slf4j简介

slf4j简介

准确的说，slf4j并不是一种具体的日志系统，而是一个用户日志系统的facade，允许用户在部署最终应用时方便的变更其日志系统。

使用方式：

在系统开发中，统一按照slf4j的API进行开发，在部署时，选择不同的日志系统包，即可自动转换到不同的日志系统上。比如：选择JDK自带的日志系统，则只需要将slf4j-api-1.5.10.jar和slf4j-jdk14-1.5.10.jar放置到classpath中即可，如果中途无法忍受JDK自带的日志系统了，想换成log4j的日志系统，仅需要用slf4j-log4j12-1.5.10.jar替换slf4j-jdk14-1.5.10.jar即可（当然也需要log4j的jar及配置文件）

使用场景：

我们开发的是类库或者嵌入式组件，可以考虑使用slf4j，因为我们并不能决定用户选择哪种日志系统（不同软件开发公司会钟情于不同的日志系统）；但是如果我们开发独立应用，面向的是最终客户，则无需考虑slf4j，因为最终客户只关心功能实现，不会在意开发公司具体使用什么日志系统的。

应用举例

我们先举个实际例子，让大家有个直观认识，建立一个简单测试类，如下：

package chb.test.slf4j;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
/**
 * @author chb
 *
 */
publicclass TestSlf4j {
 Logger log = LoggerFactory.getLogger(TestSlf4j.class);

 publicvoid testLog(){
 log.info("this is a test log");
 }
 /**
 * @param args
 */
 publicstaticvoid main(String[] args) {
 TestSlf4j slf = new TestSlf4j();
 slf.testLog();
 }
}

package chb.test.slf4j;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
/**
 * @author chb
 *
 */
public class TestSlf4j {
        Logger log = LoggerFactory.getLogger(TestSlf4j.class);
        
        public void testLog(){
                log.info("this is a test log");
        }
        /**
         * @param args
         */
        public static void main(String[] args) {
                TestSlf4j slf = new TestSlf4j();
                slf.testLog();
        }
}

1>JDK自带的log输出

首先，我们在classpath中加入slf4j-api-1.5.10.jar和slf4j-jdk14-1.5.10.jar两个包，然后运行main函数，输出信息如下：

2010-1-5 21:44:47 chb.test.slf4j.TestSlf4j testLog
信息: this is a test log

2010-1-5 21:44:47 chb.test.slf4j.TestSlf4j testLog
信息: this is a test log

2>slg4j提供的simple log

然后，我们用slf4j-simple-1.5.10.jar替换slf4j-jdk14-1.5.10.jar，选择使用slf4j提供的simple log，输出信息如下：

0 [main] INFO chb.test.slf4j.TestSlf4j - this is a test log

0 [main] INFO chb.test.slf4j.TestSlf4j - this is a test log

3>log4j日志输出

再然后，我们再用slf4j-log4j12-1.5.10.jar替换slf4j-simple-1.5.10.jar（记得classpath也需要增加log4j依赖jar包），同时增加一个log4j.properties文件，内容如下：

log4j.debug=true
log4j.rootLogger=DEBUG,stdout
log4j.appender.stdout=org.apache.log4j.ConsoleAppender
log4j.appender.stdout.Target=System.out
log4j.appender.stdout.layout=org.apache.log4j.PatternLayout
log4j.appender.stdout.layout.ConversionPattern=%d{ABSOLUTE} %5p - %m%n

log4j.debug=true
log4j.rootLogger=DEBUG,stdout
log4j.appender.stdout=org.apache.log4j.ConsoleAppender
log4j.appender.stdout.Target=System.out
log4j.appender.stdout.layout=org.apache.log4j.PatternLayout
log4j.appender.stdout.layout.ConversionPattern=%d{ABSOLUTE} %5p - %m%n

我们再稍微修改一下main函数，加载一下log4j.properties，如；

publicstaticvoid main(String[] args) {
 System.setProperty("log4j.configuration", "log4j.properties");
 TestSlf4j slf = new TestSlf4j();
 slf.testLog();
}

        public static void main(String[] args) {
                System.setProperty("log4j.configuration", "log4j.properties");
                TestSlf4j slf = new TestSlf4j();
                slf.testLog();
        }

运行main函数，输出结果如下：

log4j: Parsing for [root] with value=[DEBUG,stdout].
log4j: Level token is [DEBUG].
log4j: Category root set to DEBUG
log4j: Parsing appender named "stdout".
log4j: Parsing layout options for"stdout".
log4j: Setting property [conversionPattern] to [%d{ABSOLUTE} %5p - %m%n ].
log4j: End of parsing for"stdout".
log4j: Setting property [target] to [System.out].
log4j: Parsed "stdout" options.
log4j: Finished configuring.
22:01:40,831 INFO - this is a test log

log4j: Parsing for [root] with value=[DEBUG,stdout].
log4j: Level token is [DEBUG].
log4j: Category root set to DEBUG
log4j: Parsing appender named "stdout".
log4j: Parsing layout options for "stdout".
log4j: Setting property [conversionPattern] to [%d{ABSOLUTE} %5p - %m%n  ].
log4j: End of parsing for "stdout".
log4j: Setting property [target] to [System.out].
log4j: Parsed "stdout" options.
log4j: Finished configuring.
22:01:40,831  INFO - this is a test log

原理介绍--静态绑定

大家看到要使用哪种日志系统，只需要将对应的日志系统所需要的jar包文件（包括slf4j提供的jar包和日志系统自身依赖的jar包，例如：slf4j-log4j12-1.5.10.jar和log4j.1.2.jar）放入classpath即可，slf4j可以自动探测具体使用哪种日志系统，这种技术被称为静态绑定。

在实际使用中，我们通过LoggerFactory.getLogger()获得logger，查看LoggerFactory的源代码会发现如下两点，

• LoggerFactory通过StaticLoggerBinder.getSingleton().getLoggerFactory()获得LogFactory，然后再通过该LogFactory来获取logger的
• 但是StaticLoggerBinder类并不在slf4j-api-1.5.10.jar中，分析与具体日志系统相关的jar包，会发现每个jar包都有一个StaticLoggerBinder类的实现(如slf4j-log4j12-1.5.10.jar、slf4j-simple-1.5.10.jar、slf4j-jdk14-1.5.10.jar均有StaticLoggerBinder类实现)，这就很明白了，slf4j在启动时会动态到classpath中查找StaticLoggerBinder类，找到之后就可以生成对应日志系统的日志文件了。

这里就有一个问题了，slf4j是如何将自己的通用日志格式转成不同的日志系统的格式的呢？

我们再分析每个日志系统相关的源代码，会发现不同日志系统包都会有一个Adapter，用来在slf4j和不同日志系统之间做转换。

 

下面是LoggerFactory.java的源代码，大家可以参考一下，为了理解方便，我已经将代码顺序做了调整：

/**
 * Return a logger named corresponding to the class passed as parameter, using
 * the statically bound {@link ILoggerFactory} instance.
 *
 * @param clazz
 * the returned logger will be named after clazz
 * @return logger
 */
publicstatic Logger getLogger(Class clazz) {
 return getLogger(clazz.getName());
}


publicstatic Logger getLogger(String name) {
 ILoggerFactory iLoggerFactory = getILoggerFactory();
 return iLoggerFactory.getLogger(name);
}

/**
 * Return the {@link ILoggerFactory} instance in use.
 *
 * <p>
 * ILoggerFactory instance is bound with this class at compile time.
 *
 * @return the ILoggerFactory instance in use
 */
publicstatic ILoggerFactory getILoggerFactory() {
 if (INITIALIZATION_STATE == UNINITIALIZED) {
 INITIALIZATION_STATE = ONGOING_INITILIZATION;
 performInitialization();
 }
 switch (INITIALIZATION_STATE) {
 case SUCCESSFUL_INITILIZATION:
 return getSingleton().getLoggerFactory();
 case FAILED_INITILIZATION:
 thrownew 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;
 }
 thrownew IllegalStateException("Unreachable code");
}


privatefinalstaticvoid performInitialization() {
 bind();
 versionSanityCheck();
 singleImplementationSanityCheck();
}

privatefinalstaticvoid bind() {
 try {
 // the next line does the binding
 getSingleton();
 INITIALIZATION_STATE = SUCCESSFUL_INITILIZATION;
 emitSubstituteLoggerWarning();
 } catch (NoClassDefFoundError ncde) {
 INITIALIZATION_STATE = FAILED_INITILIZATION;
 String msg = ncde.getMessage();
 if (msg != null && msg.indexOf("org/slf4j/impl/StaticLoggerBinder") != -1) {
 Util
 .reportFailure("Failed to load class /"org.slf4j.impl.StaticLoggerBinder/".");
 Util.reportFailure("See " + NO_STATICLOGGERBINDER_URL
 + " for further details.");
 }
 throw ncde;
 } catch (Exception e) {
 INITIALIZATION_STATE = FAILED_INITILIZATION;
 // we should never get here
 Util.reportFailure("Failed to instantiate logger ["
 + getSingleton().getLoggerFactoryClassStr() + "]", e);
 }
}


privatefinalstatic StaticLoggerBinder getSingleton() {
 if (GET_SINGLETON_METHOD == GET_SINGLETON_INEXISTENT) {
 return StaticLoggerBinder.getSingleton();
 }
 if (GET_SINGLETON_METHOD == GET_SINGLETON_EXISTS) {
 return StaticLoggerBinder.getSingleton();
 }
 try {
 StaticLoggerBinder singleton = StaticLoggerBinder.getSingleton();
 GET_SINGLETON_METHOD = GET_SINGLETON_EXISTS;
 return singleton;
 } catch (NoSuchMethodError nsme) {
 GET_SINGLETON_METHOD = GET_SINGLETON_INEXISTENT;
 return StaticLoggerBinder.getSingleton();
 }
}

  /**
   * Return a logger named corresponding to the class passed as parameter, using
   * the statically bound {@link ILoggerFactory} instance.
   * 
   * @param clazz
   *          the returned logger will be named after clazz
   * @return logger
   */
  public static Logger getLogger(Class clazz) {
    return getLogger(clazz.getName());
  }
 
 
  public static Logger getLogger(String name) {
    ILoggerFactory iLoggerFactory = getILoggerFactory();
    return iLoggerFactory.getLogger(name);
  }
 
  /**
   * Return the {@link ILoggerFactory} instance in use.
   * 
   * <p>
   * 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();
    }
    switch (INITIALIZATION_STATE) {
    case SUCCESSFUL_INITILIZATION:
      return getSingleton().getLoggerFactory();
    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() {
    bind();
    versionSanityCheck();
    singleImplementationSanityCheck();
  }
 
  private final static void bind() {
    try {
      // the next line does the binding
      getSingleton();
      INITIALIZATION_STATE = SUCCESSFUL_INITILIZATION;
      emitSubstituteLoggerWarning();
    } catch (NoClassDefFoundError ncde) {
      INITIALIZATION_STATE = FAILED_INITILIZATION;
      String msg = ncde.getMessage();
      if (msg != null && msg.indexOf("org/slf4j/impl/StaticLoggerBinder") != -1) {
        Util
            .reportFailure("Failed to load class /"org.slf4j.impl.StaticLoggerBinder/".");
        Util.reportFailure("See " + NO_STATICLOGGERBINDER_URL
            + " for further details.");
      }
      throw ncde;
    } catch (Exception e) {
      INITIALIZATION_STATE = FAILED_INITILIZATION;
      // we should never get here
      Util.reportFailure("Failed to instantiate logger ["
          + getSingleton().getLoggerFactoryClassStr() + "]", e);
    }
  }
 
 
  private final static StaticLoggerBinder getSingleton() {
    if (GET_SINGLETON_METHOD == GET_SINGLETON_INEXISTENT) {
      return StaticLoggerBinder.getSingleton();
    }
    if (GET_SINGLETON_METHOD == GET_SINGLETON_EXISTS) {
      return StaticLoggerBinder.getSingleton();
    }
    try {
      StaticLoggerBinder singleton = StaticLoggerBinder.getSingleton();
      GET_SINGLETON_METHOD = GET_SINGLETON_EXISTS;
      return singleton;
    } catch (NoSuchMethodError nsme) {
      GET_SINGLETON_METHOD = GET_SINGLETON_INEXISTENT;
      return StaticLoggerBinder.getSingleton();
    }
  }