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.
* <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 {};
}@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).
* <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 {};
}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<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;
}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
原文出处:https://www.cnblogs.com/bigdataZJ/p/springboot-start.html