1.spring boot日志概述

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spring boot使用Commons Logging作为内部的日志系统，并且给Java Util Logging，Log4J2以及Logback都提供了默认的配置。
如果使用了spring boot的Starters，那么默认会使用Logback用于记录日志。

2.spring boot日志默认配置

我们启动一个空的spring-boot项目看一下控制台的日志

控制台的默认配置

logging.pattern.console=%clr(%d{${LOG_DATEFORMAT_PATTERN:-yyyy-MM-dd HH:mm:ss.SSS}}){faint} %clr(${LOG_LEVEL_PATTERN:-%5p}) %clr(${PID:- }){magenta} %clr(---){faint} %clr([%15.15t]){faint} %clr(%-40.40logger{39}){cyan} %clr(:){faint} %m%n${LOG_EXCEPTION_CONVERSION_WORD:-%wEx}
其中%clr为配置不同的颜色输出，支持的颜色有以下几种：

blue
cyan
faint
green
magenta
red
yellow
输出顺序分析：

1、日期和时间--精确到毫秒，并按照时间进行简单的排序，格式为：

%clr(%d{${LOG_DATEFORMAT_PATTERN:-yyyy-MM-dd HH:mm:ss.SSS}}){faint}

2、日志级别--ERROR,WARN,INFO,DEBUG,TRACE

%clr(${LOG_LEVEL_PATTERN:-%5p})

3、进程ID号

%clr(${PID:- })

4、日志内容，用"---"分隔符分开

%clr(---){faint}

5、线程名字--括在方括号中

%clr([%15.15t]){faint}

6、日志的名字--通常对应的是类名

%clr(%-40.40logger{39}){cyan}
注意：Logback没有FATAL级别（映射到ERROR）

不同日志级别对应的颜色如下

3.spring boot日志配置

可以通过application.properties或者application.yml查看所有配置

每个配置后面都有说明，就不一一赘述了。

4.spring boot日志实现原理

点击配置属性，可以进入LoggingApplicationListener这个类，

`/**

• An {@link ApplicationListener} that configures the {@link LoggingSystem}. If the
• environment contains a {@code logging.config} property it will be used to bootstrap the
• logging system, otherwise a default configuration is used. Regardless, logging levels
• will be customized if the environment contains {@code logging.level.*} entries and
• logging groups can be defined with {@code logging.group}.
• Debug and trace logging for Spring, Tomcat, Jetty and Hibernate will be enabled when
• the environment contains {@code debug} or {@code trace} properties that aren't set to
• {@code "false"} (i.e. if you start your application using
• {@literal java -jar myapp.jar [--debug | --trace]}). If you prefer to ignore these
• properties you can set {@link #setParseArgs(boolean) parseArgs} to {@code false}.
• By default, log output is only written to the console. If a log file is required the
• {@code logging.path} and {@code logging.file} properties can be used.
• Some system properties may be set as side effects, and these can be useful if the
• logging configuration supports placeholders (i.e. log4j or logback):
• • {@code LOG_FILE} is set to the value of path of the log file that should be written
  • (if any).
  • {@code PID} is set to the value of the current process ID if it can be determined.
• @author Dave Syer
• @author Phillip Webb
• @author Andy Wilkinson
• @author Madhura Bhave
• @since 2.0.0
• @see LoggingSystem#get(ClassLoader)
  */`

它实现了GenericApplicationListener接口，它默认定义了日志组DEFAULT_GROUP_LOGGERS和日志级别LOG_LEVEL_LOGGERS

private static final Map> DEFAULT_GROUP_LOGGERS;
    static {
        MultiValueMap loggers = new LinkedMultiValueMap<>();
        loggers.add("web", "org.springframework.core.codec");
        loggers.add("web", "org.springframework.http");
        loggers.add("web", "org.springframework.web");
        loggers.add("web", "org.springframework.boot.actuate.endpoint.web");
        loggers.add("web",
                "org.springframework.boot.web.servlet.ServletContextInitializerBeans");
        loggers.add("sql", "org.springframework.jdbc.core");
        loggers.add("sql", "org.hibernate.SQL");
        DEFAULT_GROUP_LOGGERS = Collections.unmodifiableMap(loggers);
    }

    private static final Map> LOG_LEVEL_LOGGERS;
    static {
        MultiValueMap loggers = new LinkedMultiValueMap<>();
        loggers.add(LogLevel.DEBUG, "sql");
        loggers.add(LogLevel.DEBUG, "web");
        loggers.add(LogLevel.DEBUG, "org.springframework.boot");
        loggers.add(LogLevel.TRACE, "org.springframework");
        loggers.add(LogLevel.TRACE, "org.apache.tomcat");
        loggers.add(LogLevel.TRACE, "org.apache.catalina");
        loggers.add(LogLevel.TRACE, "org.eclipse.jetty");
        loggers.add(LogLevel.TRACE, "org.hibernate.tool.hbm2ddl");
        LOG_LEVEL_LOGGERS = Collections.unmodifiableMap(loggers);
    }

你也可以自定义logging.level和logging.group,它们都是map结构。LoggingApplicationListener重写了onApplicationEvent方法，实现日志的打印

         @Override
    public void onApplicationEvent(ApplicationEvent event) {
        if (event instanceof ApplicationStartingEvent) {
            onApplicationStartingEvent((ApplicationStartingEvent) event); //1
        }
        else if (event instanceof ApplicationEnvironmentPreparedEvent) {
            onApplicationEnvironmentPreparedEvent(
                    (ApplicationEnvironmentPreparedEvent) event); //2 
        }
        else if (event instanceof ApplicationPreparedEvent) {
            onApplicationPreparedEvent((ApplicationPreparedEvent) event); //3
        }
        else if (event instanceof ContextClosedEvent && ((ContextClosedEvent) event)
                .getApplicationContext().getParent() == null) {
            onContextClosedEvent();  //4
        }
        else if (event instanceof ApplicationFailedEvent) {
            onApplicationFailedEvent();  //5
        }
    }

第一步：根据classloader里加载的依赖决定使用哪个日志系统？

主要实现有JavaLoggingSystem，Log4J2LoggingSystem，LogbackLoggingSystem

    private void onApplicationStartingEvent(ApplicationStartingEvent event) {
        this.loggingSystem = LoggingSystem
                .get(event.getSpringApplication().getClassLoader());
        this.loggingSystem.beforeInitialize();
    }

第二步：通过classpath，enviroment等获取参数初始化日志系统

/**
     * Initialize the logging system according to preferences expressed through the
     * {@link Environment} and the classpath.
     * @param environment the environment
     * @param classLoader the classloader
     */
    protected void initialize(ConfigurableEnvironment environment,
            ClassLoader classLoader) {
        new LoggingSystemProperties(environment).apply();
        LogFile logFile = LogFile.get(environment);
        if (logFile != null) {
            logFile.applyToSystemProperties();
        }
        initializeEarlyLoggingLevel(environment);
        initializeSystem(environment, this.loggingSystem, logFile);
        initializeFinalLoggingLevels(environment, this.loggingSystem);
        registerShutdownHookIfNecessary(environment, this.loggingSystem);
    }

第三步：注册springBootLoggingSystem

    private void onApplicationPreparedEvent(ApplicationPreparedEvent event) {
        ConfigurableListableBeanFactory beanFactory = event.getApplicationContext()
                .getBeanFactory();
        if (!beanFactory.containsBean(LOGGING_SYSTEM_BEAN_NAME)) {
            beanFactory.registerSingleton(LOGGING_SYSTEM_BEAN_NAME, this.loggingSystem);
        }
    }

第四步和第五步：日志系统清洗

    private void onContextClosedEvent() {
        if (this.loggingSystem != null) {
            this.loggingSystem.cleanUp();
        }
    }

    private void onApplicationFailedEvent() {
        if (this.loggingSystem != null) {
            this.loggingSystem.cleanUp();
        }
    }

5.自定义配置文件

日志系统 自定义配置文件
Logback

logback-spring.xml, logback-spring.groovy, logback.xml, or logback.groovy

Log4j2

log4j2-spring.xml or log4j2.xml

JDK (Java Util Logging)

logging.properties

6.总结

spring boot日志系统封装了logback，log4j2和java log，默认情况下使用java log，一旦使用各种starts，则默认使用Log4J2，也可以通过classpath来改变，pom.xml指定

 
 org.springframework.boot 
 spring-boot-starter 
  
   
   org.springframework.boot 
   spring-boot-starter-logging 
   
  
 
 
 org.springframework.boot 
 spring-boot-starter-log4j 
 

参考资料

【1】https://docs.spring.io/spring-boot/docs/2.1.2.RELEASE/reference/htmlsingle/#boot-features-logging-format

【2】https://www.jb51.net/article/133795.htm

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