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log4j slf4j，slf4j + log4j原理實現及源碼分析

 2023-10-07 阅读 23 评论 0

摘要：2019獨角獸企業重金招聘Python工程師標準>>> #0 系列目錄# 2種日志接口框架，4種日志實現框架jdk-logging、log4j、logback日志介紹及原理jcl與jul、log4j1、log4j2、logback的集成原理slf4j與jul、log4j1、log4j2、logback的集成原理slf4j、jcl、jul、log4j1

2019獨角獸企業重金招聘Python工程師標準>>> hot3.png

#0 系列目錄#

  • 2種日志接口框架，4種日志實現框架
  • jdk-logging、log4j、logback日志介紹及原理
  • jcl與jul、log4j1、log4j2、logback的集成原理
  • slf4j與jul、log4j1、log4j2、logback的集成原理
  • slf4j、jcl、jul、log4j1、log4j2、logback大總結
  • slf4j + log4j原理實現及源碼分析
  • Apache Log4j
  • Log4j架構分析與實戰
  • Log4J配置文件詳解
  • Apache Commons Logging
  • Commons Logging 架構分析
  • JDK Logging
  • JDK Logging 深入分析
  • Logback
  • 從Log4j遷移到LogBack的理由
  • Logback 深入分析
  • Apache Log4j2
  • Log4j2架構分析與實戰
  • Log4j2配置文件詳解

log4j slf4j。#1 Slf4j# ##1.1 介紹## SLF4J，即簡單日志門面（Simple Logging Facade for Java）。從設計模式的角度考慮，它是用來在log和代碼層之間起到門面的作用。對用戶來說只要使用slf4j提供的接口，即可隱藏日志的具體實現。這與jdbc和相似。使用jdbc也就避免了不同的具體數據庫。使用了slf4j可以對客戶端應用解耦。因為當我們在代碼實現中引入log日志的時候，用的是接口，所以可以實時的更具情況來調換具體的日志實現類。這就是slf4j的作用。

SLF4J所提供的核心API是一些接口以及一個LoggerFactory的工廠類。SLF4J提供了統一的記錄日志的接口，只要按照其提供的方法記錄即可，最終日志的格式、記錄級別、輸出方式等通過具體日志系統的配置來實現，因此可以在應用中靈活切換日志系統。

配置SLF4J是非常簡單的一件事，只要將與你打算使用的日志系統對應的jar包加入到項目中，SLF4J就會自動選擇使用你加入的日志系統。

slf4j注解log報錯、##1.2 簡單使用##

/*** Slf4j 日志門面接口 Test* @author taomk* @version 1.0* @since 15-10-15 下午3:39*/
public class Slf4jFacadeTest {private static Logger logger = LoggerFactory.getLogger(Slf4jFacadeTest.class);public static void main(String[] args){if(logger.isDebugEnabled()){logger.debug("slf4j-log4j debug message");}if(logger.isInfoEnabled()){logger.debug("slf4j-log4j info message");}if(logger.isTraceEnabled()){logger.debug("slf4j-log4j trace message");}}
}

上述Logger接口、LoggerFactory類都是slf4j自己定義的。那么，SLF4J是怎么實現日志綁定的？

##1.3 日志綁定##

  • 在應用中，通過LoggerFactory類的靜態getLogger()獲取logger，代碼如下：
    private static Logger logger = LoggerFactory.getLogger(Slf4jFacadeTest.class);
  • LoggerFactory.getLogger(Slf4jFacadeTest.class)方法的源碼如下：LoggerFactory.java
    public static Logger getLogger(String name) {ILoggerFactory iLoggerFactory = getILoggerFactory();return iLoggerFactory.getLogger(name);}

log4j2和log4j、上述獲取Log的過程大致分成2個階段：

  • 獲取ILoggerFactory的過程 (從字面上理解就是生產Logger的工廠)；
  • 根據ILoggerFactory獲取Logger的過程；

下面來詳細說明：

  • 1 獲取ILoggerFactory的過程，又可以分成3個過程：
  • 1.1 從類路徑中尋找org/slf4j/impl/StaticLoggerBinder.class類：LoggerFactory.java
    private static String STATIC_LOGGER_BINDER_PATH = "org/slf4j/impl/StaticLoggerBinder.class";private static Set<URL> findPossibleStaticLoggerBinderPathSet() {// use Set instead of list in order to deal with bug #138// LinkedHashSet appropriate here because it preserves insertion order during iterationSet<URL> staticLoggerBinderPathSet = new LinkedHashSet<URL>();try {ClassLoader loggerFactoryClassLoader = LoggerFactory.class.getClassLoader();Enumeration<URL> paths;if (loggerFactoryClassLoader == null) {paths = ClassLoader.getSystemResources(STATIC_LOGGER_BINDER_PATH);} else {paths = loggerFactoryClassLoader.getResources(STATIC_LOGGER_BINDER_PATH);}while (paths.hasMoreElements()) {URL path = (URL) paths.nextElement();staticLoggerBinderPathSet.add(path);}} catch (IOException ioe) {Util.report("Error getting resources from path", ioe);}return staticLoggerBinderPathSet;}

如果找到多個，則輸出 Class path contains multiple SLF4J bindings，表示有多個日志實現與slf4j進行了綁定。下面看下當出現多個StaticLoggerBinder的時候的輸出日志（簡化了一些內容）：LoggerFactory.java

SLF4J: Class path contains multiple SLF4J bindings.
SLF4J: Found binding in [slf4j-log4j12-1.7.12.jar!/org/slf4j/impl/StaticLoggerBinder.class]
SLF4J: Found binding in [logback-classic-1.1.3.jar!/org/slf4j/impl/StaticLoggerBinder.class]
SLF4J: Found binding in [slf4j-jdk14-1.7.12.jar!/org/slf4j/impl/StaticLoggerBinder.class]
SLF4J: See http://www.slf4j.org/codes.html#multiple_bindings for an explanation.
SLF4J: Actual binding is of type [org.slf4j.impl.Log4jLoggerFactory]
  • 1.2 “隨機選取"一個StaticLoggerBinder.class來創建一個單例：
    private final static void bind() {try {Set<URL> staticLoggerBinderPathSet = findPossibleStaticLoggerBinderPathSet();// 打印搜索到的所有StaticLoggerBinder日志reportMultipleBindingAmbiguity(staticLoggerBinderPathSet);// the next line does the binding 隨機選取綁定StaticLoggerBinder.getSingleton();INITIALIZATION_STATE = SUCCESSFUL_INITIALIZATION;// 打印最終實際綁定StaticLoggerBinder日志reportActualBinding(staticLoggerBinderPathSet);fixSubstitutedLoggers();} catch (NoClassDefFoundError ncde) {String msg = ncde.getMessage();if (messageContainsOrgSlf4jImplStaticLoggerBinder(msg)) {INITIALIZATION_STATE = NOP_FALLBACK_INITIALIZATION;Util.report("Failed to load class \"org.slf4j.impl.StaticLoggerBinder\".");Util.report("Defaulting to no-operation (NOP) logger implementation");Util.report("See " + NO_STATICLOGGERBINDER_URL + " for further details.");} else {failedBinding(ncde);throw ncde;}} catch (java.lang.NoSuchMethodError nsme) {String msg = nsme.getMessage();if (msg != null && msg.indexOf("org.slf4j.impl.StaticLoggerBinder.getSingleton()") != -1) {INITIALIZATION_STATE = FAILED_INITIALIZATION;Util.report("slf4j-api 1.6.x (or later) is incompatible with this binding.");Util.report("Your binding is version 1.5.5 or earlier.");Util.report("Upgrade your binding to version 1.6.x.");}throw nsme;} catch (Exception e) {failedBinding(e);throw new IllegalStateException("Unexpected initialization failure", e);}}
  • 1.3 根據上述創建的StaticLoggerBinder單例，返回一個ILoggerFactory實例：LoggerFactory.java
    public static ILoggerFactory getILoggerFactory() {if (INITIALIZATION_STATE == UNINITIALIZED) {INITIALIZATION_STATE = ONGOING_INITIALIZATION;performInitialization();}switch (INITIALIZATION_STATE) {case SUCCESSFUL_INITIALIZATION:// 返回綁定return StaticLoggerBinder.getSingleton().getLoggerFactory();case NOP_FALLBACK_INITIALIZATION:return NOP_FALLBACK_FACTORY;case FAILED_INITIALIZATION:throw new IllegalStateException(UNSUCCESSFUL_INIT_MSG);case ONGOING_INITIALIZATION:// support re-entrant behavior.// See also http://bugzilla.slf4j.org/show_bug.cgi?id=106return TEMP_FACTORY;}throw new IllegalStateException("Unreachable code");}

slf4j-log4j12，所以slf4j與其他實際的日志框架的集成jar包中，都會含有這樣的一個org/slf4j/impl/StaticLoggerBinder.class類文件，并且提供一個ILoggerFactory的實現。

  • 2 根據ILoggerFactory獲取Logger的過程：這就要看具體的ILoggerFactory類型了，下面與Log4j集成來詳細說明。

#2 Log4j介紹# Apache的一個開放源代碼項目，通過使用Log4j，我們可以控制日志信息輸送的目的地是控制臺、文件、GUI組件、甚至是套接口服務器、NT的事件記錄器、UNIX Syslog守護進程等；用戶也可以控制每一條日志的輸出格式；通過定義每一條日志信息的級別，用戶能夠更加細致地控制日志的生成過程。這些可以通過一個配置文件來靈活地進行配置，而不需要修改程序代碼。具體詳細介紹，參見Log4j架構分析與實戰。

#3 Slf4j與Log4j集成# ##3.1 Maven依賴##

<!-- slf4j -->
<dependency><groupId>org.slf4j</groupId><artifactId>slf4j-api</artifactId><version>1.7.12</version>
</dependency><!-- slf4j-log4j -->
<dependency><groupId>org.slf4j</groupId><artifactId>slf4j-log4j12</artifactId><version>1.7.12</version>
</dependency><!-- log4j -->
<dependency><groupId>log4j</groupId><artifactId>log4j</artifactId><version>1.2.17</version>
</dependency>

##3.2 使用案例##

  • 第一步：編寫log4j.properties配置文件,放到類路徑下
log4j.rootLogger = debug, console
log4j.appender.console = org.apache.log4j.ConsoleAppender
log4j.appender.console.layout = org.apache.log4j.PatternLayout
log4j.appender.console.layout.ConversionPattern = %-d{yyyy-MM-dd HH:mm:ss} %m%n

配置文件的詳細內容參見Log4J配置文件詳解。

  • 第二步：代碼中如下使用
private static Logger logger=LoggerFactory.getLogger(Log4jSlf4JTest.class);
public static void main(String[] args){if(logger.isDebugEnabled()){logger.debug("slf4j-log4j debug message");}if(logger.isInfoEnabled()){logger.info("slf4j-log4j info message");}if(logger.isTraceEnabled()){logger.trace("slf4j-log4j trace message");}
}
  • 補充說明：
  • 1 配置文件同樣可以隨意放置，如log4j1原生方式加載配置文件的方式log4j1原生開發
  • 2 注意兩者方式的不同：
slf4j:  Logger logger=LoggerFactory.getLogger(Log4jSlf4JTest.class);
log4j:  Logger logger=Logger.getLogger(Log4jTest.class);

slf4j的Logger是slf4j定義的接口，而log4j的Logger是類。LoggerFactory是slf4j自己的類。

##3.3 原理分析## 先來看下slf4j-log4j12包中的內容：

輸入圖片說明

  • 的確是有org/slf4j/impl/StaticLoggerBinder.class類。
  • 該StaticLoggerBinder返回的ILoggerFactory類型將會是Log4jLoggerFactory。
  • Log4jLoggerAdapter就是實現了slf4j定義的Logger接口。

來看下具體過程：

  • 1 獲取對應的ILoggerFactory：從上面的slf4j的原理中我們知道：ILoggerFactory是由StaticLoggerBinder來創建出來的，所以可以簡單分成2個過程：
  • 1.1 第一個過程：slf4j尋找綁定類StaticLoggerBinder：使用ClassLoader來加載“org/slf4j/impl/StaticLoggerBinder.class"這樣的類的url，然后就找到了slf4j-log4j12包中的StaticLoggerBinder。
  • 1.2 第二個過程：創建出StaticLoggerBinder實例，并創建出ILoggerFactory，源碼如下：
StaticLoggerBinder.getSingleton().getLoggerFactory();

以slf4j-log4j12中的StaticLoggerBinder為例，創建出的ILoggerFactory為Log4jLoggerFactory。

  • 2 根據ILoggerFactory獲取Logger的過程，來看下Log4jLoggerFactory是如何返回一個slf4j定義的Logger接口的實例的，源碼如下：
org.apache.log4j.Logger log4jLogger;
if (name.equalsIgnoreCase(Logger.ROOT_LOGGER_NAME))log4jLogger = LogManager.getRootLogger();
elselog4jLogger = LogManager.getLogger(name);
Logger newInstance = new Log4jLoggerAdapter(log4jLogger);
  • 2.1 我們可以看到是通過log4j1的原生方式，即使用log4j1的LogManager來獲取，引發log4j1的加載配置文件，然后初始化，最后返回一個org.apache.log4j.Logger log4jLogger，參見log4j1原生開發。
  • 2.2 將上述的org.apache.log4j.Logger log4jLogger封裝成Log4jLoggerAdapter，而Log4jLoggerAdapter是實現了slf4j的接口，所以我們使用的slf4j的Logger接口實例（這里即Log4jLoggerAdapter）都會委托給內部的org.apache.log4j.Logger實例。

#4 Slf4j與Log4j源碼分析# ##4.1 Slf4j初始化##

  • 初始化時機：下面這行代碼，會在當前類類加載時或者顯示得執行調用LoggerFactory.getLogger()方法時，觸發Slf4j初始化，并綁定具體日志：Slf4jFacadeTest.java
    private static Logger logger = LoggerFactory.getLogger(Slf4jFacadeTest.class);
  • 初始化步驟：
    1. 源碼1：LoggerFactory.java
    public static Logger getLogger(String name) {// 1. 初始化LoggerFactory，綁定具體日志，獲得具體日志的LoggerFactory。ILoggerFactory iLoggerFactory = getILoggerFactory();// 2. 根據具體日志的LoggerFactory，觸發具體日志的初始化并獲得具體日志的Logger對象；return iLoggerFactory.getLogger(name);}
  1. 源碼2：LoggerFactory.java
    public static ILoggerFactory getILoggerFactory() {// 1. 是否已經初始化了，否則進行初始化if (INITIALIZATION_STATE == UNINITIALIZED) {INITIALIZATION_STATE = ONGOING_INITIALIZATION;performInitialization();}switch (INITIALIZATION_STATE) {case SUCCESSFUL_INITIALIZATION:// 2. 成功初始化，則直接獲得具體日志的LoggerFactoryreturn StaticLoggerBinder.getSingleton().getLoggerFactory();case NOP_FALLBACK_INITIALIZATION:return NOP_FALLBACK_FACTORY;case FAILED_INITIALIZATION:throw new IllegalStateException(UNSUCCESSFUL_INIT_MSG);case ONGOING_INITIALIZATION:// support re-entrant behavior.// See also http://bugzilla.slf4j.org/show_bug.cgi?id=106return TEMP_FACTORY;}throw new IllegalStateException("Unreachable code");}
  1. 源碼3：LoggerFactory.java
    private final static void performInitialization() {// 1. 綁定具體的日志實現bind();if (INITIALIZATION_STATE == SUCCESSFUL_INITIALIZATION) {versionSanityCheck();}}
  1. 源碼4：LoggerFactory.java
    private final static void bind() {try {// 1. 掃描查找“org/slf4j/impl/StaticLoggerBinder.class”Set<URL> staticLoggerBinderPathSet = findPossibleStaticLoggerBinderPathSet();// 2. 打印找到多個“org/slf4j/impl/StaticLoggerBinder.class”的日志reportMultipleBindingAmbiguity(staticLoggerBinderPathSet);// the next line does the binding 隨機選擇綁定，類加載器隨機選擇。StaticLoggerBinder.getSingleton();INITIALIZATION_STATE = SUCCESSFUL_INITIALIZATION;// 3. 打印綁定具體org/slf4j/impl/StaticLoggerBinder.class的日志reportActualBinding(staticLoggerBinderPathSet);fixSubstitutedLoggers();} catch (NoClassDefFoundError ncde) {String msg = ncde.getMessage();if (messageContainsOrgSlf4jImplStaticLoggerBinder(msg)) {INITIALIZATION_STATE = NOP_FALLBACK_INITIALIZATION;Util.report("Failed to load class \"org.slf4j.impl.StaticLoggerBinder\".");Util.report("Defaulting to no-operation (NOP) logger implementation");Util.report("See " + NO_STATICLOGGERBINDER_URL + " for further details.");} else {failedBinding(ncde);throw ncde;}} catch (java.lang.NoSuchMethodError nsme) {String msg = nsme.getMessage();if (msg != null && msg.indexOf("org.slf4j.impl.StaticLoggerBinder.getSingleton()") != -1) {INITIALIZATION_STATE = FAILED_INITIALIZATION;Util.report("slf4j-api 1.6.x (or later) is incompatible with this binding.");Util.report("Your binding is version 1.5.5 or earlier.");Util.report("Upgrade your binding to version 1.6.x.");}throw nsme;} catch (Exception e) {failedBinding(e);throw new IllegalStateException("Unexpected initialization failure", e);}}
  1. 源碼5：LoggerFactory.java
    private static Set<URL> findPossibleStaticLoggerBinderPathSet() {// use Set instead of list in order to deal with bug #138// LinkedHashSet appropriate here because it preserves insertion order during iterationSet<URL> staticLoggerBinderPathSet = new LinkedHashSet<URL>();try {ClassLoader loggerFactoryClassLoader = LoggerFactory.class.getClassLoader();Enumeration<URL> paths;if (loggerFactoryClassLoader == null) {// 1. 掃包查找“org/slf4j/impl/StaticLoggerBinder.class”paths = ClassLoader.getSystemResources(STATIC_LOGGER_BINDER_PATH);} else {paths = loggerFactoryClassLoader.getResources(STATIC_LOGGER_BINDER_PATH);}while (paths.hasMoreElements()) {URL path = (URL) paths.nextElement();staticLoggerBinderPathSet.add(path);}} catch (IOException ioe) {Util.report("Error getting resources from path", ioe);}return staticLoggerBinderPathSet;}
  1. 源碼6：slf4j-log4j-1.6.2.jar中StaticLoggerBinder.java
    private static final StaticLoggerBinder SINGLETON = new StaticLoggerBinder();public static final StaticLoggerBinder getSingleton() {// 1. 返回StaticLoggerBinder實例return SINGLETON;}private StaticLoggerBinder() {// 2. StaticLoggerBinder初始化loggerFactory = new Log4jLoggerFactory();try {Level level = Level.TRACE;}cache (NoSuchFieldError nsfe) {Util.report("This version of SLF4J requires log4j version 1.2.12 or later. See also http://www.slf4j.org/codes.html#log4j_version");}}

以上就是Slf4j初始化過程的源代碼，其初始化過程就是綁定具體日志實現；

##4.2 Log4j初始化## 這里只在源碼層級做分析，不想看源碼，可直接參考具體詳細流程，請參見Log4j初始化分析。

  • 初始化時機：前一小節Slf4j已提到，在iLoggerFactory.getLogger(name)時觸發Log4j初始化。iLoggerFactory具體類型為：Log4jLoggerFactory。

  • 初始化步驟：

  1. 源碼1：Log4jLoggerFactory.java
  public Logger getLogger(String name) {Logger slf4jLogger = null;// protect against concurrent access of loggerMapsynchronized (this) {slf4jLogger = (Logger) loggerMap.get(name);if (slf4jLogger == null) {// 1. 獲取Logej具體的Logger對象。org.apache.log4j.Logger log4jLogger;if(name.equalsIgnoreCase(Logger.ROOT_LOGGER_NAME)) {log4jLogger = LogManager.getRootLogger();} else {log4jLogger = LogManager.getLogger(name);}slf4jLogger = new Log4jLoggerAdapter(log4jLogger);loggerMap.put(name, slf4jLogger);}}return slf4jLogger;}
  1. 源碼2：LogManager.java靜態代碼塊初始化Log4j
    static {// By default we use a DefaultRepositorySelector which always returns 'h'.// 1. 初始化Logger倉庫，并添加一個RootLogger實例，默認日志級別為DEBUG。Hierarchy h = new Hierarchy(new RootLogger((Level) Level.DEBUG));repositorySelector = new DefaultRepositorySelector(h);/** Search for the properties file log4j.properties in the CLASSPATH.  */String override =OptionConverter.getSystemProperty(DEFAULT_INIT_OVERRIDE_KEY, null);// 2. 檢查系統屬性log4j.defaultInitOverride，如果該屬性被設置為false，則執行初始化；否則（只要不是false，無論是什么值，甚至沒有值，都是否則），跳過初始化。// if there is no default init override, then get the resource// specified by the user or the default config file.if(override == null || "false".equalsIgnoreCase(override)) {String configurationOptionStr = OptionConverter.getSystemProperty(DEFAULT_CONFIGURATION_KEY, null);String configuratorClassName = OptionConverter.getSystemProperty(CONFIGURATOR_CLASS_KEY, null);URL url = null;// 3. 把系統屬性log4j.configuration的值賦給變量resource。如果該系統變量沒有被定義，則把resource賦值為"log4j.properties"。注意：在apache的log4j文檔中建議使用定義log4j.configuration系統屬性的方法來設置默認的初始化文件是一個好方法。// if the user has not specified the log4j.configuration// property, we search first for the file "log4j.xml" and then// "log4j.properties"if(configurationOptionStr == null) {	url = Loader.getResource(DEFAULT_XML_CONFIGURATION_FILE);if(url == null) {url = Loader.getResource(DEFAULT_CONFIGURATION_FILE);}} else {try {// 4. 試圖把resource變量轉化成為一個URL對象url。如果一般的轉化方法行不通，就調用org.apache.log4j.helpers.Loader.getResource(resource, Logger.class)方法來完成轉化。url = new URL(configurationOptionStr);} catch (MalformedURLException ex) {// so, resource is not a URL:// attempt to get the resource from the class pathurl = Loader.getResource(configurationOptionStr); }	}// If we have a non-null url, then delegate the rest of the// configuration to the OptionConverter.selectAndConfigure// method.if(url != null) {LogLog.debug("Using URL ["+url+"] for automatic log4j configuration.");try {// 5. 如果url以".xml"結尾，則調用方法DOMConfigurator.configure(url)來完成初始化；否則，則調用方法PropertyConfigurator.configure(url)來完成初始化。如果url指定的資源不能被獲得，則跳出初始化過程。OptionConverter.selectAndConfigure(url, configuratorClassName,LogManager.getLoggerRepository());} catch (NoClassDefFoundError e) {LogLog.warn("Error during default initialization", e);}} else {LogLog.debug("Could not find resource: ["+configurationOptionStr+"].");}} else {LogLog.debug("Default initialization of overridden by " + DEFAULT_INIT_OVERRIDE_KEY + "property."); }  }
  1. 源碼3：OptionConverter.java
    public static void selectAndConfigure(URL url, String clazz, LoggerRepository hierarchy) {Configurator configurator = null;String filename = url.getFile();// 1. 如果url以".xml"結尾，則調用方法DOMConfigurator.configure(url)來完成初始化；否則，則調用方法PropertyConfigurator.configure(url)來完成初始化。如果url指定的資源不能被獲得，則跳出初始化過程。if(clazz == null && filename != null && filename.endsWith(".xml")) {clazz = "org.apache.log4j.xml.DOMConfigurator";}if(clazz != null) {LogLog.debug("Preferred configurator class: " + clazz);configurator = (Configurator) instantiateByClassName(clazz,Configurator.class,null);if(configurator == null) {LogLog.error("Could not instantiate configurator ["+clazz+"].");return;}} else {configurator = new PropertyConfigurator();}configurator.doConfigure(url, hierarchy);}

##4.3 Log4j輸出日志## 這里只在源碼層級做分析，不想看源碼，可直接參考具體詳細流程，請參見Log4j輸出日志。

  • 預處理： 當調用Log4j的方法（如：debug(String, Throwable)、info(String, Throwable)）輸出日志時，首先對日志信息進行預處理。
  1. 源碼1：Category.java（Logger繼承自Category.java）
    public void info(Object message) {// 1. 根據全局日志等級threshold進行判斷，如果日志等級低于threshold，不輸出日志。if(repository.isDisabled(Level.INFO_INT))return;// 2. 根據當前logger配置的日志等級level進行判斷，如果日志等級低于當前logger配置的日志等級，不輸出日志。if(Level.INFO.isGreaterOrEqual(this.getEffectiveLevel()))// 3. 將日志信息封裝成LoggingEvent對象。forcedLog(FQCN, Level.INFO, message, null);}
  1. 源碼2：Category.java
    protected void forcedLog(String fqcn, Priority level, Object message, Throwable t) {// 1. 將LoggingEvent對象分發給所有的Appender。callAppenders(new LoggingEvent(fqcn, this, level, message, t));}public void callAppenders(LoggingEvent event) {int writes = 0;for(Category c = this; c != null; c=c.parent) {// Protected against simultaneous call to addAppender, removeAppender,...synchronized(c) {if(c.aai != null) {// 2. 將LoggingEvent對象分發給所有的Appender。writes += c.aai.appendLoopOnAppenders(event);}if(!c.additive) {break;}}}if(writes == 0) {repository.emitNoAppenderWarning(this);}}public int appendLoopOnAppenders(LoggingEvent event) {int size = 0;Appender appender;if(appenderList != null) {size = appenderList.size();for(int i = 0; i < size; i++) {appender = (Appender) appenderList.elementAt(i);appender.doAppend(event);}}    return size;}
  • 輸出日志前還有兩道工序需要處理：Filter處理和日志信息格式化。
  1. 源碼1：
    public synchronized void doAppend(LoggingEvent event) {if (closed) {LogLog.error("Attempted to append to closed appender named ["+ name + "].");return;}if (!isAsSevereAsThreshold(event.getLevel())) {return;}// Filter處理Filter f = this.headFilter;FILTER_LOOP: while (f != null) {// 1. 有三種返回值 DENY、ACCEPT、NEUTRAL，DENY表示丟棄當前日志信息，ACCEPT表示輸出當前日志信息，NEUTRAL表示繼續下一個Filter。Filter只能在XML配置文件中使用，Properties文件中不支持。switch (f.decide(event)) {case Filter.DENY:return;case Filter.ACCEPT:break FILTER_LOOP;case Filter.NEUTRAL:f = f.getNext();}}this.append(event);}public void append(LoggingEvent event) {// Reminder: the nesting of calls is:////    doAppend()//      - check threshold//      - filter//      - append();//        - checkEntryConditions();//        - subAppend();if(!checkEntryConditions()) {return;}subAppend(event);}protected void subAppend(LoggingEvent event) {// 2. 日志信息格式化：對日志進行格式化處理。this.qw.write(this.layout.format(event));if (layout.ignoresThrowable()) {String[] s = event.getThrowableStrRep();if (s != null) {int len = s.length;for (int i = 0; i < len; i++) {this.qw.write(s[i]);this.qw.write(Layout.LINE_SEP);}}}if (shouldFlush(event)) {// 3. 將日志信息輸出至目的地（文件、數據庫或網格）this.qw.flush();}}
  1. 若想了解Log4j的日志異步輸出實現，請參見Log4j性能優化。

轉載于:https://my.oschina.net/xianggao/blog/519199

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