SpringBoot内置生命周期事件详解 SpringBoot源码(十)

private static final long serialVersionUID = 5516075349620653480L;

/**

 * The object on which the Event initially occurred.
 */
protected transient Object  source;
/**
 * Constructs a prototypical Event.
 *
 * @param    source    The object on which the Event initially occurred.
 * @exception  IllegalArgumentException  if source is null.
 */
public EventObject(Object source) {
    if (source == null)
        throw new IllegalArgumentException("null source");
    this.source = source;
}
/**
 * The object on which the Event initially occurred.
 *
 * @return   The object on which the Event initially occurred.
 */
public Object getSource() {
    return source;
}
/**
 * Returns a String representation of this EventObject.
 *
 * @return  A a String representation of this EventObject.
 */
public String toString() {
    return getClass().getName() + "[source=" + source + "]";
}}
可以看到EventObject类只有一个属性source，这个属性是用来记录最初事件是发生在哪个类，举个栗子，比如在SpringBoot启动过程中会发射ApplicationStartingEvent事件，而这个事件最初是在SpringApplication类中发射的，因此source就是SpringApplication对象。

public ApplicationStartingEvent(SpringApplication application, String[] args) {

	super(application, args);

}

private final ConfigurableEnvironment environment;

/**

 * Create a new {@link ApplicationEnvironmentPreparedEvent} instance.
 * @param application the current application
 * @param args the arguments the application is running with
 * @param environment the environment that was just created
 */
public ApplicationEnvironmentPreparedEvent(SpringApplication application,
		String[] args, ConfigurableEnvironment environment) {
	super(application, args);
	this.environment = environment;
}
/**
 * Return the environment.
 * @return the environment
 */
public ConfigurableEnvironment getEnvironment() {
	return this.environment;
}}
可以看到ApplicationEnvironmentPreparedEvent事件多了一个environment属性，我们不妨想一下，多了environment属性的作用是啥？ 答案就是ApplicationEnvironmentPreparedEvent事件的environment属性作用是利用事件发布订阅机制，相应监听器们可以从ApplicationEnvironmentPreparedEvent事件中取出environment变量，然后我们可以为environment属性增加属性值或读出environment变量中的值。

private final ConfigurableApplicationContext context;

/**

 * Create a new {@link ApplicationContextInitializedEvent} instance.
 * @param application the current application
 * @param args the arguments the application is running with
 * @param context the context that has been initialized
 */
public ApplicationContextInitializedEvent(SpringApplication application,
		String[] args, ConfigurableApplicationContext context) {
	super(application, args);
	this.context = context;
}
/**
 * Return the application context.
 * @return the context
 */
public ConfigurableApplicationContext getApplicationContext() {
	return this.context;
}}
可以看到ApplicationContextInitializedEvent事件多了个ConfigurableApplicationContext类型的context属性，context属性的作用同样是为了相应监听器可以拿到这个context属性执行一些逻辑，具体作用将在3.4.4详述。

private final ConfigurableApplicationContext context;

/**

 * Create a new {@link ApplicationPreparedEvent} instance.
 * @param application the current application
 * @param args the arguments the application is running with
 * @param context the ApplicationContext about to be refreshed
 */
public ApplicationPreparedEvent(SpringApplication application, String[] args,
		ConfigurableApplicationContext context) {
	super(application, args);
	this.context = context;
}
/**
 * Return the application context.
 * @return the context
 */
public ConfigurableApplicationContext getApplicationContext() {
	return this.context;
}}
同样可以看到ApplicationPreparedEvent事件多了个ConfigurableApplicationContext类型的context属性，多了context属性的作用是能让监听该事件的监听器们能拿到context属性，监听器拿到context属性一般有如下作用：

private final ConfigurableApplicationContext context;

/**

 * Create a new {@link ApplicationStartedEvent} instance.
 * @param application the current application
 * @param args the arguments the application is running with
 * @param context the context that was being created
 */
public ApplicationStartedEvent(SpringApplication application, String[] args,
		ConfigurableApplicationContext context) {
	super(application, args);
	this.context = context;
}
/**
 * Return the application context.
 * @return the context
 */
public ConfigurableApplicationContext getApplicationContext() {
	return this.context;
}}
ApplicationStartedEvent事件将在容器刷新后但ApplicationRunner和CommandLineRunner的run方法执行前触发，标志Spring容器已经刷新，此时容器已经准备完毕了。

private final ConfigurableApplicationContext context;

/**

 * Create a new {@link ApplicationReadyEvent} instance.
 * @param application the current application
 * @param args the arguments the application is running with
 * @param context the context that was being created
 */
public ApplicationReadyEvent(SpringApplication application, String[] args,
		ConfigurableApplicationContext context) {
	super(application, args);
	this.context = context;
}
/**
 * Return the application context.
 * @return the context
 */
public ConfigurableApplicationContext getApplicationContext() {
	return this.context;
}}
ApplicationReadyEvent事件在调用完ApplicationRunner和CommandLineRunner的run方法后触发，此时标志SpringApplication已经正在运行。

private final ConfigurableApplicationContext context;

private final Throwable exception;

/**

 * Create a new {@link ApplicationFailedEvent} instance.
 * @param application the current application
 * @param args the arguments the application was running with
 * @param context the context that was being created (maybe null)
 * @param exception the exception that caused the error
 */
public ApplicationFailedEvent(SpringApplication application, String[] args,
		ConfigurableApplicationContext context, Throwable exception) {
	super(application, args);
	this.context = context;
	this.exception = exception;
}
/**
 * Return the application context.
 * @return the context
 */
public ConfigurableApplicationContext getApplicationContext() {
	return this.context;
}
/**
 * Return the exception that caused the failure.
 * @return the exception
 */
public Throwable getException() {
	return this.exception;
}}
可以看到ApplicationFailedEvent事件除了多了一个context属性外，还多了一个Throwable类型的exception属性用来记录SpringBoot启动失败时的异常。