This article is based on Spring Boot version 1.3.3, using spring-boot-starter-web.
After the configuration is completed, the code is written as follows:
@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!";
}
}Although there are only a few lines of code, this is already a complete Web program. When accessing the path part of the url When it is "/", the string "Welcome!" is returned.
First of all, it is a very ordinary java program entry, a static main method that conforms to the agreement. In this main method, the static run method of SpringApplication is called, and the Application class object and the parameter args of the main method are passed in as parameters.
Then there is a RootController class that uses two Spring annotations. We do not use this class directly in the main method.
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);
}In this static method, create a SpringApplication object and call the run method of the object.
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();
}Call the initialize method in the constructor to initialize the member variables sources, webEnvironment, initializers, listeners, and mainApplicationClass of the SpringApplication object. The assignment of sources is relatively simple, it is the parameter we pass to the SpringApplication.run method. Let’s take a look at the remaining ones one by one.
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;
}You can see that webEnvironment is a boolean, and this member variable is used to indicate whether the current application is a Web application. So how to determine whether the current application is a Web application is to check whether the class contained in the WEB_ENVIRONMENT_CLASSES array exists in the classpath. If it exists, then the current program is a Web application, and vice versa.
In the example of this article, the value of webEnvironment is true.
The initializers member variable is a collection of ApplicationContextInitializer type objects. As the name suggests, ApplicationContextInitializer is an interface that can be used to initialize ApplicationContext.
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);
}As you can see, the key is to call getSpringFactoriesInstances(ApplicationContextInitializer.class) to get the list of ApplicationContextInitializer type objects.
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 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.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。
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 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。
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。
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。
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。
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 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.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！
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：
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。
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。
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。
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 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.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。
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 = "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。
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。
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);
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