一、環境搭建

我們以log4j為例，探尋slf4j與log4j的綁定過程。

1.Java類

public class Slf4jBind {
    public static void main(String[] args) {
        Logger LOGGER = LoggerFactory.getLogger(Slf4jBind.class);
        LOGGER.info("slf4j hello world");
    }
}

2.log4j.properties文件

來自https://docs.oracle.com/cd/E29578_01/webhelp/cas_webcrawler/src/cwcg_config_log4j_file.html

log4j.rootLogger=ERROR,stdout
log4j.logger.com.endeca=INFO
# Logger for crawl metrics
log4j.logger.com.endeca.itl.web.metrics=INFO
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log4j.appender.stdout=org.apache.log4j.ConsoleAppender
log4j.appender.stdout.layout=org.apache.log4j.PatternLayout
log4j.appender.stdout.layout.ConversionPattern
 
=%p\t%d{ISO8601}\t%r\t%c\t[%t]\t%m%n

3.Maven的pom.xml中添加如下依賴

<dependency>
    <groupId>org.slf4j</groupId>
    <artifactId>slf4j-log4j12</artifactId>
    <version>1.6.4</version>
</dependency>
<dependency>
    <groupId>org.slf4j</groupId>
    <artifactId>slf4j-api</artifactId>
    <version>1.6.4</version>
</dependency>
<dependency>
    <groupId>log4j</groupId>
    <artifactId>log4j</artifactId>
    <version>1.2.16</version>
</dependency>

其中，slf4j-api定義接口的包。

slf4j-log4j12為slf4j與logj之間的橋接包。

log4j為log4j的具體實現包。

4.單步跟蹤

在main函數處打斷點，debug模式下單步跟蹤即可

二、slf4j綁定log4j流程

main函數中調用LoggerFactory.getLogger時，將依次執行下面的函數。

slf4j對LoggerFactroy的說明如下：

LoggerFactory是為各個日誌API生成Logger的幫助類。

如log4j，logback，jdk 1.4 logging，也支持NOPLogger，SimpleLogger。

LoggerFactory內部封裝了ILoggerFactory，具體的ILoggerFactory與LoggerFactory在編譯期(complile time)綁定。

public static Logger getLogger(Class clazz) {
  return getLogger(clazz.getName());
}
 
public static Logger getLogger(String name) {
  ILoggerFactory iLoggerFactory = getILoggerFactory();
  return iLoggerFactory.getLogger(name);
}
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/
 * ILoggerFactory instance is bound with this class at compile time.
 * 
 * @return the ILoggerFactory instance in use
 */
public static ILoggerFactory getILoggerFactory() {
  if (INITIALIZATION_STATE == UNINITIALIZED) {
    INITIALIZATION_STATE = ONGOING_INITILIZATION;
    performInitialization();
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  }
  switch (INITIALIZATION_STATE) {
  case SUCCESSFUL_INITILIZATION:
    return StaticLoggerBinder.getSingleton().getLoggerFactory();
  case NOP_FALLBACK_INITILIZATION:
    return NOP_FALLBACK_FACTORY;
  case FAILED_INITILIZATION:
    throw new IllegalStateException(UNSUCCESSFUL_INIT_MSG);
  case ONGOING_INITILIZATION:
    // support re-entrant behavior.
    // See also http://bugzilla.slf4j.org/show_bug.cgi?id=106
    return TEMP_FACTORY;
  }
  throw new IllegalStateException("Unreachable code");
}

private final static void performInitialization() {
  //對具體的日誌框架實現是否僅有一個進行check
  singleImplementationSanityCheck();
  //與具體的日誌框架綁定
  bind();
  if (INITIALIZATION_STATE == SUCCESSFUL_INITILIZATION) {
  //版本依賴兼容性check
    versionSanityCheck();
  }
}

1.對具體的日誌框架實現是否僅有一個進行check。

1).判斷classpath下是否有多個org/slf4j/impl/StaticLoggerBinder.class

2).如果有多個，將提示有多個SLF4J的綁定包。

private static String STATIC_LOGGER_BINDER_PATH = "org/slf4j/impl/StaticLoggerBinder.class";
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private static void singleImplementationSanityCheck() {
  try {
    ClassLoader loggerFactoryClassLoader = LoggerFactory.class
        .getClassLoader();
    Enumeration paths;
    if (loggerFactoryClassLoader == null) {
      paths = ClassLoader.getSystemResources(STATIC_LOGGER_BINDER_PATH);
    } else {
      paths = loggerFactoryClassLoader
          .getResources(STATIC_LOGGER_BINDER_PATH);
    }
    // use Set instead of list in order to deal with  bug #138
    // LinkedHashSet appropriate here because it preserves insertion order during iteration
    Set implementationSet = new LinkedHashSet();
    while (paths.hasMoreElements()) {
      URL path = (URL) paths.nextElement();
      implementationSet.add(path);
    }
    if (implementationSet.size() > 1) {
      Util.report("Class path contains multiple SLF4J bindings.");
      Iterator iterator = implementationSet.iterator();
      while(iterator.hasNext()) {
        URL path = (URL) iterator.next();
        Util.report("Found binding in [" + path + "]");
      }
      Util.report("See " + MULTIPLE_BINDINGS_URL + " for an explanation.");
    }
  } catch (IOException ioe) {
    Util.report("Error getting resources from path", ioe);
  }
}

2.綁定具體的日誌框架

根據找到的org/slf4j/impl/StaticLoggerBinder.class，執行StaticLoggerBinder.getSingleton()進行綁定。

private final static void bind() {
  try {
    // the next line does the binding
    StaticLoggerBinder.getSingleton();//綁定
    INITIALIZATION_STATE = SUCCESSFUL_INITILIZATION;
    emitSubstituteLoggerWarning();
  } catch (NoClassDefFoundError ncde) {
    String msg = ncde.getMessage();
    if (messageContainsOrgSlf4jImplStaticLoggerBinder(msg)) {
      INITIALIZATION_STATE = NOP_FALLBACK_INITILIZATION;
      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_INITILIZATION;
      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. or 2.0.x");
    }
    throw nsme;
  } catch (Exception e) {
    failedBinding(e);
    throw new IllegalStateException("Unexpected initialization failure", e);
  }
}

因為具體的日誌框架為log4j，StaticLoggerBinder返回的為Log4j的LoggerFactory。

private final ILoggerFactory loggerFactory;
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private StaticLoggerBinder() {
  loggerFactory = new Log4jLoggerFactory();
  try {
    Level level = Level.TRACE;
  } catch (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");
  }
}
  

經過以上步驟，slf4j將於具體的日誌框架log4j綁定。

如下所示，ILoggerFactory實際返回類型為Log4jLoggerFactory。

並調用Log4jLoggerFactory中的getLogger方法返回Log4jLogger用於log寫入。

slf4j日誌框架綁定機制