最近的一個項目采用Spring Boot構(gòu)建，使用了log4j2記錄日志泻肯；按照之前的習(xí)慣，通過-Dlog4j.configurationFile指定log4j2.xml路徑，啟動應(yīng)用疲吸，但意外的發(fā)現(xiàn)日志配置竟然沒有生效；

初步估計是Spring Boot對日志部分進(jìn)行了某種修改前鹅，導(dǎo)致了這個問題摘悴，查看Spring Boot的jar包，發(fā)現(xiàn)spring.factories中有這么一段配置:

org.springframework.context.ApplicationListener=\
org.springframework.boot.ClearCachesApplicationListener,\
org.springframework.boot.builder.ParentContextCloserApplicationListener,\
org.springframework.boot.context.FileEncodingApplicationListener,\
org.springframework.boot.context.config.AnsiOutputApplicationListener,\
org.springframework.boot.context.config.ConfigFileApplicationListener,\
org.springframework.boot.context.config.DelegatingApplicationListener,\
org.springframework.boot.liquibase.LiquibaseServiceLocatorApplicationListener,\
org.springframework.boot.logging.ClasspathLoggingApplicationListener,\
org.springframework.boot.logging.LoggingApplicationListener

可以看到有個LoggingApplicationListener：

public class LoggingApplicationListener implements GenericApplicationListener{}

可以看到它會監(jiān)聽Spring的事件舰绘，關(guān)鍵邏輯如下：

public void onApplicationEvent(ApplicationEvent event) {
        if (event instanceof ApplicationStartedEvent) {
            onApplicationStartedEvent((ApplicationStartedEvent) 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();
        }
    }

首先看看它的** onApplicationStartedEvent**方法：

    private void onApplicationStartedEvent(ApplicationStartedEvent event) {
        this.loggingSystem = LoggingSystem
                .get(event.getSpringApplication().getClassLoader());
        this.loggingSystem.beforeInitialize();
    }
  public void beforeInitialize() {
        LoggerContext loggerContext = getLoggerContext();
        if (isAlreadyInitialized(loggerContext)) {
            return;
        }
        super.beforeInitialize();
        loggerContext.getConfiguration().addFilter(FILTER);
    }
  private LoggerContext getLoggerContext() {
        return (LoggerContext) LogManager.getContext(false);
    }

LogManager.getContext(false)實際上會調(diào)用Log4jContextFactory的getContext方法：

public LoggerContext getContext(final String fqcn, final ClassLoader loader, final Object externalContext,
                                final boolean currentContext, final URI configLocation, final String name) {
    final LoggerContext ctx = selector.getContext(fqcn, loader, currentContext, configLocation);
    if (externalContext != null && ctx.getExternalContext() == null) {
        ctx.setExternalContext(externalContext);
    }
    if (name != null) {
        ctx.setName(name);
    }
    if (ctx.getState() == LifeCycle.State.INITIALIZED) {
        if (configLocation != null || name != null) {
            ContextAnchor.THREAD_CONTEXT.set(ctx);
            final Configuration config = ConfigurationFactory.getInstance().getConfiguration(name, configLocation);
            LOGGER.debug("Starting LoggerContext[name={}] from configuration at {}", ctx.getName(), configLocation);
            ctx.start(config);
            ContextAnchor.THREAD_CONTEXT.remove();
        } else {
            ctx.start();
        }
    }
    return ctx;
}

而ConfigurationFactory就會從log4j.configurationFile獲取配置文件路徑執(zhí)行初始化蹂喻；既然已經(jīng)加載了-Dlog4j.configurationFile指定的配置文件，那為什么沒最終生效呢捂寿？繼續(xù)看下去：

private void onApplicationEnvironmentPreparedEvent(
        ApplicationEnvironmentPreparedEvent event) {
    if (this.loggingSystem == null) {
        this.loggingSystem = LoggingSystem
                .get(event.getSpringApplication().getClassLoader());
    }
    initialize(event.getEnvironment(), event.getSpringApplication().getClassLoader());
}

protected void initialize(ConfigurableEnvironment environment,
        ClassLoader classLoader) {
    new LoggingSystemProperties(environment).apply();
    LogFile logFile = LogFile.get(environment);//讀取logging.file和logging.path參數(shù)
    if (logFile != null) {
        logFile.applyToSystemProperties();//添加系統(tǒng)屬性LOG_PATH口四、LOG_FILE
    }
    initializeEarlyLoggingLevel(environment); //如果在application.yml或application.properties中定義了debug或trace，則設(shè)置日志級別為DEBUG或TRACE
       //關(guān)鍵方法秦陋，調(diào)用LoggingSystem的initialize方法下面單獨(dú)說明
    initializeSystem(environment, this.loggingSystem, logFile);
    initializeFinalLoggingLevels(environment, this.loggingSystem);
    registerShutdownHookIfNecessary(environment, this.loggingSystem);
}

Log4J2LoggingSystem的initialize方法邏輯如下：

public void initialize(LoggingInitializationContext initializationContext,
        String configLocation, LogFile logFile) {
    LoggerContext loggerContext = getLoggerContext();
    if (isAlreadyInitialized(loggerContext)) {
        return;
    }
    loggerContext.getConfiguration().removeFilter(FILTER);
    super.initialize(initializationContext, configLocation, logFile);
    markAsInitialized(loggerContext);
}

繼續(xù)看super.initialize(initializationContext, configLocation, logFile);實現(xiàn):

public void initialize(LoggingInitializationContext initializationContext,
        String configLocation, LogFile logFile) {
    if (StringUtils.hasLength(configLocation)) {//如果通過logging.config指定了log4j2配置文件蔓彩，則采用該配置文件
        initializeWithSpecificConfig(initializationContext, configLocation, logFile);
        return;
    }
       //采用默認(rèn)配置
    initializeWithConventions(initializationContext, logFile);
}

如果配置了logging.config屬性，最終會通過如下代碼重新配置log4j2:

    protected void loadConfiguration(String location, LogFile logFile) {
        Assert.notNull(location, "Location must not be null");
        try {
            LoggerContext ctx = getLoggerContext();
            URL url = ResourceUtils.getURL(location);
            ConfigurationSource source = getConfigurationSource(url);

          //關(guān)鍵方法驳概，采用新的配置文件重新配置log4j2
            ctx.start(ConfigurationFactory.getInstance().getConfiguration(source));
        }
        catch (Exception ex) {
            throw new IllegalStateException(
                    "Could not initialize Log4J2 logging from " + location, ex);
        }
    }

如果沒有指定赤嚼，會調(diào)用initializeWithConventions方法：

private void initializeWithConventions(
        LoggingInitializationContext initializationContext, LogFile logFile) {
       //根據(jù)支持的格式，在classpath下查找log4j2.yaml顺又、log4j2.yml更卒、log4j2.json、log4j2.jsn和log4j2.xml文件待榔，如果找到則調(diào)用getLoggerContext().reconfigure()重現(xiàn)初始化log4j2,防止某些屬性文件發(fā)生變化
    String config = getSelfInitializationConfig();
    if (config != null && logFile == null) {
        // self initialization has occurred, reinitialize in case of property changes
        reinitialize(initializationContext);
        return;
    }
    if (config == null) {//如果找不到上述配置文件逞壁，則在classpath下查找log4j2-spring.yaml、log4j2-spring.yml锐锣、log4j2-spring.json腌闯、log4j2-spring.jsn和log4j2-spring.xml文件
        config = getSpringInitializationConfig();
    }
    if (config != null) {//如果找到以spring結(jié)尾的配置文件，調(diào)用ctx.start重新配置log4j2
        loadConfiguration(initializationContext, config, logFile);
        return;
    }
       //如果前面的查找都失敗了雕憔，則加載默認(rèn)配置姿骏；如果定義了log日志文件路徑，則加載log4j2-file.xml斤彼，否則加載log4j2.xml
    loadDefaults(initializationContext, logFile);
}

在Spring boot jar包的org.springframework.boot.logging.log4j2包下面分瘦，有兩個配置文件log4j2.xml和log4j2-file.xml,這是默認(rèn)的log4j2配置文件蘸泻，文件內(nèi)容如下:

<?xml version="1.0" encoding="UTF-8"?>
<Configuration status="WARN">
    <Properties>
        <Property name="PID">????</Property>
        <Property name="LOG_EXCEPTION_CONVERSION_WORD">%xwEx</Property>
        <Property name="LOG_LEVEL_PATTERN">%5p</Property>
        <Property name="LOG_PATTERN">%clr{%d{yyyy-MM-dd HH:mm:ss.SSS}}{faint} %clr{${LOG_LEVEL_PATTERN}} %clr{${sys:PID}}{magenta} %clr{---}{faint} %clr{[%15.15t]}{faint} %clr{%-40.40c{1.}}{cyan} %clr{:}{faint} %m%n${sys:LOG_EXCEPTION_CONVERSION_WORD}</Property>
    </Properties>
    <Appenders>
        <Console name="Console" target="SYSTEM_OUT" follow="true">
            <PatternLayout pattern="${LOG_PATTERN}" />
        </Console>
    </Appenders>
    <Loggers>
        <Logger name="org.apache.catalina.startup.DigesterFactory" level="error" />
        <Logger name="org.apache.catalina.util.LifecycleBase" level="error" />
        <Logger name="org.apache.coyote.http11.Http11NioProtocol" level="warn" />
        <logger name="org.apache.sshd.common.util.SecurityUtils" level="warn"/>
        <Logger name="org.apache.tomcat.util.net.NioSelectorPool" level="warn" />
        <Logger name="org.crsh.plugin" level="warn" />
        <logger name="org.crsh.ssh" level="warn"/>
        <Logger name="org.eclipse.jetty.util.component.AbstractLifeCycle" level="error" />
        <Logger name="org.hibernate.validator.internal.util.Version" level="warn" />
        <logger name="org.springframework.boot.actuate.autoconfigure.CrshAutoConfiguration" level="warn"/>
        <logger name="org.springframework.boot.actuate.endpoint.jmx" level="warn"/>
        <logger name="org.thymeleaf" level="warn"/>
        <Root level="info">
            <AppenderRef ref="Console" />
        </Root>
    </Loggers>
</Configuration>

<?xml version="1.0" encoding="UTF-8"?>
<Configuration status="WARN">
    <Properties>
        <Property name="PID">????</Property>
        <Property name="LOG_EXCEPTION_CONVERSION_WORD">%xwEx</Property>
        <Property name="LOG_LEVEL_PATTERN">%5p</Property>
        <Property name="LOG_PATTERN">%d{yyyy-MM-dd HH:mm:ss.SSS} ${LOG_LEVEL_PATTERN} ${sys:PID} --- [%t] %-40.40c{1.} : %m%n${sys:LOG_EXCEPTION_CONVERSION_WORD}</Property>
    </Properties>
    <Appenders>
        <Console name="Console" target="SYSTEM_OUT" follow="true">
            <PatternLayout pattern="${LOG_PATTERN}" />
        </Console>
        <RollingFile name="File" fileName="${sys:LOG_FILE}" filePattern="logs/$${date:yyyy-MM}/app-%d{yyyy-MM-dd-HH}-%i.log.gz">
            <PatternLayout>
                <Pattern>${LOG_PATTERN}</Pattern>
            </PatternLayout>
            <Policies>
                <SizeBasedTriggeringPolicy size="10 MB" />
            </Policies>
        </RollingFile>
    </Appenders>
    <Loggers>
        <Logger name="org.apache.catalina.startup.DigesterFactory" level="error" />
        <Logger name="org.apache.catalina.util.LifecycleBase" level="error" />
        <Logger name="org.apache.coyote.http11.Http11NioProtocol" level="warn" />
        <logger name="org.apache.sshd.common.util.SecurityUtils" level="warn"/>
        <Logger name="org.apache.tomcat.util.net.NioSelectorPool" level="warn" />
        <Logger name="org.crsh.plugin" level="warn" />
        <logger name="org.crsh.ssh" level="warn"/>
        <Logger name="org.eclipse.jetty.util.component.AbstractLifeCycle" level="error" />
        <Logger name="org.hibernate.validator.internal.util.Version" level="warn" />
        <logger name="org.springframework.boot.actuate.autoconfigure.CrshAutoConfiguration" level="warn"/>
        <logger name="org.springframework.boot.actuate.endpoint.jmx" level="warn"/>
        <logger name="org.thymeleaf" level="warn"/>
        <Root level="info">
            <AppenderRef ref="Console" />
            <AppenderRef ref="File" />
        </Root>
    </Loggers>
</Configuration>