阅读须知
- 版本：2.0.4
- 文章中使用/* */注释的方法会做深入分析
正文
@SpringBootApplication
public class BootApplication {
public static void main(String[] args) {
SpringApplication.run(BootApplication.class, args);
}
}
这段代码相信大家都很熟悉，spring boot的启动类，我们就以这段代码作为切入点，来分析spring boot的启动流程：
SpringApplication：
public static ConfigurableApplicationContext run(Class<?> primarySource,
String... args) {
return run(new Class<?>[] { primarySource }, args);
}
public static ConfigurableApplicationContext run(Class<?>[] primarySources,
String[] args) {
/* 构建SpringApplication并运行，创建并且刷新一个新的ApplicationContext */
return new SpringApplication(primarySources).run(args);
}
SpringApplication：
public SpringApplication(Class<?>... primarySources) {
this(null, primarySources);
}
public SpringApplication(ResourceLoader resourceLoader, Class<?>... primarySources) {
this.resourceLoader = resourceLoader;
Assert.notNull(primarySources, "PrimarySources must not be null");
this.primarySources = new LinkedHashSet<>(Arrays.asList(primarySources));
// 判断是否能够成功加载一些关键的类来确认web应用类型，这个类型后面会用到
this.webApplicationType = deduceWebApplicationType();
/* 获取并设置Spring上下文初始化器 */
setInitializers((Collection) getSpringFactoriesInstances(
ApplicationContextInitializer.class));
/* 获取并设置Spring应用监听器 */
setListeners((Collection) getSpringFactoriesInstances(ApplicationListener.class));
// 追述到应用主类，也就是main方法所在的类
this.mainApplicationClass = deduceMainApplicationClass();
}
这里ApplicationContextInitializer和ApplicationListener我们会在后面用到的时候说明它们的作用。
SpringApplication：
private <T> Collection<T> getSpringFactoriesInstances(Class<T> type) {
return getSpringFactoriesInstances(type, new Class<?>[] {});
}
private <T> Collection<T> getSpringFactoriesInstances(Class<T> type,
Class<?>[] parameterTypes, Object... args) {
ClassLoader classLoader = Thread.currentThread().getContextClassLoader();
/* 加载工厂名称，Set防止重复 */
Set<String> names = new LinkedHashSet<>(
SpringFactoriesLoader.loadFactoryNames(type, classLoader));
// 创建工厂实例
List<T> instances = createSpringFactoriesInstances(type, parameterTypes,
classLoader, args, names);
// 根据@Order和@Priority进行排序
AnnotationAwareOrderComparator.sort(instances);
return instances;
}
SpringFactoriesLoader：
public static List<String> loadFactoryNames(Class<?> factoryClass, @Nullable ClassLoader classLoader) {
String factoryClassName = factoryClass.getName();
/* 加载工厂配置，根据传入的factoryClass获取工厂名称集合 */
return loadSpringFactories(classLoader).getOrDefault(factoryClassName, Collections.emptyList());
}
SpringFactoriesLoader：
private static Map<String, List<String>> loadSpringFactories(@Nullable ClassLoader classLoader) {
MultiValueMap<String, String> result = cache.get(classLoader);
if (result != null) {
return result;
}
try {
// 加载资源，路径META-INF/spring.factories
Enumeration<URL> 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()) {
// value用逗号分隔组成集合
List<String> factoryClassNames = Arrays.asList(
StringUtils.commaDelimitedListToStringArray((String) entry.getValue()));
// 添加key和集合的映射
result.addAll((String) entry.getKey(), factoryClassNames);
}
}
// 结果缓存
cache.put(classLoader, result);
return result;
}
catch (IOException ex) {
throw new IllegalArgumentException("Unable to load factories from location [" +
FACTORIES_RESOURCE_LOCATION + "]", ex);
}
}
我们来看一下META-INF/spring.factories文件中有什么内容：
# 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
文件中我们看到了ApplicationContextInitializer和ApplicationListener的key value配置，还有一些其他的接口，遇到后我们在进行分析。构建好SpringApplication后，接下来就是运行它的run方法：
SpringApplication：
public ConfigurableApplicationContext run(String... args) {
// StopWatch是一个简单的秒表，允许多个任务的计时，暴露每个命名任务的总运行时间和运行时间
StopWatch stopWatch = new StopWatch();
stopWatch.start();
ConfigurableApplicationContext context = null;
Collection<SpringBootExceptionReporter> exceptionReporters = new ArrayList<>();
configureHeadlessProperty();
// 获取SpringApplicationRunListener集合，同样是从上面加载的配置中获取
SpringApplicationRunListeners listeners = getRunListeners(args);
listeners.starting();
try {
ApplicationArguments applicationArguments = new DefaultApplicationArguments(
args);
/* 准备环境 */
ConfigurableEnvironment environment = prepareEnvironment(listeners,
applicationArguments);
configureIgnoreBeanInfo(environment);
// 打印banner，就是我们在控制台看到的那个Spring的logo
Banner printedBanner = printBanner(environment);
// 根据不同的webApplicationType返回不同的应用上下文实例
context = createApplicationContext();
// 同样从上面加载的配置中获取SpringBootExceptionReporter
exceptionReporters = getSpringFactoriesInstances(
SpringBootExceptionReporter.class,
new Class[] { ConfigurableApplicationContext.class }, context);
/* 准备上下文 */
prepareContext(context, environment, listeners, applicationArguments,
printedBanner);
/* 刷新上下文 */
refreshContext(context);
// 刷新后操作，默认空实现，子类覆盖
afterRefresh(context, applicationArguments);
stopWatch.stop();
if (this.logStartupInfo) {
new StartupInfoLogger(this.mainApplicationClass)
.logStarted(getApplicationLog(), stopWatch);
}
listeners.started(context);
// 调用所有ApplicationRunner和CommandLineRunner的run方法
callRunners(context, applicationArguments);
}
catch (Throwable ex) {
// 运行失败的异常处理、日志打印和通知
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;
}
在这里我们看到很多处SpringApplicationRunListener的相关方法调用，我们来说明一下它的作用，就像它的命名一样，主要是监听SpringApplication的run方法的各个关键步骤，在上面加载的配置中，我们看到了一个实现为EventPublishingRunListener，主要作用是发布应用的启动运行、启动完成等一些关键事件，具体代码有兴趣的同学可以自行查阅。
SpringApplication：
private ConfigurableEnvironment prepareEnvironment(
SpringApplicationRunListeners listeners,
ApplicationArguments applicationArguments) {
// 创建Environment
ConfigurableEnvironment environment = getOrCreateEnvironment();
/* 配置environment */
configureEnvironment(environment, applicationArguments.getSourceArgs());
listeners.environmentPrepared(environment);
// 将environment绑定到SpringApplication
bindToSpringApplication(environment);
if (this.webApplicationType == WebApplicationType.NONE) {
environment = new EnvironmentConverter(getClassLoader())
.convertToStandardEnvironmentIfNecessary(environment);
}
// 附加一个ConfigurationPropertySource到environment
ConfigurationPropertySources.attach(environment);
return environment;
}
SpringApplication：
protected void configureEnvironment(ConfigurableEnvironment environment,
String[] args) {
// 在此应用程序的环境中添加、删除或重新排序PropertySource
configurePropertySources(environment, args);
// 配置此应用程序环境的激活（或默认为激活）配置文件
// 可以通过spring.profiles.active属性在配置文件处理期间激活其他配置文件
configureProfiles(environment, args);
}
SpringApplication：
private void prepareContext(ConfigurableApplicationContext context,
ConfigurableEnvironment environment, SpringApplicationRunListeners listeners,
ApplicationArguments applicationArguments, Banner printedBanner) {
context.setEnvironment(environment);
// 应用相应的ApplicationContext后置处理，子类可以覆盖，
// 默认实现为context设置beanNameGenerator和resourceLoader
postProcessApplicationContext(context);
// 在context刷新之前应用之前加载的ApplicationContextInitializer
// 在META-INF/spring.factories中默认配置了4个ApplicationContextInitializer，具体作用可以自行了解一下
applyInitializers(context);
listeners.contextPrepared(context);
// 日志打印
if (this.logStartupInfo) {
logStartupInfo(context.getParent() == null);
logStartupProfileInfo(context);
}
// 添加特定的boot单例bean
context.getBeanFactory().registerSingleton("springApplicationArguments",
applicationArguments);
if (printedBanner != null) {
context.getBeanFactory().registerSingleton("springBootBanner", printedBanner);
}
// 获取所有资源，示例中就是获取到我们的启动类
Set<Object> sources = getAllSources();
Assert.notEmpty(sources, "Sources must not be empty");
// 加载资源注册成为Spring的bean
load(context, sources.toArray(new Object[0]));
listeners.contextLoaded(context);
}
SpringApplication：
private void refreshContext(ConfigurableApplicationContext context) {
refresh(context);
if (this.registerShutdownHook) {
try {
context.registerShutdownHook();
}
catch (AccessControlException ex) {
}
}
}
这里上下文的刷新和注册关闭钩子我们都在之前的Spring源码分析文章中分析过，不再赘述。读者可能会有疑惑，Spring boot不是有非常厉害的自动配置么，文章中并没有看到啊，Spring boot的自动配置我们会用单独的文章来分析，本篇文章我们主要分析整个启动流程的步骤和一些扩展，到这里，整个Spring boot的启动流程就分析完了。
