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