SpringApplication是所有springboot的入口类，分析此类有助于我们了解springboot的工作机制。本文以2.0.3.REALEASE版本作分析 SpringApplication 调用实例如下 package com.example.demospringbootweb; import org.springframework.boot.SpringApplication; import org.springframework.boot.autoconfigure.SpringBootApplication; @SpringBootApplication public class DemoSpringbootWebApplication { public static void main(String[] args) { SpringApplication.run(DemoSpringbootWebApplication.class, args); } } 调用的是SpringApplication.run()方法进行应用程序的启动。代码很简单也容易让用户上手，笔者这就进入其具体的类以探瑰宝。 注释描述 先看下其官方注释，有助于我们入门。由于注释过长，笔者此处只对其主要内容作下翻译总结 可以简单的通过main()函数来辅助启动一个spring应用程序。默认情况下其会按照以下步骤来辅助我们创建的应用 创建一个关联的ApplicationContext实例 注册CommandLinePropertySource实例暴露命令行的参数作为spring的属性 刷新ApplicationContext，并加载所有的单例beans 触发实现了CommandLineRunner的实例beans SpringApplications可以读取来自不同源的beans。官方建议用户使用@Configuration注解相应的启动类，当然也支持从以下方式加载相应的beans AnnotatedBeanDefinitionReader加载指定的类 XmlBeanDefinitionReader加载XML的配置信息或者GroovyBeanDefinitionReader加载groovy脚本资源 ClassPathBeanDefinitionScanner扫描指定的包加载相应bean 过于抽象，笔者继续通过源码来对上述的内容进行回顾 构造函数 /** * 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; // 加载的主类，可指定多个 Assert.notNull(primarySources, "PrimarySources must not be null"); this.primarySources = new LinkedHashSet<>(Arrays.asList(primarySources)); // 推断是否为web环境 this.webApplicationType = deduceWebApplicationType(); // 加载ApplicationContextInitializer接口类 setInitializers((Collection) getSpringFactoriesInstances( ApplicationContextInitializer.class)); // 加载ApplicationListener接口类 setListeners((Collection) getSpringFactoriesInstances(ApplicationListener.class)); // 推断主函数类 this.mainApplicationClass = deduceMainApplicationClass(); } 对上述的注释作下简单的解释 SpringApplication#deduceWebApplicationType() 推断是否为web环境，源码如下 private WebApplicationType deduceWebApplicationType() { if (ClassUtils.isPresent(REACTIVE_WEB_ENVIRONMENT_CLASS, null) && !ClassUtils.isPresent(MVC_WEB_ENVIRONMENT_CLASS, null)) { return WebApplicationType.REACTIVE; } for (String className : WEB_ENVIRONMENT_CLASSES) { if (!ClassUtils.isPresent(className, null)) { return WebApplicationType.NONE; } } return WebApplicationType.SERVLET; } 从代码层看总共有三种应用类型，也代表了三个环境类型 WebApplicationType.REACTIVE reactive web应用(classpath环境下须有org.springframework.web.reactive.DispatcherHandler) WebApplicationType.SERVLET servlet web应用(classpath环境下存在javax.servlet.Servlet或者org.springframework.web.context.ConfigurableWebApplicationContext) WebApplicationType.NONE 简单的JAVA应用(classpath环境不存在上述的类) SpringApplication#deduceMainApplicationClass() 推断主函数类，源码如下 private Class deduceMainApplicationClass() { try { StackTraceElement[] stackTrace = new RuntimeException().getStackTrace(); for (StackTraceElement stackTraceElement : stackTrace) { if ("main".equals(stackTraceElement.getMethodName())) { return Class.forName(stackTraceElement.getClassName()); } } } catch (ClassNotFoundException ex) { // Swallow and continue } return null; } 很简单，就是寻找哪个类下含有main方法，此处和我们常用的启动类不谋而合 SpringApplication#getSpringFactoriesInstances() 找寻相应的接口实现类，源码如下 private Collection getSpringFactoriesInstances(Class type, Class[] parameterTypes, Object... args) { // 上下文classLoader ClassLoader classLoader = Thread.currentThread().getContextClassLoader(); // 通过SpringFactoriesLoader来加载相应的类 Set names = new LinkedHashSet<>( SpringFactoriesLoader.loadFactoryNames(type, classLoader)); List instances = createSpringFactoriesInstances(type, parameterTypes, classLoader, args, names); AnnotationAwareOrderComparator.sort(instances); return instances; } 进而查看相应的静态方法SpringFactoriesLoader.loadFactoryNames()，源码如下 public static List loadFactoryNames(Class factoryClass, @Nullable ClassLoader classLoader) { String factoryClassName = factoryClass.getName(); // 关键处理类 return loadSpringFactories(classLoader).getOrDefault(factoryClassName, Collections.emptyList()); } 关键处理类出来了，源码跟上 private static Map> loadSpringFactories(@Nullable ClassLoader classLoader) { // 缓存处理 MultiValueMap result = cache.get(classLoader); if (result != null) { return result; } try { // 找寻所有classpath下的"META-INF/spring.factories"文件 Enumeration urls = (classLoader != null ? classLoader.getResources(FACTORIES_RESOURCE_LOCATION) : ClassLoader.getSystemResources(FACTORIES_RESOURCE_LOCATION)); result = new LinkedMultiValueMap<>(); while (urls.hasMoreElements()) { URL url = urls.nextElement(); UrlResource resource = new UrlResource(url); Properties properties = PropertiesLoaderUtils.loadProperties(resource); for (Map.Entry entry : properties.entrySet()) { // 对含有,的进行分隔并转为list集合 List factoryClassNames = Arrays.asList( StringUtils.commaDelimitedListToStringArray((String) entry.getValue())); result.addAll((String) entry.getKey(), factoryClassNames); } } cache.put(classLoader, result); return result; } catch (IOException ex) { throw new IllegalArgumentException("Unable to load factories from location [" + FACTORIES_RESOURCE_LOCATION + "]", ex); } } 由此我们得出结论，classpath环境下所有含META-INF/spring.factories的文件，里面约定了默认的实现。笔者以spring-boot-2.0.3.REALEASE.jar为例 # PropertySource Loaders org.springframework.boot.env.PropertySourceLoader=\ org.springframework.boot.env.PropertiesPropertySourceLoader,\ org.springframework.boot.env.YamlPropertySourceLoader # Run Listeners org.springframework.boot.SpringApplicationRunListener=\ org.springframework.boot.context.event.EventPublishingRunListener # Error Reporters org.springframework.boot.SpringBootExceptionReporter=\ org.springframework.boot.diagnostics.FailureAnalyzers # Application Context Initializers org.springframework.context.ApplicationContextInitializer=\ org.springframework.boot.context.ConfigurationWarningsApplicationContextInitializer,\ org.springframework.boot.context.ContextIdApplicationContextInitializer,\ org.springframework.boot.context.config.DelegatingApplicationContextInitializer,\ org.springframework.boot.web.context.ServerPortInfoApplicationContextInitializer # Application Listeners org.springframework.context.ApplicationListener=\ org.springframework.boot.ClearCachesApplicationListener,\ org.springframework.boot.builder.ParentContextCloserApplicationListener,\ org.springframework.boot.context.FileEncodingApplicationListener,\ org.springframework.boot.context.config.AnsiOutputApplicationListener,\ org.springframework.boot.context.config.ConfigFileApplicationListener,\ org.springframework.boot.context.config.DelegatingApplicationListener,\ org.springframework.boot.context.logging.ClasspathLoggingApplicationListener,\ org.springframework.boot.context.logging.LoggingApplicationListener,\ org.springframework.boot.liquibase.LiquibaseServiceLocatorApplicationListener # Environment Post Processors org.springframework.boot.env.EnvironmentPostProcessor=\ org.springframework.boot.cloud.CloudFoundryVcapEnvironmentPostProcessor,\ org.springframework.boot.env.SpringApplicationJsonEnvironmentPostProcessor,\ org.springframework.boot.env.SystemEnvironmentPropertySourceEnvironmentPostProcessor # Failure Analyzers org.springframework.boot.diagnostics.FailureAnalyzer=\ org.springframework.boot.diagnostics.analyzer.BeanCurrentlyInCreationFailureAnalyzer,\ org.springframework.boot.diagnostics.analyzer.BeanNotOfRequiredTypeFailureAnalyzer,\ org.springframework.boot.diagnostics.analyzer.BindFailureAnalyzer,\ org.springframework.boot.diagnostics.analyzer.BindValidationFailureAnalyzer,\ org.springframework.boot.diagnostics.analyzer.UnboundConfigurationPropertyFailureAnalyzer,\ org.springframework.boot.diagnostics.analyzer.ConnectorStartFailureAnalyzer,\ org.springframework.boot.diagnostics.analyzer.NoUniqueBeanDefinitionFailureAnalyzer,\ org.springframework.boot.diagnostics.analyzer.PortInUseFailureAnalyzer,\ org.springframework.boot.diagnostics.analyzer.ValidationExceptionFailureAnalyzer,\ org.springframework.boot.diagnostics.analyzer.InvalidConfigurationPropertyNameFailureAnalyzer,\ org.springframework.boot.diagnostics.analyzer.InvalidConfigurationPropertyValueFailureAnalyzer # FailureAnalysisReporters org.springframework.boot.diagnostics.FailureAnalysisReporter=\ org.springframework.boot.diagnostics.LoggingFailureAnalysisReporter 因此SpringApplication构造函数中加载的ApplicationContextInitializer类有如下 ConfigurationWarningsApplicationContextInitializer (对ComponentScan指定的值为"org"等进行报警输出) ContextIdApplicationContextInitializer (创建默认名为application的ContextId对象，也可通过spring.application.name指定) DelegatingApplicationContextInitializer (对context.initializer.classes指定的class集合进行加载) ServerPortInfoApplicationContextInitializer (将local.server.port设置为指定的web端口，默认为8080) 而加载的ApplicationListener类有如下 ClearCachesApplicationListener (反射工具缓存清空事件) ParentContextCloserApplicationListener (父ApplicationContext关闭事件) FileEncodingApplicationListener (系统变量配置的file.encoding值是否与环境变量spring.mandatory-file-encoding一致事件) AnsiOutputApplicationListener (控制台彩色输出事件，可通过spring.output.ansi.enabled来指定) ConfigFileApplicationListener (读取spring.profile.active/spring.profile.include配置) DelegatingApplicationListener (委托事件处理类) ClasspathLoggingApplicationListener (打印classpath信息，级别为debug) LoggingApplicationListener (日志处理事件) LiquibaseServiceLocatorApplicationListener (classpath是否存在liquibase的CustomResolverServiceLocator类判断事件) 小结 由此SpringApplication构造函数完成了一些必要的初始化，重点在于ApplicationContextInitializer和ApplicationListener接口类。并且通过构造函数反射来进行实例化 限于篇幅过长，笔者将对SpringApplication#run()方法的具体解析放于下一章节来分析 作者：南柯问天 出处：http://www.cnblogs.com/question-sky/ 本文版权归本人和博客园共有，欢迎转载，但未经作者同意必须保留此段声明，且在文章页面明显位置给出原文连接，否则保留追究法律责任的权利。https://www.cnblogs.com/question-sky/p/9366500.html