dubbo2.7.x服务暴露流程总结

这篇文章主要总结一下dubbo服务端启动的时候服务暴露过程，虽然和各种博客上已经有很多介绍服务暴露的帖子，但还是想把自己跟源码过程中遇到的问题和心得记录下来，算是个总结，并且本篇文章是基于dubbo最新的2.7.6版本，和官网介绍的2.6.5版本差别还是有点大的（重构了很多模块，代码逻辑比之前清晰多了，说实话这里要吐槽下之前版本的代码，常常看见一个上百行的代码，只有屈指可数的注释，真让人看的头大。。），本文也会提到某些有差异的地方：

2.7.5版本dubbo中加入了OneTimeExecutionApplicationContextEventListener类来专门负责dubbo的启动和停止，（老版本中是直接写在serviceBean中，并且只监听ContextRefreshedEvent事件来暴露服务，而不负责服务的关闭清理），这个类监听了ContextRefreshedEvent事件和ContextClosedEvent事件分别来处理服务的启动和停止，在监听到ContextRefreshedEvent事件之后，会有一个单例的DubboBootstrap（2.7.5版本新增）被创建出来，专门负责dubbo服务的启动，这部分代码如下：

/*
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You under the Apache License, Version 2.0
 * (the "License"); you may not use this file except in compliance with
 * the License.  You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package org.apache.dubbo.config.spring.context;

import org.apache.dubbo.config.bootstrap.DubboBootstrap;

import org.springframework.context.ApplicationListener;
import org.springframework.context.event.ApplicationContextEvent;
import org.springframework.context.event.ContextClosedEvent;
import org.springframework.context.event.ContextRefreshedEvent;
import org.springframework.core.Ordered;

/**
 * The {@link ApplicationListener} for {@link DubboBootstrap}s lifecycle when the {@link ContextRefreshedEvent}
 * and {@link ContextClosedEvent} raised
 *
 * @since 2.7.5
 */
public class DubboBootstrapApplicationListener extends OneTimeExecutionApplicationContextEventListener
        implements Ordered {

    /**
     * The bean name of {@link DubboBootstrapApplicationListener}
     *
     * @since 2.7.6
     */
    public static final String BEAN_NAME = "dubboBootstrapApplicationListener";

    private final DubboBootstrap dubboBootstrap;

    public DubboBootstrapApplicationListener() {
        this.dubboBootstrap = DubboBootstrap.getInstance();
    }

    @Override
    public void onApplicationContextEvent(ApplicationContextEvent event) {
        if (event instanceof ContextRefreshedEvent) {
            //调用到下面的dubboBootstrap.start()方法
            onContextRefreshedEvent((ContextRefreshedEvent) event);
        } else if (event instanceof ContextClosedEvent) {
            onContextClosedEvent((ContextClosedEvent) event);
        }
    }

    private void onContextRefreshedEvent(ContextRefreshedEvent event) {
        dubboBootstrap.start();
    }

    private void onContextClosedEvent(ContextClosedEvent event) {
        dubboBootstrap.stop();
    }

    @Override
    public int getOrder() {
        return LOWEST_PRECEDENCE;
    }
}