SpringBoot启动源码分析及相关技巧学习

2,255 阅读7分钟

对于源码学习,我觉得我们带着问题一起看会好一点。

一、Springboot的启动原理是怎样的?

话不多说,我们首先去[start.spring.io]网站上下载一个demo,springboot版本我们选择2.1.4,然后我们一起打断点一步步了解下springboot的启动原理。

我们的工程目录如下:

一切的一切,将从我们的DemoApplication.java文件开始。代码如下:

@SpringBootApplication
public class DemoApplication {
	public static void main(String[] args) {
		SpringApplication.run(DemoApplication.class, args);
	}
}

技巧一

我经常看到有朋友在DemoApplication类中实现ApplicationContextAware接口,然后获取ApplicationContext对象,就比如下面的代码:

@SpringBootApplication
public class DemoApplication implements ApplicationContextAware {
    private static ApplicationContext applicationContext = null;
    public static void main(String[] args) {
    	SpringApplication.run(DemoApplication.class, args);
    	// 获取某个bean
    	System.out.println(applicationContext.getBean("xxxx"));
    }
    @Override
    public void setApplicationContext(ApplicationContext applicationContext) throws BeansException {
    	DemoApplication.applicationContext = applicationContext;
    }
}

当然这种方法可行,但是其实SpringApplication.run方法已经把Spring上下文返回了,我们直接用就行了~~~代码如下:

@SpringBootApplication
public class DemoApplication {
    public static void main(String[] args) {
    	ConfigurableApplicationContext applicationContext = SpringApplication.run(DemoApplication.class, args);
    	// 获取某个bean
    	System.out.println(applicationContext.getBean("xxxx"));
    }
}

代码跳至SpringApplication类第263

@SuppressWarnings({ "unchecked", "rawtypes" })
    public SpringApplication(ResourceLoader resourceLoader, Class<?>... primarySources) {
        // 1、初始化一个类加载器
    	this.resourceLoader = resourceLoader;
    	Assert.notNull(primarySources, "PrimarySources must not be null");
    	// 2、启动类集合
    	this.primarySources = new LinkedHashSet<>(Arrays.asList(primarySources));
    	// 3、当前应用类型,有三种:NONE、SERVLET、REACTIVE
    	this.webApplicationType = WebApplicationType.deduceFromClasspath();
    	// 4、初始化Initializer
    	setInitializers((Collection) getSpringFactoriesInstances(
    			ApplicationContextInitializer.class));
    	// 5、初始化Listeners
    	setListeners((Collection) getSpringFactoriesInstances(ApplicationListener.class));
    	// 6、初始化入口类
    	this.mainApplicationClass = deduceMainApplicationClass();
    }

步骤1-3没什么好讲的,就是初始化一些标识和列表啥的,重点看下第4和第5点,第4、5点帮我们加载了所有依赖的ApplicationListenerApplicationContextInitializer配置项,代码移步至SpringFactoriesLoader132行,我们可以看到springboot会去加载每个jar里边这个文件META-INF/spring.factories的内容,同时还以类加载器ClassLoader为键值,对所有的配置做了一个Map缓存。

private static Map<String, List<String>> loadSpringFactories(@Nullable ClassLoader classLoader) {
    // cache做了缓存,我们可以指定classloader,默认为Thread.currentThread().getContextClassLoader();
    // (可在ClassUtils类中getDefaultClassLoader找到答案)
    MultiValueMap<String, String> result = cache.get(classLoader);
    if (result != null) {
    	return result;
    }
    
    try {
    	Enumeration<URL> urls = (classLoader != null ?
    	        // FACTORIES_RESOURCE_LOCATION的值就是META-INF/spring.factories
    			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()) {
    			String factoryClassName = ((String) entry.getKey()).trim();
    			for (String factoryName : StringUtils.commaDelimitedListToStringArray((String) entry.getValue())) {
    				result.add(factoryClassName, factoryName.trim());
    			}
    		}
    	}
    	cache.put(classLoader, result);
    	return result;
    }
    catch (IOException ex) {
    	throw new IllegalArgumentException("Unable to load factories from location [" +
    			FACTORIES_RESOURCE_LOCATION + "]", ex);
    }
}

我们简单看下spring-boot-autoconfigure-2.1.4.RELEASE.jar下的spring.factories看下内容:

# Initializers初始化器
org.springframework.context.ApplicationContextInitializer=\
org.springframework.boot.autoconfigure.SharedMetadataReaderFactoryContextInitializer,\
org.springframework.boot.autoconfigure.logging.ConditionEvaluationReportLoggingListener

# Application Listeners监听器
org.springframework.context.ApplicationListener=\
org.springframework.boot.autoconfigure.BackgroundPreinitializer

# Auto Configure自动配置(下文将会有讲原理)
org.springframework.boot.autoconfigure.EnableAutoConfiguration=\
org.springframework.boot.autoconfigure.admin.SpringApplicationAdminJmxAutoConfiguration
......

技巧二

接下来我们看下步骤6,这里可以学习一个小技巧,我们如何获得当前方法调用链中某一个中间方法所在的类信息呢?我们看源码:

private Class<?> deduceMainApplicationClass() {
    try {
    	StackTraceElement[] stackTrace = new RuntimeException().getStackTrace();
    	// 获取运行时方法栈
    	for (StackTraceElement stackTraceElement : stackTrace) {
    	    // 根据名称找到类名
    		if ("main".equals(stackTraceElement.getMethodName())) {
    			return Class.forName(stackTraceElement.getClassName());
    		}
    	}
    }
    catch (ClassNotFoundException ex) {
    	// Swallow and continue
    }
    return null;
}

到目前为止,我们只完成了SpringApplication这个类的初始化工作,我们拥有了META-INF/spring.factories目录下配置的包括监听器、初始化器在内的所有类名,并且实例化了这些类,最后存储于SpringApplication这个类中。

代码移步至SpringApplication.java295行,代码如下

public ConfigurableApplicationContext run(String... args) {
    // 1、计时器,spring内部封装的计时器,用于计算容器启动的时间
    StopWatch stopWatch = new StopWatch();
    stopWatch.start();
    // 2、创建一个初始化上下文变量
    ConfigurableApplicationContext context = null;
    // 3、这是spring报告之类的,没深入了解
    Collection<SpringBootExceptionReporter> exceptionReporters = new ArrayList<>();
    configureHeadlessProperty();
    // 4、获取配置的SpringApplicationRunListener类型的监听器,并且启动它
    SpringApplicationRunListeners listeners = getRunListeners(args);
    listeners.starting();
    try {
    	ApplicationArguments applicationArguments = new DefaultApplicationArguments(
    			args);
    	// 5、准备spring上下文环境
    	ConfigurableEnvironment environment = prepareEnvironment(listeners,
    			applicationArguments);
    	configureIgnoreBeanInfo(environment);
    	// 6、打印banner
    	Banner printedBanner = printBanner(environment);
    	// 7、为context赋值
    	context = createApplicationContext();
    	exceptionReporters = getSpringFactoriesInstances(
    			SpringBootExceptionReporter.class,
    			new Class[] { ConfigurableApplicationContext.class }, context);
    	// 8、准备好context上下文各种组件,environment,listeners
    	prepareContext(context, environment, listeners, applicationArguments,
    			printedBanner);
    	// 9、刷新上下文
    	refreshContext(context);
    	afterRefresh(context, applicationArguments);
    	// 10、计时器关闭
    	stopWatch.stop();
    	if (this.logStartupInfo) {
    		new StartupInfoLogger(this.mainApplicationClass)
    				.logStarted(getApplicationLog(), stopWatch);
    	}
    	listeners.started(context);
    	// 11、调用runners,后面会讲到
    	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;
}

技巧三

步骤1中使用到了计时器StopWatch这个工具,这个工具我们也可以直接拿来使用的,通常我们统计一段代码、一个方法执行的时间,我们会使用System.currentTimeMillis来实现,我们也可以使用StopWatch来代替,StopWatch的强大之处在于它可以统计各个时间段的耗时占比,使用大致如下:

@SpringBootApplication
public class DemoApplication {
    public static void main(String[] args) {
    	StopWatch stopWatch = new StopWatch();
    	stopWatch.start("startContext");
    	ConfigurableApplicationContext applicationContext = SpringApplication.run(DemoApplication.class, args);
    	stopWatch.stop();
    	stopWatch.start("printBean");
    	// 获取某个bean
    	System.out.println(applicationContext.getBean(DemoApplication.class));
    	stopWatch.stop();
    	System.err.println(stopWatch.prettyPrint());
    }
}

步骤4代码移步至SpringApplication413行,代码如下:

private SpringApplicationRunListeners getRunListeners(String[] args) {
    Class<?>[] types = new Class<?>[] { SpringApplication.class, String[].class };
    return new SpringApplicationRunListeners(logger, getSpringFactoriesInstances(
		SpringApplicationRunListener.class, types, this, args));
}

可以看出,springboot依旧是去META-INF/spring.factoriesSpringApplicationRunListener配置的类,并且启动。

步骤5默认创建Spring Environment模块中的StandardServletEnvironment标准环境。

步骤7默认创建的上下文类型是AnnotationConfigServletWebServerApplicationContext,可以看出这个是Spring上下文中基于注解的Servlet上下文,因此,我们最开始的DemoApplication.java类中声明的注解@SpringBootApplication将会被扫描并解析。

步骤9刷新上下文是最核心的,看过spring源码都知道,这个refresh()方法很经典,具体可以参考小编另一篇文章Spring容器IOC初始化过程

步骤11中会执行整个上下文中,所有实现了ApplicationRunnerCommandLineRunner的bean,SpringApplication787代码如下:

private void callRunners(ApplicationContext context, ApplicationArguments args) {
    List<Object> runners = new ArrayList<>();
    runners.addAll(context.getBeansOfType(ApplicationRunner.class).values());
    runners.addAll(context.getBeansOfType(CommandLineRunner.class).values());
    // 对所有runners进行排序并执行
    AnnotationAwareOrderComparator.sort(runners);
    for (Object runner : new LinkedHashSet<>(runners)) {
    	if (runner instanceof ApplicationRunner) {
    		callRunner((ApplicationRunner) runner, args);
    	}
    	if (runner instanceof CommandLineRunner) {
    		callRunner((CommandLineRunner) runner, args);
    	}
    }
}

技巧四】 平时开发中,我们可能会想在Spring容器启动完成之后执行一些操作,举个例子,就假如我们某个定时任务需要再应用启动完成时执行一次,看了上面步骤11的源码,我们大概对下面的代码会恍然大悟,哦,原来这代码就是在SpringApplication这个类中调用的。

@Component
public class MyRunner implements ApplicationRunner {

	@Override
	public void run(ApplicationArguments args) throws Exception {
		System.err.println("执行了ApplicationRunner~");
	}

}
@Component
public class MyCommandRunner implements CommandLineRunner {

	@Override
	public void run(String... args) throws Exception {
		System.out.println("执行了commandrunner");
	}

}

注意点:

  • 1、CommandLineRunner和ApplicationRunner执行时期是在spring容器启动完成之后执行的
  • 2、整个容器生命周期只执行一次

二、注解@EnableAutoConfiguration的作用是什么?

一般情况下,java引入的jar文件中声明的bean是不会被spring扫描到的,那么我们的各种starter是如何初始化自身的bean呢?答案是在META-INF/spring.factories中声明org.springframework.boot.autoconfigure.EnableAutoConfiguration=xxx,就比如spring-cloud-netflix-zuul这个starter中申明的内容如下:

org.springframework.boot.autoconfigure.EnableAutoConfiguration=\
org.springframework.cloud.netflix.zuul.ZuulServerAutoConfiguration,\
org.springframework.cloud.netflix.zuul.ZuulProxyAutoConfiguration

这样声明是什么意思呢?就是说springboot启动的过程中,会将org.springframework.boot.autoconfigure.EnableAutoConfiguration=xxx声明的类实例成为bean,并且注册到容器当中,下面是测试用例:

我们在mydemo中声明一个bean,代码如下:

@Service
public class MyUser {

}

demo中,打印MyUser这个bean,打印如下:

Exception in thread "main" org.springframework.beans.factory.NoSuchBeanDefinitionException: No qualifying bean of type 'css.demo.user.MyUser' available
	at org.springframework.beans.factory.support.DefaultListableBeanFactory.getBean(DefaultListableBeanFactory.java:343)
	at org.springframework.beans.factory.support.DefaultListableBeanFactory.getBean(DefaultListableBeanFactory.java:335)
	at org.springframework.context.support.AbstractApplicationContext.getBean(AbstractApplicationContext.java:1123)
	at com.example.demo.DemoApplication.main(DemoApplication.java:15)

我们mydemo工程中加上该配置:

demo工程打印如下:

2019-08-02 19:31:34.814  INFO 21984 --- [           main] o.s.b.w.embedded.tomcat.TomcatWebServer  : Tomcat started on port(s): 8080 (http) with context path ''
2019-08-02 19:31:34.818  INFO 21984 --- [           main] com.example.demo.DemoApplication         : Started DemoApplication in 2.734 seconds (JVM running for 3.254)
执行了ApplicationRunner~
执行了commandrunner
css.demo.user.MyUser@589b028e

为什么配置上去就可以了呢?其实在springboot启动过程中,在AutoConfigurationImportSelector#getAutoConfigurationEntry中会去调用getCandidateConfigurations方法,该方法源码如下:

protected List<String> getCandidateConfigurations(AnnotationMetadata metadata,
			AnnotationAttributes attributes) {
    List<String> configurations = SpringFactoriesLoader.loadFactoryNames(
            // 此处会去调用EnableAutoConfiguration注解
    		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;
}

getSpringFactoriesLoaderFactoryClass方法源码如下:

protected Class<?> getSpringFactoriesLoaderFactoryClass() {
	return EnableAutoConfiguration.class;
}

本质上还是利用了META-INF/spring.factories文件中的配置,结合springboot factories机制完成的。

三、总结

本文从大致方向解析了springboot的大致启动过程,有些地方点到为止,并未做深入研究,但我们学习源码一为了吸收其编码精华,写出更好的代码,二为了解相关原理,方便更加快速定位解决问题,如有写的不对的地方,请指正,欢迎评论区留言交流,谢谢大家!