SpringBoot启动流程详解(搬运)

SpringApplication类的静态run方法

以下代码摘自：org.springframework.boot.SpringApplication

public 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.SpringApplication

public 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.SpringApplication

private 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.SpringApplication

private List> initializers;

private void initialize(Object[] sources) {
    ...
    // 为成员变量initializers赋值
    setInitializers((Collection) getSpringFactoriesInstances(ApplicationContextInitializer.class));
    ...
}

public void setInitializers(Collection> initializers) {
    this.initializers = new ArrayList>();
    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.SpringApplication

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

private <T> Collectionextends 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。

public static final String FACTORIES_RESOURCE_LOCATION = "META-INF/spring.factories";

public static List loadFactoryNames(Class factoryClass, ClassLoader classLoader) {
    String factoryClassName = factoryClass.getName();
    try {
        Enumeration urls = (classLoader != null ? classLoader.getResources(FACTORIES_RESOURCE_LOCATION) :
                ClassLoader.getSystemResources(FACTORIES_RESOURCE_LOCATION));
        List result = new ArrayList();
        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 Initializers
org.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.SpringApplication

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.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。

接下来是成员变量listeners

以下代码摘自：org.springframework.boot.SpringApplication

private List> listeners;

private void initialize(Object[] sources) {
    ...
    // 为成员变量listeners赋值
    setListeners((Collection) getSpringFactoriesInstances(ApplicationListener.class));
    ...
}

public void setListeners(Collection> listeners) {
    this.listeners = new ArrayList>();
    this.listeners.addAll(listeners);
}

listeners成员变量，是一个ApplicationListener

以下内容摘自spring-boot-1.3.3.RELEASE.jar中的资源文件META-INF/spring.factories

# Application Listeners
org.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。

最后是mainApplicationClass

以下代码摘自：org.springframework.boot.SpringApplication

private 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.SpringApplication

public 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.SpringApplication

private 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.SpringApplication

public 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 Listeners
org.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.EventPublishingRunListener

public EventPublishingRunListener(SpringApplication application, String[] args) {
    this.application = application;
    this.args = args;
    this.multicaster = new SimpleApplicationEventMulticaster();
    for (ApplicationListener listener : application.getListeners()) {
        this.multicaster.addApplicationListener(listener);
    }
}

@Override
public void started() {
    publishEvent(new ApplicationStartedEvent(this.application, this.args));
}

@Override
public void environmentPrepared(ConfigurableEnvironment environment) {
    publishEvent(new ApplicationEnvironmentPreparedEvent(this.application, this.args,
            environment));
}

@Override
public void contextPrepared(ConfigurableApplicationContext context) {
    registerApplicationEventMulticaster(context);
}

@Override
public 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));
}

@Override
public 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！

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

@Override
public 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；

@Override
public 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；

@Override
public 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：

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.SpringApplication

public 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.DefaultApplicationArguments

DefaultApplicationArguments(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.SimpleCommandLinePropertySource

public 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.SpringApplication

private 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.SpringApplication

private Map defaultProperties;

private boolean addCommandLineProperties = true;

private Set additionalProfiles = new HashSet();

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 profiles = new LinkedHashSet(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

@Override
public 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

@Override
public 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

@Override
public 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

@Override
public void onApplicationEvent(ApplicationEvent event) {
    if (event instanceof ApplicationEnvironmentPreparedEvent) {
        onApplicationEnvironmentPreparedEvent((ApplicationEnvironmentPreparedEvent) event);
    }
    if (event instanceof ApplicationPreparedEvent) {
        onApplicationPreparedEvent(event);
    }
}

private void onApplicationEnvironmentPreparedEvent(
        ApplicationEnvironmentPreparedEvent event) {
    List postProcessors = loadPostProcessors();
    postProcessors.add(this);
    AnnotationAwareOrderComparator.sort(postProcessors);
    for (EnvironmentPostProcessor postProcessor : postProcessors) {
        postProcessor.postProcessEnvironment(event.getEnvironment(),event.getSpringApplication());
    }
}

List 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 Processors
org.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.DelegatingApplicationListener

private static final String PROPERTY_NAME = "context.listener.classes";

private SimpleApplicationEventMulticaster multicaster;

@Override
public void onApplicationEvent(ApplicationEvent event) {
    if (event instanceof ApplicationEnvironmentPreparedEvent) {
        List> delegates = getListeners(((ApplicationEnvironmentPreparedEvent) event).getEnvironment());
        if (delegates.isEmpty()) {
            return;
        }
        this.multicaster = new SimpleApplicationEventMulticaster();
        for (ApplicationListener listener : delegates) {
            this.multicaster.addApplicationListener(listener);
        }
    }
    if (this.multicaster != null) {
        this.multicaster.multicastEvent(event);
    }
}

@SuppressWarnings("unchecked")
private List> getListeners(ConfigurableEnvironment env) {
    String classNames = env.getProperty(PROPERTY_NAME);
    List> listeners = new ArrayList>();
    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) 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

@Override
public 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.SpringApplication

private 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 = "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.SpringBootBanner

private static final String[] BANNER = { "",
        "  .   ____          _            __ _ _",
        " /\\\\ / ___'_ __ _ _(_)_ __  __ _ \\ \\ \\ \\",
        "( ( )\\___ | '_ | '_| | '_ \\/ _` | \\ \\ \\ \\",
        " \\\\/  ___)| |_)| | | | | || (_| |  ) ) ) )",
        "  '  |____| .__|_| |_|_| |_\\__, | / / / /",
        " =========|_|==============|___/=/_/_/_/" };

private static final String SPRING_BOOT = " :: Spring Boot :: ";

private static final int STRAP_LINE_SIZE = 42;

@Override
public 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 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。

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.BeanUtils

public static  T instantiate(Class 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.AnnotationConfigEmbeddedWebApplicationContext

public AnnotationConfigEmbeddedWebApplicationContext() {
    this.reader = new AnnotatedBeanDefinitionReader(this);
    this.scanner = new ClassPathBeanDefinitionScanner(this);
}

@Override
public 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> getInitializers() {
    return asUnmodifiableOrderedSet(this.initializers);
}

private static  Set asUnmodifiableOrderedSet(Collection elements) {
    List list = new ArrayList();
    list.addAll(elements);
    Collections.sort(list, AnnotationAwareOrderComparator.INSTANCE);
    return new LinkedHashSet(list);
}