Spring之事件发布系统

springboot应用，启动spring容器大致有如下几个过程：

• 容器开始启动
• 初始化环境变量
• 初始化上下文
• 加载上下文
• 完成

对应的Spring应用的启动器的监听器可以监听以上的过程，接口如下：

public interface SpringApplicationRunListener {

    /**
     * Called immediately when the run method has first started. Can be used for very
     * early initialization.
     */
    void started();

    /**
     * Called once the environment has been prepared, but before the
     * {@link ApplicationContext} has been created.
     * @param environment the environment
     */
    void environmentPrepared(ConfigurableEnvironment environment);

    /**
     * Called once the {@link ApplicationContext} has been created and prepared, but
     * before sources have been loaded.
     * @param context the application context
     */
    void contextPrepared(ConfigurableApplicationContext context);

    /**
     * Called once the application context has been loaded but before it has been
     * refreshed.
     * @param context the application context
     */
    void contextLoaded(ConfigurableApplicationContext context);

    /**
     * Called immediately before the run method finishes.
     * @param context the application context or null if a failure occurred before the
     * context was created
     * @param exception any run exception or null if run completed successfully.
     */
    void finished(ConfigurableApplicationContext context, Throwable exception);

}

对应几个关键事件

监听器接口中的每个方法代表容器启动过程一个阶段，每一个阶段可以自定义执行一系列任务。可以将SpringApplicationRunListener理解成一个命令模式（传入一个对象，至于当前这个时间点，如何调用，不关心）。每一个阶段执行的序列图如下所示，其中任意方法，为SpringApplicationRunListener接口中定义的，此阶段调用的方法。

SpringApplicationRunListeners是对多个SpringApplicationRunListener做了一次代理，聚合多个SpringApplicationRunListener,在任一个过程完成，会依次调用对应方法。

    private final List<SpringApplicationRunListener> listeners;

    SpringApplicationRunListeners(Log log,
            Collection<? extends SpringApplicationRunListener> listeners) {
        this.log = log;
        this.listeners = new ArrayList<SpringApplicationRunListener>(listeners);
    }

    public void started() {
        for (SpringApplicationRunListener listener : this.listeners) {
            listener.started();
        }
    }

    public void environmentPrepared(ConfigurableEnvironment environment) {
        for (SpringApplicationRunListener listener : this.listeners) {
            listener.environmentPrepared(environment);
        }
    }

SpringApplicationRunListener有一个特殊的实现EventPublishingRunListener，他的主要功能就是，在特定时间点出发特殊事件。

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

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

事件发布采用观察者模式，整个事件发布流程如下图。