深入学习 Spring Boot：Spring Boot启动分析
文接 深入学习 Spring Boot：Spring Boot启动分析（上）
2、启动 Spring 应用程序run()
Spring Boot首先帮我们实例化了一个SpringApplication对象，接下类调用这个对象的run方法，创建和刷新一个新的ApplicationContext
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);
listeners.finished(context, null);
stopWatch.stop();
if (this.logStartupInfo) {
new StartupInfoLogger(this.mainApplicationClass)
.logStarted(getApplicationLog(), stopWatch);
}
return context;
}
catch (Throwable ex) {
handleRunFailure(context, listeners, exceptionReporters, ex);
throw new IllegalStateException(ex);
}
}
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2.1 启动一个StopWatch测量程序的运行时间。StopWatch可以在开发和调试阶段验证程序的性能
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2.2 使用Headless模式：
System.setProperty("java.awt.headless",System.getProperty("java.awt.headless", true);
Headless模式是系统的一种配置模式。在该模式下，系统缺少了显示设备、键盘或鼠标。Spring Boot程序一般是服务端程序，服务器往往缺少前述硬件，但又需要使用他们提供的功能，生成相应的数据，以提供给客户端（比如在console生成spring神兽，绘制验证码之类的）。此时，我们可以在程序开始激活headless模式，告诉程序，现在你要工作在Headless mode下，就不要指望硬件帮忙了，你得自力更生，依靠系统的计算能力模拟出这些特性来。
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2.3 通过SpringFactoriesLoader加载SpringApplicationRunListener
并启动，用以监听SpringApplication的run方法产生的各类事件。SpringApplicationRunListeners是对SpringApplicationRunListener实例集合的一个封装。在这里，SpringApplication只加载了一个Listener：EventPublishingRunListener，事实上这个类是充当着事件广播器的作用，它可以将run方法中的事件（如starting、environmentPrepared）封装为Event对象，发布到我们之前加载的ApplicationListener中。 -
2.4 创建和配置ConfigurableEnvironment：
private ConfigurableEnvironment prepareEnvironment(
SpringApplicationRunListeners listeners,
ApplicationArguments applicationArguments) {
// Create and configure the environment
ConfigurableEnvironment environment = getOrCreateEnvironment();
configureEnvironment(environment, applicationArguments.getSourceArgs());
listeners.environmentPrepared(environment);
bindToSpringApplication(environment);
if (this.webApplicationType == WebApplicationType.NONE) {
environment = new EnvironmentConverter(getClassLoader())
.convertToStandardEnvironmentIfNecessary(environment);
}
ConfigurationPropertySources.attach(environment);
return environment;
}
a、根据我们之前检测出来的webApplicationType创建一个ConfigurableEnvironment对象（若this.webApplicationType == WebApplicationType.SERVLET则创建其子类StandardServletEnvironment()对象，否则创建StandardEnvironment对象）。在实例化一个ConfigurableEnvironment对象时，程序会读取运行环境的各种数据，如"java.vm.version" -> "25.73-b02"；
b、将我们在启动应用程序时带入的参数（如“--debug”），配置到ConfigurableEnvironment对象中；
c、调用listeners.environmentPrepared(environment)，发布事件。在这里，EventPublishingRunListener会封装一个ApplicationEnvironmentPreparedEvent，然后发布到各个Listener中。Listener执行的动作我们暂时不分析，明显可以看到的就是在debug模式下，console会打印第一行日志，显示应用程序的运行环境；
d、将environment绑定到SpringApplication上
e、在environment.propertySources中添加（如果没有的话）一个ConfigurationPropertySourcesPropertySource对象，使得environment管理的PropertySource对象能适配 PropertySourcesPropertyResolver，能够通过属性名get到具体的配置，详细见 ConfigurationPropertySources
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2.5 将系统属性“spring.beaninfo.ignore”设置为true，跳过扫描BeanInfo类，防止重复加载bean。详见IGNORE_BEANINFO_PROPERTY_NAME
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2.6 调用
printBanner打印“Spring神兽” -
2.7 根据webApplicationType创建一个ConfigurableApplicationContext对象。在本例中，webApplicationType为”SERVLET“，创建的对象为"org.springframework.boot.web.servlet.context.AnnotationConfigServletWebServerApplicationContext"
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2.8 加载spring.factories中的SpringBootExceptionReporter实现类
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2.9 准备Context：
private void prepareContext(ConfigurableApplicationContext context,
ConfigurableEnvironment environment, SpringApplicationRunListeners listeners,
ApplicationArguments applicationArguments, Banner printedBanner) {
context.setEnvironment(environment);
postProcessApplicationContext(context);
applyInitializers(context);
listeners.contextPrepared(context);
if (this.logStartupInfo) {
logStartupInfo(context.getParent() == null);
logStartupProfileInfo(context);
}
// Add boot specific singleton beans
context.getBeanFactory().registerSingleton("springApplicationArguments",
applicationArguments);
if (printedBanner != null) {
context.getBeanFactory().registerSingleton("springBootBanner", printedBanner);
}
// Load the sources
Set<Object> sources = getAllSources();
Assert.notEmpty(sources, "Sources must not be empty");
load(context, sources.toArray(new Object[sources.size()]));
listeners.contextLoaded(context);
}
a、setEnvironment(environment)
b、postProcessApplicationContext(context)，做一些善后工作：如果成员变量beanNameGenerator不为Null，那么为ApplicationContext对象注册beanNameGenerator bean；如果成员变量resourceLoader不为null，则为ApplicationContext对象设置ResourceLoader。
protected void postProcessApplicationContext(ConfigurableApplicationContext context) {
if (this.beanNameGenerator != null) {
context.getBeanFactory().registerSingleton(
AnnotationConfigUtils.CONFIGURATION_BEAN_NAME_GENERATOR,
this.beanNameGenerator);
}
if (this.resourceLoader != null) {
if (context instanceof GenericApplicationContext) {
((GenericApplicationContext) context)
.setResourceLoader(this.resourceLoader);
}
if (context instanceof DefaultResourceLoader) {
((DefaultResourceLoader) context)
.setClassLoader(this.resourceLoader.getClassLoader());
}
}
}
c、依次调用SpringApplication的initializers中的初始化器：
protected void applyInitializers(ConfigurableApplicationContext context) {
for (ApplicationContextInitializer initializer : getInitializers()) {
Class<?> requiredType = GenericTypeResolver.resolveTypeArgument(
initializer.getClass(), ApplicationContextInitializer.class);
Assert.isInstanceOf(requiredType, context, "Unable to call initializer.");
initializer.initialize(context);
}
}
在本例中，initializers列表及其任务为：
initializers = {ArrayList@952} size = 6
// 读取key为”context.initializer.classes“的配置，实例化Initializer并执行initialize();
0 = {DelegatingApplicationContextInitializer@1057}
// 设置context的id，本例等于”application“；
1 = {ContextIdApplicationContextInitializer@1058}
// 为context.beanFactoryPostProcessor增加一个ConfigurationWarningsPostProcessor对象，报告warning等级的错误配置
2 = {ConfigurationWarningsApplicationContextInitializer@1059}
// 为context添加一个ApplicationListener，监听WebServerInitializedEvent
3 = {ServerPortInfoApplicationContextInitializer@1060}
// 为context.beanFactoryPostProcessor增加一个CachingMetadataReaderFactoryPostProcessor
4 = {SharedMetadataReaderFactoryContextInitializer@1061}
//为context添加一个AutoConfigurationReportListener，用以监听自动配置报告
5 = {AutoConfigurationReportLoggingInitializer@1062}
d、调用监听器，报告contextPrepared事件；
e、打印启动日志：
2017-09-10 20:18:51.937 INFO 28314 --- [ main] com.zhuangqf.learn.App : Starting App on zhuangqinfa with PID 28314 (/home/zhuangqf/workspace/spring/SpringBootDemo/target/classes started by zhuangqf in /home/zhuangqf/workspace/spring/SpringBootDemo)
2017-09-10 20:18:51.939 INFO 28314 --- [ main] com.zhuangqf.learn.App : No active profile set, falling back to default profiles: default
f、注册指定的bean："springApplicationArguments"和"springBootBanner"：
// Add boot specific singleton beans
context.getBeanFactory().registerSingleton("springApplicationArguments", applicationArguments);
if (printedBanner != null) {
context.getBeanFactory().registerSingleton("springBootBanner", printedBanner);
}
简单而言， context的beanFactory是一个DefaultListableBeanFactory对象，其内部有多个Map<String, Object> 用来存放注册的bean。
g、加载context.primarySources，在本例中为App.class，其主要是加载App上的注解或默认的配置文件。
- 2.10 refreshContext(context)
加载或刷新持久化形式的配置（如xml文件、properties文件，和数据库信息）。
由于这是一个启动方法, 如果它失败了, 它应该销毁已经创建的单例, 以避免悬空的资源。换言之, 在调用该方法之后, 所有的单例bean都不应实例化。
@Override
public void refresh() throws BeansException, IllegalStateException {
synchronized (this.startupShutdownMonitor) {
// Prepare this context for refreshing.
prepareRefresh();
// Tell the subclass to refresh the internal bean factory.
ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();
// Prepare the bean factory for use in this context.
prepareBeanFactory(beanFactory);
try {
// Allows post-processing of the bean factory in context subclasses.
postProcessBeanFactory(beanFactory);
// Invoke factory processors registered as beans in the context.
invokeBeanFactoryPostProcessors(beanFactory);
// Register bean processors that intercept bean creation.
registerBeanPostProcessors(beanFactory);
// Initialize message source for this context.
initMessageSource();
// Initialize event multicaster for this context.
initApplicationEventMulticaster();
// Initialize other special beans in specific context subclasses.
onRefresh();
// Check for listener beans and register them.
registerListeners();
// Instantiate all remaining (non-lazy-init) singletons.
finishBeanFactoryInitialization(beanFactory);
// Last step: publish corresponding event.
finishRefresh();
}
catch (BeansException ex) {
if (logger.isWarnEnabled()) {
logger.warn("Exception encountered during context initialization - " +
"cancelling refresh attempt: " + ex);
}
// Destroy already created singletons to avoid dangling resources.
destroyBeans();
// Reset 'active' flag.
cancelRefresh(ex);
// Propagate exception to caller.
throw ex;
}
finally {
// Reset common introspection caches in Spring's core, since we
// might not ever need metadata for singleton beans anymore...
resetCommonCaches();
}
}
}
整个方法比较复杂，我们一点点解析：
a、准备刷新prepareRefresh()，完成了以下任务：
设置context的startupDate为当前时间；
设置closed标志位为false，active标志为true；
读取Property配置到environment中；
检查environment必设的配置是否为null；
初始化this.earlyApplicationEvents作为存放发布时间的Set
b、obtainFreshBeanFactory() ：销毁原有的beanFactory类，并新建一个beanFactory返回
c、prepareBeanFactory(beanFactory)：设置beanFactory的各个属性，由AbstractApplicationContext实现；
d、postProcessBeanFactory(beanFactory)：对beanFactory根据具体的Context子类设置不同的属性，例如，本例中context的具体类型为ServletWebServerApplicationContext，会在beanFactory中注册一个ServletContextAwareProcessor。
e、invokeBeanFactoryPostProcessors(beanFactory);调用注册到beanFactory中的postRrocessors。
