再談springboot啓動爲什麼不打印日誌?

首先看下日誌的啓動流程，說一下前提，日誌啓動是基於spring事件機制，包括dubbo服務導出的啓動也是基於spring事件機制
我們看看spring事件監聽器的定義

/*
 * Copyright 2002-2016 the original author or authors.
 *
 * Licensed 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
 *
 *      https://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.springframework.context;

import java.util.EventListener;

/**
 * Interface to be implemented by application event listeners.
 * Based on the standard {@code java.util.EventListener} interface
 * for the Observer design pattern.
 *
 * <p>**As of Spring 3.0, an ApplicationListener can generically declare the event type
 * that it is interested in. When registered with a Spring ApplicationContext, events
 * will be filtered accordingly, with the listener getting invoked for matching event
 * objects only.**
 *
 * @author Rod Johnson
 * @author Juergen Hoeller
 * @param <E> the specific ApplicationEvent subclass to listen to
 * @see org.springframework.context.event.ApplicationEventMulticaster
 */
@FunctionalInterface
public interface ApplicationListener<E extends ApplicationEvent> extends EventListener {

	/**
	 * Handle an application event.
	 * @param event the event to respond to
	 */
	void onApplicationEvent(E event);
}

圖片截圖不全，我們看看源碼中的註釋，就可知道了。我們聲明一個事件類型，spring啓動完成後，會進行分發事件，我們匹配自己定義的事件類型。接下來看看啓動流程，當然是關於日誌的加載流程

日誌系統類有個靜態代碼塊，當類加載的時候，靜態代碼塊也會被加載，靜態代碼塊如下

題外話，判斷一個對象是否是同一個對象？1.由同一個classloader加載，並且是一個同類名。倆個classloader加載一個類，在虛擬機內部會認爲是倆個類。
綜上所述，是加載日誌組件的流程。接下來我們自定義個日誌配置文件，然後看看我們的自定義的xml文件是否生效，以及模擬控制檯不打印日誌的情況
首先演示我們的配置是生效的

接下來把依賴日誌文件更改成log4j2，pom文件以及xml文件如下

xml文件如下

<?xml version="1.0" encoding="UTF-8"?>

<Configuration status="error">
    <Appenders>
        <Console name="Console" target="SYSTEM_OUT">
            <PatternLayout pattern="%d{HH:mm:ss.SSS} [%t] %-5level %logger{36} - %msg%n"/>
        </Console>
    </Appenders>

    <Loggers>
        <Root level="info">
            <AppenderRef ref="Console"/>
        </Root>
    </Loggers>
</Configuration>

先來我們來看配置是否生效

說明配置已經生效。看看打印日誌的模式樣式是否與原來有區別

現在模擬不打印日誌的情況，log4j2基礎組件截圖發下

不打印情況下,主要是修改log4j2的配置文件，如下圖