SpringApplication#run()
main函数经常调用的run()方法是我们分析的关键，先上源码
public ConfigurableApplicationContext run(String... args) {
StopWatch stopWatch = new StopWatch();
stopWatch.start();
ConfigurableApplicationContext context = null;
Collection<SpringBootExceptionReporter> exceptionReporters = new ArrayList<>();
// 读取java.awt.headless系统变量，默认为true.常用于linux图片的渲染
configureHeadlessProperty();
// 获取SpringApplicationRunListener接口集合并实例化调用公共接口starting()
SpringApplicationRunListeners listeners = getRunListeners(args);
listeners.starting();
try {
ApplicationArguments applicationArguments = new DefaultApplicationArguments(
args);
// environment configuration
ConfigurableEnvironment environment = prepareEnvironment(listeners,
applicationArguments);
// spring.beaninfo.ignore属性读取
configureIgnoreBeanInfo(environment);
// springboot的banner样图
Banner printedBanner = printBanner(environment);
// 创建spring应用上下文(尚未刷新)
context = createApplicationContext();
// SpringBootExceptionReporter接口集合读取
exceptionReporters = getSpringFactoriesInstances(
SpringBootExceptionReporter.class,
new Class[] { ConfigurableApplicationContext.class }, context);
// applicationContext configuration
prepareContext(context, environment, listeners, applicationArguments,
printedBanner);
// 刷新spring应用上下文
refreshContext(context);
afterRefresh(context, applicationArguments);
stopWatch.stop();
if (this.logStartupInfo) {
new StartupInfoLogger(this.mainApplicationClass)
.logStarted(getApplicationLog(), stopWatch);
}
// SpringApplicationRunListener接口的started()方法调用
listeners.started(context);
callRunners(context, applicationArguments);
}
catch (Throwable ex) {
handleRunFailure(context, ex, exceptionReporters, listeners);
throw new IllegalStateException(ex);
}
try {
// SpringApplicationRunListener接口的running()方法调用
listeners.running(context);
}
catch (Throwable ex) {
handleRunFailure(context, ex, exceptionReporters, null);
throw new IllegalStateException(ex);
}
return context;
}
以上的代码注释有点多，笔者分块来进行罗列分析
SpringApplication#getRunListeners()
获取SpringApplicationRunListener接口集合并实例化，根据前文得知，读取的是META\spring.factories文件中的对应属性，
此处笔者以org.springframework.boot.context.event.EventPublishingRunListener为例。
先观察下其构造函数
public EventPublishingRunListener(SpringApplication application, String[] args) {
this.application = application;
this.args = args;
this.initialMulticaster = new SimpleApplicationEventMulticaster();
for (ApplicationListener<?> listener : application.getListeners()) {
this.initialMulticaster.addApplicationListener(listener);
}
}
注意application.getListeners()方法，根据前文得知，其会拿到类型为ApplicationListener的集合并存入至SimpleApplicationEventMulticaster广播类中；
其余方法，比如starting()/started()/environmentPrepared()等等方法均是由其统一调用所有的ApplicationListener接口的对应事件。笔者此处以starting()为例
@Override
public void starting() {
this.initialMulticaster.multicastEvent(
new ApplicationStartingEvent(this.application, this.args));
}
其会找寻支持响应ApplicationStartingEvent事件的Listeners，并执行相应的事件方法。响应的监听器有LoggingApplicationListener和LiquibaseServiceLocatorApplicationListener等
SpringApplication#prepareEnvironment()
Environment环境准备工作
private ConfigurableEnvironment prepareEnvironment(
SpringApplicationRunListeners listeners,
ApplicationArguments applicationArguments) {
// Create and configure the environment
ConfigurableEnvironment environment = getOrCreateEnvironment();
// 解析args参数和spring.profiles.active配置读取
configureEnvironment(environment, applicationArguments.getSourceArgs());
// 触发ApplicationEnvironmentPreparedEvent事件
listeners.environmentPrepared(environment);
bindToSpringApplication(environment);
if (this.webApplicationType == WebApplicationType.NONE) {
environment = new EnvironmentConverter(getClassLoader())
.convertToStandardEnvironmentIfNecessary(environment);
}
ConfigurationPropertySources.attach(environment);
return environment;
}
内含代码内容过多，笔者此处针对自己的阅读作下小结
args参数是会被包装为SimpleCommandLinePropertySource属性源，对应key为
commandLineArgs。用户可通过系统变量设定spring.config.location/spring.config.name等属性spring.profiles.active系统属性读取，用于不同条件的配置ApplicationEnvironmentPreparedEvent事件触发，主要的有ConfigFileApplicationListener监听类(其会默认读取application.properties/application.xml/application.yaml配置文件)
PS:ConfigFileApplicationListener这个类比较重要，其也会去读取配置文件中的
spring.profiles.active属性加载Profile；
另外也去读取EnvironmentPostProcessor接口来统一调用。有兴趣的读者可好好分析一下将Environment含有的属性源绑定至key为
configurationProperties的PropertySources类型中，方便springboot全局搜索上下文所含有的属性
SpringApplication#configureIgnoreBeanInfo()
读取spring.beaninfo.ignore属性，默认为true，主要是用于忽略bean的基本信息
SpringApplication#printBanner()
创建banner打印对象，其默认打印的样图如下(作用于console)。具体的读者感兴趣可自行阅读原理
. ____ _ __ _ _
/\\ / ___'_ __ _ _(_)_ __ __ _ \ \ \ \
( ( )\___ | '_ | '_| | '_ \/ _` | \ \ \ \
\\/ ___)| |_)| | | | | || (_| | ) ) ) )
' |____| .__|_| |_|_| |_\__, | / / / /
=========|_|==============|___/=/_/_/_/
:: Spring Boot :: (v2.0.3.RELEASE)
SpringApplication#createApplicationContext()
创建应用上下文对象，其会根据判断出来的应用类型来创建相应的上下文。
| SERVLET | org.springframework.boot.web.servlet.context.AnnotationConfigServletWebServerApplicationContext |
| REACTIVE | org.springframework.boot.web.servlet.context.AnnotationConfigReactiveWebServerApplicationContext |
| NONE | org.springframework.context.annotation.AnnotationConfigApplicationContext |
很明显，全部会应用注解方式来加载上下文。
SpringApplication#prepareContext()
对已创建的上下文对象作下预备工作
SpringApplication#applyInitializers()
启动相应的初始化类，这些初始化类均是ApplicationContextInitializer接口的实现类
protected void applyInitializers(ConfigurableApplicationContext context) {
for (ApplicationContextInitializer initializer : getInitializers()) {
// 检查相应的ApplicationContextInitializer实体类所接收的泛型实体class
Class<?> requiredType = GenericTypeResolver.resolveTypeArgument(
initializer.getClass(), ApplicationContextInitializer.class);
// 只对接收的泛型为ConfigurableApplicationContext.class进行相应的初始化
Assert.isInstanceOf(requiredType, context, "Unable to call initializer.");
initializer.initialize(context);
}
}
针对上面的代码注释，初始化类基本有ConfigurationWarningsApplicationContextInitializer/ContextIdApplicationContextInitializer/DelegatingApplicationContextInitializer/ServerPortInfoApplicationContextInitializer等。
有兴趣的可自行分析相应的初始化类都执行了哪些操作
SpringApplication#load()
加载beans到相应的上下文对象ApplicationContext中
// 此处的source一般为main函数所在的class，也可通过SpringApplication#setSource()来新增
protected void load(ApplicationContext context, Object[] sources) {
if (logger.isDebugEnabled()) {
logger.debug(
"Loading source " + StringUtils.arrayToCommaDelimitedString(sources));
}
BeanDefinitionLoader loader = createBeanDefinitionLoader(
getBeanDefinitionRegistry(context), sources);
if (this.beanNameGenerator != null) {
loader.setBeanNameGenerator(this.beanNameGenerator);
}
if (this.resourceLoader != null) {
loader.setResourceLoader(this.resourceLoader);
}
if (this.environment != null) {
loader.setEnvironment(this.environment);
}
// 加载beans
loader.load();
}
笔者此处最关心这个BeanDefinitionLoader会耍什么花样，继续往下
先从构造函数开始
BeanDefinitionLoader(BeanDefinitionRegistry registry, Object... sources) {
Assert.notNull(registry, "Registry must not be null");
Assert.notEmpty(sources, "Sources must not be empty");
this.sources = sources;
// 注解解析类
this.annotatedReader = new AnnotatedBeanDefinitionReader(registry);
// xml解析类
this.xmlReader = new XmlBeanDefinitionReader(registry);
if (isGroovyPresent()) {
this.groovyReader = new GroovyBeanDefinitionReader(registry);
}
// classpath扫描类，并屏蔽指定的sources类
this.scanner = new ClassPathBeanDefinitionScanner(registry);
this.scanner.addExcludeFilter(new ClassExcludeFilter(sources));
}
再看下处理方法load()
public int load() {
int count = 0;
for (Object source : this.sources) {
count += load(source);
}
return count;
}
private int load(Object source) {
Assert.notNull(source, "Source must not be null");
if (source instanceof Class<?>) {
return load((Class<?>) source);
}
if (source instanceof Resource) {
return load((Resource) source);
}
if (source instanceof Package) {
return load((Package) source);
}
if (source instanceof CharSequence) {
return load((CharSequence) source);
}
throw new IllegalArgumentException("Invalid source type " + source.getClass());
}
// 笔者重点关注此处
private int load(Class<?> source) {
if (isGroovyPresent()
&& GroovyBeanDefinitionSource.class.isAssignableFrom(source)) {
// Any GroovyLoaders added in beans{} DSL can contribute beans here
GroovyBeanDefinitionSource loader = BeanUtils.instantiateClass(source,
GroovyBeanDefinitionSource.class);
load(loader);
}
// 查看class是否被@Component修饰过。此处多用于加载main类
if (isComponent(source)) {
// 注册bean
this.annotatedReader.register(source);
return 1;
}
return 0;
}
此处的load()方法主要是对设置的sources类进行判断是否被@Component注解，是则注入至bean工厂。即我们常写的main()函数类一般会加上@SpringApplication注解，其最终会被AnnotatedBeanDefinitionReader处理。
SpringApplication#refresh()
刷新上下文对象，笔者此处就不展开了，可参考之前的文章Spring源码情操陶冶-AbstractApplicationContext
异常处理
springboot的异常处理机制是通过读取SpringBootExceptionReporter接口类来对不同的异常进行不同的输出，感兴趣的可自行阅读
小结
通过上述的分析，基本对springboot的工作原理有了一定的了解，最主要的其实还是其会将公共的配置放置于META\spring.factories文件中，我们以后只要多关注此文件就会明白的更多。
至于@SpringBootApplication注解是如何为springboot服务的，笔者后续再分析