spring-boot源码解析之应用启动

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spring-boot源码解析之应用启动

spring boot 项目使用默认配置的思想，极大的简化了 spring 项目的开发。下面的代码就是一个最简单的 spring 项目:

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

代码很简洁，只调用了SpringApplication的 run 方法。接下来，我们就要深入源码，分析下，spring boot的启动过程。

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public static ConfigurableApplicationContext run(Class<?> primarySource, String... args) {
    return run(new Class<?>[] { primarySource }, args);
}

public static ConfigurableApplicationContext run(Class<?>[] primarySources, String[] args){
    return new SpringApplication(primarySources).run(args);
}

在 SpringApplication的静态方法 run里，其实上是构造了SpringApplication的实例，并调用了实例的 run(类方法)。

先看看 SpringApplication 的构造方法干了什么？

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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));

    /* 1.1
     * 此处的webApplicationType 标识当前程序的类型, 判断方式是当前路径下是否有不同类型项目对应的class
     * NONE: The application should not run as a web application and should not start an embedded web server.
     * SERVLET: servlet-based web application and should start an embedded servlet web server.例如 spring mvc
     * REACTIVE: reactive web application and should start an embedded reactive web server. 例如 spring web flux
     */
    this.webApplicationType = WebApplicationType.deduceFromClasspath();
    /* 1.2
     * 设置初始化器(Initializer)
     */
    setInitializers((Collection) getSpringFactoriesInstances(ApplicationContextInitializer.class));
    /* 1.2
     * 设置监听器(Listener)
     */
    setListeners((Collection) getSpringFactoriesInstances(ApplicationListener.class));
    // 1.3
    this.mainApplicationClass = deduceMainApplicationClass();
}

使用deduceFromClasspath 判断当前程序类型

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private static final String[] SERVLET_INDICATOR_CLASSES = { "javax.servlet.Servlet",
			"org.springframework.web.context.ConfigurableWebApplicationContext" };

private static final String WEBMVC_INDICATOR_CLASS = "org.springframework."
			+ "web.servlet.DispatcherServlet";

private static final String WEBFLUX_INDICATOR_CLASS = "org."
			+ "springframework.web.reactive.DispatcherHandler";

private static final String JERSEY_INDICATOR_CLASS = "org.glassfish.jersey.servlet.ServletContainer";

static WebApplicationType deduceFromClasspath() {
	if (ClassUtils.isPresent(WEBFLUX_INDICATOR_CLASS, null)
		&& !ClassUtils.isPresent(WEBMVC_INDICATOR_CLASS, null)
		&& !ClassUtils.isPresent(JERSEY_INDICATOR_CLASS, null)) {

    	return WebApplicationType.REACTIVE;
	}		
    for (String className : SERVLET_INDICATOR_CLASSES) {
    	if (!ClassUtils.isPresent(className, null)) {
			return WebApplicationType.NONE;
		}
	}
	return WebApplicationType.SERVLET;
}

设置初始化器(Initializer) 和监听器(listener)

可以看到初始化器和监听器的获取方法都是getSpringFactoriesInstances。

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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) {
    // 获取当前资源的加载器
    ClassLoader classLoader = getClassLoader();
    // Use names and ensure unique to protect against duplicates
    Set<String> names = new LinkedHashSet<>(SpringFactoriesLoader.loadFactoryNames(type, classLoader));
    List<T> instances = createSpringFactoriesInstances(type, parameterTypes,classLoader, args, names);
    AnnotationAwareOrderComparator.sort(instances); // 根据 @Order和@Priority注解排序
    return instances;
}

当入参是”ApplicationContextInitializer.class”时,factoryClassName为”org.springframework.context.ApplicationContextInitializer”。

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public static List<String> loadFactoryNames(Class<?> factoryClass, @Nullable ClassLoader classLoader) {
    String factoryClassName = factoryClass.getName();
    return loadSpringFactories(classLoader).getOrDefault(factoryClassName, Collections.emptyList();
}
private static Map<String, List<String>> loadSpringFactories(@Nullable ClassLoader classLoader) {
    MultiValueMap<String, String> result = cache.get(classLoader);
    if (result != null) {
    	return result;
    }
    try {
        /** groupId: org.springframework.boot
         *  artifactId: spring-boot
         *  路径 /META-INF/spring.factories
         *  key 是接口，value是实现类，多个实现类逗号隔开
         */
    	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()) {
                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);
    }
}

文件中的一段内容:

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# 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

# 在Spring上下文被刷新之前进行初始化的操作。

# 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

#ApplicationListener 监听ApplicationEvent类型的子类

上述配置文件中的实现类会通过createSpringFactoriesInstances方法获取对应的实例。

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private <T> List<T> createSpringFactoriesInstances(Class<T> type,
			Class<?>[] parameterTypes, ClassLoader classLoader, Object[] args,
			Set<String> names) {
    List<T> instances = new ArrayList<>(names.size());
    for (String name : names) {
        try {
            Class<?> instanceClass = ClassUtils.forName(name, classLoader);
            Assert.isAssignable(type, instanceClass);
            // 获得class的构造器
            Constructor<?> constructor = instanceClass.getDeclaredConstructor(parameterTypes);
            // 对于java class 使用 Constructor.newInstance获取实例。
		    T instance = (T) BeanUtils.instantiateClass(constructor, args);
		    instances.add(instance);
	    }
	    catch (Throwable ex) {
		throw new IllegalArgumentException("Cannot instantiate " + type + " : " + name, ex);
	    }
    }
	return instances;
}

推断应用入口类

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// 通过构造一个运行时异常，通过异常栈中方法名为main的栈帧来得到入口类的名字。
// 巧妙的实现
private Class<?> deduceMainApplicationClass() {
    try {
	    StackTraceElement[] stackTrace = new RuntimeException().getStackTrace();
	    for (StackTraceElement stackTraceElement : stackTrace) {
		    if ("main".equals(stackTraceElement.getMethodName())) {
			    return Class.forName(stackTraceElement.getClassName());
		    }
		}
	}
	catch (ClassNotFoundException ex) {
		// Swallow and continue
	}
	return null;
}

至此，对于SpringApplication实例的初始化过程就结束了。

SpringApplication.run方法

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public ConfigurableApplicationContext run(String... args) {
    // 初始化并启动一个计时工具
    StopWatch stopWatch = new StopWatch();
    stopWatch.start();
    ConfigurableApplicationContext context = null;
    Collection<SpringBootExceptionReporter> exceptionReporters = new ArrayList<>();
    configureHeadlessProperty();
    // 2.1 获取SpringApplicationRunListeners
    SpringApplicationRunListeners listeners = getRunListeners(args);
    // 发出ApplicationStartingEvent事件
    listeners.starting();
	try {
        ApplicationArguments applicationArguments = new DefaultApplicationArguments(args); // 注意，此时参数为空
        // 2.2 根据SpringApplicationRunListeners以及参数来准备环境
        ConfigurableEnvironment environment = prepareEnvironment(listeners,applicationArguments);
        configureIgnoreBeanInfo(environment);
        // 准备Banner打印器 - 就是启动Spring Boot的时候打印在console上的ASCII艺术字体
        Banner printedBanner = printBanner(environment);
        // 2.3 创建Spring上下文
        context = createApplicationContext();
        // 准备异常报告器
        exceptionReporters = getSpringFactoriesInstances(SpringBootExceptionReporter.class,
            new Class[] { ConfigurableApplicationContext.class }, context);
        // 2.4 Spring上下文前置处理
        prepareContext(context, environment, listeners, applicationArguments,printedBanner);
        // 2.5 Spring上下文刷新
        refreshContext(context);
        // 2.6 Spring上下文后置处理
        afterRefresh(context, applicationArguments);
        // 停止计时器
        stopWatch.stop();
        if (this.logStartupInfo) {
            new StartupInfoLogger(this.mainApplicationClass).logStarted(getApplicationLog(), stopWatch);
        }
        // 发出ApplicationStartedEvent事件
        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;
}
// 设置java.awt.headless系统属性为true - 没有图形化界面
private void configureHeadlessProperty() {
	System.setProperty(SYSTEM_PROPERTY_JAVA_AWT_HEADLESS, System.getProperty(
        SYSTEM_PROPERTY_JAVA_AWT_HEADLESS, Boolean.toString(this.headless)));
}

获取 run listeners

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

这里仍然利用了getSpringFactoriesInstances方法来获取实例.

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# Run Listeners
org.springframework.boot.SpringApplicationRunListener=\
org.springframework.boot.context.event.EventPublishingRunListener 

# EventPublishingRunListener主要是负责发布SpringApplicationEvent事件的。通过内置的initialMulticaster 发布事件。

准备环境 prepareEnvironment

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private ConfigurableEnvironment prepareEnvironment(
    SpringApplicationRunListeners listeners,
    ApplicationArguments applicationArguments) {
    // Create and configure the environment
    // 使用已有环境(refresh)或新建环境
    ConfigurableEnvironment environment = getOrCreateEnvironment();
    configureEnvironment(environment, applicationArguments.getSourceArgs());
    // 发布ApplicationEnvironmentPreparedEvent 事件
    listeners.environmentPrepared(environment);
    bindToSpringApplication(environment);
    if (!this.isCustomEnvironment) {
        environment = new EnvironmentConverter(getClassLoader()).convertEnvironmentIfNecessary(environment, deduceEnvironmentClass());
	}
    ConfigurationPropertySources.attach(environment);
    return environment;
}
private ConfigurableEnvironment getOrCreateEnvironment() {
    if (this.environment != null) {
        return this.environment;
    }
    switch (this.webApplicationType) {
        case SERVLET:
            return new StandardServletEnvironment(); // spring boot web
        case REACTIVE:
            return new StandardReactiveWebEnvironment();
        default:
            return new StandardEnvironment();
    }
}
protected void configureEnvironment(ConfigurableEnvironment environment,String[] args) {
    if (this.addConversionService) {
        // 初始化属性转换服务
        ConversionService conversionService = ApplicationConversionService.getSharedInstance();
		environment.setConversionService((ConfigurableConversionService) conversionService);
	}
    //配置Property Sources
	configurePropertySources(environment, args);
    //配置Profiles
	configureProfiles(environment, args);
}

创建Spring上下文

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public static final String DEFAULT_CONTEXT_CLASS = "org.springframework.context."
			+ "annotation.AnnotationConfigApplicationContext";
public static final String DEFAULT_SERVLET_WEB_CONTEXT_CLASS = "org.springframework.boot."
			+ "web.servlet.context.AnnotationConfigServletWebServerApplicationContext";
public static final String DEFAULT_REACTIVE_WEB_CONTEXT_CLASS = "org.springframework."
			+ "boot.web.reactive.context.AnnotationConfigReactiveWebServerApplicationContext";
protected ConfigurableApplicationContext createApplicationContext() {
    Class<?> contextClass = this.applicationContextClass;
    if (contextClass == null) {
        try {
            switch (this.webApplicationType) {
            case SERVLET:
                contextClass = Class.forName(DEFAULT_SERVLET_WEB_CONTEXT_CLASS);
                break;
            case REACTIVE:
                contextClass = Class.forName(DEFAULT_REACTIVE_WEB_CONTEXT_CLASS);
                break;
            default:
                contextClass = Class.forName(DEFAULT_CONTEXT_CLASS);
            }
        }
        catch (ClassNotFoundException ex) {
            throw new IllegalStateException(
                "Unable create a default ApplicationContext, "
                    + "please specify an ApplicationContextClass",
            ex);
        }
    }
    return (ConfigurableApplicationContext) BeanUtils.instantiateClass(contextClass);
}

Spring上下文前置处理

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private void prepareContext(ConfigurableApplicationContext context,
    ConfigurableEnvironment environment, SpringApplicationRunListeners listeners,
    ApplicationArguments applicationArguments, Banner printedBanner) {
    // 将环境和上下文关联起来
    context.setEnvironment(environment);
    // 为上下文配置Bean生成器以及资源加载器(如果它们非空)
    postProcessApplicationContext(context);
    // 调用初始化器
    applyInitializers(context);
    // 触发Spring Boot启动过程的contextPrepared事件
    listeners.contextPrepared(context);
    if (this.logStartupInfo) {
        logStartupInfo(context.getParent() == null);
        logStartupProfileInfo(context);
    }
    // Add boot specific singleton beans
    // 添加两个Spring Boot中的特殊单例Beans - springApplicationArguments以及springBootBanner
    ConfigurableListableBeanFactory beanFactory = context.getBeanFactory();
    beanFactory.registerSingleton("springApplicationArguments", applicationArguments);
    if (printedBanner != null) {
        beanFactory.registerSingleton("springBootBanner", printedBanner);
    }
    if (beanFactory instanceof DefaultListableBeanFactory) {
        ((DefaultListableBeanFactory) beanFactory)
            .setAllowBeanDefinitionOverriding(this.allowBeanDefinitionOverriding);  
    }
    // Load the sources
    // 加载sources
    Set<Object> sources = getAllSources();
    Assert.notEmpty(sources, "Sources must not be empty");
    // 构造BeanDefinitionLoader并完成Bean定义的加载
    load(context, sources.toArray(new Object[0]));
    // 触发Spring Boot启动过程的contextLoaded事件
    listeners.contextLoaded(context);
}

protected void postProcessApplicationContext(ConfigurableApplicationContext context) {
    // 为上下文配置Bean生成器以及资源加载器(如果它们非空)
    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());
        }
    }
    if (this.addConversionService) {
        context.getBeanFactory().setConversionService(ApplicationConversionService.getSharedInstance());
    }
}
// 调用初始化器
protected void applyInitializers(ConfigurableApplicationContext context) {
    // 在创建SpringApplication实例时设置的初始化器
    for (ApplicationContextInitializer initializer : getInitializers()) {
        Class<?> requiredType = GenericTypeResolver.resolveTypeArgument(
            initializer.getClass(), ApplicationContextInitializer.class);
        Assert.isInstanceOf(requiredType, context, "Unable to call initializer.");
		initializer.initialize(context);
    }
}

Spring上下文刷新

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private void refreshContext(ConfigurableApplicationContext context) {
    refresh(context);
    // shutdown 钩子
    if (this.registerShutdownHook) {
        try {
            context.registerShutdownHook();
        }
        catch (AccessControlException ex) {
            // Not allowed in some environments.
        }
    }
}
protected void refresh(ApplicationContext applicationContext) {
	Assert.isInstanceOf(AbstractApplicationContext.class, applicationContext);
	((AbstractApplicationContext) applicationContext).refresh();
}

Spring上下文后置处理

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protected void afterRefresh(ConfigurableApplicationContext context,
			ApplicationArguments args) {
}

callRunners

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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());
    // 应用顺序
    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);
        }
    }
}

private void callRunner(ApplicationRunner runner, ApplicationArguments args) {
    try {
        (runner).run(args);
    }
    catch (Exception ex) {
        throw new IllegalStateException("Failed to execute ApplicationRunner", ex);
    }
}

private void callRunner(CommandLineRunner runner, ApplicationArguments args) {
    try {
        (runner).run(args.getSourceArgs());
    }
    catch (Exception ex) {
        throw new IllegalStateException("Failed to execute CommandLineRunner", ex);
    }
}

调用注册的runner,下面的接口的实现类，区别是参数不同：

  1. org.springframework.boot.ApplicationRunner
  2. org.springframework.boot.CommandLineRunner
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public interface ApplicationRunner {

    void run(ApplicationArguments args) throws Exception;

}

public interface CommandLineRunner {

    void run(String... args) throws Exception;
}
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