Spring Boot 探索之旅（三）——Boot项目启动运行原理

╰半橙微兮° 2022-05-10 03:44 154阅读 0赞

# 一、boot项目核心注解@SpringBootApplication #

**首先进入**SpringBootApplication**源码分析**

@Target(ElementType.TYPE)//注解生效地点：类
    @Retention(RetentionPolicy.RUNTIME)//注解生效时间：运行期
    @Documented//生成文档
    @Inherited//该注解可以被继承
    @SpringBootConfiguration//表明自身是一个配置类
    @EnableAutoConfiguration//开启自动配置（核心关键）
    @ComponentScan(excludeFilters = {
    		@Filter(type = FilterType.CUSTOM, classes = TypeExcludeFilter.class),
    		@Filter(type = FilterType.CUSTOM, classes = AutoConfigurationExcludeFilter.class) })//默认扫描该类同包及其子包所有类的所有注解
    public @interface SpringBootApplication {
    
    	/**
    	 * 按类排除
    	 */
    	@AliasFor(annotation = EnableAutoConfiguration.class)
    	Class<?>[] exclude() default {};
    
    	/**
    	 * 按类名排除
    	 */
    	@AliasFor(annotation = EnableAutoConfiguration.class)
    	String[] excludeName() default {};
    
    	/**
    	 * 扫描具有componenScan注解的类名
    	 */
    	@AliasFor(annotation = ComponentScan.class, attribute = "basePackages")
    	String[] scanBasePackages() default {};
    
    	/**
    	 * 扫描具有componenScan注解的类
    	 */
    	@AliasFor(annotation = ComponentScan.class, attribute = "basePackageClasses")
    	Class<?>[] scanBasePackageClasses() default {};
    
    }

**再进入EnableAutoConfiguration源码分析**

@Target(ElementType.TYPE)
    @Retention(RetentionPolicy.RUNTIME)
    @Documented
    @Inherited
    @AutoConfigurationPackage
    @Import(AutoConfigurationImportSelector.class)//导入配置选择器
    public @interface EnableAutoConfiguration {
    
    	String ENABLED_OVERRIDE_PROPERTY = "spring.boot.enableautoconfiguration";
    
    	/**
    	 * 按类解除自动配置
    	 */
    	Class<?>[] exclude() default {};
    
    	/**
    	 *按类名解除自动配置
    	 */
    	String[] excludeName() default {};
    
    }

**对导入的AutoConfigurationImportSelector分析**

public class AutoConfigurationImportSelector
    		implements DeferredImportSelector, BeanClassLoaderAware, ResourceLoaderAware,
    		BeanFactoryAware, EnvironmentAware, Ordered {
    
    	private static final String[] NO_IMPORTS = {};
    
    	private static final Log logger = LogFactory
    			.getLog(AutoConfigurationImportSelector.class);
    
    	private static final String PROPERTY_NAME_AUTOCONFIGURE_EXCLUDE = "spring.autoconfigure.exclude";
    
    	private ConfigurableListableBeanFactory beanFactory;
    
    	private Environment environment;
    
    	private ClassLoader beanClassLoader;
    
    	private ResourceLoader resourceLoader;
    
    	@Override
    	public String[] selectImports(AnnotationMetadata annotationMetadata) {
        //如果开关关闭，则不做任何自动配置
    		if (!isEnabled(annotationMetadata)) {
    			return NO_IMPORTS;
    		}
        //通过类加载器获取默认配置数据
    		AutoConfigurationMetadata autoConfigurationMetadata = AutoConfigurationMetadataLoader
    				.loadMetadata(this.beanClassLoader);
    		AnnotationAttributes attributes = getAttributes(annotationMetadata);
    		List<String> configurations = getCandidateConfigurations(annotationMetadata,
    				attributes);
    		configurations = removeDuplicates(configurations);
    		Set<String> exclusions = getExclusions(annotationMetadata, attributes);
    //排除解除自动配置的类
    		checkExcludedClasses(configurations, exclusions);
    		configurations.removeAll(exclusions);
    		configurations = filter(configurations, autoConfigurationMetadata);
    		fireAutoConfigurationImportEvents(configurations, exclusions);
    		return StringUtils.toStringArray(configurations);
    	}
    
    	@Override
    	public Class<? extends Group> getImportGroup() {
    		return AutoConfigurationGroup.class;
    	}
    
    	protected boolean isEnabled(AnnotationMetadata metadata) {
    		if (getClass() == AutoConfigurationImportSelector.class) {
    			return getEnvironment().getProperty(
    					EnableAutoConfiguration.ENABLED_OVERRIDE_PROPERTY, Boolean.class,
    					true);
    		}
    		return true;
    	}
    
    	/**
    	 * Return the appropriate {@link AnnotationAttributes} from the
    	 * {@link AnnotationMetadata}. By default this method will return attributes for
    	 * {@link #getAnnotationClass()}.
    	 * @param metadata the annotation metadata
    	 * @return annotation attributes
    	 */
    	protected AnnotationAttributes getAttributes(AnnotationMetadata metadata) {
    		String name = getAnnotationClass().getName();
    		AnnotationAttributes attributes = AnnotationAttributes
    				.fromMap(metadata.getAnnotationAttributes(name, true));
    		Assert.notNull(attributes,
    				() -> "No auto-configuration attributes found. Is "
    						+ metadata.getClassName() + " annotated with "
    						+ ClassUtils.getShortName(name) + "?");
    		return attributes;
    	}
    
    	/**
    	 * Return the source annotation class used by the selector.
    	 * @return the annotation class
    	 */
    	protected Class<?> getAnnotationClass() {
    		return EnableAutoConfiguration.class;
    	}
    
    	/**
    	 * Return the auto-configuration class names that should be considered. By default
    	 * this method will load candidates using {@link SpringFactoriesLoader} with
    	 * {@link #getSpringFactoriesLoaderFactoryClass()}.
    	 * @param metadata the source metadata
    	 * @param attributes the {@link #getAttributes(AnnotationMetadata) annotation
    	 * attributes}
    	 * @return a list of candidate configurations
    	 */
    	protected List<String> getCandidateConfigurations(AnnotationMetadata metadata,
    			AnnotationAttributes attributes) {
    		List<String> configurations = SpringFactoriesLoader.loadFactoryNames(
    				getSpringFactoriesLoaderFactoryClass(), getBeanClassLoader());
    		Assert.notEmpty(configurations,
    				"No auto configuration classes found in META-INF/spring.factories. If you "
    						+ "are using a custom packaging, make sure that file is correct.");
    		return configurations;
    	}
    
    。。。。

**        探究AutoConfigurationMetadataLoader.loadMetadata(this.beanClassLoader)**

final class AutoConfigurationMetadataLoader {
    
    //在autoconfiguration依赖中
    	protected static final String PATH = "META-INF/"
    			+ "spring-autoconfigure-metadata.properties";
    
    	private AutoConfigurationMetadataLoader() {
    	}
    
    	public static AutoConfigurationMetadata loadMetadata(ClassLoader classLoader) {
    		return loadMetadata(classLoader, PATH);
    	}
    
    	static AutoConfigurationMetadata loadMetadata(ClassLoader classLoader, String path) {
    //加载默认配置
    		try {
    			Enumeration<URL> urls = (classLoader != null) ? classLoader.getResources(path)
    					: ClassLoader.getSystemResources(path);
    			Properties properties = new Properties();
    			while (urls.hasMoreElements()) {
    				properties.putAll(PropertiesLoaderUtils
    						.loadProperties(new UrlResource(urls.nextElement())));
    			}
    			return loadMetadata(properties);
    		}
    		catch (IOException ex) {
    			throw new IllegalArgumentException(
    					"Unable to load @ConditionalOnClass location [" + path + "]", ex);
    		}
    	}
    
    	static AutoConfigurationMetadata loadMetadata(Properties properties) {
    		return new PropertiesAutoConfigurationMetadata(properties);
    	}
    
    	/**
    	 * {@link AutoConfigurationMetadata} implementation backed by a properties file.
    	 */
    	private static class PropertiesAutoConfigurationMetadata
    			implements AutoConfigurationMetadata {
    
    		private final Properties properties;
    
    		PropertiesAutoConfigurationMetadata(Properties properties) {
    			this.properties = properties;
    		}
    
    		@Override
    		public boolean wasProcessed(String className) {
    			return this.properties.containsKey(className);
    		}
    
    		@Override
    		public Integer getInteger(String className, String key) {
    			return getInteger(className, key, null);
    		}
    
    		@Override
    		public Integer getInteger(String className, String key, Integer defaultValue) {
    			String value = get(className, key);
    			return (value != null) ? Integer.valueOf(value) : defaultValue;
    		}
    
    		@Override
    		public Set<String> getSet(String className, String key) {
    			return getSet(className, key, null);
    		}
    
    		@Override
    		public Set<String> getSet(String className, String key,
    				Set<String> defaultValue) {
    			String value = get(className, key);
    			return (value != null) ? StringUtils.commaDelimitedListToSet(value)
    					: defaultValue;
    		}
    
    		@Override
    		public String get(String className, String key) {
    			return get(className, key, null);
    		}
    
    		@Override
    		public String get(String className, String key, String defaultValue) {
    			String value = this.properties.getProperty(className + "." + key);
    			return (value != null) ? value : defaultValue;
    		}
    
    	}
    
    }

**        查看spring-autoconfigure-metadata.properties文件**

![70][]

# 二、Spring Boot核心Main方法 #

**  分析SpringApplication.run方法**

public ConfigurableApplicationContext run(String... args) {
    //StopWatch检查项目启动所需时间
    		StopWatch stopWatch = new StopWatch();
    		stopWatch.start();
    //创建项目上下文对象
    		ConfigurableApplicationContext context = null;
    		Collection<SpringBootExceptionReporter> exceptionReporters = new ArrayList<>();
    		configureHeadlessProperty();
     // 从配置中获取SpringApplicationRunListener实现类并实例化，
     // 最终把所有这些实例包装到一个SpringApplicationRunListeners实例来使用
    		SpringApplicationRunListeners listeners = getRunListeners(args);
    		listeners.starting();
    		try {
    			ApplicationArguments applicationArguments = new DefaultApplicationArguments(
    					args);
    			ConfigurableEnvironment environment = prepareEnvironment(listeners,
    					applicationArguments);
    			configureIgnoreBeanInfo(environment);
    			Banner printedBanner = printBanner(environment);
    // 这里创建应用上下文
    			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;
    	}

[70]: /images/20220510/7b4ed74418be438cafe10e19646bd784.png