Spring Boot系列——Spring Boot如何启动

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。

<pre style="margin: 0.5em 0px; padding: 0.4em 0.6em; border-radius: 8px; background: rgb(255, 255, 255); color: rgb(0, 0, 0); font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-align: start; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial; font-family: Menlo; font-size: 9pt;">/**
 * 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. * <p>  * 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 {};

}</pre>

@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。

<pre style="margin: 0.5em 0px; padding: 0.4em 0.6em; border-radius: 8px; background: rgb(255, 255, 255); color: rgb(0, 0, 0); font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-align: start; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial; font-family: Menlo; font-size: 9pt;">/**
 * 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).
 * <p>  * When using {@link SpringBootApplication}, the auto-configuration of the context is
 * automatically enabled and adding this annotation has therefore no additional effect. * <p>  * 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. * <p>  * 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. * <p>  * 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 {};

}</pre>

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。

image

@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。

<pre style="margin: 0.5em 0px; padding: 0.4em 0.6em; border-radius: 8px; background: rgb(255, 255, 255); color: rgb(0, 0, 0); font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-align: start; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial; font-family: Menlo; font-size: 9pt;">@SpringBootApplication public class TutorialApplication {

    public static void main(String[] args) {
        SpringApplication.run(TutorialApplication.class, args);
    }
}
</pre>

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

<pre style="margin: 0.5em 0px; padding: 0.4em 0.6em; border-radius: 8px; background: rgb(255, 255, 255); color: rgb(0, 0, 0); font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-align: start; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial; font-family: Menlo; font-size: 9pt;">public static ConfigurableApplicationContext run(Class<?>[] primarySources,
      String[] args) {
   return new SpringApplication(primarySources).run(args);
}</pre>

进入SpringApplication构造方法，可以看到

<pre style="margin: 0.5em 0px; padding: 0.4em 0.6em; border-radius: 8px; background: rgb(255, 255, 255); color: rgb(0, 0, 0); font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-align: start; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial; font-family: Menlo; font-size: 9pt;">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();
}</pre>

主要做了如下几件事：

• 加载Source，这里只有只有Application

image

• 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

image

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

image

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

<pre style="margin: 0.5em 0px; padding: 0.4em 0.6em; border-radius: 8px; background: rgb(255, 255, 255); color: rgb(0, 0, 0); font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-align: start; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial; font-family: Menlo; font-size: 9pt;">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;
}</pre>

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。

<pre style="margin: 0.5em 0px; padding: 0.4em 0.6em; border-radius: 8px; background: rgb(255, 255, 255); color: rgb(0, 0, 0); font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-align: start; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial; font-family: Menlo; font-size: 9pt;">public ConfigurableApplicationContext run(String... args) {
   StopWatch stopWatch = new StopWatch();
   stopWatch.start();
   ConfigurableApplicationContext context = null;
   Collection<SpringBootExceptionReporter> 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;
}</pre>

• 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