slf4j中橋接器運(yùn)行的原理是什么

<dependency>
 <groupId>org.slf4j</groupId>
 <artifactId>slf4j-api</artifactId>
</dependency>
<dependency>
 <groupId>org.slf4j</groupId>
 <artifactId>slf4j-log4j12</artifactId>
</dependency>

public interface SLF4JServiceProvider {

  /**
   * 返回ILoggerFactory的實(shí)現(xiàn)類，用于LoggerFactory類的綁定
   */
  ILoggerFactory getLoggerFactory();

  /**
   * 返回IMarkerFactory實(shí)例
   */
  IMarkerFactory getMarkerFactory();

  /**
   * 返回MDCAdapter實(shí)例
   */
  MDCAdapter getMDCAdapter();

  /**
   * 獲取請(qǐng)求版本
   */
  String getRequesteApiVersion();

  /**
   * 初始化，實(shí)現(xiàn)類中一般用于初始化ILoggerFactory等
   */
  void initialize();
}

public class Log4j12ServiceProvider implements SLF4JServiceProvider {

  public static String REQUESTED_API_VERSION = "1.8.99"; 

  private ILoggerFactory loggerFactory; 
  private IMarkerFactory markerFactory; 
  private MDCAdapter mdcAdapter;
  
  public Log4j12ServiceProvider() {
    try {
      @SuppressWarnings("unused")
      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");
    }
  }

  @Override
  public void initialize() {
    loggerFactory = new Log4jLoggerFactory();
    markerFactory = new BasicMarkerFactory();
    mdcAdapter = new Log4jMDCAdapter();
  }
  
  @Override
  public ILoggerFactory getLoggerFactory() {
    return loggerFactory;
  }

  @Override
  public IMarkerFactory getMarkerFactory() {
    return markerFactory;
  }

  @Override
  public MDCAdapter getMDCAdapter() {
    return mdcAdapter;
  }

  @Override
  public String getRequesteApiVersion() {
    return REQUESTED_API_VERSION;
  }
}

public class Log4jLoggerFactory implements ILoggerFactory {

  private static final String LOG4J_DELEGATION_LOOP_URL = "http://www.slf4j.org/codes.html#log4jDelegationLoop";

  // check for delegation loops
  static {
    try {
      Class.forName("org.apache.log4j.Log4jLoggerFactory");
      String part1 = "Detected both log4j-over-slf4j.jar AND bound slf4j-log4j12.jar on the class path, preempting StackOverflowError. ";
      String part2 = "See also " + LOG4J_DELEGATION_LOOP_URL + " for more details.";

      Util.report(part1);
      Util.report(part2);
      throw new IllegalStateException(part1 + part2);
    } catch (ClassNotFoundException e) {
      // this is the good case
    }
  }

  ConcurrentMap<String, Logger> loggerMap;

  public Log4jLoggerFactory() {
    loggerMap = new ConcurrentHashMap<>();
    // force log4j to initialize
    org.apache.log4j.LogManager.getRootLogger();
  }

  @Override
  public Logger getLogger(String name) {
    Logger slf4jLogger = loggerMap.get(name);
    if(slf4jLogger != null) {
      return slf4jLogger;
    } else {
      org.apache.log4j.Logger log4jLogger;
      if(name.equalsIgnoreCase(Logger.ROOT_LOGGER_NAME)) {
        log4jLogger = LogManager.getRootLogger();
      } else {
        log4jLogger = LogManager.getLogger(name);
      }

      Logger newInstance = new Log4jLoggerAdapter(log4jLogger);
      Logger oldInstance = loggerMap.putIfAbsent(name, newInstance);
      return oldInstance == null &#63; newInstance : oldInstance;
    }
  }
}

public final class Log4jLoggerAdapter extends LegacyAbstractLogger implements LocationAwareLogger, Serializable {

  final transient org.apache.log4j.Logger logger;

  Log4jLoggerAdapter(org.apache.log4j.Logger logger) {
    this.logger = logger;
    this.name = logger.getName();
    traceCapable = isTraceCapable();
  }

  @Override
  public boolean isDebugEnabled() {
    return logger.isDebugEnabled();
  }


  @Override
  public void log(Marker marker, String callerFQCN, int level, String msg, Object[] arguments, Throwable t) {
    Level log4jLevel = toLog4jLevel(level);
    NormalizedParameters np = NormalizedParameters.normalize(msg, arguments, t);
    String formattedMessage = MessageFormatter.basicArrayFormat(np.getMessage(), np.getArguments());
    logger.log(callerFQCN, log4jLevel, formattedMessage, np.getThrowable());
  }

  public void log(LoggingEvent event) {
    Level log4jLevel = toLog4jLevel(event.getLevel().toInt());
    if (!logger.isEnabledFor(log4jLevel))
      return;

    org.apache.log4j.spi.LoggingEvent log4jevent = toLog4jEvent(event, log4jLevel);
    logger.callAppenders(log4jevent);

  }
  
  // 省略其他方法
}