jar!\META-INF\spring.factories
            Enumeration<URL> urls = classLoader != null ? classLoader.getResources("META-INF/spring.factories") : ClassLoader.getSystemResources("META-INF/spring.factories");
            LinkedMultiValueMap result = new LinkedMultiValueMap();

            while(urls.hasMoreElements()) {
                URL url = (URL)urls.nextElement();
                UrlResource resource = new UrlResource(url);
                Properties properties = PropertiesLoaderUtils.loadProperties(resource);
                Iterator var6 = properties.entrySet().iterator();

                while(var6.hasNext()) {
                    Entry<?, ?> entry = (Entry)var6.next();
                    String factoryTypeName = ((String)entry.getKey()).trim();
                    String[] var9 = StringUtils.commaDelimitedListToStringArray((String)entry.getValue());
                    int var10 = var9.length;

                    for(int var11 = 0; var11 < var10; ++var11) {
                        String factoryImplementationName = var9[var11];
                        result.add(factoryTypeName, factoryImplementationName.trim());
                    }
                }
            }

            cache.put(classLoader, result);
            return result;
        } catch (IOException var13) {
            throw new IllegalArgumentException("Unable to load factories from location [META-INF/spring.factories]", var13);
        }
    }
}

spring.factories部分內容

　　

# Initializers
org.springframework.context.ApplicationContextInitializer=\
org.springframework.boot.autoconfigure.SharedMetadataReaderFactoryContextInitializer,\
org.springframework.boot.autoconfigure.logging.ConditionEvaluationReportLoggingListener

# Application Listeners
org.springframework.context.ApplicationListener=\
org.springframework.boot.autoconfigure.BackgroundPreinitializer

# Auto Configuration Import Listeners
org.springframework.boot.autoconfigure.AutoConfigurationImportListener=\
org.springframework.boot.autoconfigure.condition.ConditionEvaluationReportAutoConfigurationImportListener

# Auto Configuration Import Filters
org.springframework.boot.autoconfigure.AutoConfigurationImportFilter=\
org.springframework.boot.autoconfigure.condition.OnBeanCondition,\
org.springframework.boot.autoconfigure.condition.OnClassCondition,\
org.springframework.boot.autoconfigure.condition.OnWebApplicationCondition

# Auto Configure
org.springframework.boot.autoconfigure.EnableAutoConfiguration=\
org.springframework.boot.autoconfigure.admin.SpringApplicationAdminJmxAutoConfiguration,\
org.springframework.boot.autoconfigure.aop.AopAutoConfiguration,\
。。。

　　鍵值對，值對應的都是org.springframework.boot.autoconfigure包中的內容，上圖

　　

在這個包下可以找到所有的默認配置

　　比如org.springframework.boot.autoconfigure.web.servlet.WebMvcAutoConfiguration

　　

/**
 * {@link EnableAutoConfiguration Auto-configuration} for {@link EnableWebMvc Web MVC}.
 *
 * @author Phillip Webb
 * @author Dave Syer
 * @author Andy Wilkinson
 * @author Sébastien Deleuze
 * @author Eddú Meléndez
 * @author Stephane Nicoll
 * @author Kristine Jetzke
 * @author Bruce Brouwer
 * @author Artsiom Yudovin
 * @since 2.0.0
 */
//有很多@Conditional開頭的注解，表示在滿足條件下才會生效

@Configuration(proxyBeanMethods = false)
@ConditionalOnWebApplication(type = Type.SERVLET)//滿足類型web應用
@ConditionalOnClass({ Servlet.class, DispatcherServlet.class, WebMvcConfigurer.class })//項目中存在這些類，引入了相關jar包就會有
@ConditionalOnMissingBean(WebMvcConfigurationSupport.class)//沒有該類型的bean在容器中
@AutoConfigureOrder(Ordered.HIGHEST_PRECEDENCE + 10)
@AutoConfigureAfter({ DispatcherServletAutoConfiguration.class, TaskExecutionAutoConfiguration.class,
      ValidationAutoConfiguration.class })
public class WebMvcAutoConfiguration {

找到InternalResourceViewResolver

　　

@Bean
@ConditionalOnMissingBean//如果自定義配置了，就不會生效
public InternalResourceViewResolver defaultViewResolver() {
   InternalResourceViewResolver resolver = new InternalResourceViewResolver();
   resolver.setPrefix(this.mvcProperties.getView().getPrefix());
   resolver.setSuffix(this.mvcProperties.getView().getSuffix());
   return resolver;
}

如果只想修改其中的屬性值，一般也都只需要改變屬性值即可，無需自己配置bean，追getter方法就可以找到WebMvcProperties.java

/**
 * {@link ConfigurationProperties properties} for Spring MVC.
 *
 * @author Phillip Webb
 * @author Sébastien Deleuze
 * @author Stephane Nicoll
 * @author Eddú Meléndez
 * @author Brian Clozel
 * @since 2.0.0
 */
@ConfigurationProperties(prefix = "spring.mvc")//這是一個配置類，前綴是spring.mvc

public class WebMvcProperties {

prefix和suffix是內部類View的屬性

　　

public static class View {

   /**
    * Spring MVC view prefix.
    */
   private String prefix;

   /**
    * Spring MVC view suffix.
    */
   private String suffix;

   public String getPrefix() {
      return this.prefix;
   }

   public void setPrefix(String prefix) {
      this.prefix = prefix;
   }

   public String getSuffix() {
      return this.suffix;
   }

   public void setSuffix(String suffix) {
      this.suffix = suffix;
   }

}

所以要覆蓋的話，只需要在application.yaml中配置spring.mvc.view.prefix和suffix即可。其他想要覆蓋的

spring-boot-autoconfigure-2.2.2.RELEASE.jar!\META-INF\additional-spring-configuration-metadata.json和spring-configuration-metadata.json
中有英文解釋，這兩個文件用於idea在application.yaml文件中提示用，參考鏈接https://www.jianshu.com/p/4488ea10ab2f

spring:
  mvc:
    view:
      prefix: /WEB-INF/pages
      suffix: .html