slf自己主动绑定实现类过程推断

依照绑定实现类的方式是基于约定原则：推断分下面几个步骤

1.LoggerFactory扫描实现类路径有几个实现类，即在org/slf4j/impl/下有几个StaticLoggerBinder.class

2.假设有多个实现类，向开发者报告多个实现类的路径

3.假设有多个实现类，向开发者报告真正绑定的是哪一个实现类

4.假设没有实现类，怎么办？

详细代码实现

//要扫描的文件路径 

  private static String STATIC_LOGGER_BINDER_PATH = "org/slf4j/impl/StaticLoggerBinder.class";

//扫描全部的StaticLoggerBinder.class路径放入到set集合
  private static Set findPossibleStaticLoggerBinderPathSet() {
    // use Set instead of list in order to deal with  bug #138
    // LinkedHashSet appropriate here because it preserves insertion order during iteration
    Set staticLoggerBinderPathSet = new LinkedHashSet();
    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);
      }
      while (paths.hasMoreElements()) {
        URL path = (URL) paths.nextElement();
        staticLoggerBinderPathSet.add(path);
      }
    } catch (IOException ioe) {
      Util.report("Error getting resources from path", ioe);
    }
    return staticLoggerBinderPathSet;
  }

//报告全部的实现类路径

  private static void reportMultipleBindingAmbiguity(Set staticLoggerBinderPathSet) {
    if (isAmbiguousStaticLoggerBinderPathSet(staticLoggerBinderPathSet)) {
      Util.report("Class path contains multiple SLF4J bindings.");
      Iterator iterator = staticLoggerBinderPathSet.iterator();
      while (iterator.hasNext()) {
        URL path = (URL) iterator.next();
        Util.report("Found binding in [" + path + "]");
      }
      Util.report("See " + MULTIPLE_BINDINGS_URL + " for an explanation.");
    }
  }

//报告真正用到的实现类名称

private static void reportActualBinding(Set staticLoggerBinderPathSet) {
    if (isAmbiguousStaticLoggerBinderPathSet(staticLoggerBinderPathSet)) {
      Util.report("Actual binding is of type ["+StaticLoggerBinder.getSingleton().getLoggerFactoryClassStr()+"]");
    }
  }

//去拿到实现类的实例，由于StaticLoggerBinder类不一定存在，所以要捕获NoClassDefFoundError异常

private final static void bind() {
    try {
      Set staticLoggerBinderPathSet = findPossibleStaticLoggerBinderPathSet();
      reportMultipleBindingAmbiguity(staticLoggerBinderPathSet);
      // the next line does the binding
      StaticLoggerBinder.getSingleton();
      INITIALIZATION_STATE = SUCCESSFUL_INITIALIZATION;
      reportActualBinding(staticLoggerBinderPathSet);
      emitSubstituteLoggerWarning();
    } 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);
    }
  }

到此，就真正拿到实现类的实例了