原始碼分析

本文原始碼基於 Springboot 2.1.0

package org.springframework.boot.builder;

import java.lang.ref.WeakReference;

import org.springframework.beans.BeansException;
import org.springframework.boot.builder.ParentContextApplicationContextInitializer.
	ParentContextAvailableEvent;
import org.
 
springframework.context.ApplicationContext;
import org.springframework.context.ApplicationContextAware;
import org.springframework.context.ApplicationListener;
import org.springframework.context.ConfigurableApplicationContext;
import org.springframework.context.event.ContextClosedEvent;
import org.springframework.
 
core.Ordered;
import org.springframework.util.ObjectUtils;

/**
 * Listener that closes the application context if its parent is closed. It listens for
 * refresh events and grabs the current context from there, and then listens for closed
 * events and propagates it down the hierarchy.
 *
 * 在一個應用上下文的雙親應用上下文關閉時關閉該應用上下文。這個監聽器監聽應用上下文重新整理事件並從中取出
 * 應用上下文，然後監聽關閉事件在應用上下文的層級結構中往下層傳播該事件。
 * 
 * @author Dave Syer
 * @author Eric Bottard
 */
 

public class ParentContextCloserApplicationListener
		implements ApplicationListener<ParentContextAvailableEvent>,
		ApplicationContextAware, Ordered {

	private int order = Ordered.LOWEST_PRECEDENCE - 10;

	private ApplicationContext context;

	@Override
	public int getOrder() {
		return this.order;
	}

	@Override
	public void setApplicationContext(ApplicationContext context) throws BeansException {
		this.context = context;
	}

	// 當一個應用上下文中發出ParentContextAvailableEvent 事件時，表明該應用上下文被設定了雙親
	// 應用上下文並且自己已經可用(refreshed),現在在該應用上下文的雙親上下文中登記一個事件監聽器，
	// 用於監聽雙親上下文的關閉事件，從而關閉該子應用上下文。
	@Override
	public void onApplicationEvent(ParentContextAvailableEvent event) {
		maybeInstallListenerInParent(event.getApplicationContext());
	}

	private void maybeInstallListenerInParent(ConfigurableApplicationContext child) {
		if (child == this.context
				&& child.getParent() instanceof ConfigurableApplicationContext) {
			ConfigurableApplicationContext parent = (ConfigurableApplicationContext) child
					.getParent();
			parent.addApplicationListener(createContextCloserListener(child));
		}
	}

	/**
	 * Subclasses may override to create their own subclass of ContextCloserListener. This
	 * still enforces the use of a weak reference.
	 * @param child the child context
	 * @return the {@link ContextCloserListener} to use
	 */
	protected ContextCloserListener createContextCloserListener(
			ConfigurableApplicationContext child) {
		return new ContextCloserListener(child);
	}

	/**
	 * ApplicationListener to close the context.
	 * 
	 * 定義一個應用事件監聽器，關注事件ContextClosedEvent，當該事件發生時，關閉指定的孩子應用上下文。
	 * 對於指定的孩子應用上下文使用WeakReference弱引用保持。
	 * 事件ContextClosedEvent發生時，如果弱引用中保持的孩子應用上下文還在(意思就是弱引用尚未釋放),
	 * 並且孩子上下文處於活躍狀態，則關閉它。
	 * 注意 ： 在關閉之前會再次檢查當前上下文和孩子上下文之間的父子關係，僅在父子關係成立的情況下才真正
	 * 做相應的關閉動作。
	 *  
	 * 該靜態內部類僅被ParentContextCloserApplicationListener用於向雙親應用上下文登記一個關閉孩子
	 * 引用上下文的ApplicationListener
	 */
	protected static class ContextCloserListener
			implements ApplicationListener<ContextClosedEvent> {

		private WeakReference<ConfigurableApplicationContext> childContext;

		public ContextCloserListener(ConfigurableApplicationContext childContext) {
			this.childContext = new WeakReference<>(childContext);
		}

		@Override
		public void onApplicationEvent(ContextClosedEvent event) {
			ConfigurableApplicationContext context = this.childContext.get();
			if ((context != null)
					&& (event.getApplicationContext() == context.getParent())
					&& context.isActive()) {
				context.close();
			}
		}

		@Override
		public boolean equals(Object obj) {
			if (this == obj) {
				return true;
			}
			if (obj == null) {
				return false;
			}
			if (obj instanceof ContextCloserListener) {
				ContextCloserListener other = (ContextCloserListener) obj;
				return ObjectUtils.nullSafeEquals(this.childContext.get(),
						other.childContext.get());
			}
			return super.equals(obj);
		}

		@Override
		public int hashCode() {
			return ObjectUtils.nullSafeHashCode(this.childContext.get());
		}

	}

}

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