Springboot加载日志模块顺序

Springboot加载日志模块

Spring jcl

Spring jcl 是Springboot连接SLF4J的基础包，实现了对Log4j 2.x API是否存在，以及Spring框架类路径中的SLF4J 1.7api。

ServiceLoader

通过配置寻找接口的实现类，实现类的jar包的META-INF下新建一个文件夹services，并在services下新建一个文件，以接口的全限定名为文件名，内容为实现类的全限定名。

参考链接：
https://www.jianshu.com/p/7601ba434ff4
https://blog.csdn.net/shi2huang/article/details/80308531

Log4j获取ConfigurationFactory

核心代码

String ALL_TYPES = "*"; for (final ConfigurationFactory factory : getFactories()) { final String[] types = factory.getSupportedTypes(); if (types != null) { for (final String type : types) { // 只有 XmlConfigurationFactory 符合条件 if (type.equals(ALL_TYPES)) { final Configuration config = factory.getConfiguration(loggerContext, name, configLocation); if (config != null) { return config; } } } } } 

Springboot加载log4j2的流程

Springboot 加载log4j2主要分为两个阶段，第一阶段成为start， 第二阶段为initalize

start阶段

• LogAdapter利用Class.forName加载不同日志的实现，使用内部类进行调用
• log4j 通过PluginProcess对 @Plugin注释识别ConfigurationFactory，具体可参考Springboot配置log4j2爬坑

initalize阶段

初始化阶段，SpringApplication.run()方法的 SpringApplicationRunListeners

• beforeInitialize()， 判断是否以slf4j桥接。

if (isBridgeJulIntoSlf4j()) { removeJdkLoggingBridgeHandler(); SLF4JBridgeHandler.install(); } // ClassUtils.isPresent() Sping 实现的类似Class.forName功能 ClassUtils.isPresent(BRIDGE_HANDLER, getClassLoader()); 

• LoggingApplicationListener 代码配置onApplicationEvent

public void onApplicationEvent(ApplicationEvent event) { // beforeInitalize 阶段 if (event instanceof ApplicationStartingEvent) { onApplicationStartingEvent((ApplicationStartingEvent) event); } // 环境配置，即initalizeing阶段 else if (event instanceof ApplicationEnvironmentPreparedEvent) { onApplicationEnvironmentPreparedEvent( (ApplicationEnvironmentPreparedEvent) event); } // 程序运行起来后，调用日志阶段 else if (event instanceof ApplicationPreparedEvent) { onApplicationPreparedEvent((ApplicationPreparedEvent) event); } // 程序关闭 else if (event instanceof ContextClosedEvent && ((ContextClosedEvent) event) .getApplicationContext().getParent() == null) { onContextClosedEvent(); } // 失败 else if (event instanceof ApplicationFailedEvent) { onApplicationFailedEvent(); } } 

• Log4j2判断日志是否为同一实例，利用了identityHashCoder给AppClassLoader做身份哈希判重
• shutdownhook 学习(有时间在研究)

private void registerShutdownHookIfNecessary(Environment environment, LoggingSystem loggingSystem) { boolean registerShutdownHook = environment .getProperty(REGISTER_SHUTDOWN_HOOK_PROPERTY, Boolean.class, false); if (registerShutdownHook) { Runnable shutdownHandler = loggingSystem.getShutdownHandler(); if (shutdownHandler != null && shutdownHookRegistered.compareAndSet(false, true)) { registerShutdownHook(new Thread(shutdownHandler)); } } } void registerShutdownHook(Thread shutdownHook) { Runtime.getRuntime().addShutdownHook(shutdownHook); }