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Spring Boot启动流程详解（一）

2024-10-17 23:11:38   213人阅读

环境

本文基于Spring Boot版本1.3.3, 使用了spring-boot-starter-web。

配置完成后，编写了代码如下：

@SpringBootApplicationpublic class Application {    public static void main(String[] args) {        SpringApplication.run(Application.class, args);    }}@RestControllerpublic class RootController {    public static final String PATH_ROOT = "/";    @RequestMapping(PATH_ROOT)    public String welcome() {        return "Welcome!";    }}

虽然只有几行代码，但是这已经是一个完整的Web程序，当访问url的path部分为"/"时，返回字符串"Welcome!"。

首先是一个非常普通的java程序入口，一个符合约定的静态main方法。在这个main方法中，调用了SpringApplication的静态run方法，并将Application类对象和main方法的参数args作为参数传递了进去。

然后是一个使用了两个Spring注解的RootController类，我们在main方法中，没有直接使用这个类。

SpringApplication类的静态run方法

以下代码摘自：org.springframework.boot.SpringApplicationpublic static ConfigurableApplicationContext run(Object source, String... args) {    return run(new Object[] { source }, args);}public static ConfigurableApplicationContext run(Object[] sources, String[] args) {    return new SpringApplication(sources).run(args);}

在这个静态方法中，创建SpringApplication对象，并调用该对象的run方法。

构造SpringApplication对象

以下代码摘自：org.springframework.boot.SpringApplicationpublic SpringApplication(Object... sources) {    initialize(sources);}private void initialize(Object[] sources) {    // 为成员变量sources赋值    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();}

构造函数中调用initialize方法，初始化SpringApplication对象的成员变量sources，webEnvironment，initializers，listeners，mainApplicationClass。sources的赋值比较简单，就是我们传给SpringApplication.run方法的参数。剩下的几个，我们依次来看看是怎么做的。

首先是webEnvironment:

以下代码摘自：org.springframework.boot.SpringApplicationprivate boolean webEnvironment; private static final String[] WEB_ENVIRONMENT_CLASSES = { "javax.servlet.Servlet",            "org.springframework.web.context.ConfigurableWebApplicationContext" };private void initialize(Object[] sources) {    ...        // 为成员变量webEnvironment赋值        this.webEnvironment = deduceWebEnvironment();    ...}private boolean deduceWebEnvironment() {    for (String className : WEB_ENVIRONMENT_CLASSES) {        if (!ClassUtils.isPresent(className, null)) {            return false;        }    }    return true;}

可以看到webEnvironment是一个boolean，该成员变量用来表示当前应用程序是不是一个Web应用程序。那么怎么决定当前应用程序是否Web应用程序呢，是通过在classpath中查看是否存在WEB_ENVIRONMENT_CLASSES这个数组中所包含的类，如果存在那么当前程序即是一个Web应用程序，反之则不然。
在本文的例子中webEnvironment的值为true。

然后是initializers:

initializers成员变量，是一个ApplicationContextInitializer类型对象的集合。 顾名思义，ApplicationContextInitializer是一个可以用来初始化ApplicationContext的接口。

以下代码摘自：org.springframework.boot.SpringApplicationprivate List<ApplicationContextInitializer<?>> initializers;private void initialize(Object[] sources) {    ...    // 为成员变量initializers赋值    setInitializers((Collection) getSpringFactoriesInstances(            ApplicationContextInitializer.class));    ...}public void setInitializers(        Collection<? extends ApplicationContextInitializer<?>> initializers) {    this.initializers = new ArrayList<ApplicationContextInitializer<?>>();    this.initializers.addAll(initializers);}

可以看到，关键是调用getSpringFactoriesInstances(ApplicationContextInitializer.class)，来获取ApplicationContextInitializer类型对象的列表。

以下代码摘自：org.springframework.boot.SpringApplicationprivate <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;}

在该方法中，首先通过调用SpringFactoriesLoader.loadFactoryNames(type, classLoader)来获取所有Spring Factories的名字，然后调用createSpringFactoriesInstances方法根据读取到的名字创建对象。最后会将创建好的对象列表排序并返回。

以下代码摘自：org.springframework.core.io.support.SpringFactoriesLoaderpublic 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);    }}

可以看到，是从一个名字叫spring.factories的资源文件中，读取key为org.springframework.context.ApplicationContextInitializer的value。而spring.factories的部分内容如下：

以下内容摘自spring-boot-1.3.3.RELEASE.jar中的资源文件META-INF/spring.factories# Application Context Initializersorg.springframework.context.ApplicationContextInitializer=org.springframework.boot.context.ConfigurationWarningsApplicationContextInitializer,org.springframework.boot.context.ContextIdApplicationContextInitializer,org.springframework.boot.context.config.DelegatingApplicationContextInitializer,org.springframework.boot.context.web.ServerPortInfoApplicationContextInitializer

可以看到，最近的得到的，是ConfigurationWarningsApplicationContextInitializer，ContextIdApplicationContextInitializer，DelegatingApplicationContextInitializer，ServerPortInfoApplicationContextInitializer这四个类的名字。

接下来会调用createSpringFactoriesInstances来创建ApplicationContextInitializer实例。

以下代码摘自：org.springframework.boot.SpringApplicationprivate <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.getConstructor(parameterTypes);            T instance = (T) constructor.newInstance(args);            instances.add(instance);        }        catch (Throwable ex) {            throw new IllegalArgumentException(                    "Cannot instantiate " + type + " : " + name, ex);        }    }    return instances;}

所以在我们的例子中，SpringApplication对象的成员变量initalizers就被初始化为，ConfigurationWarningsApplicationContextInitializer，ContextIdApplicationContextInitializer，DelegatingApplicationContextInitializer，ServerPortInfoApplicationContextInitializer这四个类的对象组成的list。

下图画出了加载的ApplicationContextInitializer，并说明了他们的作用。至于何时应用他们，且听后面慢慢分解。

<iframe id="iframe_0.2849134769446755" src="data:text/html;charset=utf8,%3Cstyle%3Ebody%7Bmargin:0;padding:0%7D%3C/style%3E%3Cimg%20id=%22img%22%20src=%22http://zhaox.github.io/assets/images/SpringBootApplicationContextInitializer.png?_=5551828%22%20style=%22border:none;max-width:1052px%22%3E%3Cscript%3Ewindow.onload%20=%20function%20()%20%7Bvar%20img%20=%20document.getElementById(‘img‘);%20window.parent.postMessage(%7BiframeId:‘iframe_0.2849134769446755‘,width:img.width,height:img.height%7D,%20‘http://www.cnblogs.com‘);%7D%3C/script%3E" frameborder="0" scrolling="no" width="320" height="240"></iframe>

接下来是成员变量listeners

以下代码摘自：org.springframework.boot.SpringApplicationprivate List<ApplicationListener<?>> listeners;private void initialize(Object[] sources) {    ...    // 为成员变量listeners赋值    setListeners((Collection) getSpringFactoriesInstances(ApplicationListener.class));    ...}public void setListeners(Collection<? extends ApplicationListener<?>> listeners) {    this.listeners = new ArrayList<ApplicationListener<?>>();    this.listeners.addAll(listeners);}

listeners成员变量，是一个ApplicationListener<?>类型对象的集合。可以看到获取该成员变量内容使用的是跟成员变量initializers一样的方法，只不过传入的类型从ApplicationContextInitializer.class变成了ApplicationListener.class。

看一下spring.factories中的相关内容：

以下内容摘自spring-boot-1.3.3.RELEASE.jar中的资源文件META-INF/spring.factories# Application Listenersorg.springframework.context.ApplicationListener=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.liquibase.LiquibaseServiceLocatorApplicationListener,org.springframework.boot.logging.ClasspathLoggingApplicationListener,org.springframework.boot.logging.LoggingApplicationListener

也就是说，在我们的例子中，listener最终会被初始化为ParentContextCloserApplicationListener，FileEncodingApplicationListener，AnsiOutputApplicationListener，ConfigFileApplicationListener，DelegatingApplicationListener，LiquibaseServiceLocatorApplicationListener，ClasspathLoggingApplicationListener，LoggingApplicationListener这几个类的对象组成的list。

下图画出了加载的ApplicationListener，并说明了他们的作用。至于他们何时会被触发，等事件出现时，我们再说明。

<iframe id="iframe_0.5443945319860284" src="data:text/html;charset=utf8,%3Cstyle%3Ebody%7Bmargin:0;padding:0%7D%3C/style%3E%3Cimg%20id=%22img%22%20src=%22http://zhaox.github.io/assets/images/SpringBootApplicationListener.png?_=5551828%22%20style=%22border:none;max-width:1052px%22%3E%3Cscript%3Ewindow.onload%20=%20function%20()%20%7Bvar%20img%20=%20document.getElementById(‘img‘);%20window.parent.postMessage(%7BiframeId:‘iframe_0.5443945319860284‘,width:img.width,height:img.height%7D,%20‘http://www.cnblogs.com‘);%7D%3C/script%3E" frameborder="0" scrolling="no" width="320" height="240"></iframe>

最后是mainApplicationClass

以下代码摘自：org.springframework.boot.SpringApplicationprivate Class<?> mainApplicationClass;private void initialize(Object[] sources) {    ...    // 为成员变量mainApplicationClass赋值    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;}

在deduceMainApplicationClass方法中，通过获取当前调用栈，找到入口方法main所在的类，并将其复制给SpringApplication对象的成员变量mainApplicationClass。在我们的例子中mainApplicationClass即是我们自己编写的Application类。

SpringApplication对象的run方法

经过上面的初始化过程，我们已经有了一个SpringApplication对象，根据SpringApplication类的静态run方法一节中的分析，接下来会调用SpringApplication对象的run方法。我们接下来就分析这个对象的run方法。

以下代码摘自：org.springframework.boot.SpringApplicationpublic ConfigurableApplicationContext run(String... args) {    StopWatch stopWatch = new StopWatch();    stopWatch.start();    ConfigurableApplicationContext context = null;    configureHeadlessProperty();    SpringApplicationRunListeners listeners = getRunListeners(args);    listeners.started();    try {        ApplicationArguments applicationArguments = new DefaultApplicationArguments(                args);        context = createAndRefreshContext(listeners, applicationArguments);        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, ex);        throw new IllegalStateException(ex);    }}

  • 可变个数参数args即是我们整个应用程序的入口main方法的参数，在我们的例子中，参数个数为零。

  • StopWatch是来自org.springframework.util的工具类，可以用来方便的记录程序的运行时间。

SpringApplication对象的run方法创建并刷新ApplicationContext，算是开始进入正题了。下面按照执行顺序，介绍该方法所做的工作。

headless模式

以下代码摘自：org.springframework.boot.SpringApplicationprivate static final String SYSTEM_PROPERTY_JAVA_AWT_HEADLESS = "java.awt.headless";private boolean headless = true;public ConfigurableApplicationContext run(String... args) {    ...    //设置headless模式        configureHeadlessProperty();    ...}private void configureHeadlessProperty() {    System.setProperty(SYSTEM_PROPERTY_JAVA_AWT_HEADLESS, System.getProperty(            SYSTEM_PROPERTY_JAVA_AWT_HEADLESS, Boolean.toString(this.headless)));}

实际上是就是设置系统属性java.awt.headless，在我们的例子中该属性会被设置为true，因为我们开发的是服务器程序，一般运行在没有显示器和键盘的环境。关于java中的headless模式，更多信息可以参考这里。

SpringApplicationRunListeners

以下代码摘自：org.springframework.boot.SpringApplicationpublic ConfigurableApplicationContext run(String... args) {    ...    SpringApplicationRunListeners listeners = getRunListeners(args);    listeners.started();    /**         * 创建并刷新ApplicationContext         * context = createAndRefreshContext(listeners, applicationArguments);         **/    listeners.finished(context, null);    ...}private SpringApplicationRunListeners getRunListeners(String[] args) {    Class<?>[] types = new Class<?>[] { SpringApplication.class, String[].class };    return new SpringApplicationRunListeners(logger, getSpringFactoriesInstances(            SpringApplicationRunListener.class, types, this, args));}

run方法中，加载了一系列SpringApplicationRunListener对象，在创建和更新ApplicationContext方法前后分别调用了listeners对象的started方法和finished方法, 并在创建和刷新ApplicationContext时，将listeners作为参数传递到了createAndRefreshContext方法中，以便在创建和刷新ApplicationContext的不同阶段，调用listeners的相应方法以执行操作。所以，所谓的SpringApplicationRunListeners实际上就是在SpringApplication对象的run方法执行的不同阶段，去执行一些操作，并且这些操作是可配置的。

同时，可以看到，加载SpringApplicationRunListener时，使用的是跟加载ApplicationContextInitializer和ApplicationListener时一样的方法。那么加载了什么，就可以从spring.factories文件中看到了：

以下内容摘自spring-boot-1.3.3.RELEASE.jar中的资源文件META-INF/spring.factories# Run Listenersorg.springframework.boot.SpringApplicationRunListener=org.springframework.boot.context.event.EventPublishingRunListener

可以看到，在我们的例子中加载的是org.springframework.boot.context.event.EventPublishingRunListener。我们看一看这个SpringApplicationRunListener究竟做了点什么工作了？

以下代码摘自：org.springframework.boot.context.event.EventPublishingRunListenerpublic EventPublishingRunListener(SpringApplication application, String[] args) {    this.application = application;    this.args = args;    this.multicaster = new SimpleApplicationEventMulticaster();    for (ApplicationListener<?> listener : application.getListeners()) {        this.multicaster.addApplicationListener(listener);    }}@Overridepublic void started() {    publishEvent(new ApplicationStartedEvent(this.application, this.args));}@Overridepublic void environmentPrepared(ConfigurableEnvironment environment) {    publishEvent(new ApplicationEnvironmentPreparedEvent(this.application, this.args,            environment));}@Overridepublic void contextPrepared(ConfigurableApplicationContext context) {    registerApplicationEventMulticaster(context);}@Overridepublic void contextLoaded(ConfigurableApplicationContext context) {    for (ApplicationListener<?> listener : this.application.getListeners()) {        if (listener instanceof ApplicationContextAware) {            ((ApplicationContextAware) listener).setApplicationContext(context);        }        context.addApplicationListener(listener);    }    publishEvent(new ApplicationPreparedEvent(this.application, this.args, context));}@Overridepublic void finished(ConfigurableApplicationContext context, Throwable exception) {    publishEvent(getFinishedEvent(context, exception));}

EventPublishingRunListener在对象初始化时，将SpringApplication对象的成员变量listeners全都保存下来，然后在自己的public方法被调用时，发布相应的事件，或执行相应的操作。可以说这个RunListener是在SpringApplication对象的run方法执行到不同的阶段时，发布相应的event给SpringApplication对象的成员变量listeners中记录的事件监听器。

下图画出了SpringApplicationRunListeners相关的类结构，虽然我们的例子中只有一个SpringApplicationRunListener，但在这样的设计下，想要扩展是非常容易的！

<iframe id="iframe_0.09906402624392485" src="data:text/html;charset=utf8,%3Cstyle%3Ebody%7Bmargin:0;padding:0%7D%3C/style%3E%3Cimg%20id=%22img%22%20src=%22http://zhaox.github.io/assets/images/SpringBootSpringApplicationRunListener.png?_=5551828%22%20style=%22border:none;max-width:1052px%22%3E%3Cscript%3Ewindow.onload%20=%20function%20()%20%7Bvar%20img%20=%20document.getElementById(‘img‘);%20window.parent.postMessage(%7BiframeId:‘iframe_0.09906402624392485‘,width:img.width,height:img.height%7D,%20‘http://www.cnblogs.com‘);%7D%3C/script%3E" frameborder="0" scrolling="no" width="320" height="240"></iframe>

接下来，我们看一下在调用listeners的started方法。在我们的例子中，也就是发布了ApplicationStartedEvent时，我们已经加载的事件监听器都做了什么操作。至于其它事件的发布，我们按照代码执行的顺序在后面的章节在介绍。

  • ParentContextCloserApplicationListener不监听ApplicationStartedEvent，没有操作；
  • FileEncodingApplicationListener不监听ApplicationStartedEvent，没有操作；
  • AnsiOutputApplicationListener不监听ApplicationStartedEvent，没有操作；
  • ConfigFileApplicationListener不监听ApplicationStartedEvent，没有操作；
  • DelegatingApplicationListener不监听ApplicationStartedEvent，没有操作；
  • LiquibaseServiceLocatorApplicationListener监听ApplicationStartedEvent，会检查classpath中是否有liquibase.servicelocator.ServiceLocator并做相应操作；
以下代码摘自：org.springframework.boot.liquibase.LiquibaseServiceLocatorApplicationListener@Overridepublic void onApplicationEvent(ApplicationStartedEvent event) {    if (ClassUtils.isPresent("liquibase.servicelocator.ServiceLocator", null)) {        new LiquibasePresent().replaceServiceLocator();    }}

我们的例子中，classpath中不存在liquibase，所以不执行任何操作。

  • ClasspathLoggingApplicationListener监听ApplicationStartedEvent，会打印classpath到debug日志；
@Overridepublic void onApplicationEvent(ApplicationEvent event) {    if (event instanceof ApplicationStartedEvent) {        if (this.logger.isDebugEnabled()) {            this.logger.debug("Application started with classpath: " + getClasspath());    }    ...}private String getClasspath() {    ClassLoader classLoader = Thread.currentThread().getContextClassLoader();    if (classLoader instanceof URLClassLoader) {        return Arrays.toString(((URLClassLoader) classLoader).getURLs());    }    return "unknown";}

因为是debug级别的日志，而SpringBoot的默认日志级别是info级，所以我们在控制台不会看到classpath的输出。

  • LoggingApplicationListener监听ApplicationStartedEvent，会根据classpath中的类情况创建相应的日志系统对象，并执行一些初始化之前的操作；
@Overridepublic void onApplicationEvent(ApplicationEvent event) {    if (event instanceof ApplicationStartedEvent) {        onApplicationStartedEvent((ApplicationStartedEvent) event);    }    ...}private void onApplicationStartedEvent(ApplicationStartedEvent event) {    this.loggingSystem = LoggingSystem            .get(event.getSpringApplication().getClassLoader());    this.loggingSystem.beforeInitialize();}

我们的例子中，创建的是org.springframework.boot.logging.logback.LogbackLoggingSystem类的对象，Logback是SpringBoot默认采用的日志系统。下图画出了SpringBoot中的日志系统体系：

<iframe id="iframe_0.9601398515445843" src="data:text/html;charset=utf8,%3Cstyle%3Ebody%7Bmargin:0;padding:0%7D%3C/style%3E%3Cimg%20id=%22img%22%20src=%22http://zhaox.github.io/assets/images/SpringBootLoggingSystem.png?_=5551828%22%20style=%22border:none;max-width:1052px%22%3E%3Cscript%3Ewindow.onload%20=%20function%20()%20%7Bvar%20img%20=%20document.getElementById(‘img‘);%20window.parent.postMessage(%7BiframeId:‘iframe_0.9601398515445843‘,width:img.width,height:img.height%7D,%20‘http://www.cnblogs.com‘);%7D%3C/script%3E" frameborder="0" scrolling="no" width="320" height="240"></iframe>

好了，ApplicationStartedEvent事件的处理这样就结束了。以后在介绍事件处理的时候，我们只介绍监听该事件的监听器的操作，而不监听的，就不再说明了。

创建并刷新ApplicationContext

以下代码摘自：org.springframework.boot.SpringApplicationpublic ConfigurableApplicationContext run(String... args) {    ...    try {        ApplicationArguments applicationArguments = new DefaultApplicationArguments(                args);        context = createAndRefreshContext(listeners, applicationArguments);        afterRefresh(context, applicationArguments);        ...    }    catch (Throwable ex) {        handleRunFailure(context, listeners, ex);        throw new IllegalStateException(ex);    }}

首先是创建一个DefaultApplicationArguments对象，之后调用createAndRefreshContext方法创建并刷新一个ApplicationContext，最后调用afterRefresh方法在刷新之后做一些操作。

先来看看DefaultApplicationArguments吧：

以下代码摘自：org.springframework.boot.DefaultApplicationArgumentsDefaultApplicationArguments(String[] args) {    Assert.notNull(args, "Args must not be null");    this.source = new Source(args);    this.args = args;}private static class Source extends SimpleCommandLinePropertySource {    Source(String[] args) {        super(args);    }    ...}以下代码摘自：org.springframework.core.env.SimpleCommandLinePropertySourcepublic SimpleCommandLinePropertySource(String... args) {    super(new SimpleCommandLineArgsParser().parse(args));}

可以看到是把main函数的args参数当做一个PropertySource来解析。我们的例子中，args的长度为0，所以这里创建的DefaultApplicationArguments也没有实际的内容。

创建并配置ApplicationConext的Environment

以下代码摘自：org.springframework.boot.SpringApplicationprivate ConfigurableEnvironment environment;private boolean webEnvironment;private ConfigurableApplicationContext createAndRefreshContext(        SpringApplicationRunListeners listeners,        ApplicationArguments applicationArguments) {    ConfigurableApplicationContext context;    // 创建并配置Environment    ConfigurableEnvironment environment = getOrCreateEnvironment();    configureEnvironment(environment, applicationArguments.getSourceArgs());    listeners.environmentPrepared(environment);    if (isWebEnvironment(environment) && !this.webEnvironment) {        environment = convertToStandardEnvironment(environment);    }    ...    return context;}private ConfigurableEnvironment getOrCreateEnvironment() {    if (this.environment != null) {        return this.environment;    }    if (this.webEnvironment) {        return new StandardServletEnvironment();    }    return new StandardEnvironment();}

Spring Application的Environment代表着程序运行的环境，主要包含了两种信息，一种是profiles，用来描述哪些bean definitions是可用的；一种是properties，用来描述系统的配置，其来源可能是配置文件、JVM属性文件、操作系统环境变量等等。

首先要调用getOrCreateEnvironment方法获取一个Environment对象。在我们的例子中，执行到此处时，environment成员变量为null，而webEnvironment成员变量的值为true，所以会创建一个StandardServletEnvironment对象并返回。

之后是调用configureEnvironment方法来配置上一步获取的Environment对象，代码如下：

以下代码摘自：org.springframework.boot.SpringApplicationprivate Map<String, Object> defaultProperties;private boolean addCommandLineProperties = true;private Set<String> additionalProfiles = new HashSet<String>();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()]));}

configureEnvironment方法先是调用configurePropertySources来配置properties，然后调用configureProfiles来配置profiles。

configurePropertySources首先查看SpringApplication对象的成员变量defaultProperties，如果该变量非null且内容非空，则将其加入到Environment的PropertySource列表的最后。然后查看SpringApplication对象的成员变量addCommandLineProperties和main函数的参数args，如果设置了addCommandLineProperties=true，且args个数大于0，那么就构造一个由main函数的参数组成的PropertySource放到Environment的PropertySource列表的最前面(这就能保证，我们通过main函数的参数来做的配置是最优先的，可以覆盖其他配置）。在我们的例子中，由于没有配置defaultProperties且main函数的参数args个数为0，所以这个函数什么也不做。

configureProfiles首先会读取Properties中key为spring.profiles.active的配置项，配置到Environment，然后再将SpringApplication对象的成员变量additionalProfiles加入到Environment的active profiles配置中。在我们的例子中，配置文件里没有spring.profiles.active的配置项，而SpringApplication对象的成员变量additionalProfiles也是一个空的集合，所以这个函数没有配置任何active profile。

到现在，Environment就算是配置完成了。接下来调用SpringApplicationRunListeners类的对象listeners发布ApplicationEnvironmentPreparedEvent事件：

以下代码摘自：org.springframework.boot.context.event.EventPublishingRunListener@Overridepublic void environmentPrepared(ConfigurableEnvironment environment) {    publishEvent(new ApplicationEnvironmentPreparedEvent(this.application, this.args,            environment));}

好，现在来看一看我们加载的ApplicationListener对象都有哪些响应了这个事件，做了什么操作：

  • FileEncodingApplicationListener响应该事件，检查file.encoding配置是否与spring.mandatory_file_encoding一致：
以下代码摘自：org.springframework.boot.context.FileEncodingApplicationListener@Overridepublic void onApplicationEvent(ApplicationEnvironmentPreparedEvent event) {    RelaxedPropertyResolver resolver = new RelaxedPropertyResolver(            event.getEnvironment(), "spring.");    if (resolver.containsProperty("mandatoryFileEncoding")) {        String encoding = System.getProperty("file.encoding");        String desired = resolver.getProperty("mandatoryFileEncoding");        if (encoding != null && !desired.equalsIgnoreCase(encoding)) {            logger.error("System property ‘file.encoding‘ is currently ‘" + encoding                    + "‘. It should be ‘" + desired                    + "‘ (as defined in ‘spring.mandatoryFileEncoding‘).");            logger.error("Environment variable LANG is ‘" + System.getenv("LANG")                    + "‘. You could use a locale setting that matches encoding=‘"                    + desired + "‘.");            logger.error("Environment variable LC_ALL is ‘" + System.getenv("LC_ALL")                    + "‘. You could use a locale setting that matches encoding=‘"                    + desired + "‘.");            throw new IllegalStateException(                    "The Java Virtual Machine has not been configured to use the "                            + "desired default character encoding (" + desired                            + ").");        }    }}

在我们的例子中，因为没有spring.mandatory_file_encoding的配置，所以这个响应方法什么都不做。

  • AnsiOutputApplicationListener响应该事件，根据spring.output.ansi.enabled和spring.output.ansi.console-available对AnsiOutput类做相应配置:
以下代码摘自：org.springframework.boot.context.config.AnsiOutputApplicationListener@Overridepublic void onApplicationEvent(ApplicationEnvironmentPreparedEvent event) {    RelaxedPropertyResolver resolver = new RelaxedPropertyResolver(            event.getEnvironment(), "spring.output.ansi.");    if (resolver.containsProperty("enabled")) {        String enabled = resolver.getProperty("enabled");        AnsiOutput.setEnabled(Enum.valueOf(Enabled.class, enabled.toUpperCase()));    }    if (resolver.containsProperty("console-available")) {        AnsiOutput.setConsoleAvailable(                resolver.getProperty("console-available", Boolean.class));    }}

我们的例子中，这两项配置都是空的，所以这个响应方法什么都不做。

  • ConfigFileApplicationListener加载该事件，从一些约定的位置加载一些配置文件，而且这些位置是可配置的。
以下代码摘自：org.springframework.boot.context.config.ConfigFileApplicationListener@Overridepublic void onApplicationEvent(ApplicationEvent event) {    if (event instanceof ApplicationEnvironmentPreparedEvent) {        onApplicationEnvironmentPreparedEvent(                (ApplicationEnvironmentPreparedEvent) event);    }    if (event instanceof ApplicationPreparedEvent) {        onApplicationPreparedEvent(event);    }}private void onApplicationEnvironmentPreparedEvent(        ApplicationEnvironmentPreparedEvent event) {    List<EnvironmentPostProcessor> postProcessors = loadPostProcessors();    postProcessors.add(this);    AnnotationAwareOrderComparator.sort(postProcessors);    for (EnvironmentPostProcessor postProcessor : postProcessors) {        postProcessor.postProcessEnvironment(event.getEnvironment(),                event.getSpringApplication());    }}List<EnvironmentPostProcessor> loadPostProcessors() {    return SpringFactoriesLoader.loadFactories(EnvironmentPostProcessor.class,            getClass().getClassLoader());}以下内容摘自spring-boot-1.3.3.RELEASE.jar中的资源文件META-INF/spring.factories# Environment Post Processorsorg.springframework.boot.env.EnvironmentPostProcessor=org.springframework.boot.cloud.CloudFoundryVcapEnvironmentPostProcessor,org.springframework.boot.env.SpringApplicationJsonEnvironmentPostProcessor

可以看到，ConfigFileApplicationListener从META-INF/spring.factories文件中读取EnvironmentPostProcessor配置，加载相应的EnvironmentPostProcessor类的对象，并调用其postProcessEnvironment方法。在我们的例子中，会加载CloudFoundryVcapEnvironmentPostProcessor和SpringApplicationJsonEnvironmentPostProcessor并执行，由于我们的例子中没有CloudFoundry和Json的配置，所以这个响应，不会加载任何的配置文件到Environment中来。

  • DelegatingApplicationListener响应该事件，将配置文件中key为context.listener.classes的配置项，加载在成员变量multicaster中：
以下内容摘自：org.springframework.boot.context.config.DelegatingApplicationListenerprivate static final String PROPERTY_NAME = "context.listener.classes";private SimpleApplicationEventMulticaster multicaster;@Overridepublic void onApplicationEvent(ApplicationEvent event) {    if (event instanceof ApplicationEnvironmentPreparedEvent) {        List<ApplicationListener<ApplicationEvent>> delegates = getListeners(                ((ApplicationEnvironmentPreparedEvent) event).getEnvironment());        if (delegates.isEmpty()) {            return;        }        this.multicaster = new SimpleApplicationEventMulticaster();        for (ApplicationListener<ApplicationEvent> listener : delegates) {            this.multicaster.addApplicationListener(listener);        }    }    if (this.multicaster != null) {        this.multicaster.multicastEvent(event);    }}@SuppressWarnings("unchecked")private List<ApplicationListener<ApplicationEvent>> getListeners(        ConfigurableEnvironment env) {    String classNames = env.getProperty(PROPERTY_NAME);    List<ApplicationListener<ApplicationEvent>> listeners = new ArrayList<ApplicationListener<ApplicationEvent>>();    if (StringUtils.hasLength(classNames)) {        for (String className : StringUtils.commaDelimitedListToSet(classNames)) {            try {                Class<?> clazz = ClassUtils.forName(className,                        ClassUtils.getDefaultClassLoader());                Assert.isAssignable(ApplicationListener.class, clazz, "class ["                        + className + "] must implement ApplicationListener");                listeners.add((ApplicationListener<ApplicationEvent>) BeanUtils                        .instantiateClass(clazz));            }            catch (Exception ex) {                throw new ApplicationContextException(                        "Failed to load context listener class [" + className + "]",                        ex);            }        }    }    AnnotationAwareOrderComparator.sort(listeners);    return listeners;}

我们的例子中，因为没有key为context.listener.classes的Property，所以不会加载任何listener到该监听器中。

  • LoggingApplicationListener响应该事件，并对在ApplicationStarted时加载的LoggingSystem做一些初始化工作：
以下代码摘自：org.springframework.boot.logging.LoggingApplicationListener@Overridepublic void onApplicationEvent(ApplicationEvent event) {    if (event instanceof ApplicationStartedEvent) {        onApplicationStartedEvent((ApplicationStartedEvent) event);    }    else if (event instanceof ApplicationEnvironmentPreparedEvent) {        onApplicationEnvironmentPreparedEvent(                (ApplicationEnvironmentPreparedEvent) event);    }    else if (event instanceof ApplicationPreparedEvent) {        onApplicationPreparedEvent((ApplicationPreparedEvent) event);    }    else if (event instanceof ContextClosedEvent && ((ContextClosedEvent) event)            .getApplicationContext().getParent() == null) {        onContextClosedEvent();    }}private void onApplicationEnvironmentPreparedEvent(        ApplicationEnvironmentPreparedEvent event) {    if (this.loggingSystem == null) {        this.loggingSystem = LoggingSystem                .get(event.getSpringApplication().getClassLoader());    }    initialize(event.getEnvironment(), event.getSpringApplication().getClassLoader());}protected void initialize(ConfigurableEnvironment environment,        ClassLoader classLoader) {    LogFile logFile = LogFile.get(environment);    setSystemProperties(environment, logFile);    initializeEarlyLoggingLevel(environment);    initializeSystem(environment, this.loggingSystem, logFile);    initializeFinalLoggingLevels(environment, this.loggingSystem);    registerShutdownHookIfNecessary(environment, this.loggingSystem);}

在我们的例子中，是对加载的LogbackLoggingSystem做一些初始化工作。关于日志系统更详细的讨论，值得再写一篇文章，就不在这里展开讨论了。

打印banner

以下代码摘自：org.springframework.boot.SpringApplicationprivate Banner banner;private Banner.Mode bannerMode = Banner.Mode.CONSOLE;public static final String BANNER_LOCATION_PROPERTY = "banner.location";public static final String BANNER_LOCATION_PROPERTY_VALUE = http://www.mamicode.com/"banner.txt";private static final Banner DEFAULT_BANNER = new SpringBootBanner();private ConfigurableApplicationContext createAndRefreshContext(        SpringApplicationRunListeners listeners,        ApplicationArguments applicationArguments) {    ...    if (this.bannerMode != Banner.Mode.OFF) {        printBanner(environment);    }    ...}protected void printBanner(Environment environment) {    Banner selectedBanner = selectBanner(environment);    if (this.bannerMode == Banner.Mode.LOG) {        try {            logger.info(createStringFromBanner(selectedBanner, environment));        }        catch (UnsupportedEncodingException ex) {            logger.warn("Failed to create String for banner", ex);        }    }    else {        selectedBanner.printBanner(environment, this.mainApplicationClass,                System.out);    }}private Banner selectBanner(Environment environment) {    String location = environment.getProperty(BANNER_LOCATION_PROPERTY,            BANNER_LOCATION_PROPERTY_VALUE);    ResourceLoader resourceLoader = this.resourceLoader != null ? this.resourceLoader            : new DefaultResourceLoader(getClassLoader());    Resource resource = resourceLoader.getResource(location);    if (resource.exists()) {        return new ResourceBanner(resource);    }    if (this.banner != null) {        return this.banner;    }    return DEFAULT_BANNER;}private String createStringFromBanner(Banner banner, Environment environment)        throws UnsupportedEncodingException {    ByteArrayOutputStream baos = new ByteArrayOutputStream();    banner.printBanner(environment, this.mainApplicationClass, new PrintStream(baos));    String charset = environment.getProperty("banner.charset", "UTF-8");    return baos.toString(charset);}

printBanner方法中，首先会调用selectBanner方法得到一个banner对象，然后判断bannerMode的类型，如果是Banner.Mode.LOG，那么将banner对象转换为字符串，打印一条info日志，否则的话，调用banner对象的printbanner方法，将banner打印到标准输出System.out。

在我们的例子中，bannerMode是Banner.Mode.Console，而且也不曾提供过banner.txt这样的资源文件。所以selectBanner方法中得到到便是默认的banner对象，即SpringBootBanner类的对象：

以下代码摘自：org.springframework.boot.SpringBootBannerprivate static final String[] BANNER = { "",        "  .   ____          _            __ _ _",        " /\\\\ / ___‘_ __ _ _(_)_ __  __ _ \\ \\ \\ \\",        "( ( )\\___ | ‘_ | ‘_| | ‘_ \\/ _` | \\ \\ \\ \\",        " \\\\/  ___)| |_)| | | | | || (_| |  ) ) ) )",        "  ‘  |____| .__|_| |_|_| |_\\__, | / / / /",        " =========|_|==============|___/=/_/_/_/" };private static final String SPRING_BOOT = " :: Spring Boot :: ";private static final int STRAP_LINE_SIZE = 42;@Overridepublic void printBanner(Environment environment, Class<?> sourceClass,        PrintStream printStream) {    for (String line : BANNER) {        printStream.println(line);    }    String version = SpringBootVersion.getVersion();    version = (version == null ? "" : " (v" + version + ")");    String padding = "";    while (padding.length() < STRAP_LINE_SIZE            - (version.length() + SPRING_BOOT.length())) {        padding += " ";    }    printStream.println(AnsiOutput.toString(AnsiColor.GREEN, SPRING_BOOT,            AnsiColor.DEFAULT, padding, AnsiStyle.FAINT, version));    printStream.println();}

先打印个Spring的图形，然后打印个Spring Boot的文本，再然后打印一下Spring Boot的版本。会在控制台看到如下输出：

以下内容是程序启动后在console的输出：  .   ____          _            __ _ _ /\\ / ___‘_ __ _ _(_)_ __  __ _ \ \ \ ( ( )\___ | ‘_ | ‘_| | ‘_ \/ _` | \ \ \  \\/  ___)| |_)| | | | | || (_| |  ) ) ) )  ‘  |____| .__|_| |_|_| |_\__, | / / / / =========|_|==============|___/=/_/_/_/ :: Spring Boot ::        (v1.3.3.RELEASE)

我的天。分析启动流程这么久，终于在屏幕有一行输出了，不容易。

创建ApplicationContext

private Class<? extends ConfigurableApplicationContext> applicationContextClass;public static final String DEFAULT_CONTEXT_CLASS = "org.springframework.context."        + "annotation.AnnotationConfigApplicationContext";public static final String DEFAULT_WEB_CONTEXT_CLASS = "org.springframework."        + "boot.context.embedded.AnnotationConfigEmbeddedWebApplicationContext";private ConfigurableApplicationContext createAndRefreshContext(        SpringApplicationRunListeners listeners,        ApplicationArguments applicationArguments) {    ConfigurableApplicationContext context;    ...    context = createApplicationContext();    context.setEnvironment(environment);    postProcessApplicationContext(context);    applyInitializers(context);    listeners.contextPrepared(context);    if (this.logStartupInfo) {        logStartupInfo(context.getParent() == null);        logStartupProfileInfo(context);    }    ...    return context;}protected ConfigurableApplicationContext createApplicationContext() {    Class<?> contextClass = this.applicationContextClass;    if (contextClass == null) {        try {            contextClass = Class.forName(this.webEnvironment                    ? DEFAULT_WEB_CONTEXT_CLASS : DEFAULT_CONTEXT_CLASS);        }        catch (ClassNotFoundException ex) {            throw new IllegalStateException(                    "Unable create a default ApplicationContext, "                            + "please specify an ApplicationContextClass",                    ex);        }    }    return (ConfigurableApplicationContext) BeanUtils.instantiate(contextClass);}

createAndRefreshContext中调用createApplicationContext获取创建ApplicationContext，可以看到，当检测到本次程序是一个web应用程序（成员变量webEnvironment为true）的时候，就加载类DEFAULT_WEB_CONTEXT_CLASS，否则的话加载DEFAULT_CONTEXT_CLASS。我们的例子是一个web应用程序，所以会加载DEFAULT_WEB_CONTEXT_CLASS，也就是org.springframework.boot.context.embedded.AnnotationConfigEmbeddedWebApplicationContext。我们先来看一看这个AnnotationConfigEmbeddedWebApplicationContext具体有什么功能。下图画出了它的继承体系。

<iframe id="iframe_0.6571715840106904" src="data:text/html;charset=utf8,%3Cstyle%3Ebody%7Bmargin:0;padding:0%7D%3C/style%3E%3Cimg%20id=%22img%22%20src=%22http://zhaox.github.io/assets/images/SpringBootApplicationContext.png?_=5551828%22%20style=%22border:none;max-width:1052px%22%3E%3Cscript%3Ewindow.onload%20=%20function%20()%20%7Bvar%20img%20=%20document.getElementById(‘img‘);%20window.parent.postMessage(%7BiframeId:‘iframe_0.6571715840106904‘,width:img.width,height:img.height%7D,%20‘http://www.cnblogs.com‘);%7D%3C/script%3E" frameborder="0" scrolling="no" width="320" height="240"></iframe>

可以看到我们加载的这个AnnotationConfigEmbeddedWebApplicationContext类，从名字就可以看出来，首先是一个WebApplicationContext实现了WebApplicationContext接口，然后是一个EmbeddedWebApplicationContext，这意味着它会自动创建并初始化一个EmbeddedServletContainer，同时还支持AnnotationConfig，会将使用注解标注的bean注册到ApplicationContext中。更详细的过程，后面在例子中再一一剖析。

可以看到在加载类对象AnnotationConfigEmbeddedWebApplicationContext之后，createApplicationContext方法中紧接着调用BeanUtils的instantiate方法来创建ApplicationContext对象，其代码如下：

以下代码摘自：org.springframework.beans.BeanUtilspublic static <T> T instantiate(Class<T> clazz) throws BeanInstantiationException {    Assert.notNull(clazz, "Class must not be null");    if (clazz.isInterface()) {        throw new BeanInstantiationException(clazz, "Specified class is an interface");    }    try {        return clazz.newInstance();    }    catch (InstantiationException ex) {        throw new BeanInstantiationException(clazz, "Is it an abstract class?", ex);    }    catch (IllegalAccessException ex) {        throw new BeanInstantiationException(clazz, "Is the constructor accessible?", ex);    }}

通过调用Class对象的newInstance()方法来实例化对象，这等同于直接调用类的空的构造方法，所以我们来看AnnotationConfigEmbeddedWebApplicationContext类的构造方法：

以下代码摘自：org.springframework.boot.context.embedded.AnnotationConfigEmbeddedWebApplicationContextpublic AnnotationConfigEmbeddedWebApplicationContext() {    this.reader = new AnnotatedBeanDefinitionReader(this);    this.scanner = new ClassPathBeanDefinitionScanner(this);}@Overridepublic void setEnvironment(ConfigurableEnvironment environment) {    super.setEnvironment(environment);    this.reader.setEnvironment(environment);    this.scanner.setEnvironment(environment);}

构造方法中初始化了两个成员变量，类型分别为AnnotatedBeanDefinitionReader和ClassPathBeanDefinitionScanner用以加载使用注解的bean定义。

这样ApplicationContext对象就创建出来了，在createAndRefreshContext方法中创建了ApplicationContext对象之后会紧接着调用其setEnvironment将我们之前准备好的Environment对象赋值进去。之后分别调用postProcessApplicationContext和applyInitializers做一些处理和初始化的操作。

先来看看postProcessApplicationContext：

protected void postProcessApplicationContext(ConfigurableApplicationContext context) {    if (this.webEnvironment) {        if (context instanceof ConfigurableWebApplicationContext) {            ConfigurableWebApplicationContext configurableContext = (ConfigurableWebApplicationContext) context;            if (this.beanNameGenerator != null) {                configurableContext.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());        }    }}

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。我们的例子中，这两个成员变量都为Null，所以什么都不做。

之后是applyInitializers方法：

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);    }}public Set<ApplicationContextInitializer<?>> getInitializers() {    return asUnmodifiableOrderedSet(this.initializers);}private static <E> Set<E> asUnmodifiableOrderedSet(Collection<E> elements) {    List<E> list = new ArrayList<E>();    list.addAll(elements);    Collections.sort(list, AnnotationAwareOrderComparator.INSTANCE);    return new LinkedHashSet<E>(list);}

（写到这里，发现篇幅已经不短，就到这里作为第一篇吧。下篇继续。）

Spring Boot启动流程详解（一）


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