spring boot 啓動過程-簡述

spring boot

啓動

spring boot 的啓動代碼很簡單，最精簡的代碼以下。

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

注意：spring啓動的main方法必須包含在包內

啓動代碼很簡單，但在初始化過程當中，spring作了不少工做，打開run方法，咱們能夠看到以下代碼

public static ConfigurableApplicationContext run(Object[] sources, String[] args) {
    return new SpringApplication(sources).run(args);
}

大體分爲兩部分。

1. 加載配置信息
2. 啓動

加載配置信息

初始化的主要工做都在這個方法中完成。

private void initialize(Object[] sources) {
    if (sources != null && sources.length > 0) {
        this.sources.addAll(Arrays.asList(sources));
    }
    this.webEnvironment = deduceWebEnvironment();
    setInitializers((Collection) getSpringFactoriesInstances(
            ApplicationContextInitializer.class));
    setListeners((Collection) getSpringFactoriesInstances(ApplicationListener.class));
    this.mainApplicationClass = deduceMainApplicationClass();
}

經過判斷是否包含WEB_ENVIRONMENT_CLASSES中定義的類來決定是不是web項目。

咱們應用的是compile("org.springframework.boot:spring-boot-starter-web")， 因此間接引用了org.springframework.web.context.ConfigurableWebApplicationContext，推論爲web環境。

若是隻引用org.springframework.boot:spring-boot-starter，就不是web項目，不會啓動內置的tomcat。

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

private boolean deduceWebEnvironment() {
    for (String className : WEB_ENVIRONMENT_CLASSES) {
        if (!ClassUtils.isPresent(className, null)) {
            return false;
        }
    }
    return true;
}

下面這段代碼比較長，目的是經過META-INF/spring.factories中的org.springframework.context.ApplicationContextInitializer 讀取配置並初始化，排序。

public void setInitializers(
        Collection<? extends ApplicationContextInitializer<?>> initializers) {
    this.initializers = new ArrayList<ApplicationContextInitializer<?>>();
    this.initializers.addAll(initializers);
}

private <T> Collection<? extends T> getSpringFactoriesInstances(Class<T> type) {
    return getSpringFactoriesInstances(type, new Class<?>[] {});
}

private <T> Collection<? extends T> getSpringFactoriesInstances(Class<T> type,
			Class<?>[] parameterTypes, Object... args) {
    ClassLoader classLoader = Thread.currentThread().getContextClassLoader();
    // Use names and ensure unique to protect against duplicates
    Set<String> names = new LinkedHashSet<String>(
            SpringFactoriesLoader.loadFactoryNames(type, classLoader));
    List<T> instances = createSpringFactoriesInstances(type, parameterTypes,
            classLoader, args, names);
    AnnotationAwareOrderComparator.sort(instances);
    return instances;
}

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


//SpringFactoriesLoader
/**
 * The location to look for factories.
 * <p>Can be present in multiple JAR files.
 */
public static final String FACTORIES_RESOURCE_LOCATION = "META-INF/spring.factories";
public static List<String> loadFactoryNames(Class<?> factoryClass, ClassLoader classLoader) {
    String factoryClassName = factoryClass.getName();
    try {
        Enumeration<URL> urls = (classLoader != null ? classLoader.getResources(FACTORIES_RESOURCE_LOCATION) :
                ClassLoader.getSystemResources(FACTORIES_RESOURCE_LOCATION));
        List<String> result = new ArrayList<String>();
        while (urls.hasMoreElements()) {
            URL url = urls.nextElement();
            Properties properties = PropertiesLoaderUtils.loadProperties(new UrlResource(url));
            String factoryClassNames = properties.getProperty(factoryClassName);
            result.addAll(Arrays.asList(StringUtils.commaDelimitedListToStringArray(factoryClassNames)));
        }
        return result;
    }
    catch (IOException ex) {
        throw new IllegalArgumentException("Unable to load [" + factoryClass.getName() +
                "] factories from location [" + FACTORIES_RESOURCE_LOCATION + "]", ex);
    }
}

初始化ApplicationListener的過程和ApplicationContextInitializer同樣，就不在贅述了。

setListeners((Collection) getSpringFactoriesInstances(ApplicationListener.class));

經過調用棧獲取啓動main函數

this.mainApplicationClass = deduceMainApplicationClass();

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;
}

上面初始化的配置信息在啓動的時候會在相應的位置起做用，下面咱們分解啓動。

啓動

通過上面的初始化過程，咱們已經有了一個SpringApplication對象，根據SpringApplication類的靜態run方法分析， 接下來會調用SpringApplication對象的run方法。咱們接下來就分析這個對象的run方法。

public ConfigurableApplicationContext run(String... args) {
    // 啓動計時器
    StopWatch stopWatch = new StopWatch();
    stopWatch.start();
    ConfigurableApplicationContext context = null;
    FailureAnalyzers analyzers = null;
    configureHeadlessProperty();
    //
    SpringApplicationRunListeners listeners = getRunListeners(args);
    listeners.started();
    try {
        //建立並刷新ApplicationContext
        ApplicationArguments applicationArguments = new DefaultApplicationArguments( args);
        ConfigurableEnvironment environment = prepareEnvironment(listeners, applicationArguments);
        // print spring boot banner and version
        Banner printedBanner = printBanner(environment);
        context = createApplicationContext();
        analyzers = new FailureAnalyzers(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, analyzers, ex);
        throw new IllegalStateException(ex);
    }
}

上面的方法經過名字很明確它實現的功能。

有關java.awt.headless的內容參考： Using Headless Mode in the Java SE Platform

咱們來詳細的觀察一下第一個事件，啓動事件。

listeners.started();

//EventPublishingRunListener.java
public void started() {
    this.initialMulticaster
            .multicastEvent(new ApplicationStartedEvent(this.application, this.args));
}
public void multicastEvent(ApplicationEvent event) {
    multicastEvent(event, resolveDefaultEventType(event));
}
public void multicastEvent(final ApplicationEvent event, ResolvableType eventType) {
    ResolvableType type = (eventType != null ? eventType : resolveDefaultEventType(event));
    for (final ApplicationListener<?> listener : getApplicationListeners(event, type)) {
        Executor executor = getTaskExecutor();
        if (executor != null) {
            executor.execute(new Runnable() {
                @Override
                public void run() {
                    invokeListener(listener, event);
                }
            });
        }
        else {
            invokeListener(listener, event);
        }
    }
}

protected Collection<ApplicationListener<?>> getApplicationListeners(
			ApplicationEvent event, ResolvableType eventType) {

    Object source = event.getSource();
    Class<?> sourceType = (source != null ? source.getClass() : null);
    ListenerCacheKey cacheKey = new ListenerCacheKey(eventType, sourceType);

    // Quick check for existing entry on ConcurrentHashMap...
    ListenerRetriever retriever = this.retrieverCache.get(cacheKey);
    if (retriever != null) {
        return retriever.getApplicationListeners();
    }

    if (this.beanClassLoader == null ||
            (ClassUtils.isCacheSafe(event.getClass(), this.beanClassLoader) &&
                    (sourceType == null || ClassUtils.isCacheSafe(sourceType, this.beanClassLoader)))) {
        // Fully synchronized building and caching of a ListenerRetriever
        synchronized (this.retrievalMutex) {
            retriever = this.retrieverCache.get(cacheKey);
            if (retriever != null) {
                return retriever.getApplicationListeners();
            }
            retriever = new ListenerRetriever(true);
            Collection<ApplicationListener<?>> listeners =
                    retrieveApplicationListeners(eventType, sourceType, retriever);
            this.retrieverCache.put(cacheKey, retriever);
            return listeners;
        }
    }
    else {
        // No ListenerRetriever caching -> no synchronization necessary
        return retrieveApplicationListeners(eventType, sourceType, null);
    }
}

private Collection<ApplicationListener<?>> retrieveApplicationListeners(
			ResolvableType eventType, Class<?> sourceType, ListenerRetriever retriever) {

    LinkedList<ApplicationListener<?>> allListeners = new LinkedList<ApplicationListener<?>>();
    Set<ApplicationListener<?>> listeners;
    Set<String> listenerBeans;
    synchronized (this.retrievalMutex) {
        listeners = new LinkedHashSet<ApplicationListener<?>>(this.defaultRetriever.applicationListeners);
        listenerBeans = new LinkedHashSet<String>(this.defaultRetriever.applicationListenerBeans);
    }
    for (ApplicationListener<?> listener : listeners) {
        if (supportsEvent(listener, eventType, sourceType)) {
            if (retriever != null) {
                retriever.applicationListeners.add(listener);
            }
            allListeners.add(listener);
        }
    }
    if (!listenerBeans.isEmpty()) {
        BeanFactory beanFactory = getBeanFactory();
        for (String listenerBeanName : listenerBeans) {
            try {
                Class<?> listenerType = beanFactory.getType(listenerBeanName);
                if (listenerType == null || supportsEvent(listenerType, eventType)) {
                    ApplicationListener<?> listener =
                            beanFactory.getBean(listenerBeanName, ApplicationListener.class);
                    if (!allListeners.contains(listener) && supportsEvent(listener, eventType, sourceType)) {
                        if (retriever != null) {
                            retriever.applicationListenerBeans.add(listenerBeanName);
                        }
                        allListeners.add(listener);
                    }
                }
            }
            catch (NoSuchBeanDefinitionException ex) {
                // Singleton listener instance (without backing bean definition) disappeared -
                // probably in the middle of the destruction phase
            }
        }
    }
    AnnotationAwareOrderComparator.sort(allListeners);
    return allListeners;
}

protected boolean supportsEvent(ApplicationListener<?> listener, ResolvableType eventType, Class<?> sourceType) {
		GenericApplicationListener smartListener = (listener instanceof GenericApplicationListener ?
            (GenericApplicationListener) listener : new GenericApplicationListenerAdapter(listener));
    return (smartListener.supportsEventType(eventType) && smartListener.supportsSourceType(sourceType));
}

方法比較多，咱們一點一點的分析。

首先咱們注意到事件被封裝成了 ResolvableType 類，在經過 getApplicationListeners 方法過濾支持此事件的listener（初始化時加載的）， 當中還增長了緩存處理。

在繼續往下跟蹤，發現最後是經過 supportsEvent 方法判斷是否支持事件，繼承關係以下。

public interface GenericApplicationListener extends ApplicationListener<ApplicationEvent>, Ordered
ApplicationListener<E extends ApplicationEvent>

繼承自 GenericApplicationListener 的 GenericApplicationListenerAdapter是一個適配器， 他代理真正的 Listener，並實現了擴展方法來判斷是否支持此事件。以下圖：

這裏的判斷是個亮點！ 首先在此看 ResolvableType ，他是4.0新加入的類。從doc上能夠看出，它是一個工具類，可以匹配父類，接口，甚至泛型，功能仍是至關強大的。 事件的判斷全靠它來完成關鍵的判斷。咱們看兩個初始化時加載的 Listener：

• 0 = {ConfigFileApplicationListener@2018} public class ConfigFileApplicationListener implements EnvironmentPostProcessor, ApplicationListener<ApplicationEvent>, Ordered
• 1 = {AnsiOutputApplicationListener@1322} public class AnsiOutputApplicationListener implements ApplicationListener<ApplicationEnvironmentPreparedEvent>, Ordered
• 2 = {LoggingApplicationListener@1323} public class LoggingApplicationListener implements GenericApplicationListener {
• 3 = {ClasspathLoggingApplicationListener@2019} public final class ClasspathLoggingApplicationListener implements GenericApplicationListener {
• 4 = {BackgroundPreinitializer@2020} public class BackgroundPreinitializer implements ApplicationListener<ApplicationEnvironmentPreparedEvent> {
• 5 = {DelegatingApplicationListener@2021} public class DelegatingApplicationListener implements ApplicationListener<ApplicationEvent>, Ordered {
• 6 = {ParentContextCloserApplicationListener@1327} public class ParentContextCloserApplicationListener implements ApplicationListener<ParentContextAvailableEvent>, ApplicationContextAware, Ordered {
• 7 = {ClearCachesApplicationListener@2022} class ClearCachesApplicationListener implements ApplicationListener<ContextRefreshedEvent> {
• 8 = {FileEncodingApplicationListener@2023} public class FileEncodingApplicationListener implements ApplicationListener<ApplicationEnvironmentPreparedEvent>, Ordered {
• 9 = {LiquibaseServiceLocatorApplicationListener@2024} public class LiquibaseServiceLocatorApplicationListener implements ApplicationListener<ApplicationStartedEvent> {

從類聲明能夠看出Event泛型是不同的，這就決定了該Listener支持什麼事件。 啓動時的事件是 ApplicationStartedEvent ，根據 Listener 能夠很清楚 ApplicationStartedEvent，ApplicationEvent 會被調用。 其餘的的事件也是相同原理。

接下來是配置，由下面代碼方法實現。

ApplicationArguments applicationArguments = new DefaultApplicationArguments( args);
ConfigurableEnvironment environment = prepareEnvironment(listeners, applicationArguments);

private ConfigurableEnvironment prepareEnvironment( SpringApplicationRunListeners listeners,
        ApplicationArguments applicationArguments) {
    // Create and configure the environment
    ConfigurableEnvironment environment = getOrCreateEnvironment();
    configureEnvironment(environment, applicationArguments.getSourceArgs());
    listeners.environmentPrepared(environment);
    if (isWebEnvironment(environment) && !this.webEnvironment) {
        environment = convertToStandardEnvironment(environment);
    }
    return environment;
}

// configureEnvironment 方法先是調用configurePropertySources來配置properties，
// 而後調用 configureProfiles 來配置profiles
protected void configureEnvironment(ConfigurableEnvironment environment,
        String[] args) {
    configurePropertySources(environment, args);
    configureProfiles(environment, args);
}

protected void configurePropertySources(ConfigurableEnvironment environment,
        String[] args) {
    MutablePropertySources sources = environment.getPropertySources();
    if (this.defaultProperties != null && !this.defaultProperties.isEmpty()) {
        sources.addLast(
                new MapPropertySource("defaultProperties", this.defaultProperties));
    }
    if (this.addCommandLineProperties && args.length > 0) {
        String name = CommandLinePropertySource.COMMAND_LINE_PROPERTY_SOURCE_NAME;
        if (sources.contains(name)) {
            PropertySource<?> source = sources.get(name);
            CompositePropertySource composite = new CompositePropertySource(name);
            composite.addPropertySource(new SimpleCommandLinePropertySource(
                    name + "-" + args.hashCode(), args));
            composite.addPropertySource(source);
            sources.replace(name, composite);
        }
        else {
            sources.addFirst(new SimpleCommandLinePropertySource(args));
        }
    }
}

protected void configureProfiles(ConfigurableEnvironment environment, String[] args) {
    environment.getActiveProfiles(); // ensure they are initialized
    // But these ones should go first (last wins in a property key clash)
    Set<String> profiles = new LinkedHashSet<String>(this.additionalProfiles);
    profiles.addAll(Arrays.asList(environment.getActiveProfiles()));
    environment.setActiveProfiles(profiles.toArray(new String[profiles.size()]));
}

listeners.environmentPrepared(environment); 廣播 ApplicationEnvironmentPreparedEvent 事件。

打印banner

打印的內容是spring圖形和spring boot的版本。就不詳細說明了，有興趣的能夠自行瀏覽。

createApplicationContext

根據 webEnvironment 建立 DEFAULT_WEB_CONTEXT_CLASS 或 DEFAULT_CONTEXT_CLASS。

/**
* The class name of application context that will be used by default for non-web
* environments.
*/
public static final String DEFAULT_CONTEXT_CLASS = "org.springframework.context."
+ "annotation.AnnotationConfigApplicationContext";

/**
* The class name of application context that will be used by default for web
* environments.
*/
public static final String DEFAULT_WEB_CONTEXT_CLASS = "org.springframework."
+ "boot.context.embedded.AnnotationConfigEmbeddedWebApplicationContext";

refreshContext

真正的啓動如今纔開始。咱們看到最終調用的是refresh方法。

在查看代碼以前，先說明一下當前 Context 對象的繼承關係，請看下圖：

上面的分析已經得知當前啓動的是一個web項目， 因此在createApplicationContext()方法中建立的ApplicationContext是DEFAULT_WEB_CONTEXT_CLASS，也就是AnnotationConfigEmbeddedWebApplicationContext。 從圖中咱們能夠得知 AnnotationConfigEmbeddedWebApplicationContext 的繼承關係。因爲繼承自 EmbeddedWebApplicationContext ， 賦予了它內嵌 servlet container 的能力。啓動容器的代碼以下：

@Override
protected void onRefresh() {
    super.onRefresh();
    try {
        createEmbeddedServletContainer();
    }
    catch (Throwable ex) {
        throw new ApplicationContextException("Unable to start embedded container",
                ex);
    }
}

啓動過程來自模板類 AbstractApplicationContext，和spring mvc 是同樣的，這裏就不過多解釋了。

@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.
            // 初始化並啓動 Servlet Container
            onRefresh();

            // Check for listener beans and register them.
            registerListeners();

            // Instantiate all remaining (non-lazy-init) singletons.
            finishBeanFactoryInitialization(beanFactory);

            // Last step: publish corresponding event.
            // start embed servlet container
            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();
        }
    }
}

啓動完成後，會廣播事件，中止計時，整個啓動過程結束。

afterRefresh(context, applicationArguments);
listeners.finished(context, null);
stopWatch.stop();