springboot默认日志框架选择源码解析(推荐) springboot默认日志框架选择源码

背景：
今天新生成一个springboot项目，然而启动日志，还有mybatis的详细日志无法打印出来，自写程序中打印的日志可以输出；网上找了很多资料，都没法解决问题；于是决定跟一下源码，弄清springboot日志相关的逻辑。
环境配置：macbook； intellij idea community edition 2020.03 ； gradle 6.8.3 jdk1.8 ；
gradle引用包如下：

dependencies {compile "com.alibaba:fastjson:1.2.75"compile "mysql:mysql-connector-java" //springcompile("org.springframework.boot:spring-boot-starter")compile("org.mybatis.spring.boot:mybatis-spring-boot-starter:2.1.4")compile("org.springframework.boot:spring-boot-starter-web")compile("org.springframework.boot:spring-boot-starter-actuator") //lombokcompileOnly 'org.projectlombok:lombok'annotationProcessor 'org.projectlombok:lombok' //testtestCompile('org.springframework.boot:spring-boot-starter-test')testImplementation 'io.projectreactor:reactor-test'}

springboot 默认日志使用的是logback(引入spring-boot-starter包后，就自动引入了logback-core,logback-classic两个包，当然还有slf4j的包)，当springboot启动时，org.springframework.boot.context.logging.LoggingApplicationListener，该类211行注册的监控事件会被ApplicationStartingEvent触发；如下代码所示，会调用onApplicationStartingEvent初始化loggingSystem，而使用哪个日志组件，就要看loggingSystem初始化的值了

@Overridepublic void onApplicationEvent(ApplicationEvent event) {if (event instanceof ApplicationStartingEvent) {onApplicationStartingEvent((ApplicationStartingEvent) event); }else if (event instanceof ApplicationEnvironmentPreparedEvent) {onApplicationEnvironmentPreparedEvent((ApplicationEnvironmentPreparedEvent) event); }else if (event instanceof ApplicationPreparedEvent) {onApplicationPreparedEvent((ApplicationPreparedEvent) event); }else if (event instanceof ContextClosedEvent&& ((ContextClosedEvent) event).getApplicationContext().getParent() == null) {onContextClosedEvent(); }else if (event instanceof ApplicationFailedEvent) {onApplicationFailedEvent(); }} private void onApplicationStartingEvent(ApplicationStartingEvent event) {this.loggingSystem = LoggingSystem.get(event.getSpringApplication().getClassLoader()); this.loggingSystem.beforeInitialize(); }

如下图是org.springframework.boot.logging.LoggingSystem类里面get函数的内容：首先会从system.getProperty中获取className，新生成的项目，取到的这个值都为空，SYSTEM_PROPERTY是一个固定值，就是该类的名字；那么loggingSystem的值就是从SYSTEM_FACTORY.getLoggingSystem(classLoader); 获取到的；接下来我们得看LoggingSystemFactory.fromSpringFactories.getLoggingSystem取的值是什么了；

public static final String SYSTEM_PROPERTY = LoggingSystem.class.getName(); private static final LoggingSystemFactory SYSTEM_FACTORY = LoggingSystemFactory.fromSpringFactories(); public static LoggingSystem get(ClassLoader classLoader) {String loggingSystemClassName = System.getProperty(SYSTEM_PROPERTY); if (StringUtils.hasLength(loggingSystemClassName)) {if (NONE.equals(loggingSystemClassName)) {return new NoOpLoggingSystem(); }return get(classLoader, loggingSystemClassName); }LoggingSystem loggingSystem = SYSTEM_FACTORY.getLoggingSystem(classLoader); Assert.state(loggingSystem != null, "No suitable logging system located"); return loggingSystem; } private static LoggingSystem get(ClassLoader classLoader, String loggingSystemClassName) {try {Class systemClass = ClassUtils.forName(loggingSystemClassName, classLoader); Constructor constructor = systemClass.getDeclaredConstructor(ClassLoader.class); constructor.setAccessible(true); return (LoggingSystem) constructor.newInstance(classLoader); }catch (Exception ex) {throw new IllegalStateException(ex); }}

LoggingSystemFactory是一个接口，它的实现类在spring-boot-start有4个，其中3个是在内部内类实现，DelegatingLoggingSystemFactory（JavaLoggingSystem，Log4J2LoggingSystem，LogbackLoggingSystem，内部类实现）。上面SYSTEM_FACTORY的实现就是DelegatingLoggingSystemFactory这个类，如下代码中delegates的值为JavaLoggingSystem，Log4J2LoggingSystem，LogbackLoggingSystem；三个类具体的加载逻辑在SpringFactoriesLoader.loadFactories函数中，最终返回的loggingSystem就是前面函数返回列表中的第一个；SpringFactoriesLoader.loadFactories 才是决定springboot默认会使用哪个日志组件关键：该类是spring的核心组件类，在spring-core包中，org.springframework.core.io.support.SpringFactoriesLoader；loggingSystem的值=LogbackLoggingSystem

public interface LoggingSystemFactory { /*** Return a logging system implementation or {@code null} if no logging system is* available.* @param classLoader the class loader to use* @return a logging system*/LoggingSystem getLoggingSystem(ClassLoader classLoader); /*** Return a {@link LoggingSystemFactory} backed by {@code spring.factories}.* @return a {@link LoggingSystemFactory} instance*/static LoggingSystemFactory fromSpringFactories() {return new DelegatingLoggingSystemFactory((classLoader) -> SpringFactoriesLoader.loadFactories(LoggingSystemFactory.class, classLoader)); } }class DelegatingLoggingSystemFactory implements LoggingSystemFactory { private final Function> delegates; /*** Create a new {@link DelegatingLoggingSystemFactory} instance.* @param delegates a function that provides the delegates*/DelegatingLoggingSystemFactory(Function> delegates) {this.delegates = delegates; } @Overridepublic LoggingSystem getLoggingSystem(ClassLoader classLoader) {List delegates = (this.delegates != null) ? this.delegates.apply(classLoader) : null; if (delegates != null) {for (LoggingSystemFactory delegate : delegates) {LoggingSystem loggingSystem = delegate.getLoggingSystem(classLoader); if (loggingSystem != null) {return loggingSystem; }}}return null; } }

总结：虽然springboot会加载JavaLoggingSystem，Log4J2LoggingSystem，LogbackLoggingSystem三个日志实现类，但在选择时，还是会使用LogbackLoggingSystem作为它的日志框架
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