Spring Boot系列——Spring Boot如何启动-白红宇

Spring Boot系列——Spring Boot如何启动

阅读量：6994 次

发布时间：2019-06-27

本文共 9697 字，大约阅读时间需要 32 分钟。

Spring Boot启动过程

上篇《Spring Boot系列——5分钟构建一个应用》介绍了如何快速创建一个Spring Boot项目并运行。虽然步骤少流程简单，为开发者省去了很多重复性的配置工作，但是其底层实现并没有这么简单。

这篇，我们就通过入口类TutorialApplication看看Spring Boot是如何启动的。

注解

写过Spring Boot都知道需要有一个入口类，就是本例子中的TutorialApplication，而这个类上面必不可上的需要有一个@SpringBootApplication注解。

点击进入该注解，我们可以发现其是一个复合注解，包括@SpringBootConfiguration、@EnableAutoConfiguration和@ComponentScan。

/** * Indicates a {@link Configuration configuration} class that declares one or more * {@link Bean @Bean} methods and also triggers {@link EnableAutoConfiguration * auto-configuration} and {@link ComponentScan component scanning}. This is a convenience * annotation that is equivalent to declaring {@code @Configuration}, * {@code @EnableAutoConfiguration} and {@code @ComponentScan}. * * @author Phillip Webb * @author Stephane Nicoll * @since 1.2.0 */@Target(ElementType.TYPE)@Retention(RetentionPolicy.RUNTIME)@Documented@Inherited@SpringBootConfiguration@EnableAutoConfiguration@ComponentScan(excludeFilters = {      @Filter(type = FilterType.CUSTOM, classes = TypeExcludeFilter.class),      @Filter(type = FilterType.CUSTOM, classes = AutoConfigurationExcludeFilter.class) })public @interface SpringBootApplication {   /**    * Exclude specific auto-configuration classes such that they will never be applied.    * @return the classes to exclude    */   @AliasFor(annotation = EnableAutoConfiguration.class)   Class
    [] exclude() default {};   /**    * Exclude specific auto-configuration class names such that they will never be    * applied.    * @return the class names to exclude    * @since 1.3.0    */   @AliasFor(annotation = EnableAutoConfiguration.class)   String[] excludeName() default {};   /**    * Base packages to scan for annotated components. Use {@link #scanBasePackageClasses}    * for a type-safe alternative to String-based package names.    * @return base packages to scan    * @since 1.3.0    */   @AliasFor(annotation = ComponentScan.class, attribute = "basePackages")   String[] scanBasePackages() default {};   /**    * Type-safe alternative to {@link #scanBasePackages} for specifying the packages to    * scan for annotated components. The package of each class specified will be scanned.    *     * Consider creating a special no-op marker class or interface in each package that    * serves no purpose other than being referenced by this attribute.    * @return base packages to scan    * @since 1.3.0    */   @AliasFor(annotation = ComponentScan.class, attribute = "basePackageClasses")   Class
     [] scanBasePackageClasses() default {};}

@SpringBootConfiguration

该注解底层其实就是@Configuration注解。熟悉Spring的发展里程碑就知道这是Java Config的配置形式。

通过该注解修饰，表示该类是一个配置类。

@EnableAutoConfiguration

该注解其实也是一个复合注解。

/** * Enable auto-configuration of the Spring Application Context, attempting to guess and * configure beans that you are likely to need. Auto-configuration classes are usually * applied based on your classpath and what beans you have defined. For example, if you * have {@code tomcat-embedded.jar} on your classpath you are likely to want a * {@link TomcatServletWebServerFactory} (unless you have defined your own * {@link ServletWebServerFactory} bean). *  * When using {@link SpringBootApplication}, the auto-configuration of the context is * automatically enabled and adding this annotation has therefore no additional effect. * 
 * Auto-configuration tries to be as intelligent as possible and will back-away as you * define more of your own configuration. You can always manually {@link #exclude()} any * configuration that you never want to apply (use {@link #excludeName()} if you don't * have access to them). You can also exclude them via the * {@code spring.autoconfigure.exclude} property. Auto-configuration is always applied * after user-defined beans have been registered. * 
 * The package of the class that is annotated with {@code @EnableAutoConfiguration}, * usually via {@code @SpringBootApplication}, has specific significance and is often used * as a 'default'. For example, it will be used when scanning for {@code @Entity} classes. * It is generally recommended that you place {@code @EnableAutoConfiguration} (if you're * not using {@code @SpringBootApplication}) in a root package so that all sub-packages * and classes can be searched. * 
 * Auto-configuration classes are regular Spring {@link Configuration} beans. They are * located using the {@link SpringFactoriesLoader} mechanism (keyed against this class). * Generally auto-configuration beans are {@link Conditional @Conditional} beans (most * often using {@link ConditionalOnClass @ConditionalOnClass} and * {@link ConditionalOnMissingBean @ConditionalOnMissingBean} annotations). * * @author Phillip Webb * @author Stephane Nicoll * @see ConditionalOnBean * @see ConditionalOnMissingBean * @see ConditionalOnClass * @see AutoConfigureAfter * @see SpringBootApplication */@Target(ElementType.TYPE)@Retention(RetentionPolicy.RUNTIME)@Documented@Inherited@AutoConfigurationPackage@Import(AutoConfigurationImportSelector.class)public @interface EnableAutoConfiguration {   String ENABLED_OVERRIDE_PROPERTY = "spring.boot.enableautoconfiguration";   /**    * Exclude specific auto-configuration classes such that they will never be applied.    * @return the classes to exclude    */   Class
     [] exclude() default {};   /**    * Exclude specific auto-configuration class names such that they will never be    * applied.    * @return the class names to exclude    * @since 1.3.0    */   String[] excludeName() default {};}

其实现也是通过类似@Import的方式注入AutoConfigurationImportSelector类，并借助该类将所有符合条件的Configuration注解修饰的配置类加载到Spring Boot容器中。从classpath中搜索所有的META-INF/spring.factories配置文件，将其中org.springframework.boot.autoconfigure.EnableAutoConfiguration对应配置项通过反射的形式实例化为标注了@Configuration和javaconfig形式的IOC容器配置类，然后汇总为一个并加载到ioc容器中。

@ComponentScan

这个注解就不需要多介绍了吧，其作用自动扫描加载符合条件的bean。

SpringApplication

从项目的入口第一个碰到的就是SpringApplication类。

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

进入该类的静态方法run，可以看到其在构造SpringApplication对象

public static ConfigurableApplicationContext run(Class
    [] primarySources,      String[] args) {   return new SpringApplication(primarySources).run(args);}

进入SpringApplication构造方法，可以看到

public SpringApplication(ResourceLoader resourceLoader, Class
    ... primarySources) {   this.resourceLoader = resourceLoader;   Assert.notNull(primarySources, "PrimarySources must not be null");   this.primarySources = new LinkedHashSet<>(Arrays.asList(primarySources));   this.webApplicationType = deduceWebApplicationType();   setInitializers((Collection) getSpringFactoriesInstances(         ApplicationContextInitializer.class));   setListeners((Collection) getSpringFactoriesInstances(ApplicationListener.class));   this.mainApplicationClass = deduceMainApplicationClass();}

主要做了如下几件事：

加载Source，这里只有只有Application

推断WebApplicationType，该枚举有三种类型NONE、SERVLET、REACTIVE。

设置初始化器变量setInitializers，初始化后得到6个初始化变量，这些类在上面提到的spring.factories中可以找到

设置监听器，与上面setInitializers实现类似，最终得到如下10个listeners

最后推断带有main函数的所在类，即入口类，这里就是TutorialApplication

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;}

run方法

看完SpringApplication是如何初始化后，我们来看下这个后面的run方法具体做了哪些事。

public ConfigurableApplicationContext run(String... args) {   StopWatch stopWatch = new StopWatch();   stopWatch.start();   ConfigurableApplicationContext context = null;   Collection
    
      exceptionReporters = new ArrayList<>();   configureHeadlessProperty();   SpringApplicationRunListeners listeners = getRunListeners(args);   listeners.starting();   try {      ApplicationArguments applicationArguments = new DefaultApplicationArguments(            args);      ConfigurableEnvironment environment = prepareEnvironment(listeners,            applicationArguments);      configureIgnoreBeanInfo(environment);      Banner printedBanner = printBanner(environment);      context = createApplicationContext();      exceptionReporters = getSpringFactoriesInstances(            SpringBootExceptionReporter.class,            new Class[] { ConfigurableApplicationContext.class }, context);      prepareContext(context, environment, listeners, applicationArguments,            printedBanner);      refreshContext(context);      afterRefresh(context, applicationArguments);      stopWatch.stop();      if (this.logStartupInfo) {         new StartupInfoLogger(this.mainApplicationClass)               .logStarted(getApplicationLog(), stopWatch);      }      listeners.started(context);      callRunners(context, applicationArguments);   }   catch (Throwable ex) {      handleRunFailure(context, ex, exceptionReporters, listeners);      throw new IllegalStateException(ex);   }   try {      listeners.running(context);   }   catch (Throwable ex) {      handleRunFailure(context, ex, exceptionReporters, null);      throw new IllegalStateException(ex);   }   return context;}

StopWatch，这是一个spring-core中的工具类，用来给程序运行计时（对于经常遇到需要计算某方法或接口耗时的场景，这个比System.currentTimeMillis好用）

configureHeadlessProperty配置，设置系统属性 java.awt.headless，这里设置为 true，表示运行在服务器端，在没有显示器和鼠标键盘的模式下工作，模拟输入输出设备功能。

遍历listeners并启动

封装入参args为AppliationArguments对象

打印banner（就是我们启动时看到的spring标识）

后面就是初始化上下文并加载上下文，具体实现就不进去看了

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